CREVICE CORROSION
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CREVICE CORROSION
Narrow openings, gaps, spaces, pores etc. between metal-metal
components or metal-non-metal components may provoke localized
corrosion.
NOTE: unintentional crevices (seams, cracks etc.) can also act in the same
way
Passive alloys (especially stainless steels) are more vulnerable than more
active alloys.
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Crevice corrosion at a metal-to-metal crevice site formed between
components of type 304 stainless steel fastener in seawater
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Crevice Corrosion occurring on a Test
specimen of Type 316 SS (Stainless Steel) in
Acid Condensate Zone of a Model SO2
Scrubber.
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• Crevice corrosion is caused by the existence of
small volumes of stagnant (corrosive) solution.
Small holes, gasket surfaces, lap joints, bolt or
rivet heads, nuts, washers, surface deposits all
can cause C.C. (Crevice Corrosion).
• Type 304 SS sheet can be cut by stretching a
rubber band around it, immersing it in seawater
(Fontana). The crevice between the rubber and
the metal acts as the cutting zone.
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MECHANISM
• Consider riveted metal section, immersed in aerated
seawater (pH 7) e.g. stainless steel
• Although O2 within crevice is rapidly used up, corrosion
continues, controlled by overall cathodic reaction
outside the crevice.
• Tendency to build up M+ within the crevice must be
balanced by -ve charge diffusing in.
• Some OH- diffuses in, alot of Cl- diffuses in (OH- more
mobile, c.f. FONTANA* … less of it).
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MECHANISM
Oxidation:
M  M+ + e-
Reduction:
O2 + 2 H2O + 4 e-  4 OH-
*FONTANA says Cl- more mobile … this is wrong.
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• Initial stages …
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• Most metal ions (except alkali metals) hydrolyze:
M+ + H2O  MOH  + H+
• Both Cl- and H+ accelerate metal dissolution (Cl- breaks
down oxide, also H+ partly responsible).
• NOTE: there is now the possibility for H2 evolution within
the crevice . . . . maybe!
• NOTE: solution within crevices exposed to neutral dilute
NaCl has been seen to have 3-10x [Cl-] in bulk, pH of 2-3.
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• Later stages …
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Factors that can affect Crevice Corrosion resistance of stainless
steels
Geometrical
Type of crevice:
metal to metal
nonmetal to metal
Crevice gap (tightness)
Crevice depth
Exterior to interior surface
area ratio
Electrochemical reactions:
Metal dissolution
O2 reduction
H2 evolution
Metallurgical
Alloy composition:
major elements
minor elements
impurities
Passive film characteristics
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Environmental
Bulk solution:
O2 content
pH
chloride level
temperature
agitation
Crevice solution:
hydrolysis equilibria
Biological influences
Mass transport, migration
Diffusion and convection
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CREVICE CORROSION OF CU ALLOYS different from that of SS; attack
occurs OUTSIDE the crevice
“Crevice corrosion” of Alloy
400 (70 Ni - 30 Cu) after 45
Days in Natural Seawater
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• Cathodic conditions inside crevices of Cu alloy have been inferred from
observations of deposited Cu metal (where?).
• “Differential metal-ion concentration” has been invoked as the
mechanism; greater concentration inside the crevice raises the potential
and makes it more noble than the outside.
• Also, variant of the oxygen depletion mechanism has been invoked.
• Clearly the situation is complex.
• NOTE: ALL MATERIALS ARE SUSCEPTIBLE TO C.C. (CREVICE CORROSION),
GIVEN NARROW CREVICES, POSSIBILITY OF CONCENTRATING IONS,
DIFFERENTIAL AERATION CELLS, DIFFERENTIAL METAL CONCENTRATION
CELLS, ETC.
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COMBATING CREVICE CORROSION (after Fontana)
Methods and procedures for combating or minimizing crevice
corrosion are as follows:
1. Use welded butt joints instead of riveted or bolted
joints in new equipment. Sound welds and complete
penetration are necessary to avoid porosity and
crevices on the inside (if welded only from one side).
2. Close crevices in existing lap joints by continuous
welding, caulking, or soldering.
3. Design vessels for complete drainage; avoid sharp
corners and stagnant areas. Complete draining
facilitates washing and cleaning and tends to prevent
solids from settling on the bottom of the vessel.
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COMBATING CREVICE CORROSION (after Fontana)
4. Inspect equipment and remove deposits
frequently.
5. Remove solids in suspension early in the process,
if possible.
6. Remove wet packing materials during long
shutdowns.
7. Provide uniform environments, if possible, as in
the case of backfilling a pipeline trench.
8. Use "solid," nonabsorbent gaskets, such as
Teflon, wherever possible.
9. Weld instead of rolling in tubes in tube sheets.
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• FlLIFORM CORROSION
• Although not immediately apparent, filiform corrosion (filamentary
corrosion occurring on metal surfaces) is a special type of crevice
corrosion.
• In most instances it occurs under protective films, and for this
reason it is often referred to as underfilm corrosion.
• This type of corrosion is quite common; the most frequent example
is the attack of enameled or lacquered surfaces of food and
beverage cans that have been exposed to the atmosphere. The redbrown corrosion filaments are readily visible.
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• Filiform corrosion usually on metals covered with thin
organic film (0.1 mm) BUT also reported for:
–
–
–
–
–
–
–
–
steel;
magnesium;
aluminum;
tin;
silver;
gold;
phosphate;
enamel;
• as well as with organics (lacquer, paper; seen on paperbacked Al foil, between metal and paper).
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A lacquered steel can lid exhibiting filiform corrosion showing both
large and small filaments partially oriented in the rolling direction of
the steel sheet. Without this 10 x magnification by a light microscope,
the filiforms look like fine striations or minute tentacles; (often
mistaken for biologically – induced corrosion).
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• Attack usually occurs when relative humidity between 65
and 90%; has been seen at lower R.H. (Relative Humidity).
• Average filament width  0.05 - 3 mm .. depending on
coating (thickness, porosity, etc.), R.H., and corrosiveness of
environment (presence of SO2, H2S, etc.).
• Filament height  20 m. Growth rates observed between
0.01 mm/d and 0.85 mm/d.
• Filaments are like minute tunnels, full of corrosion
products.
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• Liquid in “head” is typically acidic. . . . pH 1- 4.
• IN ALL CASES.... O2 (or air) and water are needed
to sustain filiform corrosion … indicates a form of
a DIFFERENTIAL AERATION CELL.
Discuss:
differential aeration cell
concentration cell.
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An aside on differential aeration . .
remember example:
(differential) concentration cell
remember Cu - alloy crevice:
(potential  log [Cu+])
Attack usually begins at imperfections in coating e.g., cuts, knicks, pores, etc.
CO2 can stimulate process by dissolving in water  carbonic acid.
Chlorides, SO42-, S2- which can dissolve in condensing atmospheric moisture also
increase attack.
Optimum temperature for filiform attack between 20 & 35C.
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Effect of humidity on filiform corrosion of
enameled steel
Relative humidity, %
Appearance
0-65
No corrosion
65-80
80-90
93
95
100
Very thin filaments
Wide corrosion filaments
Very wide filaments
Mostly blisters, scattered filiform
Blisters
Source: M. Van Loo, D. D. Laiderman, and R. R. Bruhn, Corrosion, 9:2 (1953).
Appearance:
Schematic diagram of a corrosion
filament growing on an
iron surface (magnified).
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Schematic diagrams illustrating the
interaction between corrosion
filaments:
(a) Reflection of a corrosion
filament;
(b) splitting of a corrosion
filament;
(c) joining of corrosion filaments;
(d) “death trap”.
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Mechanism
Head supplied with H2O by osmosis (high concentration of Fe2+ inside)
through coating and from precipitated hydroxide/oxide. Oxygen reduction
creates hydroxide; precipitation creates corrosion product tail. . . . further
oxidation to Fe3+ oxide etc. Hydrolysis of salts in head creates acidic
conditions.
Details of mechanism
not understood………….
e.g., why do filaments
“reflect” off other filaments? etc.
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Filiform Corrosion on PVC-Coated Al Foil
Advancing head and cracked tail of a
filiform cell.
Gelatinous corrosion products oozing
out of porous tail section.
Scale:
Scale:
0.125 mm
1.25 m
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Close-up of the advancing head shown in
adjacent figure. Minute cracks can be seen
at the head/tail interface of a filiform
corrosion cell. These cracks are entry points
for water and air to provide a source of
hydroxyl ions and an electrolyte.
Intermediate corrosion products are just
beginning to form in the head, and they
undergo further reaction to form an
expanded tail. The tail region is a
progressive reaction zone that ultimately
forms spent corrosion products. Between
the head and porous end, ions gradually
react with water and oxygen and are slowly
transported in the direction of the tail to
form final corrosion products.
Scale:
15 m
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Prevention NOT EASY....
• store coated metals in dry air;
• use more brittle films . . . these should
crack at head and destroy differential
aeration;
• use impermeable coatings.
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Pitting Corrosion
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Pitting Corrosion
PITTING: Extreme localized attack, may perforate metal sheet/plate . . . etc.
“Pitting factor” =
p
d
d = average penetration from weight loss;
p = deepest penetration
“Undercutting”
pit opening usually < 1 mm.
Pits may overlap to give the
appearance of rough, general wastage.
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Pitting is an insidious and destructive form of corrosion:
– difficult to detect (pits may be small on surface, but
extensive below surface from undercutting; may be
covered with deposit);
– can cause equipment to fail (by perforation) with very
little weight loss;
– difficult to measure as pit depth and distribution vary
widely under (nominally) identical conditions;
– “incubation” period may be months or years.
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Pitting of 18-8 stainless steel by acid-chloride solution.
Pitting of stainless steel condenser tube.
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Pits usually occur on upward-facing horizontal surfaces,
pit growth
and less frequently on vertical surfaces;
pit growth
Gravity is involved.
rarely on downward-facing surfaces;
pit growth
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Mechanism: Has some features in common with CREVICE CORROSION....
consider metal M being pitted by aerated NaCl solution...
Autocatalytic processes
occurring in a corrosion pit.
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Remember:
•
inside pit - anodic, rapid dissolution;
•
outside pit - cathodic, O2 reduction;
•
most M+ will hydrolyse, form H+;
•
positive charges attract Cl- ions;
•
H+ and Cl- accelerate metal dissolution;
•
high ionic concentrations in pit make O2 solubility very low;
•
high density of solution within pits means pits are more stable
when growing downwards;
•
static environment accelerates process.
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At high pH (i.e., high OHconcentration), precipitation of iron
hydroxides and oxidation to Fe3+
oxides can lead to corrosion
product caps or tubes around pits
on steels.
Corrosion tube growth mechanism.
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Metals Susceptible to Pitting
Most often, passivating metals, especially stainless steels, often in
passivating environments (e.g., containing oxygen) but with agents such as
Cl- that attack the passive oxide film.
SENSITIZED SS particularly vulnerable (its heat treatment has depleted the
grain boundaries of Cr by precipitating chromium carbide).
COLD WORKING increases pitting attack, perhaps dislocation pattern is
important.
DISCUSS
ETCHED or GROUND surfaces more likely to pit than polished surfaces.
Stainless Steel more susceptible than Carbon Steel (though CS will have more
rapid GENERAL CORROSION).
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Some alloys developed especially to resist pitting.
Effects of alloying on pitting resistance of stainless steel alloys
Element
Effect on pitting resistance
Chromium
Nickel
Molybdenum
Silicon
Titanium and niobium
Increases
Increases
Increases
Decreases; increases when present with molybdenum
Decreases resistance in FeCI3, other mediums no
effect
Decreases
Decreases, especially in sensitized condition
Increases
Sulfur and selenium
Carbon
Nitrogen
Source: N. D. Greene and M. G. Fontana, Corrosion 15:25t (1959).
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Pitting Environments
Usually, solutions containing chloride or chlorine-containing ions (e.g.,
hypochlorites [bleaches]) have strong pitting tendencies.
Bromides are also aggressive, but fluorides and iodides are not.
Cupric, ferric and mercuric ions promote pitting . . . easily reduced
cathodically and do not require dissolved O2; CuCl2 and FeCl3 are
extremely aggressive (latter used as a test solution).
Thiosulphate ion (S2O32-) may also promote pitting.
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Evaluating Pitting Attack
Weight loss of test specimens no good ( . . . why ?).
Measurement of pit depth complicated because of statistical variations.
Relationship between pit depth
and the number of pits
appearing on a corroded surface.
Average pit depth of little use, since it is the deepest pit that causes failure.
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MAXIMUM PIT DEPTH can be a useful way of expressing pitting corrosion, and for
comparing pitting resistance of standard test samples.
HOWEVER, statistical nature of pitting means that sample size is important.
Pit depth as a function of
exposed area.
Should never predict lifetime of plant components from tests on small samples.
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Examples of pitting corrosion:
– Alloy-800 SG tubes with phosphate
chemistry…pitting  severe pitting 
wastage.
Point Lepreau had some pitting, switched to AVT.
– SS cooling water H.X. left static under silted
conditions…severe pitting; replaced with Ti
plate-type.
– Others?
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