Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
ASTM A262 is the specification which governs five practices used to determine if the Austenitic structure is susceptible to intergranular attack (IGA). This is not for determining resistance to Stress Corrosion Cracking (SCC) which is a transgranular attack. There is several cause for SCC, Chloride is the leading cause of transgranular cracking or attack. This specification is for detecting susceptibility to intergranular attack (IGA). Intergranular corrosion is the proper use, though the specification calls it attack, corrosion is what is taking place. Intergranular simply means that the corrosion is taking place between the grains or crystals, which is where sigma phase or chromium carbides are going to form which makes the material susceptible to IGA. All austenitic stainless steels should meet this requirement, if proper annealing took place.
Practice A—Oxalic Acid Etch Test for Classification of Etch Structures of Austenitic Stainless Steels. Practice A is a rapid screening examination of the microstructure to quickly determine if the structure is certain to be free of susceptibility to rapid intergranular attack. The samples are etched after metallographic preparation for cross-sectional examination which is thoroughly viewed with a traverse from inside to outside diameters of rods and tubes, from face to face on plates, and across all zones such as weld metal, weld-affected zones, and base plates on specimens containing welds. Typical examination magnification is 200X to 500X. Classification of structure then provides either acceptance or further testing required, which typically moves you to one of the next practices. If the structure is acceptable no additional testing is required.
Practice B—Ferric Sulfate–Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels. The ferric sulfate-sulfuric acid test does reveal susceptibility associated with a sigma-like phase constituent in stabilized stainless steels like AISI 321 or 347, as well as cast chromium-nickel-molybdenum stainless steels (CF-8M, CF-3M, C6-8M, and CG-3M). This practice does not reveal susceptibility to intergranular attack associated with sigma phase in wrought chromium-nickel-molybdenum alloys like 316/316L which materials have been known to cause rapid attack in some nitric acid environments.
Practice C—Nitric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels.
Practice E—Copper–Copper Sulfate–Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels.
Practice F—Copper–Copper Sulfate–50 % Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Molybdenum-Bearing Cast Austenitic Stainless Steel.
CMS provides testing for all of these practices; See Price List for costs.