DESIGN FOR CORROSION RESISTANCE
 Failure of metal components by corrosion is as common as failure due to mechanical
causes, such as brittle fracture and fatigue.
 The National Institute of Standards and Technology estimates that corrosion annually
costs the United States $70 billion, of which at least $10 billion could be prevented by
better selection of materials and design procedures.
 Although corrosion failures are minimized by proper materials selection and careful
attention to control of metallurgical structure through heat treatment and processing,
many corrosion-related failures can be minimized by proper understanding of the
interrelation of the fundamental causes of corrosion and design details.
Basic Forms of Corrosion
 Corrosion of metals is driven by the basic thermodynamic force of a metal to return to
the oxide or sulfide form, but it is more related to the electrochemistry of the reactions
of a metal in an electrolytic solution (electrolyte).
 There are eight basic forms of corrosion.
Uniform attack.
 The most common form of corrosion is uniform attack.
 It is characterized by a chemical or electrochemical reaction that proceeds uniformly
over the entire exposed surface area.
 The metal becomes thinner, leading to eventual failure.
Galvanic corrosion.
 The potential difference that exists when two dissimilar metals are immersed in a
corrosive or conductive solution is responsible for galvanic corrosion.
 The less-resistant (anodic) metal is corroded relative to the cathodic metal.
 Table below gives a brief galvanic series for some commercial alloys immersed in
seawater.
A Brief Galvanic Series for Commercial Metals and Alloys
Noble (cathodic) Platinum
Gold
Titanium
Silver
316 stainless steel
304 stainless steel
410 stainless steel
Nickel
Monel
Cupronickel
Cu-Sn bronze
Copper
Cast iron
Steel
Active (anodic)
Aluminum
Zinc
Magnesium
 In this table, for any two metals or alloys in contact in seawater, the metal that is more
anodic (lower in the series) will be corroded.
 Note that the relative position in a galvanic series depends on the electrolytic
environment as well as the metal’s surface chemistry (presence of passive surface
films).
 Use pairs of metals that are close together in the galvanic series to minimize galvanic
corrosion and avoid situations in which a small anodic area of metal is connected to a
larger surface area of more noble metal.
 If two metals far apart in the series must be used in contact, they should be electrically
insulated from each other.
Crevice corrosion.
 An intense localized corrosion frequently occurs within crevices and other shielded
areas on metal surfaces exposed to corrosive attack.
 This type of corrosion usually is associated with small volumes of stagnant liquid
trapped in design features such as holes, gasket surfaces, lap joints, and crevices under
bolt and rivet heads.
Pitting.
 Pitting is a form of extremely localized corrosive attack that produces holes in the
metal.
 It is an especially insidious form of corrosion because it causes equipment to fail after
exposure to only a small percentage of the designed-for weight loss.
Intergranular corrosion.
 Localized attack along the grain boundaries with only slight attack of the grain faces is
called intergranular corrosion.
 It is especially common in austenitic stainless steel that has been sensitized by heating
to the range 950 to 1450°F.
 It can occur either during heat treatment for stress relief or during welding.
 When it occurs during welding it is known as weld decay.
Selective leaching.
 The removal of one element from a solid-solution alloy by corrosion processes is
called selective leaching.
 The most common example of it is the selective removal of zinc from brass
(dezincification), but aluminum, iron, cobalt, and chromium also can be removed.
 When selective leaching occurs, the alloy is left in a weakened, porous condition.
Erosion-corrosion.
 Deterioration at an accelerated rate is caused by relative movement between a
corrosive fluid and a metal surface; it is called erosion- corrosion.
 Generally the fluid velocity is high and mechanical wear and abrasion may be
involved, especially when the fluid contains suspended solids.
 Erosion destroys protective surface films and exacerbates chemical attack.
 A special kind of erosion-corrosion is cavitation, which arises from the formation and
collapse of vapor bubbles near the metal surface
 Another special form of erosion-corrosion is fretting corrosion. It occurs between two
surfaces under load that are subjected to cycles of relative motion.
Stress-corrosion cracking.
 Cracking caused by the simultaneous action of a tensile stress and contact with a
specific corrosive medium is called stress-corrosion cracking (SCC).
 The stress may be a result of applied loads or “locked-in” residual stress.
 Only specific combinations of alloys and chemical environments lead to stress
corrosion cracking. However, many occur commonly, such as aluminum alloys and
seawater, copper alloys and ammonia, mild steel and caustic soda, and austenitic steel
and salt water.
Corrosion Prevention
Material Selection.
 Selecting a material with a low rate of corrosion in the environment of concern is the
obvious first step to preventing corrosion.
 In general, the more noble the metal the slower it will corrode.
 Metals are the most susceptible to corrosion, while plastics in general have much
corrosion resistance.
 Some polymers absorb moisture, which causes swelling and degradation of
mechanical properties.
 While some aspects of material selection for corrosion are straightforward—for
example, avoiding materials that are attacked in the corrosive environment of interest
or are subject to SCC in the environment—other aspects of corrosion can be quite
subtle.
 Microscopic galvanic cells can be created in metallic alloys due to such things as
micro segregation of alloying elements (especially at grain boundaries), local cold
worked regions, or differences in galvanic potential between phases in multiphase
alloys.
 The behavior of a material in a corrosive environment can be significantly changed by
seemingly small changes in the corrosive environment. Such change factors are
temperature, the amount of dissolved oxygen, or impurities in the liquid.
Cathodic Protection.
 Cathodic protection reduces galvanic corrosion by supplying electrons to the anodic
metal that needs to be protected.
 This can be done by connecting the anodic metal to a sacrificial anode of an even
more anodic potential, such as Mg or Zn.
 The sacrificial anode must be in close proximity to the protected metal.
 It will be gradually corroded away, so it must be replaced periodically.
 Alternatively, cathodic protection can be achieved by applying a DC voltage to the
corrosion site that will oppose the one caused by the electrochemical reaction of
galvanic corrosion.
Corrosion Inhibitors.
 Specific chemical compounds can be added to the corrosive solution to reduce the
diffusion of ions to the metal-electrolyte interface.
 In many cases the inhibitor forms an impervious, insulating film covering either the
anode or cathode.
 The chromate salts added as inhibitors to radiator antifreeze are good examples. Other
inhibitors act as scavengers to reduce the amount of dissolved oxygen in the
electrolyte.
Protective Coatings.
 A common way to minimize corrosion is to provide a protective coating to the metal
to provide a barrier to the corrosive environment.
 Common examples are porcelain enamel, paint, and polymer coating.
 Electroplated metal coatings such as chromium are used both for corrosion protection
and for decorative purposes. Grease, oil, and wax are used as temporary coatings
during shipment or storage.
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Corrosion Prevention by Design
The essential strategy is to prevent a corrosive solution from coming in contact with a
vulnerable surface.
Tanks and containers should be designed for easy draining and easy cleaning.
Welded rather than riveted tanks will provide less opportunity for crevice
corrosion.
When possible, design to exclude air; if oxygen is eliminated, corrosion can often be
reduced or prevented.
Exceptions to that rule are titanium and stainless steel, which are more resistant to
acids that contain oxidizers than to those that do not.
An important factor in the design of metal parts to resist corrosion is the ratio of the
area of the anode to the cathode.
To minimize corrosion of the anode, its surface area should be as large as possible in
comparison with the surface area of the cathode. This will result in lower current
density at the anode, and a lower corrosion rate. For example, in galvanized steel the
zinc coating (anode) on the steel (cathode) will give protection to the steel
Great attention needs to be given to minimizing the ways that the electrolyte can come
in contact with the metal.
This can occur by direct immersion, by exposure to spray or mist, by alternate periods
of wetting and drying, as when it rains, by contact with moist earth, or by humidity in
the atmosphere.
It is important that design details minimize crevices, and that pipes drain out all
liquids so there is no residual liquid to cause corrosion.
Unavoidable crevices should be sealed with an elastomeric material.
Surfaces that are smooth collect less fluid and corrode less than rough surfaces.
Allow provisions in the design to clean equipment exposed to such things as mud, dirt,
corrosive atmospheres, and salt spray.
Combinations of corrosives with dirt or mud can cause galvanic action.
The severity of corrosion increases exponentially with temperature.
Steep temperature gradients and high fluid velocities can also increase corrosion
severity.
Designing with a corrosion allowance is another strategy.
In many situations, as in chemical plant design, it is more economical to select a
material with poorer corrosion resistance than a more expensive material with better
corrosion resistance and to make the part with larger dimensions (corrosion
allowance).
The part is kept in service until a critical dimension, such as the wall thickness, has
corroded to a predetermined limit, and then it is replaced.
This design approach requires that the design provides for easy replacement, and that a
rigorous inspection and maintenance program is in effect.
COST ANALYSIS
INTRODUCTION
 An engineering design is not complete until we have a good idea of the cost required
to build the design or manufacture the product.
 Generally, among functionally equivalent alternatives, the lowest-cost design will be
successful in a free marketplace.
 Understanding the elements that make up cost is vital because competition between
companies and between nations is fiercer than ever.
 The world is becoming a single gigantic marketplace in which newly developing
countries with very low labor costs are acquiring technology and competing
successfully with the well-established industrialized nations.
 Maintaining markets requires a detailed knowledge of costs and an understanding of
how new technology can lower costs.
 Decisions made in the design process commit 70 to 80 percent of the cost of a product.
Cost estimates are used in the following ways:
 To provide information to establish the selling price of a product or a quotation for a
good or service.
 To determine the most economical method, process, or material for manufacturing a
product. To become a basis for a cost-reduction program.
 To determine standards of production performance that may be used to control costs.
 To provide input concerning the profitability of a new product.
CATEGORIES OF COSTS
We can divide all costs into two broad categories: product costs and period costs.
Product costs are those costs that vary with each unit of product made.
 Material cost and labor cost are good examples.
 Another name for a product cost is variable cost , because the cost varies with the
volume of product made.
Period costs derive their name from the fact that they occur over a period of time regardless
of the amount (volume) of product that is made or sold.
 An example would be the insurance on the factory equipment or the expenses
associated with selling the product.
 Another name for period cost is fixed cost , because the costs remain the same
regardless of the volume of product made.
 Fixed costs cannot be readily allocated to any particular product or service that is
produced.
Yet another way of categorizing costs is by direct cost and indirect cost.
A direct cost is one that can be directly associated with a particular unit of product that is
manufactured.
 In most cases, a direct cost is also a variable cost, like materials cost.
 Advertising for a product would be a direct cost when it is assignable to a specific
product or product line, but it is not a variable cost because the cost does not vary with
the quantity produced.
An indirect cost cannot be identified with any particular product.
 Examples are rent on the factory building, cost of utilities, or wages of the shop floor
supervisors.
Often the line between direct costs and indirect costs is CONFUSING. For example,
equipment maintenance would be considered a direct cost if the machines are used
exclusively for a single product line, but if many products were manufactured with the
equipment, their maintenance would be considered an indirect cost.
The cost classifications of fixed and variable costs, examples are:
Fixed costs
1. Indirect plant cost
(a) Investment costs
Depreciation on capital investment
Interest on capital investment and inventory
Property taxes
Insurance
(b) Overhead costs (burden)
Technical services (engineering)
Product design and development
1. Nontechnical services (offi ce personnel, security, etc.)
General supplies
Rental of equipment
2. Management and administrative expenses
(a) Share of cost of corporate executive staff
(b) Legal staff
(c) Share of corporate research and development staff
(d) Marketing staff
3. Selling expenses
(a) Sales force
(b) Delivery and warehouse costs
(c) Technical service staff
Variable costs
 Materials
 Direct labor (including fringe benefi ts)
 Direct production supervision
 Maintenance costs
 Power and utilities
 Quality-control staff
 Royalty payments
 Packaging and storage costs
 Scrap losses and spoilage
Fix
xed costs such
s
as marketing
m
and sales costs, leggal expensse, securitty costs, financial
f
s
staff
exp
pense, andd adminisstrative coosts are often
o
lum
mped into an overaall categorry knownn as
gen
neral and aadministraative expeenses (G&
&A expenses).
Thee precedinng list off fixed annd variab
ble costs is meant to be illlustrative of the chief
cateegories off costs, butt it is not exhaustive
e
e.
Thee way the elements of cost buuild up to establish
e
a selling price
p
is shoown in
Fig
g. below.
e
o direct material,
of
m
d
direct
labo
or, and anyy other dirrect expen
nses
 The chhief cost elements
determ
mine the prrime cost .
 To it m
must be added
a
indiirect manu
ufacturingg costs succh as lightt, power, maintenan
nce,
suppliees, and facctory indirrect labor.. This is thhe factory cost .
 The m
manufacturring cost is made up
u of the factory cost
c
plus general
g
fixxed expen
nses
such as depreciaation, engiineering, taxes,
t
officce staff, annd purchaasing.
 The tootal cost iss the manuufacturing cost plus the sales expense.
e
 Finallyy, the selliing price is
i establish
hed by addding a proofit to the total
t
cost.
Breeak-Even Point
Sep
parating costs into fixed andd variable costs leadds to the concept of
o the breeakeven point
(BE
EP).
Thee break-evven point is the salles or prooduction volume
v
at which saales and costs
c
balannce.
Opeerating beeyond the BEP
B resullts in profi
fits; operatting below
w the BEP results in losses.
b the variiable cost ($/unit), and f be the
t fixed cost
c
Lett P be the unit saless price ($//unit), v be
($).. Q is thee number of producction unitss, or the sales
s
volu
ume of pro
oducts solld. The grross
pro
ofit Z is givven by 2
TEST DESIGN AND SELECTION OF MATERIALS (MM-402)
NAME:
ROLL # _________________
1) Defend the following statement (max 50 words).
It is becoming essential to develop a much closer working relationship between those who design a component and
those who advise the designer on materials selection.
2) Microscopic galvanic cells can be created in metallic alloys:
 Yes.

3) What do you mean by “tribology”.
 Contact mechanics.
 Triple system of forces.
No

The scientific study of friction, wear, and
lubrication
4) What do you mean by “analysis” wrt design/selection of materials?
 Cost modeling.
 Performance index.
 Decomposing the system into manageable parts
and their evaluation.
5) If we want to reduce the weight of following products, write functions, constraints & objectives and of them:
function
Oars.
constraint
Objective
function
Table Legs. constraint
Objective
function
Springs
constraint
Objective
function
Heat
constraint
Exchangers
Objective
6) Match the columns for Factors affecting Selection of materials.
Operating parameters
a) Biological effect
b) Special treatment
c) Aesthetic look
Manufacturing processes
d) Surface finish
e) Weldability
f) Taxes
Functional requirements
g) Inventory
h) Environment
Cost considerations
i) Corrosion requirement
j) Tooling
7) Categorise/Select a best possible method for manufacturing of BOLTS.
EFFECT ON THREAD STREGTH COST
FLEXIBILITY FINAL ANSWER
CASTING
POWDER
DEFORMATION
MACHINING
8) Consider the figure and answer below.
A
Your answer
(write only A or B)
B
The material index
is used here
for a beam of specified stiffness and minimum
weight
For a beam of specified stiffness and
minimum weight, obvious choice wrt material
would be Composite
The materials above the selection line with the
largest values of M, and it is these that are the
The material index
is used here for a
beam of specified stiffness and maximum weight
best choice wrt
best choice wr t
For a beam of specified stiffness and maximum
weight, obvious choice wrt material would be
Composite
The materials below the selection line with the
smallest values of M, and it is these that are the
9) Fill n the blanks.(chose appropriate word from the box below)
analysis
Rolling
Synthesis
forging
Heat treatment
inoculation
fretting
sliding
galvanic corrosion
hardness
fracture toughness
cavitation
Material index
Corrosion requirement
a) ___________contact is preferred to ____________contact.
b) Hard materials usually have low _______________
c) The potential difference that exists when two dissimilar metals are immersed in a corrosive or
conductive solution is responsible for _______________
d) ___________ involves the identification of the design elements that will comprise the product, its
decomposition into parts, and the combination of the part solutions into a total workable system.
e) _______________ allows manipulation of strength, ductility and toughness
f) _______________ is a combination of materials properties that characterises the Performance of a
material in a given application.
g) A special kind of erosion-corrosion is _______________, which arises from the formation and collapse
of vapor bubbles near the metal surface
10) Summarise the following figures in few words.
DESIGN OF A MILD STEEL
The yield strength of mild steel with an average grain size of 0.05 mm is 20,000
psi. The yield stress of the same steel with a grain size of 0.007 mm is 40,000 psi.
What will be the average grain size of the same steel with a yield stress of 30,000
psi? Assume the Hall-Petch equation is valid and that changes in the observed
yield stress are due to changes in grain size.
Solution:
σy = σ0+ Kd-1/2
Where σy is the yield strength, d is the average dia of the grains, and
σ0 and K are the constants for the metal.
Thus, for a grain size of 0.05 mm the yield stress is
20 x 6.895 MPa = 137.9 MPa.
(Note: 1,000 psi = 6.895 MPa). Using the Hall-Petch equation
137.9 = σ0 + K / √0.05
For the grain size of 0.007 mm, the yield stress is 40 x 6.895 MPa = 275.8
MPa. Therefore, again using the Hall-Petch equation:
275.8 = σ0 + K / √0.007
Solving these two equations K = 18.43 MPa_mmI/2, and σ0 = 55.5 MPa.
Now we have the Hall-Petch equation as
σy = 55.5 + Kd-1/2
If we want a yield stress of 30,000 psi or 30 x 6.895 = 206.9 MPa, the grain
size will be 0.0148 mm or 14.8 µ-m.
The Effect of grain size on the yield strength of steel at room temperature
1