3.1 Galvanic or Two-Metal CorrosionGalvanic or Two-Metal Corrosion
also called ' dissimilar metal corrosion‘.
Takes place when two metals are in physical contact with each other and are
immersed in a conducting fluid.
corrosion damage induced when two dissimilar materials are coupled in a
1. Plate and screw of different electrical potentials due to differences in
2. Multiple component implant using different metals for each component
3. Copper and steel tubing are joined in a domestic water heater, the
steel will corrode in the vicinity of the junction
The following fundamental requirements have to be met for galvanic
1. Dissimilar metals (or other conductors, such a graphite).
2. Electrical contact between the dissimilar conducting materials (can be
direct contact or a secondary connection such as a common grounding
3. Electrolyte (the corrosive medium) in contact with the dissimilar
Noble, cathodic end
18-8 austenitic stainless steels (passive condition)
Iron-chromium alloys (passive condition)
18-8 Austenitic stainless steels (active)
13% Chromium stainless steel (active)
Mild steel and iron
Magnesium and magnesium alloys
Active, anodic end
The relative nobility of a material can be predicted by measuring
its corrosion potential. The well known galvanic series lists the
relative nobility of certain materials in sea water. A small
anode/cathode area ratio is highly undesirable. In this case, the
galvanic current is concentrated onto a small anodic area. Rapid
thickness loss of the dissolving anode tends to occur under these
conditions. Galvanic corrosion problems should be solved by
designing to avoid these problems in the first place.
Fig. Galvanic corrosion between
stainless steel screw and
Fig. Galvanic corrosion between
Steel and Brass.
Fig. Anodic- cathodic behavior of steel with zinc
and tin outside layers exposed to the
(a) zinc is anodic to steel and corrodes
(b) steel is anodic to tin and corrodes (the tin
layer was perforated before the corrosion
Note that the area ratio of the anode: cathode
is an important variable affecting the dissolution
current density (and hence corrosion rate)
pertaining to the anode. The area ratio is also
important when considering the relative amount
of current "available" from the cathodic reaction.
The following have been described as "main factorsmain factors"
influencing galvanic corrosion rates in Skanaluminium's on-
line publication "Alubook - Lexical knowledge about
• Potential Difference between materials
• Cathode Efficiency
• Surface areas of connected materials (area ratio)
• Electrical resistance of the connection between the
materials and of the electrolyte.
Fig. Brass on Weathering SteelBrass on Weathering Steel - rust forms in discrete
crystallites that are fine, red and diffusely reflecting, like
hematite. The massive re-crystallized layer is a shiny blue,
approaching the blue-black of secular hematite. 5
3.2 Crevice CorrosionCrevice Corrosion
Crevice corrosion is a localized form of corrosion usually associated with a stagnant
solution on the micro-environmental level.
Such stagnant microenvironments tend to occur in crevices (shielded areas) such as
those formed under gaskets, washers, insulation material, fastener heads, surface
deposits, disbonded coatings, threads, lap joints and clamps.
Occurs under gaskets, rivets and bolts, between valve disks and seats.
Well-known examples of such geometries including flanges, gaskets, disbonded
linings/coatings, fasteners, lap joints and surface deposits.
Crevice corrosion is initiated by changes in local chemistry
within the crevice:
• Depletion of inhibitor in the crevice
• Depletion of oxygen in the crevice
• A shift to acid conditions in the crevice
• Build-up of aggressive ion species (e.g.
chloride) in the crevice
Rivets : a metal pin for passing through holes in two or more plates or pieces to hold them
together, usually made with a head at one end, the other end being hammered into a head
Depletion : to decrease seriously or exhaust the abundance or supply of
Mechanism- Chronology of Crevice Corrosion
At time zero, the oxygen
content in the water
occupying a crevice is equal
to the level of soluble
oxygen and is the same
Because of the difficult access
caused by the crevice geometry,
oxygen consumed by normal
uniform corrosion is very soon
depleted in the crevice. The
corrosion reactions now specialize
in the crevice (anodic) and on the
open surface (cathodic).
The crevice development a few more accelerating factors fully develop:
1. The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons
(acid) and forming corrosion products. The pH in a crevice can reach very acidic values,
sometimes equivalent to pure acids.
2. The acidification of the local environment can produce a serious increase in the corrosion rate of
most metals. See, for example, how the corrosion of steel is affected as a function of water pH.
3. The corrosion products seal even further the crevice environment.
4. The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in
the environment, such as chlorides and sulfates, that can be corrosive in their own right.
Fig. Schematic illustration (initial stage) of the mechanism for
crevice corrosion between two riveted sheets.
Fig. Pack rust is a form a localized corrosion typical of steel
components that develop a crevice into an open atmospheric
environment. This expression is often used in relation to bridge
inspection to describe built-up members of steel bridges which
are showing signs of rust packing between steel plates.
Zebra mussels- an example of marine environment
Underside of panel where severe corrosion was found
Close-up picture showing the severity of corrosion9
Crevice Corrosion Testing
ASTM G78ASTM G78
Standard Guide for Crevice Corrosion TestingStandard Guide for Crevice Corrosion Testing
A good example of how crevice corrosion can be reproduced and accelerated in a laboratory environment is the
formation of occluded cells with multiple crevice assemblies (MCAs), as described in the ASTM G78 Standard Guide for
Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing
In this test, washers make a number of contact sites on either side of the specimens. The number of sites showing
attack in a given time can be related to the resistance of a material to initiation of localized corrosion, and the average or
maximum depth of attack can be related to the rate of propagation. The large number of sites in duplicate or triplicate
specimens is amenable to probabilistic evaluation.
The susceptibility to localized corrosion becomes quite visible once a specimen equipped with these Teflon washers has
been exposed to a corrosive environment for an extended period of time.
... after 30 days in 0.5 FeCl3 + 0.05 M NaCl10
3.3 Pitting CorrosionPitting Corrosion
Pitting corrosion is a localized form of corrosive attack that produces holes or small
pits in a metal.
the bulk of the surface remains unattacked.
Pitting is often found in situations where resistance against general corrosion is
conferred by passive surface films.
Localized pitting attack is found where these passive films have broken down.
Pitting attack induced by microbial activity, such as sulfate reducing bacteria (SRB)
also deserves special mention.
Pitting can be one of the most dangerous forms of corrosion because it is difficult to
anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates
a metal without causing it to lose a significant amount of weight.
Special case of crevice corrosion:
1. Initiated by inclusions, scratches, or handling damage instead of
2. The presence of static flow conditions and reduced oxygen
availability are less important than in crevice corrosion 12
Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable
membrane of corrosion products. Pits can be either hemispherical or cup-shaped.
Pitting is initiated byPitting is initiated by:
1. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which
can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend
to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)
2. Localized damage to, or poor application of, a protective coating.
3. The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic
A local cell that leads to the initiation of a pit canA local cell that leads to the initiation of a pit can
be caused by an abnormal anodic sitebe caused by an abnormal anodic site
surrounded by normal surface which acts as asurrounded by normal surface which acts as a
cathodecathode, or by the presence of an abnormal, or by the presence of an abnormal
cathodic site surrounded by a normal surface incathodic site surrounded by a normal surface in
which a pit will have disappeared due towhich a pit will have disappeared due to
Fig. Local Cathode on a corroded piece of material
FIG The pitting of a 304 stainless
steel plate by an acid-chloride solution.
Alloying can have a significant impact on the pitting resistance of stainless steels.
Conventional steel has a greater resistance to pitting than stainless steels, but is still susceptible,
especially when unprotected.
Aluminum in an environment containing chlorides and aluminum brass (Cu-20Zn-2Al) in contaminated
or polluted water are usually susceptible to pitting.
Titanium is strongly resistant to pitting corrosion.
Proper material selection is very effective in preventing the occurrence of pitting corrosion. Another
option for protecting against pitting is to mitigate aggressive environments and environmental
components (e.g. chloride ions, low pH, etc.).
Inhibitors may sometimes stop pitting corrosion completely.
Further efforts during design of the system can aid in preventing pitting corrosion, for example, by
eliminating stagnant solutions or by the inclusion of cathodic protection.
Fig. Corrosion Pits are the primary source
of leaks in water handling systems
Sewer Explosion due to Pitting Corrosion
An example of corrosion damages with shared responsibilities was
the sewer explosion that killed 215 people in Guadalajara, Mexico,
in April 1992. Besides the fatalities, the series of blasts damaged
1,600 buildings and injured 1,500 people.
This example of a pitted
surface was produced
by exposing a specimen
of aluminum A92519
to 3.5% NaCl during
seven days. The width
of the picture is
approximately 1 mm.
corrosion of a metal surface, confined to a
point or small area, that takes the form of
ratio of the depth of the deepest pit
resulting from corrosion divided by the
average penetration as calculated from
Pitting resistance equivalent number
an empirical relationship to predict the
pitting resistance of austenitic and duplex
stainless steels. It is expressed as PREN =
Cr + 3.3 (Mo + 0.5 W) + 16N
*Sewer: an artificial conduit, usually underground, for carrying off waste
water and refuse, as in a town or city.
Aircraft Location of
and pitting present
Serious AR, USA 1997 NTSB
DC-6 Engine, master
Corrosion pitting Fatal AK, USA 1996 NTSB
Piper PA-23 Engine,
Corrosion pitting Fatal AL, USA 1996 NTSB
Boeing 75 Rudder Control Corrosion pitting Substantial
damage to plane
WI, USA 1996 NTSB
Embraer 120 Propeller
Corrosion pitting Fatal and
serious, loss of
GA, USA 1995 NTSB
Hydraulic Line Corrosion pitting Loss of plane, no
AZ, USA 1994 NTSB
Corrosion pitting Loss of plane, no
AK, USA 1994 NTSB
Embraer 120 Propeller
Corrosion pitting Damage to plane,
Canada 1994 NTSB
Embraer 120 Propeller
Corrosion pitting Damage to plane,
Brazil 1994 NTSB
Loss of TEF Australi
3.43.4 Intergranular CorrosionIntergranular Corrosion
Intergranular corrosion refers to preferential (localized) corrosion along grain
or immediately adjacent to grain boundaries, while the bulk of the grains remain largely
This form of corrosion is usually associated with chemical segregation effects (impurities
have a tendency to be enriched at grain boundaries) or specific phases precipitated on
the grain boundaries.
This selective dissolution may lead to the dislodgement of grains.
Intergranular corrosion in sensitized stainless steels and exfoliation in aluminum
alloys represent industrially significant examples of this form of damage.
Also known as “knife- line attack”
classic example is the sensitization of
stainless steels or weld decay
FIG. Intergranular corrosion of a failed aircraft
component made of 7075-T6 aluminum (picture
width = 500 mm)
Fig. Severe problem in the welding of stainless steels,
when it is termed weld decay.