Intergranular Corrosion,Evaluation of Intergranular Corrosion
IN THE ARTICLE "Localized Corrosion" in this Volume, intergranular corrosion is defined and the mechanisms are
described. It is the purpose of this article to discuss when to evaluate for susceptibility to intergranular attack and how to
determine which of the various evaluation tests are applicable. However, it may first be necessary to review the
methodology of intergranular corrosion and its effect on the various alloy families.
Most alloys are susceptible to intergranular attack when exposed to specific environments. This is because grain
boundaries are sites for precipitation and segregation, which makes them chemically and physically different from the
grains themselves. Intergranular attack is defined as the selective dissolution of grain boundaries, or closely adjacent
regions, without appreciable attack of the grains themselves. This is caused by potential differences between the grainboundary
region and any precipitates, intermetallic phases, or impurities that form at the grain boundaries. The actual
mechanism differs with each alloy system.
Precipitates that form as a result of the exposure of metals at elevated temperatures (for example, during production,
fabrication, and welding) often nucleate and grow preferentially at grain boundaries. If these precipitates are rich in
alloying elements that are essential for corrosion resistance, the regions adjacent to the grain boundary are depleted of
these elements. The metal is thus sensitized and is susceptible to intergranular attack in a corrosive environment. For
example, in austenitic stainless steels such as AISI type 304, the cause of intergranular attack is the precipitation of
chromium-rich carbides [(Cr, Fe)23C6] at grain boundaries. These chromium-rich precipitates are surrounded by metal that
is depleted in chromium; therefore, they are more rapidly attacked at these zones than on undepleted metal surfaces.
Impurities that segregate at grain boundaries may promote galvanic action in a corrosive environment by serving as
anodic or cathodic sites. Therefore, this would affect the rate of dissolution of the alloy matrix in the vicinity of the grain
boundary. An example of this is found in aluminum alloys when they contain intermetallic compounds, such as Mg5Al8
and CuAl2, at the grain boundaries. During exposures to chloride solutions, the galvanic couples formed between these
precipitates and the alloy matrix can lead to severe intergranular attack. Susceptibility to intergranular attack depends on
the corrosive solution and on the extent of intergranular precipitation, which is a function of alloy composition,
fabrication, and heat treatment parameters.
Corrosion tests for evaluating the susceptibility of an alloy to intergranular attack are typically classified as either
simulated-service or accelerated tests. The first laboratory tests for detecting intergranular attack were simulated-service
exposures. These were first observed and used in 1926 when intergranular attack was detected in an austenitic stainless
steel in a copper sulfate-sulfuric acid (CuSO4-H2SO4) pickling tank (Ref 1). Another simulated-service test for alloys
intended for service in nitric acid (HNO3) plants is described in Ref 2. In this case, for accelerated results, iron-chromium
alloys were tested in a boiling 65% HNO3 solution.
Over the years, specific tests have been developed and standardized for evaluating the susceptibility of various alloys to
intergranular attack. For example, tests for the low-alloy austenitic stainless steels have been standardized by the
American Society for Testing and Materials (ASTM) in Standard A 262, with its various practices (A to E). Practice A is
a screening test that uses an electrolytic oxalic acid etch combined with metallographic examination. The other practices
involve exposing the material (possibly after a sensitizing treatment) to boiling solutions of 65% HNO3, acidified ferric
sulfate (Fe2(SO4)3) solution, nitric-hydrofluoric acid (HNO3-HF) solution, or acidified CuSO4 solution, depending on the
specific alloy and its application. Similar ASTM tests have been developed for other higher-alloyed stainless steels,
ferritic stainless steels, high nickel-base alloys, and aluminum alloys (Table 1

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