Intergranular corrosion in stainless steels

Intergranular corrosion of Fe-Ni-Cr alloys has been the subject of a comprehensive review by Cowan and Tedmon who summarised the various tests used for determining susceptibility (Table 19.3). Of these tests. Nos. 1-5, which are regarded as reliable test procedures by the ASTM, have been incorporated into ASTM A262 1986 Recommended Practice for Detecting Susceptibility to Intergranular Attack in Stainless Steel as follows ... 

Corradi and Gasperini claimed that the potentiostatic method was more effective and simpler than the Strauss test for determining intergranular corrosion of stainless steels, and suggested that the method may lend itself for use on finished equipment in service as a non-destructive test. 

Fig. 28. Micrographs of a fractured specimen, showing numerous small cracks on the gauge length (enlarged in (b)), and intergranular fracture (c) enlargement of fracture surface [57]. Copyright 1991. Electric Power Research Institute/EPRl ER-7247. Intergranular Corrosion of Stainless Steel. Vol. 1 Mechanism of Crack Initiation. Reprinted with permission.

Intergranular corrosion of stainless steel does not occur in all environments. Thus inferior corrosion resistance at the grain boundaries does not necessarily lead to practical problems. However, a reasonable aim is to obtain corrosion properties at the welds equal to those elsewhere on the steel surface, in order to utilize the material as much as possible. 

Dundas HJ, Bond AP. Niobium and titanium requirements for stabilization of ferritic stainless steels. In Steigerwald RF, editor. Intergranular Corrosion of Stainless Steel. STP 656. Philadephia ASTM, 1978. 

Precipitation processes of this kind are always caused by heat treatments, snch as sensitizing annealing, that are inappropriate for the alloy in question. For the austenitic chronuum-nickel-molybdenum steels used for the fabrication of chemical plant equipment, the critical tanperature range is 400-800°C. Chromium depletion through formation of chromium-rich carbides, mostly of the type (M23Cg), is the main cause of intergranular corrosion in these steels. The precipitation of chromium nitrides of importance only that the chromium-rich nitride (CrjN) can initiate intergranular corrosion, especially in ferritic steels. Since the intermetalUc phases in stainless steels contain appreciably less chromium than carbides and nitrides and their deposition is far slower, the chromium depletion related to these phases is minimal. 

A low carbon concentration reduces the risk of intergranular corrosion of stainless steels that results firom the precipitation of chromium carbides at the grain boundaries (Chapter 7). The presence of Ti, Nb or Ta in small amounts (stabilized stainless steels) also works this way, because these elements form particularly stable carbides. 

Streicher, M. A., Theory and Application of Evaluation Tests for Detecting Susceptibility to Intergranular Attack in Stainless Steels and Related Alloys—Problems and Opportunities, Intergranular Corrosion of Stainless Alloys, ASTM STP 656, R. F. Steigerwald, Ed., ASTM International, West Conshohocken, PA, 1978, pp. 3-84. 

As described before, intergranular corrosion of stainless steels occurs when the material is sensitized. This condition produces a chromium-depleted envelope around each grain, which is less corrosion-resistant. This results in intergranular corrosion of the stainless steel in certain environments. The two ASTM standards that describe how to test stainless steels for susceptibility to this form of corrosion are ASTM A 262 and A 763. A third standard, G 28, is applicable to nickel-rich, chromium-bearing alloys. 

Austenitic Stainless Steels—Determination of resistance to intergranular corrosion of stainless steels— Part 1 Austenitic and ferritic-austenitic (duplex) stainless steels—Corrosion test in nitric acid medium by measurement of loss in mass (Huey test)... 

AFNOR (1998) NF EN ISO 3651-1 1998. Determination of Resistance to Intergranular Corrosion of Stainless Steels. Part 1 Austenitic and Ferritic-Austenitic (Duplex) Stainless Steels. Corrosion Test in Nitric Acid Medium by Measurement of Loss in Mass (Huey Test), AFNOR, Paris. 

Huey test Corrosion testing in a boiling solution of nitric acid. This test is mainly used to detect the susceptibility to intergranular corrosion of stainless steel. 

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Doshi, C. P. and Austin, W. W., Effect of Grain Size on Carbide Precipitation and Intergranular Corrosion in AISI Type 201 Stainless Steel , Corrosion, 21, 332 (1965)... 

Hodges, R. J., Intergranular Corrosion in High Purity Ferritic Stainless Steel. Isothermal Time-Temp. Sensitisation Measurements , Corrosion, 27, 164 (1971)... 

Practices for detecting susceptibility to intergranular attack in austenitic stainless steels Recommended practice for detection of susceptibility to intergranular corrosion in severely sensitised austenitic stainless steel (intent to withdraw)... 

The austenitic stainless steels that are not stabilized or that are not of the extra-low-carbon types, when heated in the temperature range of 450 to 843°C (850 to 1,550°F), have chromium-rich compounds (chromium carbides) precipitated in the grain boundaries. This causes grain-boundary impoverishment of chromium and makes the affected metal susceptible to intergranular corrosion in many environments. Hot nitric acid is one environment which causes severe... 

Intergranular corrosion (weld decay) and stress corrosion cracking are problems associated with the use of stainless steels, and must be considered when selecting types suitable for use in a particular environment. Stress corrosion cracking in stainless steels can be caused by a few ppm of chloride ions (see Section 7.4.5). 

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