Contradictory Results from Single Loop Electrochemical Potentiokinetic Reactivation Test and Oxalic Acid Test for Intergranular Corrosion in 304L Stainless Steels Attributed to Si Grain-Boundary Segregation

Abstract

The intergranular corrosion susceptibility of AISI 304L stainless steel can be measured in the field using the single loop electrochemical potentiokinetic reactivation test. However, the present work questions the reliability of the test, with nominally the same as-received samples given a wide range of degrees of sensitization, enough for opposite conclusions and potentially false positive results. More worryingly, when comparing the results to the oxalic acid etch test, a false negative result of the SL-EPR test was obtained on some heat-treated samples. This could entail serious repercussions and justifies a thorough investigation into this issue. The most likely explanation for the unexpected behavior of one batch of 304L, is its moderate silicon enrichment at 0.4 wt%. As long as the stainless steel does not exceed the passive region, as is the case for the SL-EPR test, the moderate silicon additions help suppress intergranular corrosion, but in the strongly oxidizing environment of the oxalic acid test the stainless steel becomes transpassive and the silicon results in accelerated corrosion of the grain boundaries.