scope:

These recommended practices cover the following five tests:

Practice A-Oxalic Acid Etch Test for Classification of Etch Structures of Stainless Steels (Sections 2 to 6, incl),

Practice B-Ferric Sulfate-Sulfuric Acid Test for detecting susceptibility to intergranular attack in stainless steels (see Sections 7 to 13, incl),

Practice C-Nitric Acid Test for detecting susceptibility to intergranular attack in stainless steels (see Sections 14 to 20, incl),

Practice D-Nitric-Hydrofluoric Acid Test for detecting susceptibility to intergranular attack in molybdenum-bearing austenitic stainless steels (see Sections 21 to 27, incl), and

Practice E-Copper-Copper Sulfate-Sulfuric Acid Test for detecting susceptibility to intergranular attack in stainless steels (see Section 28).

The following factors govern the application of these recommended practices:

Susceptibility to intergranular attack associated with the precipitation of chromium carbides is readily detected in all five tests.

Sigma phase in wrought chromium-nickel-molybdenum steels, which may or may not be visible in the microstructure, can result in high corrosion rates only in nitric acid.

Sigma phase in titanium or columbium stabilized alloys, which may or may not be visible in the microstructure, can result in high corrosion rates in both the nitric acid and ferric sulfate-sulfuric acid solutions.

The oxalic acid etch test is a rapid method of identifying, by simple etching, those specimens of certain stainless steel grades which are essentially free of susceptibility to intergranular attack associated With chromium carbide precipitates. these. specimens will have low corrosion rates in certain corrosion tests and therefore can be eliminated (screened) from testing as "acceptable."

The ferric sulfate-sulfuric acid test, the nitric acid test, and the nitric-hydrofluoric acid test are based on weight loss determinations and thus provide a quantitative measure of the relative performance of specimens evaluated. In contrast, the copper-copper sulfate-sulfuric acid test is based on visual examination of bend specimens and therefore classifies the specimens only as acceptable or non-acceptable.

In most cases either the 24-h copper-copper sulfate-sulfuric acid test or the 120-h ferric sulfate-sulfuric acid test, combined with the oxalic acid etch test, will provide the required information in the shortest time. All stainless grades listed in the accompanying table may be evaluated in these combinations of screening and corrosion tests, except those specimens of molybdenum-bearing grades (for example 316, 316L, 317, and 317L), which represents steel intended for use in nitric acid environments.

For AISI Grades 316, 316L, 317, and 317L only, the nitric-hydrofluoric acid test may be used to provide test result in 4 h.

The 240-h nitric acid test must be applied to stabilized and molybdenum-bearing grades intended for service in nitric acid and to all stainless steel grades which might be subject to end grain corrosion in nitric acid service.

Only those stainless steel grades are listed in Table 1 for which data on the application of the oxalic acid etch test and on their performance in various quantitative evaluation tests are available.