Potentiodynamic Reactivation Method for Detecting Sensitization in AISI 304 and 304L Stainless Steels

Electrochemical methods have been proposed as a nondestructive, quantitative means for determining the degree of sensitization associated with the chromium-depleted zones surrounding precipitates of chromium carbides in stainless steels. The use of one such method, the electrochemical potentiokinetic reactivation (EPR) test, has been investigated on AISI 304 and 304L stainless steels. Its reproducibility, the effects of scan rate, solution temperature, surface finish, and nonmetallic inclusions were examined. Measurements made with the EPR method were compared with those obtained with the oxalic acid etch test, the ferric sulfate - 50% sulfuric acid test, and the copper sulfate -16% sulfuric acid test of ASTM A 262. Both the EPR and the oxalic acid etch test are nondestructive and can therefore be applied to equipment in the field. It was found that the reproducibility of the EPR test is good and that comparable results can be obtained with a variety of instruments. Results obtained from a round robin ASTM test program are included. The EPR test provides a sensitive, nondestructive, quantitative measure of relatively small degrees of sensitization to intergranular attack. However, when at least some of the grains are completely surrounded by carbide precipitates, the EPR test is less sensitive (it “saturates”) in differentiating between a range of degrees of sensitization than are the ferric sulfate or copper sulfate tests. Even though it is not a quantitative test, the oxalic acid etch test can also be used as a nondestructive method for identifying material which is free of carbide precipitates, i.e., completely free of sensitization.