TABLE OF CONTENTS
LIST OF FIGURES ................................................................................................................................. 2
LIST OF TABLES .................................................................................................................................. 2
1. ABSTRACT ....................................................................................................................................... 3
2. INTRODUCTION .............................................................................................................................. 4
3. EFFECTS OF CORROSION .............................................................................................................. 5
4. GENERAL/UNIFORM CORROSION ............................................................................................... 5
4.1. Controlling Parameters................................................................................................................. 6
4.2. Preventative Methods ................................................................................................................... 7
5. CORROSION FATIGUE ................................................................................................................... 7
5.1. Controlling Parameters................................................................................................................. 8
5.2. Preventative Methods ................................................................................................................... 9
6. CREVICE CORROSION ................................................................................................................. 10
6.1. Controlling Parameters............................................................................................................... 11
6.2. Preventative Methods ................................................................................................................. 11
7. DEALLOYING OR SELECTIVE LEACHING ................................................................................ 12
7.1. Dezincification........................................................................................................................... 12
7.2. Graphitic Corrosion.................................................................................................................... 13
7.3. Controlling Parameters............................................................................................................... 13
7.4. Preventative Methods ................................................................................................................. 14
8. EROSION-CORROSION ................................................................................................................. 15
8.1. Controlling Parameters............................................................................................................... 16
8.2. Preventative Methods ................................................................................................................. 16
9. FRETTING CORROSION ............................................................................................................... 17
9.1. Controlling Parameters............................................................................................................... 18
9.2. Preventative Methods ................................................................................................................. 19
10. GALVANIC CORROSION ............................................................................................................ 19
10.1. Controlling Parameters............................................................................................................. 20
10.2. Preventative Methods ............................................................................................................... 21
11. HYDROGEN DAMAGE................................................................................................................ 22
11.1. High Temperature Hydrogen Attack ......................................................................................... 22
11.2. Hydrogen Blistering ................................................................................................................. 22
11.3. Hydrogen Embrittlement .......................................................................................................... 22
11.4. Controlling Parameters............................................................................................................. 23
11.5. Preventative Methods ............................................................................................................... 24
12. INTERGRANULAR CORROSION ............................................................................................... 25
12.1. Controlling Parameters............................................................................................................. 26
12.2. Preventative Methods ............................................................................................................... 26
13. PITTING CORROSION ................................................................................................................. 27
13.1. Controlling Parameters............................................................................................................. 28
13.2. Preventative Methods ............................................................................................................... 28
14. STRESS CORROSION CRACKING ............................................................................................. 29
14.1. Controlling Parameters............................................................................................................. 29
14.2. Preventative Methods ............................................................................................................... 31
15. STRAY CURRENT CORROSION................................................................................................. 31
15.1. Controlling Parameters............................................................................................................. 32
15.2. Preventative Methods ............................................................................................................... 33
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16. CONCLUSION .............................................................................................................................. 33
REFERENCES...................................................................................................................................... 35
LIST OF FIGURES
Figure (1)
Figure (2)
Figure (3)
Figure (4)
Figure (5)
Figure (6)
Figure (7)
Figure (8)
Figure (9)
Figure (10)
Figure (11)
Figure (12)
Figure (13)
Figure (14)
Figure (15)
Figure (16)
The Corrosion Cycle of Steel (1).
Uniform Corrosion Progression (5).
Fatigue and Corrosion Fatigue Curves for an Aluminum Alloy (4).
Corrosion Fatigue Progression (7).
Crevice Corrosion Progression (8).
Types of Dealloying (8).
Erosion-Corrosion Progression (8).
Techniques to Prevent Erosion Corrosion (6).
Fretting Corrosion Progression (8).
Galvanic Corrosion (8).
Hydrogen Damage Progression (8).
Intergranular Corrosion (8).
Typical Cross-Sectional Shapes of Corrosion Pits (4).
Pitting Corrosion Progression (4).
Stress Corrosion Cracking Progression (8).
Stray Current Corrosion Progression (8).
LIST OF TABLES
Table (1)
Table (2)
Table (3)
Combinations of Alloys and Environments Subject to Dealloying and Elements
Preferentially Removed (6).
Metals’ Susceptibilities to Hydrogen Damage (6).
Environments that May Cause Stress Corrosion Cracking in Certain Alloys (6).
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1. ABSTRACT
For my honors research project, I plan on researching the different forms of corrosion
that occur in metallic materials. Corrosive behavior can be predicted with the use of previous
data and modeling programs. From this behavior, performance criteria can be created that
determines whether a product has reached its failure threshold. However, there are multiple
forms of corrosion that exist. In order to make sure the correct performance criteria are being
established for a product, the correct form of corrosion must be identified.
I will focus on three major categories involving corrosion. First, I want to research the
definition of corrosion. From this definition, I will discuss the different types of corrosion that
are found in nature. Each type of corrosion will be classified and the cause of that type of
corrosion will be explained. Next, the different controlling parameters that affect the rates of
corrosion will be evaluated. From these controlling parameters, research will be conducted in
order to determine what types of preventative methods are used against these controlling
parameters in order to reduce the corrosion procession. This research will be conducted from
previous case studies and the data will be presented in tables and figures for clarity.
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2. INTRODUCTION
Corrosion is a natural process that occurs in the environment. Like all natural processes,
it tends towards the lowest states of energy possible. One example of this occurs with iron and
steel. In order to reach their lowest states of energy, iron and steel mix with oxygen and water,
which both naturally occur in the environment. These combinations create iron oxides, which
are commonly known as rust. The iron oxides that are created reach a chemical makeup that is
similar to iron ore. Iron ore is the original constituent that is used to produce finished steel
products (1). Figure (1) demonstrates the corrosion cycle of steel in the steel production process.
Figure (1) The Corrosion Cycle of Steel (1).
The word corrosion comes from the Latin base corrodere, which means “to gnaw to
pieces.” Generally, as described by most people, corrosion is the wearing away or eating into
through a gradual procession. Scientifically speaking, corrosion is defined as “a chemical or
electrochemical reaction between a material, usually a metal, and its environment that produces a
deterioration of the material and its properties” (1). The environment that is described in the
definition of corrosion consists of everything that is surrounding and in contact with the specific
object. Environments all over the world have a wide variety of characteristics. Some are dry
and hot, others are cold and wet, while several are abrasive from sand or salt water. As a result,
environments are often described due to their primary factors that affect corrosion. The first
environmental factor that affects corrosion rates is the physical state of the environment. The
physical state can range from gas, like outer space, liquid, like the ocean, or solid, like in the
earth’s surface. The next environmental factor affecting corrosion rates is the chemical
composition of that environment. These chemical compositions focus on the different elements
and concentrations of those elements that are in contact with the object in question. Finally,
temperature is a very important environmental factor that affects the rate of corrosion for specific
objects. In certain environments, higher or lower temperatures can impact the environment’s
constituents increasing or decreasing the rates of corrosion (1).
In simple terms, when a metal deteriorates after it reacts with its environment, it is called
corrosion. When discussing corrosion, it is critical to consider the combination of both the
environment and the material. Without the knowledge of what type of environment a material is
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going to be exposed to, the behavior of the corrosion, including rates and severity, cannot be
expressed. Similarly, without the knowledge of what type of material is being exposed to a
specific environment, the corrosive behavior of that material cannot be described. Therefore, the
corrosivity of a material is dependent upon the type of environment it is exposed to. Along the
same lines, the corrosivity of an environment is dependent upon the material that is being
exposed (1).
3. EFFECTS OF CORROSION
Corrosion effects society in all aspects of life. The immediate effects of corrosion
include the impact on the service life of our personal possessions. Items that we own, including
cars, grills, metallic basketball hoops, and even metallic carpentry tools, have their service lives
decreased as a result of corrosion. One form of preventative maintenance used to increase the
service lives of these items consists of painting (1). The paint reduces the impact of oxygen and
water on the metallic surface decreasing the rate of corrosion.
The secondary effects of corrosion include the impact of service life in public property.
Steel reinforcing bars are used in concrete for a wide variety of applications. In these
applications, the procession of corrosion in the rebar goes unnoticed since it cannot be seen. As
a result, failure occurs in an instant without warning. The most common instances of rebar use
in the public sector include highway bridges, reinforced concrete buildings, and parking decks.
These sudden collapses result in large repair costs and the endangerment of the safety of public
civilians (1). Professional engineers are expected to demonstrate integrity and honesty to the
highest standards due to their direct impact on the enhancement of the quality of life for society.
As stated by the first canon of the American Society of Civil Engineers (ASCE) Code of Ethics,
“Engineers shall hold paramount the safety, health and welfare of the public and shall strive to
comply with the principles of sustainable development in the performance of their professional
duties” (2). Therefore, it is extremely important to inspect these structures that include rebar in
order to prevent dangerous collapses and to protect the safety of the public.
The final, and possibly the most dangerous, effect of corrosion occurs in major industrial
plants. Corrosion in these facilities often leads to the plants being shutdown in order to repair the
damages and replace the corroded equipment. This corrosion failure affects both the industrial
plants themselves and the surrounding community to the plants. The plants get the burden of
having to repair and replace the equipment that has been damaged by the corrosion. This is a
large expense to the industrial plants. When the damage from the corrosion occurs, there is often
an explosion and possible structure collapse. Depending on the size of the explosion and
collapse, nearby homes can be damaged and destroyed. This places the safety of the community
at risk. Their safety is also placed in jeopardy due to the toxins that are released and exposed to
the nearby environment (1). These toxins can contaminate and pollute water sources creating
health issues for civilians. Often times, as a result of the health issues and damages caused by
the corroded equipment, industrial plants are forced to pay out large settlements for the pain and
suffering they have caused nearby communities.
4. GENERAL/UNIFORM CORROSION
Uniform corrosion refers to corrosion that occurs throughout an entire steel structure.
The corrosion is not a localized attack and does not penetrate very deep past the surface. In this
5
type of corrosion, there is a uniform distribution of cathodic reactants over the entire exposed
metal surface. Reactions take place in a uniform manner resulting in no exact location for
cathodic or anodic reaction. The cathodes and anodes are randomly located and alternate
locations with time. The most common example of uniform corrosion occurs when steel rusts
due to the oxygen in the air (3). This type of corrosion leads to the most metal weight loss when
compared to all other forms of corrosion. When inspecting corrosion, uniform corrosion is quite
detectable and predictable. As a result, it is considered one of the least problematic forms of
corrosion. The only way that uniform corrosion becomes troublesome is if the area that the
corrosion is occurring is out of sight. This could lead to serious damage in a specific structure.
One example of this occurs with corrosion on the inside of a pipeline. It is impossible to see the
corrosion procession on the inside of a steel pipe. Other instances of uniform corrosion that
occur with immerged or buried structures demonstrate that even the simplest forms of corrosion
procession require monitoring (4). The progression of uniform corrosion is illustrated in Figure
(2) below.
Figure (2) Uniform Corrosion Progression (5).
4.1. Controlling Parameters
The most important factor affecting uniform corrosion is the type of environment it is
exposed to. The use of corrosion inhibitors, or removal of corrosive species from the
environment, can affect the rate at which uniform corrosion proceeds. The most important
aspect about the environment is the type of atmosphere that is surrounding the material. One
type of environment that can induce uniform corrosion is a dry atmosphere. If traces of sulfur
compounds or pollutants with hydrogen sulfide, H2 S, are present in the atmosphere, corrosion
can initiate without any moisture. An example of this occurs with the tarnishing of silver. When
silver tarnishes in a dry environment, the presence of hydrogen sulfide is often found in the air
(3).
The more common type of environment where uniform corrosion is often discovered is a
humid, wet, damp atmosphere. When the atmosphere reaches approximately 70% humidity,
uniform corrosion of a material can occur. This 70% coincides with the equilibrium value of
saturated sodium chloride, NaCl. The sodium chloride solution is often found on the surface of
metallic materials. When the humidity reaches 70%, a thin film of moisture forms on the
metallic surface. This thin film of moisture acts as an electrolyte allowing for current to pass to
the metal. As a result, any metallic structures that are exposed to the atmospheric air are
susceptible to corrosion. From previous studies, it has been determined that as relative humidity
increases above 70%, the rate of corrosion increases significantly (3).
The last major common environment that is susceptible to uniform corrosion is a wet,
water atmosphere. If water is found on the surface of a metallic material, corrosion will
6
commence. Seawater, rain, and drops of dew provide a wet surface that forms on the metal. The
wet surface acts as an electrolyte that allows current to flow through it to the metal. In the case
of dew, if sulfur dioxide, SO2 , is present, the dew becomes acidic and increases the rate of
corrosion. A common example of this occurs with cars. If cars are left outside overnight, acidic
dew can form on them in the morning causing corrosion to initiate (3).
4.2. Preventative Methods
When attempting to prevent uniform corrosion from occurring, the most important factor
to take into consideration is the type of metallic material being used. It is critical that materials
be selected, depending on the type of environment that will be encountered, which prevent
against uniform corrosion most efficiently. Once a material has been selected, it is key to make
sure that this material is coated wherever possible on the surface to prevent the atmosphere from
acting against the metallic material. If coatings are not used, surface treatments are essential in
protecting the metallic surface from the atmosphere. These surface treatments produce a metal
oxide layer of uniform thickness which can be controlled to protect the metallic material. In
other cases, surface treatments involve the use of additional metallic elements. For instance, in
some cases, chromium is incorporated into the metallic materials in order to serve as a form of
corrosion resistance. The last effective method to prevent uniform corrosion involves the use of
vapor phase inhibitors. Vapor phase inhibitors deposit from a vapor phase on the metal surface
to be protected. These inhibitors adjust the pH levels of the surrounding atmospheric
environment in order to resist the corrosive elements attacking the metallic surface (6).
5. CORROSION FATIGUE
Fatigue occurs from the failure of a metal by cracking due to cyclical stress. The most
common cases result from rapid fluctuating stresses that are normally very small compared to the
tensile strength of the metal. The stress applied to the metal has an inverse relationship when
compared to the cycles required to cause a fracture. As the stress applied increases, the number
of cycles required to cause a fracture decreases. Similarly, as the stress applied decreases, the
number of cycles required to cause a fracture increases. In the case of steel, the property of an
endurance limit provides a small amount of safety against fatigue. The endurance limit of steel
refers to the stress level at which no failure will occur. As long as the stress is below that level,
despite an infinite number of cycles, the steel will not fail as a result of fatigue. In order to
determine that stress level, one million cycles is commonly run on steel products. If no failure
occurs in one million cycles, the endurance limit is identified as the maximum stress level.
Typically, a stress versus number of cycles, S-N, curve is attained by plotting the maximum
stress applied in a cycle against the number of cycles necessary to cause failure (4).
When a metal is applied with cyclical stresses in a corrosive environment, the number of
cycles required to cause a fracture decreases dramatically compared to the same metal in air.
This acceleration to fracture of the metal in a corrosive environment is known as corrosion
fatigue and is shown in Figure (3) by comparing the dashed and solid curves. As seen in Figure
(3), the dashed line on the S-N curve represents the fatigue curve in air. The solid line represents
the corrosion fatigue curve in a corrosive environment, tap water in this instance. The solid
curve shows that the life span of a particular metal is much lower in a corrosive environment
compared to air. The S-N curve for a corrosive environment continually decreases as the number
7
of cycles increases, even at low stress levels. As a result, a large decline or complete elimination
of the endurance limit occurs. This is opposite of the S-N curve for air which levels off at a
certain stress level. This stress level becomes the endurance limit for a particular metal (4).
Figure (4), shown below, illustrates the progression of corrosion fatigue with time.
Figure (3) Fatigue and Corrosion Fatigue Curves for an Aluminum Alloy (4).
Figure (4) Corrosion Fatigue Progression (7).
5.1. Controlling Parameters
In order to prevent against corrosion fatigue, it is important to select a metallic material
that has high fracture toughness. However, a material that has high fracture toughness has
reduced strength. In the same manner, a material with high strength has reduced fracture
toughness. This inverse relationship between strength and fracture toughness provides a
8
challenge for engineers and manufacturers to select and produce a metallic material that is strong
with high fracture toughness. One solution to this problem is reducing the grain size of the
material. By reducing the grain size, the strength is maintained while the fracture toughness is
improved (6).
The texture of the surface also affects the initiation of corrosion fatigue. A rough surface
provides little resistance against the initiation of cracks. However, a highly polished surface
provides a great surface for resisting cracking. Temperature is another important factor that
affects crack initiation. A higher temperature climate protects very little against the initiation of
cracks. On the other hand, a lower temperature provides some resistance against crack initiation
and propagation (6).
Along with these controlling parameters, the two most important factors that affect the
initiation and propagation of corrosion fatigue are the stress applied to the metallic surface and
the environment surrounding the metallic material. If the stress applied is large, the number of
cycles required to cause a fracture is small. If the stress applied is small, the number of cycles
required to cause a fracture is large. Once cracks have initiated, the environment which
surrounds the metallic material controls the rate at which corrosion fatigue propagation will
occur. The resistance to corrosion of the metallic material in that particular environment will
determine the rate at which corrosion fatigue proceeds (6).
5.2. Preventative Methods
The goal of preventing corrosion fatigue is not to stop the cracks from developing once
they have initiated. Instead, the goal is to prevent corrosion fatigue cracks from starting. One
way to prevent corrosion fatigue cracks from starting is to introduce compressive stresses. The
compressive stresses act against the cyclical stresses retarding the cracks from initiating. These
compressive stresses can be applied by shot-peening, shot-blasting, nitriding, and carburizing
(3).
Another form of prevention against the initiation of corrosion fatigue is through the use
of cathodic protection. This is achieved by having a more active metal attacked than the current
metal. In order to do this, the metals affected are often coated by metals that are more resistant
to corrosion fatigue. The two coatings that have been found to be most productive in this
protection are zinc and cadmium coatings. These coatings introduce compressive stresses that
occur during electrodeposition (3).
Opposite of cathodic protection, anodic protection is another form of corrosion fatigue
prevention. Stainless steels and carbon steels have been improved with the formation of passive
layers on the metallic surface. The anodic protection polarizes the steel by increasing the
potential to the passive region. This technique of corrosion prevention improves the fatigue life
of the metal best in oxidizing environments (3).
The last effective preventative method involves changing either the metallic composition
of the alloy or modifying the environment that the metal is located in. When discussing the
composition of the alloy, a small change in the content of one of the metals can make a huge
difference. For instance, stainless steels fatigue life can be improved with an increase in the
nickel, Ni, content. Mild steels can be improved in the same manner with the addition of 1-2%
of titanium to the alloy. If it has been decided that the metallic composition will not be changed,
inhibitors can be added to the environment in order to improve the corrosion fatigue life. An
example of an inhibitor includes adding sodium dichromate to a chloride-sulfate solution in order
9
to increase the corrosion fatigue resistance of steel. These inhibitors form a protective film on
the metallic surface that is being affected by corrosion fatigue. The combination of the proper
inhibitor and its effectiveness against a specific environment is extremely important when
determining the prevention of corrosion fatigue. The setback to these two types of corrosive
protection is the cost involved. Modifying the metallic compositions and using the large amount
of inhibitors required to form the protective film on the metallic surface can be very expensive
and be prohibitive for making such modifications (3).
6. CREVICE CORROSION
Crevice corrosion is a localized form of corrosion that occurs in the formation of a
differential aeration cell. This leads to the limit of certain materials, such as steel, in marine
environments, chemical industries, and petrochemical industries. The formation of crevice
corrosion stems from four major causes. The first, and most common, cause is the result of tight
spaces between two components. These components can either be metal-to-metal or non-metal
to metal, depending on the situation. The second cause occurs from the presence of cracks,
cavities, and other defects that are present on the surface of a metal. These defects allow for
oxygen or water to enter the metal creating larger cracks and rust to form. The third cause
occurs when materials are used in marine environments. If barnacles, biofouling organisms, or
similar deposits are present with the material, crevice corrosion is likely to occur. The last cause
occurs when materials are used in a land environment. If dirt, mud, or other deposits are found
on the metal surface, crevice corrosion will begin to develop (3).
The process of crevice corrosion is initiated due to the differential aeration cell. Under
these conditions, the oxygen starved areas located in the crevices, cracks, and areas affected by
dirt are anodic, while the areas with free access to oxygen are cathodic (4). The rate of crevice
corrosion procession can be increased by a couple different factors. One of these factors is the
width and depth of the crack. From previous studies, it has been determined that the smaller the
crevice, the quicker crevice corrosion will set in. This is due to the fact that when the ratio of
crevice volume to crevice area is small, the acidity is increased. Crevice corrosion rates also
increase when the ratio of the cathodic area to the anodic area is large. On the other hand, the
rate of crevice corrosion procession can be decreased. It has been determined that a differential
concentration of oxygen is set up when the crevice consumes oxygen. This accelerates the
oxygen accelerating the rate of corrosion. However, if the temperature is increased, the
solubility of the oxygen decreases which delays the procession of crevice corrosion (3).
Figure (5) Crevice Corrosion Progression (8).
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6.1. Controlling Parameters
There are a number of important factors that affect the progression of crevice corrosion in
metallic materials. The most common factor is the crevice type. The type of crevice refers to
whether surface contact areas are metal-to-metal, metal to non-metal, or a marine growth, such
as barnacles, that attach onto the surface of the metal. Knowing what type of crevice is present
and whether the factors affecting that crevice are man-made or natural is an important factor
when determining what type or corrosion prevention method to apply to the material. Another
important factor involves knowing the composition of the alloys that are being used. Knowing
whether or not these materials are resistant to crevice corrosion could have a major impact on life
cycle costs. The elements of an alloy affect electrochemical and chemical processes, passive
film formation, passive current density, and metal dissolution. An example of this occurs with
the presence of molybdenum, Mo. Stainless steels with increased contents of molybdenum have
an increased resistance to crevice corrosion (3).
The characteristics of the passive film formed on the surface of the metallic material also
affect the progression of crevice corrosion. The faster a passive film breaks down, the quicker
the onset of crevice corrosion to occur. From previous studies, it has been determined that as the
seawater temperature is raised to 70°C, the rate of crevice corrosion of certain steels decreases.
Underneath the passive film, the geometry of the crevices in the surface of the material also
affects the rate of crevice corrosion. The depth, width, number of crevices, and ratio of interior
to exterior crevice all affect the rate of crevice corrosion. The smaller these crevices are, the
faster the initiation of crevice corrosion. This is a result of the increased acidity of the crevice.
When the crevice volume to crevice area ratio is small, the acidity increases initiating the start of
crevice corrosion to occur (3).
The onset of crevice corrosion is closely linked to the passive film on the metallic
surface. If a stable passive film exists, the initiation of crevice corrosion is prevented. In order
for the passive film to form on the metallic surface, oxygen is needed. Therefore, the onset of
crevice corrosion is highly dependent on oxygen. When the crevices begin to consume oxygen,
a potential difference is created between the open areas exposed to oxygen and the crevice areas.
The oxygen concentration differential accelerates the affects of the oxygen initiating crevice
corrosion. However, as the temperature is increased, the oxygen becomes less soluble. This
decreases the progression of crevice corrosion (3).
6.2. Preventative Methods
The preventative methods used against crevice corrosion often start by eliminating the
characteristics that induce crevice corrosion. Since crevice corrosion can initiate at spaces
between components, the use of welded joints is often preferred to bolted or riveted joints. If
bolted or riveted joints are used, these spaces that are created can be sealed with non-corrosive
materials. These issues can also be avoided by minimizing the contact areas between the metalto-metal and non-metal to metal contact areas. Along the same lines, dirt, mud, and other debris
collect in these small spaces and cracks. By eliminating as many sharp corners, edges, and
spaces where dirt and debris can be collected, crevice corrosion can be reduced to a certain
extent. This can also be accomplished by removing the dirt and debris deposits from time to
time. In areas where sharp corners, edges, and spaces must occur, weld overlays can be placed
with a highly resistant alloy (3).
11
Besides avoiding spaces created by joints, the type of material used can have a major
impact on the prevention of crevice corrosion. Certain alloys, such as titanium, are highly
resistant to crevice corrosion. Stainless steels with contents of molybdenum, Mo, up to 4.5%
dramatically increase the susceptibility to crevice corrosion. Another common form of corrosion
prevention is cathodic protection. This is applied by attaching stainless steels to an adjacent mild
steel structure. If this is not possible, painting the cathodic surface is a temporary corrosion
prevention method. As long as the surface is painted routinely, the resistance to crevice
corrosion will persist. In certain situations, painting the material will not be a cost effective form
of corrosion prevention. To save money, avoiding hygroscopic, or moisture retaining materials,
is an easy form of corrosion prevention. However, in certain instances, materials are placed in a
marine environment. In this case, it is important to maintain the seawater at a high velocity.
This high velocity keeps the solids in the water in suspension, preventing them from entering
cracks and cervices in the material (3).
7. DEALLOYING OR SELECTIVE LEACHING
Dealloying and selective leaching refer to a type of localized corrosion which involves
the removal of one of the elements of an alloy by corrosion. This removal occurs either through
a preferential attack or by dissolution of the matrix material. There are a variety of types of
selective dissolution when discussing dealloying. These types of dealloying are named after the
alloy that is being affected, in particular, the element of the alloy that is being dissolved by the
corrosion (4).
7.1. Dezincification
Dezincification refers to the dealloying or selective leaching of zinc in alloys, such as
brass, that contain more than 15% zinc. When an item has undergone dezincification, the
appearance is rarely changed except for a copper hue that has formed. Underneath the
appearance, however, the item will have become weak and susceptible to failure at any moment.
One way to tell that dezincification has occurred is to look at the color of the item. When the
item is taken apart, it is easy to tell the difference between the red copper without zinc and the
yellow brass that has been unaffected (4).
The environments where dezincification normally occurs are areas that include water,
especially seawater. Other environments include slightly acidic conditions, the presence of
ammonia species or carbon dioxide, and appreciable oxygen. In these environments, two types
of dezincification occur. The most common type is uniform dezincification. This form of
corrosion occurs uniformly throughout the surface in a general manner (4). The other type of
dezincification is plug dezincification. This form of corrosion is localized and only affects
specific areas of the surface. The area around these localized attacks is unaffected (3). The
location of plugs may be distinguished in some cases. If there is a deposit of brownish-white
zinc-rich corrosion product existing on the item, the plug will be found directly below that
deposit (4).
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7.2. Graphitic Corrosion
Graphitic corrosion refers to a form of selective leaching in which gray cast iron is
affected. The only material left from the gray cast iron after selective leaching is graphite. Often
times, the term graphitization is used to describe graphitic corrosion. This term is not preferred
due to the fact that graphitization is used to describe the decomposition of carbide to graphite in
metallurgy. One common occurrence of graphitic corrosion is in gray cast iron pipe. The gray
cast iron goes through the process of selective dissolution of the ferrite alloy. The only material
remaining is the 4 to 4.5 percent of graphite in the original alloy (3).
Gray cast iron was only commonly used in water pipes up until the 1950’s when ductile
iron pipe became available. Later on in the century, plastic pipe became an option which further
prevented corrosion in piping. After the pipe has undergone graphitic corrosion, the pipe looks
relatively unchanged in shape. The pipe may look a little dirty but the original mill markings can
often be seen. There are two methods of telling when graphitic corrosion has set in to gray cast
iron pipes. One method is to strike it with a metal object, such as a hammer. If the pipe
responds with a dull “thunk” compared to a sharp “clang” usually heard when two pieces of
metal strike one another, the pipe has most likely been affected by graphitic corrosion. The
second method is performed with a sharp object. Since graphitized pipe is rather soft, the pipe
can easily be carved or cut with a knife or chisel (4).
Figure (6) Types of Dealloying (8).
7.3. Controlling Parameters
The initiation and propagation of dealloying, or selective leaching, are controlled by two
major factors. The first factor involves the contents of the metallic alloy in a surrounding
environment. Certain metallic elements are resistant to corrosion in specific environments. For
instance, gray irons possess a high content of graphite, making them more susceptible to
graphitic corrosion. Ductile cast iron contains a lower content of graphite making it less
susceptible to graphitic corrosion. White cast iron, which contains essentially no graphite, is
practically resistant to graphitic corrosion. This shows that the composition of a metallic alloy
determines whether or not that alloy is susceptible to dealloying. An alloy could be resistant to
corrosion in one circumstance but susceptible to corrosion in another, depending on what
environment is surrounding the metallic material (1).
The result of different elements reacting differently to the surrounding environment
creates the second major factor that affects dealloying. The environment has a key impact on
13
whether or not an alloy is resistant to corrosion. High concentrations of aggressive anions, such
as chloride and sulfate, increase the possibility of dealloying to initiate. These aggressive anions
raise the pH of the water which increases the likelihood of a corrosion attack to set in. Besides
the anions that are present in the atmosphere, oxygen plays an important role in dealloying
initiation and propagation. Even though dealloying can occur without the presence of oxygen in
some cases, oxygen creates an aggressive environment that acts as an accelerant to the rate of
corrosion. If high temperatures are also included in an environment, the rate of corrosion
propagation continues to increase. The last important aggressive environmental aspect that
should be avoided is stagnant water conditions. Stagnant water conditions allow the elements in
the water to react with the metallic surface, removing a specific element from an alloy (1). By
looking at Table (1) below, a variety of alloys are shown with the environment they are affected
by and the element that is removed from that specific alloy as a result of the environmental
conditions.
Table (1) Combinations of Alloys and Environments Subject to Dealloying and Elements
Preferentially Removed (6).
7.4. Preventative Methods
The preventative methods used against dealloying and selective leaching often refer to
the specific type of dealloying or selective leaching being discussed. For instance,
dezincification refers to the dealloying or selective leaching of zinc in alloys that contain more
than 15% zinc. The best preventative method against this type of dealloying is to use alloys that
possess more than 85% of the other metallic element in the alloy. One of the most common
alloys comprised of zinc is brass. One conventional technique used to decrease the content of
14
zinc in this alloy is the addition of tin, arsenic, or antimony. The addition of these elements
decreases the content of zinc in the brass alloy which reduces the possibility for dealloying to
initiate. The other common preventative method used against dealloying involves the
environment in which the metallic material is placed. In most cases, dealloying or selective
leaching occurs in environments where water or acidic conditions are present. Avoidance of
these environments, which may cause the solution to become stagnant creating deposits that can
accumulate on the metallic surface, is an essential step in reducing the likelihood of the onset of
dealloying or selective leaching (3).
8. EROSION-CORROSION
When discussing the resistance of a metallic material to erosion-corrosion, it is important
to understand the properties of the films that naturally form on surface of the metal or alloy.
There are two types of naturally occurring films that form on metal surfaces. One type is a
relatively thick and porous diffusion barrier. Examples of this type of diffusion barrier include
red dust on carbon steel and copper oxide on copper. The second type is a thin and invisible
passive film. These types of film are formed on stainless steels, nickel alloys, and titanium (4).
These naturally occurring films protect the metals and alloys from corrosion well until
the layer is worn away from the metal or alloy. If a liquid is flowing on the surface of these
metals and the flow is turbulent, the random liquid motion can remove the protective film from
the metal. With the protective film eroded away and liquid in contact with the surface of an
unprotected metal, additional oxidation begins to occur accelerating the rate of corrosion. This
erosive attack can be uniform, but it is more commonly found producing localized pitted areas all
over the surface of the metal that even produces small holes (4).
In some instances, when the liquid flowing on the surface of the metal becomes turbulent,
cavitation can occur. When the liquid becomes turbulent, voids or cavities in the metal surface
can collapse leading to a localized corrosive attack. Along with turbulent liquid flow, cavitation
can also occur as a result of high vibrations. The corrosive damage caused by cavitation is very
similar to the damage caused by erosion-corrosion. The corrosion is localized and takes form in
small areas that create a pitted surface (4).
Figure (7) Erosion-Corrosion Progression (8).
15
8.1. Controlling Parameters
It is difficult to define what factors affect the resistance of a metallic material to erosion
corrosion. The most important property that affects erosion corrosion is hardness. A harder
material provides a better resistance to erosion corrosion than a softer material. However, there
are exceptions to this which make it difficult to state that a harder material will provide a better
resistance to corrosion. Another factor affecting the initiation and propagation of erosion
corrosion is the relative surface roughness of the material. In general, a smoother surface will
provide a better resistance to erosion corrosion since the fluid will be able to flow effortlessly
across the metallic surface creating very little friction. This decrease in friction decreases the
rate at which the naturally occurring protective film on the material is eroded away. Some other
properties that affect erosion corrosion are fluid velocity, fluid density, angle of impact, and the
general resistance to corrosion of the metallic material in a given environment. Equation (1)
below predicts the rate of erosion of a metallic material using some of the factors that were
previously mentioned. The downfall to this equation is that it does not take into account the
effects of corrosion on the rate of erosion. With the added effects of corrosion, the rate of
erosion would be expected to take place at a higher rate (6).
(1)
Where:
C is a system constant
φ is the angle of impact
ρ is the density of the erodent
v is the erodent velocity
HV is the hardness of the metal
8.2. Preventative Methods
The techniques used to prevent erosion corrosion involve both the environment and the
design of the metallic material. One of the most important environmental factors to consider is
the flow of the fluid on the metallic surface. It is important to avoid fluid flow that is turbulent
since cavitation can occur as a result. This turbulent flow can collapse voids or cavaties in the
metallic surface which can lead to the initiation of corrosion (4).
The other techniques used to prevent erosion corrosion involve the design of the
metallic materials. An important location that often leads to erosion corrosion initiation is at
welded joints. Welded joints act as surface defects that allow for the flowing fluid to initiate
erosion. In order to prevent this erosion initiation, protective plates are added along welded
areas. This protective plate inhibits the fluid stream from flowing across the welded joints which
decreases the likelihood for erosion corrosion initiation. Another location where erosion
corrosion occurs is at flow impingements. When one flow impinges another flow, the location of
the impingement often leads to erosion. By placing a deflector plate at this location, the metallic
surface is protected from the impinging flow. These techniques for erosion corrosion prevention
are shown in Figure (8) below. The last major technique used to prevent erosion corrosion
involves concentrate additions. If a concentrate addition is to be added to a container, the piping
for the concentrate should be placed vertically into the center of the container. This allows the
16
already present fluid to dissipate the velocity at which the concentrate is entering the container,
reducing the effects of erosion corrosion (6).
Figure (8) Techniques to Prevent Erosion Corrosion (6).
9. FRETTING CORROSION
Fretting corrosion refers to the corrosion damage that is caused from two metal surfaces
being in contact with one another at their asperities, or surface protrusions. In order for fretting
corrosion to occur, there are two key components that must exist. First, there must be a load that
is applied to these two surfaces. Second, there must be the presence of a repeated surface
motion, which may be caused by vibration or repeated slippage. The damage that results from
fretting corrosion may show up as pits, grooves, or oxide debris. In most cases, this damage
occurs in machinery, bolted connections, and ball or roller bearings (4).
In most instances, fretting corrosion occurs at the contact points between two loaded
surfaces that are not designed to move against one another. As the surfaces rub along one
another, they begin to wear away their protective film on the metal surface exposing the metal to
the corrosive atmospheric environment. The corrosive atmosphere creates a metal loss where the
two surfaces were in contact with one another, oxide debris, galling, seizing, fatiguing, or
cracking of the metal (4).
17
Figure (9) Fretting Corrosion Progression (8).
9.1. Controlling Parameters
The two major aspects that affect the initiation of fretting corrosion include the load that
is applied between the two metal surfaces and the environment in which the metallic surface is
located. The application of the load can be broken down into three major categories. First is the
contact load itself. The wear that occurs between the two metallic surfaces is a linear
relationship between the load and fretting corrosion. As the load applied to the metallic surfaces
increases, the progression of fretting corrosion would increase as well. The second aspect of the
load refers to the amplitude at which the load is being applied. From previous studies, it has
been determined that no amplitude lower limit exists where fretting corrosion will not occur.
Amplitude oscillations as low as 3 or 4 nm create enough friction to induce fretting corrosion.
However, it has been determined that there is a limit, depending on the type of metallic material,
at which the progression rate of fretting corrosion will dramatically increase. The third and final
aspect of the load refers to the number of cycles at which the load is applied. As the number of
cycles increases, the degree of fretting corrosion increases as well. During this time period, the
surface proceeds through a number of changes. At first, the damage is considered negligible.
This is known as the incubation period. Then, the fretting corrosion acts at a constant rate. This
is called the steady-state period. Finally, the rate of fretting corrosion is increased during the
final stage of the load cycles (3).
The environmental characteristics that effect the initiation of fretting corrosion include
the temperature and humidity of the surrounding environment. The effect from temperature
depends on the type of oxide that is produced in the environment. Some oxides that are formed
produce a protective, adherent oxide which prevents any metal-to-metal contact. As a result, the
rate of fretting corrosion is decreased. Therefore, the temperature does not control the rate of
fretting corrosion. The formation of a particular oxide on the metallic surface is what causes the
decrease in rate of fretting corrosion. When referring to humidity of the environment, fretting
corrosion reacts completely opposite to uniform or general corrosion. In the case of uniform or
18
general corrosion, as the humidity increases, the rate of corrosion increases and as the humidity
decreases, the rate of corrosion decreases. In the case of fretting corrosion, as the humidity
increases, the rate of corrosion decreases and as the humidity decreases, the rate of corrosion
increases. This is directly related to the debris that is formed as a result of the wear between the
two metallic surfaces. In a dry environment, this debris stays on the surface not allowing the two
metallic surfaces to make complete contact with one another. If the air is humid instead, the
debris particles become mobile and may move from the metallic surface allowing for complete
meta-to-metal contact between the two surfaces. This decreases the wear rate between the two
metallic surfaces which decreases the progression rate of fretting corrosion (3).
9.2. Preventative Methods
The methods used to prevent fretting corrosion from initiating involve minimizing the
slippage that occurs between two metallic surfaces. One prevention method to this problem is
increasing the load that occurs at the material surfaces. By increasing the load, the frictional
forces increase reducing the likelihood that the materials will move across one another. Another
method involves decreasing the amplitude at which the load repeatedly acts on the surface
contact points. Even though a lower limit for the amplitude does not exist, decreasing the
amplitude of the load as much as possible will decrease the possibility of fretting corrosion to
occur. If the amplitude of the load cannot be reduced, using metallic materials that produce
protective oxide films are another option to prevent the initiation of fretting corrosion. At certain
temperatures, some metallic surfaces produce protective films from the oxides in the surrounding
environment. This protective film eliminates any metal-to-metal contact which reduces the
possibility of fretting corrosion to occur (3).
Some other preventative methods against fretting corrosion include applying other
materials to the two metallic surfaces. One of these types of methods includes the use of gaskets.
A gasket is a mechanical seal that fills the space between two mating surfaces. By applying a
gasket, any vibrations caused from the oscillating load are absorbed and not transferred between
the metallic surfaces. If a gasket cannot be added, the two metallic surfaces in contact can be
hardened by shot-peening. Shot-peening is a cold working process used to produce a
compressive residual stress layer and modify mechanical properties of metals. This compressive
residual stress layer increases the resistance against fretting corrosion of the metals in contact.
Finally, if this is not an option, the use of low viscosity lubricating oils is advised. The layer of
lubricating oil acts as protective film preventing any metal-to-metal contact. Along the same
lines of lubricating oils, the metallic surfaces can be separated with the application of wrapping
paper. By wrapping the two surfaces, the metal-to-metal contact is eliminated decreasing the
wear on the metallic surfaces which prevents the initiation of fretting corrosion (3).
10. GALVANIC CORROSION
Galvanic corrosion can occur as a result of one of the following two combinations. The
first combination must contain two metals that possess differential electrochemical potentials.
The second combination must include two metals that have two different tendencies when it
comes to corrosion. In both cases, the two metals must be in contact with one another in a
corrosive electrolyte (3). An electrolyte is any substance that contains free ions that make the
substance electrically conductive.
19
When two metallic materials of different electrochemical potentials are joined together, a
galvanic cell is created. A galvanic cell is a cell that has its source of energy come from the
chemical change in the cell (3). In a galvanic cell, the less noble material becomes the anode.
The more noble material will become the cathode of the galvanic cell. In almost all cases, since
the anode is less noble, it will be the material that corrodes in the galvanic cell. The cathode will
most likely have its corrosion potential reduced as a result of the anode (4).
In order to determine the tendency of a metal to corrode in a galvanic cell, the galvanic
series is used. The galvanic series is a list of metals and alloys that are arranged by their
electrochemical potentials. On one end of the spectrum are the anodic metals and alloys. On the
other end are the cathodic metals and alloys. When pairing metals and alloys together with one
another, it is important to use the galvanic series in order to try and prevent galvanic corrosion.
By looking at the list, pairing an anodic metal with a metal that is more cathodic to it will only
increase the rate of galvanic corrosion in the anodic material. By joining metals that are close to
one another in the galvanic series, the risk of galvanic corrosion setting in is minimized (3).
Figure (10) Galvanic Corrosion (8).
10.1. Controlling Parameters
There are a few factors that can be altered which affect the extent at which galvanic
corrosion will attack a metallic material. The first of these factors is the position of these metals
in the galvanic series. As stated earlier, the galvanic series is a list of metals and alloys that are
arranged by their electrochemical potentials. The anodic metals and alloys are on one end of the
spectrum while the cathodic metals and alloys are on the other end. To prevent the initiation of
galvanic corrosion, it is important to pair metals that are close to one another in the galvanic
series to eliminate an anodic and cathodic metal being in contact with one another (3).
The next major factor that affects galvanic corrosion is the nature of the environment
surrounding the metallic material. One example of this occurs with seawater. If seawater is in
contact with a metallic surface, a galvanic cell is likely to form. If this seawater contains salt or
is acidic, the rate of galvanic corrosion progression is dramatically increased due to the
ionization of the electrolyte. When the environment surrounding the material is underground in
soil, the galvanic corrosion of the material depends on the temperature of the climate. If the
climate is cold, the rate of galvanic corrosion is reduced due to the resistivity of the soil. In
20
warmer climates, the rate of galvanic corrosion is increased due to the decreased resistivity of the
soil (3).
The last major factor affecting galvanic corrosion includes the area of the material, the
distance at which the current must travel through the material, and the geometric shape of the
material. When a current is flowing through a galvanic couple, the density of the current in the
anode or cathode controls the rate of corrosion. When a material has a small area, the current
density is increased which increases the damage caused by galvanic corrosion. Since the anode
is the metallic material that is less noble, a large cathodic to anodic area ratio will result in more
oxygen reduction increasing the galvanic corrosion rate (3).
The distance of the material in which the current must travel through also greatly affects
the rate of galvanic corrosion. It is commonly known that the conductivity of a material has an
inverse relationship with the length at which the current must travel through. As a result, a
shorter path that the current travels through creates the most galvanic corrosion. This leads to
the greatest galvanic corrosion occurring at the connection between the two metallic materials.
The severity of this galvanic corrosion can be decreased by increasing the length of the materials
which increases the distance which the current must travel. If the two materials are far apart
from one another, the risk of galvanic corrosion is eliminated due to the fact that very little
current will flow between the materials (3).
The geometry of the metallic materials is the last major factor that can prevent the
process of galvanic corrosion. When current is flowing through a material, it does not travel
around the corners or sharp edges. This factor reduces the conductivity of the material which
decreases the severity of galvanic corrosion (3).
10.2. Preventative Methods
The main methods used to prevent the initiation of galvanic corrosion involve retarding
the current flow from one metallic material to the other. The two most common prevention
methods involve focusing on the galvanic series and controlling the cathode to anode area ratios.
By pairing metals that are close to one another in the galvanic series, the contact between an
anodic and cathodic metal is eliminated which reduces the effects of galvanic corrosion. If
anodic and cathodic materials are in contact with one another, it is important to reduce the
cathodic to anodic area ratio. Since the anode is the metallic material that is less noble, a large
anodic to cathodic area ratio will result in an increased prevention against the rate of galvanic
corrosion progression (3).
One way to reduce the current flow between the cathodic and anodic materials is with the
application of coatings. By coating the cathodic material in the galvanic couple, the cathodic to
anodic area is reduced, increasing the prevention against galvanic corrosion. By coating the
more noble metal, the electrons being consumed in the cathodic reaction is prevented which
controls the rate of galvanic corrosion. With all these prevention methods, galvanic corrosion
can still occur. In order to plan ahead for this corrosion, it is a smart idea in the designing phase
to make the components of a product replaceable. Replacing an entire metallic system can
become very expensive if it is damaged by galvanic corrosion. By designing replaceable parts,
only the corroded sections of the system need to be replaced, saving the owner money when it
comes to maintenance expenses (3).
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11. HYDROGEN DAMAGE
Hydrogen damage includes three types of attacks on metallic materials. These attacks
include high temperature hydrogen attack, hydrogen blistering, and hydrogen embrittlement (3).
11.1. High Temperature Hydrogen Attack
High temperature hydrogen attack requires the presence of atomic hydrogen. If
molecular hydrogen is present, it will not permeate the steel at atmospheric temperatures and will
not cause the hydrogen attack to occur. At temperatures exceeding 230°C with a hydrogen
partial pressure greater than 100 psi, atomic hydrogen will react with the carbon constituent of
the steel creating methane. This can be seen in Equation (2) below (3):
Fe3 C + 4H → 3Fe + CH4
(2)
This removal of carbon from the steel causes a large loss in strength of the steel. The
methane that has been created inside of the steel now increases the internal pressure. This
creates fissures at the non-metallic inclusions or the grain boundaries. The methane and
molecular hydrogen cannot diffuse through steel, so they begin to build up. This build up of
methane and molecular hydrogen is what causes the hydrogen attack at high temperatures. The
steel may also be seen containing blisters filled with methane. These blisters are different from
hydrogen blistering blisters which are filled with hydrogen (3).
11.2. Hydrogen Blistering
Hydrogen blistering normally occurs in low-strength steel alloys. The atomic hydrogen
diffuses into the steel alloy through defects, such as nonmetallic inclusions or at grain
boundaries, where it recombines into molecular hydrogen. As the molecular hydrogen is formed
inside the grain boundaries or at the inclusions, an enormous amount of internal pressure is
created causing splits, fissures, and even blisters at the steel surface (4). The cracks formed are
parallel to the metallic surface along the original laminations that have been formed at various
depths throughout the piece of steel. Stepwise cracking can also occur when multiple blisters
join together in the steel at various depths to form a series of steps in the crack (3).
11.3. Hydrogen Embrittlement
Hydrogen embrittlement differs from high temperature hydrogen attack and hydrogen
blistering due to the fact that hydrogen embrittlement involves gaseous hydrogen. Gaseous
hydrogen differs from the hydrogen released by a cathodic reaction in two major aspects. First,
when discussing cathodic hydrogen, the metallic surface absorbs the hydrogen as atomic
hydrogen. Gaseous hydrogen, on the other hand, is absorbed in the molecular form and
dissipates throughout the steel material forming atomic hydrogen. Second, the pressure
generated by the gaseous hydrogen is much lower than the pressure created by the cathodic
hydrogen (3).
Hydrogen embrittlement occurs during the plastic deformation of the steel alloys that are
in contact with hydrogen gas. It affects high strength steels and is strain rate dependent resulting
22
in the highest degradation when the hydrogen pressure is high and the strain rate is low. The
local stresses that are exerted on the steel alloy lead to the embrittlement of the material (3).
Since the pressures created by the gaseous hydrogen are much lower than those produced by
cathodic hydrogen, extreme cracking and blisters are not formed. Instead, the steel alloy is
simply more brittle with a significant loss is strength.
Figure (11) Hydrogen Damage Progression (8).
11.4. Controlling Parameters
Hydrogen damage of metals occurs when hydrogen is absorbed in metal and thus
weakens the load-carrying capacity of the metal. Metals absorb hydrogen when in an
environment that contains or generates hydrogen. This can be during the production, processing,
or the service of the metal. Hydrogen can be absorbed during the production of forged steel. In
steelmaking, hydrogen comes from the moisture in the atmosphere and from additives used
during processing. The pressure of the hydrogen gas creates what are called “flakes” in the
metal, causing embrittlement. Similar damage can occur when steel is welded in an environment
that contains hydrogen, which can be introduced through atmospheric or surface contamination.
As the welded metal containing hydrogen cools, it becomes super-saturated, and hydrogen
diffuses into the surrounding metal heated during the process. As a result, the area around the
weld becomes embrittled by subsequent corrosion (6).
Hydrogen can also be absorbed by metal during aqueous corrosion or cathodic charging.
When corrosion occurs in a low-pH solution, some of the reduced hydrogen does not form H2,
instead, diffusing into the metal as atomic hydrogen. Substances such as arsenic, antimony,
sulfur, selenium, tellurium, and cyanide ions prevent the hydrogen atoms from forming H2, and
are called cathodic poisons. Cathodic poisons facilitate contamination by keeping hydrogen in
atomic form, in which hydrogen more readily diffuses into the metal. Environments containing
hydrogen sulfide, which contains both hydrogen and the cathodic poison sulfur, are especially
dangerous for alloys and metals. Hydrogen sulfide is often encountered in the petroleum
industry during the drilling and completion of oil and gas wells, along with the storage and
piping of petroleum products containing hydrogen sulfide (6).
23
If steel is in a hydrogen environment of high temperature and pressure, hydrogen attack
can occur. Hydrogen is absorbed into the steel, and although the steel may appear fine for a
while, it will suddenly lose its strength and ductility. The hydrogen reacts with the carbides
inside the steel. This delayed attack is encountered in petrochemical plants, where hydrogen and
hydrocarbon streams are at high temperatures and pressures. The hydrogen can react directly
with the metal to form hydrides. This can occur in metals such as titanium, zirconium, uranium,
and their alloys. The formation of hydrides can severely weaken these metals. For example,
zirconium hydride is so brittle and weak that it can be crushed into powder. The hydrogen can
be absorbed into the metallic material during melting, welding, or from water vapor and
hydrocarbons (6).
Aside from hydrogen attack, which occurs at high temperatures, the majority of corrosion
occurs at ambient temperature. This is because at high temperatures, the mobility of hydrogen is
great enough that there is little accumulation of it at any one location in the crystal lattice. Once
ambient conditions are met, solubility decreases and causes pockets of hydrogen to form. This,
compounded with the decrease in mobility, traps the hydrogen within the metal lattice. As
shown in Table (2) below, a variety of metals that are susceptible to certain types of hydrogen
damage are listed (6).
Table (2) Metals’ Susceptibilities to Hydrogen Damage (6).
11.5. Preventative Methods
The main factors that affect hydrogen damage are the hydrogen diffusion through the
metallic material and the hydrogen in the surrounding environment. Since most of the hydrogen
that diffuses into the metallic material occurs through surface defects, such as nonmetallic
inclusions or grain boundaries, it is important to reduce these defects to a minimum, if not
eliminate them. This reduction will lead to less hydrogen entering the material resulting in less
damage. It is also important to limit the amount of hydrogen in the operating environment. With
less hydrogen surrounding the metallic material, the probability of hydrogen damage initiation is
less likely. Fewer hydrogen atoms and molecules will be able to diffuse into the metal leading to
fewer blisters and cracks forming from the internal pressures that are created from the hydrogen
build up (6).
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It is also important to maintain a low amount of hydrogen content in the metallic material
during the metallic processing stage before the metal is put into service. High temperatures
during processing can lead to severe hydrogen contamination. Slow baking while in the bodycentered cubic (BCC) ferrite form is the first step to making a metal resistant to hydrogen
induced cracking (HIC). Heat treatment is also recommended before and after welding. This
heat treatment forces the hydrogen to stay mobile prohibiting accumulation, which prevents HIC
from occurring. If the metallic material is welded at any point, it is important to make sure the
welding rod has low hydrogen content as well. After a metal is welded, it is important to keep it
in a dry place. This is essential to avoid hydrogen absorption from water vapor (6).
Some substances can also be combined with the metal during processing in order to
reduce the likelihood of corrosion initiation. Corrosion inhibitors can be added to the process
fluids, thus reducing the general corrosion rate of the metal. Their presence slows the generation
of hydrogen ions at the surface, reducing the concentration that drives the hydrogen inward. On
the contrary, it is critical to avoid the use of certain popular corrosion prevention methods used
for other forms of corrosion. One example is the use of cathodic protection. Cathodic protection
techniques create a source of hydrogen that can diffuse into the protected metal. By avoiding
cathodic protection, a source of hydrogen is eliminated, reducing the possibility for hydrogen
damage initiation (6).
12. INTERGRANULAR CORROSION
Intergranular corrosion is a form of localized corrosion that attacks the grains of the
metal. The corrosion follows a preferential narrow path along the grains. In most cases,
intergranular corrosion starts at the surface and progresses in the immediate vicinity of the grain
boundary. The effects from intergranular corrosion damage the mechanical properties of the
metal, including the loss of strength and ductility. This localized attack can even lead to the
grain being completely dislodged from the metal. The corrosion works its way inward through
the grains and the resulting loss of strength is greater than the same loss of material that would
have been uniformly distributed over the surface of the metal. It will occur throughout the entire
surface of the metal, evenly distributed among the grains (3).
Compared to stress corrosion, intergranular corrosion is much less dangerous. It attacks
the crystalline structure of the material and breaks it down. The crystalline structure is
comprised of millions of tiny grains. As the corrosion progresses along these grain boundaries,
the grains become weaker and weaker. Eventually, given enough time, the grains will
disintegrate from one another creating weaknesses in the metallic material (3). These
weaknesses could develop into failures that could become very dangerous and harmful.
Figure (12) Intergranular Corrosion (8).
25
12.1. Controlling Parameters
Intergranular corrosion can occur in many forms of metallic alloys. The two most
common alloys that are affected by this form of corrosion are stainless steel and aluminum
alloys. In the case of stainless steel alloys, the metal becomes sensitized after welding (6). The
effect of time and temperature play a very important role in the welding process. Electric arc
welding produces an intense heat that is required for a short period of time. Gas welding
produces a less intense heat and requires a longer period of time to complete the weld. Since gas
welding takes a longer period of time, the steel is in the sensitizing zone for a longer period of
time allowing a larger amount of carbide to precipitate (3). When the welding takes place, the
precipitate chromium carbide is formed. This formation occurs at the grain boundaries in the
heat affected zone of the metallic material. The chromium carbide precipitate that is formed
causes intergranular corrosion to transpire within the heat affected zone of the stainless steel (6).
This leads to the fact that gas welding makes the steel more sensitive to an intergranular
corrosion attack than electric arc welding.
Aluminum alloys also experience intergranular corrosion as a result of precipitates that
form at the grain boundaries. In most cases, these precipitates are more active than the ones
produced in stainless steel. However, aluminum alloys are often affected more commonly by
exfoliation corrosion, a form of corrosion considered to be a type of intergranular corrosion.
Exfoliation corrosion affects metals that have been mechanically deformed to produce elongated
grains in one direction. The metallic material is deformed most commonly through extrusion or
rolling. In a majority of cases, the intergranular corrosion attack initiates at the exposed end
grains of the metallic material. These end grains do not possess the same protective film as the
rest of the metallic surface and are more susceptible to intergranular corrosion (6).
12.2. Preventative Methods
When it comes to preventing intergranular corrosion, there are two major methods
commonly used. The most conventional preventative method involves keeping the impurity
levels of the metallic material. This refers to impurities in the naturally formed protective film
on the metallic surface. Any cracks or pits in this surface should be tended to immediately in
order to reduce the time the metal is exposed to the surrounding atmosphere. In certain cases,
these impurities are impossible to avoid. This occurs with end grains. When the metallic piece
is cut against the grain, an impurity is formed which can lead to cracks. This leads to design
engineers reducing the amount of end grains in a product which will reduce the probability for
intergranular corrosion initiation (6).
The other method involves the precipitates that are formed from heat treatments. It is
important to know the reactions formed by the type of heat treatment used on a metallic material.
For instance, when welding, a person wants to use a high heat intensity that will require a shorter
period of time to weld. This reduces the sensitivity time of the metal which reduces the amount
of precipitates formed at the grain boundary. As a result, it is encouraged to use electric arc
welding whenever possible due to increased temperature and shorter application time compared
to gas welding. If this is not possible, the addition of stabilizing elements to the alloy may
produce more stable precipitates as long as the increased cost is within the budget of the project.
In the case of stainless steels, titanium, niobium, or tantalum can be added to the alloy to reduce
26
the carbon content of the precipitates. These elements are more stable forming more stable
carbide precipitates compared to chromium carbide (6).
13. PITTING CORROSION
Pitting corrosion is the most common type of localized corrosion and it targets metals that
form passive films on their surface, making it easier for the corrosion process to start. Pitting
corrosion causes small areas of metals to corrode leaving the formation of small craters or pits on
the surface. The rest of the metallic surface is left unharmed. In most cases, the procession of
pitting corrosion starts on the surface of the metal in a stagnant or slow-moving liquid. The
progression is one of the more gradual, but culminating, forms of corrosion in existence. It can
create penetration into a metallic surface with only a small percentage of weight loss in the
material (4).
Even though pitting corrosion affects a smaller metallic area, it is considered to be a more
dangerous form of corrosion compared to uniform corrosion. The reason for this is that pitting
corrosion is much harder to detect, predict, and design against compared to uniform corrosion.
Another reason it is more dangerous is due to the destructive nature of the corrosion itself. If a
small pit creates a hole in a piece of equipment, that piece of equipment can malfunction creating
a lot of damage. Even if the piece does not create a lot of damage, it must still be replaced since
a malfunctioning piece of equipment could lead to the failure in an entire engineering system (4).
Figure (13) Typical Cross-Sectional Shapes of Corrosion Pits (4).
Figure (14) Pitting Corrosion Progression (4).
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13.1. Controlling Parameters
Some of the main factors affecting pitting corrosion deal with the surface of the metallic
material. Certain metals form passive films on their surface which protect the metal from the
corrosive environment surrounding it. However, if defects are formed in this passive film, pits
can begin to initiate. For carbon and stainless steels, is has been determined form previous
studies that sulfide inclusions are responsible for the formation of pits. The passive film can
attain some defects due to the severe cold of the surrounding environment. The severe sold
reduces the strength of the passive film creating defects which allows pitting corrosion to initiate
(3).
One way to prevent deficiencies from forming in the passive film of the metal is by
maintaining a smooth surface finish. This smooth surface finish does not allow accumulation of
impurities to form or stagnancy to build up on the metallic surface. This minimizes the
formation of differential aeration cells which reduces the risk of pitting corrosion to initiate. The
formation of differential aeration cells can also be reduced by maintaining a high velocity of
water flow. For instance, a stainless steel pump that is pumping seawater can produce a good
service as long as it is run continuously. If the pump is stopped, differential aeration cells are
able to form allowing pits to form, which are normally associated with stagnant conditions. With
a high velocity, the passive film is able to be maintained with the greater control of oxygen on
the steel surface (3).
The last important factor affecting pitting corrosion involves contamination from the
surrounding environment. When a lot of dust particles are present in an environment, they settle
on the metallic surface. During this time, the dust particles are absorbing moisture from the
surrounding environment. This moisture creates a differential aeration cell to be formed on the
metal surface which produces a more conductive condition for pitting corrosion to initiate. If the
environment is a marine environment, the salt from the water can lead to the initiation of pitting
corrosion. Appliances and equipment in contact with the salt will begin to corrode as a result of
differential aeration cells that are created on the metallic surface (3).
13.2. Preventative Methods
The most common solution to preventing pitting corrosion from occurring is using
materials that are resistant to pitting corrosion. An example of this occurs with stainless steel. In
a stainless steel alloy, molybdenum is added to the alloy to increase the resistance of the metal to
pitting corrosion. Once this has been accomplished, it is important to maintain a uniform surface
on the metal. This can be accomplished by cleaning, heat treating, and surface finishing. By
keeping the uniform surface smooth and shiny, impurities are prevented from forming on the
metallic surface to break down the passive film (3).
Some other solutions to preventing the initiation of pitting corrosion involve minimizing
the effects of the environment surrounding the metallic material. One way of doing this involves
reducing the concentration of aggressive species in the test medium. Examples of these
aggressive species include chlorides and sulfates. The effects from the surrounding environment
may also be reduced by minimizing the external factors that lead to the localized attack of pitting
corrosion. Certain design features of a metallic material, such as crevices and sharp corners, can
lead to pitting corrosion. If these crevices and sharp corners are eliminated or reduced, the
possibility of pitting corrosion to occur may be reduced as well (3).
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14. STRESS CORROSION CRACKING
Stress corrosion cracking is a mechanical-chemical process that results in the cracking of
certain metallic alloys at stresses that are well below their tensile strengths. In order for stress
corrosion cracking to occur, a susceptible metal or alloy, proper chemical environment, and an
enduring tensile or residual stress are required. It is most likely true that no alloy is totally
resistant to stress corrosion cracking in all types of environments. This is due to the fact that
there is usually an initiation period where the metal begins to crack at a microscopic level. This
stage can last from a few months to a few years depending on the environment, metal or alloy,
and stress being applied. Once this time period has elapsed, the propagation period can begin.
By viewing the morphology of the cracks that have been produced, the natural cause of the
cracks can be classified (4). The two major forms of stress corrosion cracking are intergranular
and transgranular. Intergranular stress corrosion cracking progresses along the grain boundaries.
Transgranular stress corrosion cracking is not restricted along grain boundaries, but penetrates
the grains (6). The failures that occur as a result of these cracks are often abrupt and unexpected,
occurring after only a few months or years of adequate service (4).
Figure (15) Stress Corrosion Cracking Progression (8).
14.1. Controlling Parameters
Stress corrosion cracking occurs in a corrosive environment under a tensile stress, which
is either residual or applied. Therefore, the material itself and the environment surrounding the
material are the key factors that affect stress corrosion cracking initiation and propagation.
Stress corrosion cracking can initiate and propagate even without the evidence of corrosion
occurring. More times than not, they initiate at pre-existing flaws or blemishes that have
occurred throughout the service life of the material. One common example of this is the
discontinuities at the surface of the metallic material. Cracks can occur at irregularities in the
surface, such as grooves or defects that exist during the fabrication process (3).
Another site for crack initiation to occur at is corrosion pits. Stress corrosion cracking
has the ability to initiate at pits formed on the surface as a result of breakdown by chloride ions.
In most cases, stress corrosion cracking initiate at the base of these pits through intergranular
corrosion. The initiation of cracks at these pits is mostly determined by the electrochemistry at
the base of the pit. The final major site for crack initiation is along the grain boundaries of the
metallic material. Intergranular corrosion results from the sensitization of impurities at the grain
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boundaries. This sensitivity makes the grain boundaries highly reactive to stress corrosion
cracking (3).
The environment is the main key to the initiation and propagation of stress corrosion
cracking. In order for stress corrosion cracking to occur, the specific environment for the
metallic material in question must be present (3). The main environmental factors that affect the
rate of stress corrosion cracking include temperature and solution. However, not all
environments affect metals in the same manner. Certain metallic alloys are susceptible to stress
corrosion cracking in one environment but more resistant in another environment. Some factors
will actually increase the rate of stress corrosion cracking that occurs in a metallic material. For
instance, when the temperature of the environment is increased, the rate of stress corrosion
cracking is often accelerated. Also, when oxygen and chlorides are present in the environment,
the rate of stress corrosion cracking can also be accelerated. The main cause for concern is the
surface film that is produced on the metallic material. This surface film may protect the alloy
from other forms of corrosion in a specific environment, but not against stress corrosion
cracking. As shown below in Table (3), some examples of environments that may cause stress
corrosion cracking of certain metallic alloys are listed (6).
Table (3) Environments that May Cause Stress Corrosion Cracking in Certain Alloys (6).
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14.2. Preventative Methods
Stress corrosion cracking can be managed by a number of different preventative methods.
The most popular methods involve altering the metal or alloy, the environment, and the tensile or
residual stress applied to the metallic material. Stress corrosion cracking can only occur with a
susceptible metal or alloy, proper chemical environment, and an enduring tensile or residual
stress. One way to prevent this from occurring is by choosing your metal or alloy carefully. By
selecting a metallic material that is not susceptible to stress corrosion cracking in a specific
environment, the possibility of corrosion initiation is reduced. If the metallic material cannot be
modified, the possibility of altering the environment is often discussed. By reducing the oxygen
and chloride content in the environment, the initiation and propagation rate of stress corrosion
cracking are reduced dramatically. In addition to these factors, a reduction in temperature will
decrease the thermal stress applied to the metal or alloy reducing the likelihood of stress
corrosion cracking initiation. The same effect can be seen with a reduction in the tensile or
residual stress applied to the metallic material (6).
Along with these common forms of prevention, surface treatments and protective
coatings are commonly applied to metals and alloys as a defense mechanism against stress
corrosion cracking. Shot-peening is a cold working process used to produce a compressive
residual stress layer and modify mechanical properties of metals. This compressive residual
stress layer increases the hardness of the metallic material which increases the resistance to stress
corrosion cracking. Any protective coatings that are applied to the metallic surface as a form of
corrosion prevention remain effective as long as the protective coating is not damaged by
defects, exposing the metal directly to the surrounding environment (6).
The last two popular forms of stress corrosion cracking prevention involve the design of
the metal product. When designing a product made out of metal, it is important to reduce the
number of end grains to as small of an amount as possible. End grains act as an initiation site for
cracks for two reasons. First off, they are not protected as well by the natural protective film
formed by the metal. Second, they have an increased local stress concentration which increases
the susceptibility of crack initiation. By reducing the amount of these end grains, the number of
vulnerable sites for stress corrosion cracking to occur is reduced as well. It is also important to
include design features that resist against other forms of corrosion. Stress corrosion cracking can
initiate from pits that are formed form pitting corrosion. By preventing pitting corrosion with a
proper design, the design engineer is preventing against stress corrosion cracking initiation (6).
15. STRAY CURRENT CORROSION
Stray current corrosion that results from stray currents that appear from external sources
is similar to galvanic corrosion. However, the magnitude of the currents produced by the stray
currents can be much higher than those from galvanic cells. This leads to the consequence of a
more rapid procession of the corrosion. Another difference between stray currents and galvanic
currents is the distance at which they operate. Stray currents can operate at long distances since
the anode and cathode are almost always separated from one another. Galvanic currents always
run through two pieces of metal that are in contact with one another. The stray currents always
seek the path of least resistance and will travel along random pieces of metallic material causing
severe corrosion at the location where the current leaves the piece of metallic material (4).
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In a normal cathodic protection system, the current flows from the ground-bed through
the earth towards the metallic structure. If a metallic material is come across by the current, the
current is picked up by the metallic material, transmitted throughout the material, and discharged
from the material through the earth and back to the cathode. If this metallic material has the
current run through it, the point where the current leaves the material to go to the ground is the
anode and corrodes. In general terms, stray currents are uncontrolled currents that originate
mostly from DC systems. These currents are classified as either static, steady state, or dynamic,
unsteady state. The metallic material that receives the currents is known as the cathode and does
not corrode. The metallic material which the current leaves from is known as the anode and
corrodes (3).
Figure (16) Stray Current Corrosion Progression (8).
15.1. Controlling Parameters
Unlike the other forms of corrosion that have previously been discussed, stray current
corrosion initiates and propagates independently from the surrounding environmental conditions.
The unintended electric current that flows through the underground metallic material initiates the
corrosive attack. The severity of this attack depends on the type of current that is flowing
through the metal. A direct current, which is usually caused by dc rail transit systems, dc
welding equipment, and cathodic protection systems, causes a more severe form of stray current
corrosion. The direct current flow is continuous allowing stray current corrosion to propagate at
a more rapid pace. Alternating currents, which can originate from overhead ac power lines,
cause a less severe form of corrosion. As the frequency of the alternating current increases, the
amplitude decreases which decreases the damage caused by the stray current (6).
There is one instance in which stray current corrosion can initiate as a result of
environmental factors. Telluric currents, which are a form of stray currents, can be induced by
transient geomagnetic activity. The potential and current distribution of buried structures can be
influenced by such disturbances in the earth’s magnetic fields. Such effects, often assumed to be
of the greatest significance in close proximity to the poles, have been observed to be more
intense during periods of intensified sun spot activities. In general, harmful influences on
structures are of limited duration and do not remain highly localized to specific current pickup
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and discharge areas. Major corrosion problems as a direct result of telluric effects are therefore
relatively rare and cause much less corrosion damage compared to uncontrolled man-made stray
currents (4).
15.2. Preventative Methods
The most effective preventative method against stray current corrosion is preventing the
current from initially flowing. Without a current flowing between the anodic and cathodic
metallic materials, stray current corrosion cannot occur. However, in most instances, complete
prevention of the current is not possible due to the fact that it is generated from underground
power lines, electric trains, and welding generators. A few stray currents escape entering the
earth and are picked up by nearby buried metallic structures. If it is known that a nearby,
underground, metallic structure is being affected by stray current corrosion, it is important to
avoid the use of protective coatings. These coatings provide no protection for the metallic
material against stray current corrosion. If anything, the protective coating may accelerate the
rate of stray current corrosion if the coating contains any surface defects (6).
If preventing the current is not possible, there are a few methods to reduce the likelihood
of stray current corrosion initiation. The first option is to ground the stray current. By
grounding the current, a common return path for the current is created reducing the possibility of
stray current formation. Thus, the possibility of stray current corrosion in nearby underground
metallic materials is reduced. A second possible option is the creation of a sacrificial anode. If
the stray current cannot be prevented, the formation of a sacrificial anode will provide a path for
the stray current to flow through. Since this material is of no importance, the stray current
corrosion that will initiate is of no concern since the metallic material of greater importance is
not corroding. The last major method to control stray current corrosion is through the use of
insulation. An insulator or insulating material will have a very high resistance to the flow of
electrical current and is used to confine or control the flow of current in electrical circuits. By
insulting underground metallic materials, the possibility of a stray current flowing through them
is decreased which reduces the risk of stray current corrosion initiation and propagation (6).
16. CONCLUSION
By its definition, corrosion is the reaction between a material, which is usually metallic,
and the surrounding environment which causes the metal and its properties to deteriorate. By
looking at the major forms of corrosion that have been discussed throughout this report, it has
been determined that the key factors that affect corrosion initiation and propagation are the type
of metallic material being used and the environment surrounding that material. In order for the
onset of corrosion to be possible, a corrosive environment must be paired with a metal that is
susceptible to corrosion in that environment. If this is not the case, corrosion will not affect the
metal. The important issue to take into consideration is that environments vary around the
world. Different climates produce different environmental characteristics that will affect the
products surrounded by it. This leads to a variety of metallic materials being used around the
world in order to prevent corrosive attacks on critical structures and pieces of machinery.
As an engineer, it is crucial to investigate and research the environment that will be
surrounding the product that is being designed. Without the data and information about a
particular environment, the progression rates and severity of a corrosive attack cannot be
33
calculated. This lack of knowledge could lead to a material selection that promotes the onset of
corrosive behavior. Along the same lines, it is important for an engineer to understand the
properties of the materials that are being taken into consideration for construction use. Certain
materials react to environments in different ways. A material could promote corrosive behavior
in one environment but remain unaffected in another environment. Therefore, it is important to
look at the corrosivity of a material being exposed due to the surrounding environment as well as
the corrosivity of the surrounding environment due to the material being exposed.
Engineers need to pay close attention to the dependent relationship between the metallic
material being exposed and the environment surrounding that material when dealing with
corrosion. In almost all cases, the onset of corrosion will occur sometime sooner or later. By
designing a product with the proper material in a specific environment, the onset of a corrosive
attack can be delayed, if not prevented. However, once corrosion has initiated, preventative
forms of maintenance need to be executed in order to retard the progressive rate of corrosion and
extend the service life of certain products. It is important that these preventative forms of
maintenance are developed to be cost effective in order to ensure their continual use throughout
the service life of a product. If the cost of preventative maintenance is too great, their use will be
deterred putting the safety of the public in jeopardy. As an engineer, standards have been
developed in order to ensure that structures or mechanical pieces of equipment are safe for public
use. This requires engineers to inspect highway bridges, buildings, cars, ships, and planes on a
regular basis in order to ensure that these structures and pieces of equipment are safe. If it is
determined that a product is structurally inefficient, then that product must be shut down. Using
computer analysis programs, it can be determined that shutting down a product could prevent a
collapse or malfunction, which could have led to serious injury, or even death.
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