Double-Loop Electrochemical Potentiodynamic Reactivation (DLEPR) is a useful technique to measure the degree of sensitization (DoS) in austenitic stainless steels, as it is fast, nondestructive, and accurate in identifying low to medium DoS. While its use has been well-established for wrought austenitic stainless steels, the same is not true for their cast equivalents. A modified version, with more aggressive conditions in order to more selectively cause reactivation in regions that had residual ferrite from solidification, was tested with cast austenitic stainless steel spanning both UNS J92500 and J92600 compositions. Testing included microstructural evaluation of polished surfaces after the electrochemical tests and DoS measurements. The modified DLEPR consistently measured higher DoS values, but the trends in the data were the same for both versions of the test. Microscopy was more difficult with the modified DLEPR, as the surfaces were more severely corroded, somewhat masking smaller features such as inclusions and twin boundaries.
Stainless steels are susceptible to sensitization in the temperature range of 500-900°C.1-3 This is caused by the formation of Cr-rich M23C6 (carbide) particles on the grain boundaries (GBs), resulting in a local depletion of Cr in the surrounding area, leaving it vulnerable to corrosive attack in the form of intergranular corrosion (IGC).4-9
Electrochemical potentiodynamic reactivation (EPR) is a common method to evaluate the susceptibility of stainless steels to IGC caused by sensitization. ASTM(1) G108 is a standard method of EPR for use with wrought austenitic stainless steels, but it requires a 1μm surface finish to give reliable results and is strongly affected by the grain size of the sample.10,11 A more robust EPR method is the Double-Loop EPR (DLEPR) test and is insensitive to surface finish and only slightly sensitive to grain size.10,12 These methods are stated to only apply to wrought austenitic stainless steels, such as UNS S30400 (304) and S30403 (304L).10-12 In the literature, a modified DLEPR method exists for the cast equivalents of 304 and 304L, UNS J92600 (CF-8) and J92500 (CF-3) respectively. It is more aggressive in both its electrolyte and test parameters in order to compensate for the variations in microstructure that occur from casting.13 The cast materials often have residual ferrite due to the sequence of solidification, and no cold- or hot-working is done to homogenize the structure to pure austenite.14-17 As a result of the presence of ferrite, which contains higher Cr concentrations than austenite, the cast materials can passivate more strongly, necessitating the use of more aggressive corrosion tests.13