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GUIDELINES FOR CORROSION PREVENTION
LESSON 1: FORMS OF STAINLESS STEEL CORROSION
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LESSON 1: FORMS OF STAINLESS STEEL CORROSION
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Forms of Stainless Steel Corrosion
Ex
If stainless steel is selected, installed, and maintained correctly, it does not suffer corrosion. However, if
the environment exceeds the corrosion resistance of a particular stainless steel in a specific location,
some corrosion may occur. Only certain types of corrosion may affect stainless steels.
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TARNISHING
Tarnishing is a fairly uniform discolouration of a metal’s surface. With exterior stainless steel applications,
there may be a slight yellow tarnishing of the surface and some loss of brightness, especially if fine
particles of dirt are incorporated into the surface deposit. Some improvement may be obtained from
washing but the overall effect on appearance is small and may not be apparent when viewed from a
distance.
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PITTING
If a stainless steel corrodes, pitting is the most likely form of corrosion. If the environment overwhelms
the capability of the stainless steel, the protective, passive film is disrupted and cannot heal itself. This is
shown schematically in Figure 1a. (See Atmospheric Corrosion Section.) Pitting starts as tiny points of
attack and is usually black or dark brown in colour. In the most severe cases, the number and depth of
the pits can increase to give an extensively corroded appearance. If the attack is mild, the pits may not
detract from the general appearance but the area below them may be stained as rust leaches out.
Selecting an appropriate stainless steel and cleaning regularly to remove surface deposits reduce the
potential of pitting damage.
Figure 1a Pitting corrosion
Solution
Figure 1a
Passive film
Ex
Stainless steel
Figure 1b
Ex
Figure 1c
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CREVICE CORROSION
Crevice corrosion is similar to pitting but occurs over a larger area when deposits or other materials
block the oxygen access needed to maintain the passive film. Corrosion can occur if chloride-containing
rainwater or condensation is present in a tight crevice and conditions are created which exceed the
corrosion resistance of a particular stainless steel. It is more likely with lower-alloyed stainless steels,
particularly where the crevice gap is very small (a tight washer) and the path to free oxygen is long (a
large diameter washer). Correct design reduces the potential for crevice corrosion. In areas exposed to
water, designers should minimize crevices, seal them, or consider a more corrosion-resistant, higheralloyed grade. See Figure 2a.
Figure 2a Crevice corrosion
Metal or non-metal
Solution
Figure 2a
Passive film
Stainless steel
Ex
Figure 2b
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GALVANIC CORROSION
Galvanic or "bimetallic" corrosion can occur when two metals of differing electrochemical potential are
electrically coupled in a conducting liquid, usually called an electrolyte. Several factors determine
galvanic corrosion potential: the electrochemical potential difference, the presence of moisture to
connect the metals on a regular basis, and the relative surface area ratio of the metals. If no moisture is
present or an inert, electrically insulating material prevents electrical contact, galvanic corrosion cannot
occur.
Figure 3 illustrates galvanic corrosion.
Figure 3 Galvanic corrosion
Stainless steel
Current
Solution
Figure 3
Less noble metal
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Figure 4a
Figure 4b
Figure 4c
Figure 4d
Figure 4a shows examples of when galvanic
corrosion can and cannot occur. Figure 4b
illustrates electrochemical cell components
while Figures 4c and 4d illustrate anode and
cathode processes respectively. Figure 5
shows the galvanic series in seawater. The
metals are arranged in order from the least
noble (least corrosion-resistant) to the most
noble (most corrosion-resistant). Environment
affects a metal’s potential. Some metals are
shown twice along with the terms "active" or
"passive". In extremely severe corrosion
environments like strong acids, a stainless
steel may not be able to maintain its protective
passive film and may begin to corrode actively.
This might occur in an industrial equipment
application. These conditions are not present
in architectural applications and stainless
steels can be considered to be in the passive
state.
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Figure 5 Galvanic series in seawater at 77°F (25°C)
Least noble = anodic = most susceptible to corrosion
Magnesium and its alloys
Zinc
Galvanized steel or galvanized wrought iron
Aluminum alloys 3004, 3003, 1100, in this order
Cadmium
Low carbon steel
Wrought iron
Cast iron
Nickel cast irons
Stainless steel, Type 410 (active)
50-50 lead-tin solder
Stainless steel, Type 304 (active)
Stainless steel, Type 316 (active)
Lead
Tin
Muntz metal, C28000
Alloy 200 (active)
Alloy 600 (active)
Yellow brass, C27000
Aluminum bronzes, C61400
Red brass, C23000
Commercially pure copper, C11000
Silicon bronze, C65500
Alloy 200 (passive)
Alloy 600 (passive)
Alloy 400
Stainless steel, Type 410 (passive)
Stainless steel, Type 304 (passive)
Stainless steel, Type 316 (passive)
Alloy 825
Alloy 625
Alloy C
Silver
Titanium
Gold
Most noble = cathodic = most corrosion-resistant
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Galvanic corrosion may be a concern if there is a significant difference in electrochemical potential and
the metals are not electrically isolated from one another. If two metals are close together in the galvanic
series (e.g., two stainless steels or copper and stainless steel), the potential for galvanic corrosion is low
in all but the most aggressive environments.
The relative surface area of the two metals is important. When the surface area of the more corrosionresistant metal is large relative to the less corrosion-resistant metal, an unfavourable ratio exists and
there is an increase in the corrosion rate of the less corrosion-resistant metal. For example, coupling a
small piece of carbon steel to a large piece of stainless steel could cause rapid corrosion of the carbon
steel. If the ratio is reversed and the less corrosion-resistant material has a large surface area, the
corrosion rate of the less corrosion-resistant metal is only slightly increased.
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Dissimilar metal combinations should be avoided in areas where moisture is likely to accumulate and
remain for long periods. In well-drained exterior applications, dissimilar metals can be used together if a
favourable surface ratio exists, but they should be electrically insulated from one another.
Tip
Ex
Neoprene washers, roofing felt, paint, and other inert materials or coatings are effective barriers. When
painted carbon steel and stainless steel are welded together in an exterior application, the welded joint
should be painted. Hidden and exposed stainless steel fasteners with neoprene or other inert washers
are used regularly in aluminum, zinc, and painted galvanized steel roof applications. The inert washer
separates the metals in case water is frequently present or infiltrates under the head of the fastener.
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EMBEDDED OR TRANSFERRED IRON
Ex
Iron or carbon steel can become transferred to or embedded in the surface of stainless steel and other
architectural metals and begin to rust within a few hours or days. This can give the incorrect impression
that the material underneath is rusting. However, in severe cases, the rusting steel may actually cause
the stainless steel under it to corrode because the protective passive film cannot re-form.
The source of iron can be steel tools, abrasive polishing or blasting media or fabrication areas previously
used on carbon or low-alloy steels, use of carbon steel wool or carbon steel brushes during cleaning,
and accidental scratching. Ideally, the fabrication area should be dedicated to stainless steel. If that is not
possible, the area should be cleaned prior to stainless steel fabrication to remove residual iron particles.
To prevent accidental contamination, the stainless steel surface should be protected with protective
paper or strippable plastic films during fabrication, handling, storage and transport.
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The purchaser can specify that stainless steel products pass one of several non-destructive tests for
detection of embedded iron such as ASTM A 967. A particularly simple and straightforward test is to
thoroughly wet the surface with clean water and wait for 24 hours to see if rust appears. Additional
information about preventing, detecting and removing embedded iron and steel can be found in the
Nickel Development Institute publication, Fabrication and post-fabrication cleanup of stainless steels, No.
10 004.
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EROSION-CORROSION
?
Q
Erosion-corrosion is accelerated metal loss caused by a flowing corrosive liquid which contains abrasive
particles such as sand or debris. It can be a problem with aluminum, copper and other susceptible
materials in applications like piping and roof drainage systems. Resistance to erosion-corrosion is not
related to hardness or strength, but flow velocity, high turbulence, or changes in flow direction can have a
significant impact on performance in susceptible metals. Stainless steels are virtually immune to erosioncorrosion because they form thin, tightly adherent, protective passive films. High flow velocities are
beneficial to stainless steel corrosion performance because they help keep the stainless steel surface
clean.
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CHLORIDE STRESS CORROSION CRACKING, (SCC)
Chloride stress corrosion cracking (SCC) may occur in Types 304 and 316 exposed to chlorides and
tensile residual stress at temperatures above about 150°F (65°C). These conditions are unlikely in most
architectural applications.
Ex
SCC has occurred at lower temperatures in unusually severe indoor environments, such as swimming
pool suspended ceilings. The Nickel Development Institute publication No. 12 010, Stainless steel in
swimming pool buildings, provides additional information about appropriate grades for this application.
The potential for SCC in an aggressive marine environment was evaluated in a five-year study of 300series stainless steels in three metallurgical conditions: annealed (the normal as-delivered mill
condition), as welded, and cold-worked. The site for these tests, Kure Beach, North Carolina, U.S.A.,
experiences hot summers. The underside of the panels reached temperatures of about 120°F (50°C) and
the exposed side 140°F (60°C). None of the samples experienced stress corrosion cracking.
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Choose the most accurate statement:
A. Stainless steel never tarnishes or corrodes in an architectural application
B. If the proper grade of stainless steel is selected, it will never tarnish or corrode
C. If corrosion is seen on stainless, it is likely the material was made "cheaply"
D. Stainless will maintain its proper appearance if the proper grade is chosen, and it is
installed and maintained appropriately
E. Always use the same grade of stainless for all parts of a building
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Choose one of the following that best completes the sentence "Pitting is possible if the
stainless steel is exposed to..."
A. chlorides
B. acids
C. carbon monoxide
D. other metals
E. heat
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Which one of the following will not cause pitting on stainless steel (Choose one):
A. Urine
B. Road salt
C. Marine environment
D. Laundry bleach
E. Soap
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Crevice corrosion can occur underneath what kinds of crevices? (Choose one):
A. Metal to metal crevices
B. Glass or plastic to metal crevices
C. Crevices that are sometimes wet
D. Crevices that are always wet
E. All the above
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If I use 2 different metals that are in contact with each other, one will always get
galvanic corrosion.
A. True
B. False
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Choose one of the following that best completes the sentence "If galvanized (zinc coated steel)
fasteners are used to hold stainless steel panels in an external application,..."
A. that is bad because zinc is more active that than stainless steel, and the area ratio
effect is bad
B. that is not so bad because the area ratio is good even if zinc is more active than stainless
C. that is OK because stainless is more active than zinc, and the area ratio effect is good
D. that is not so bad because the iron in the fastener will protect the zinc coating
E. that cannot be so bad because other architects do it regularly
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I should design to ensure that components in the lower alloyed 304 and the higher alloyed 316
stainless never come in contact to avoid to prevent galvanic corrosion between these two metals.
A. True
B. False
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Some components in 304 stainless steel are now "rusting" even before they have been installed.
The most likely reason for this is (choose one):
A. The wrong grade was specified
B. Cheap imported 304 stainless steel
C. The stainless was contaminated with iron during fabrication or shipping or handling
D. The passive oxide layer didn't form
E. The wrong grade was delivered
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Choose one of the following that best completes the sentence "Stainless steels are very resistant
to flowing liquids even if they contain some abrasive particles because..."
A. stainless steels are form a thin, tightly adherent passive oxide layer that protects the metal
B. stainless steels are hard
C. stainless steels are soft
D. stainless steel contains hard wear-resistant particles
E. the statement is false
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Indoor swimming pool environments create special conditions where stainless steels may be
used but must be carefully selected.
A. True
B. False
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