Electrochemical characterization of sigma phase in superduplex stainless steels: LSV-KOH as a promising methodology over DL-EPR

Abstract

This work analyzed the volume fraction of the sigma phase in a superduplex stainless steel UNS S32760. Two electrochemical testing methodologies were applied for this purpose. Firstly the DL-EPR test was performed in three different electrolytic solutions. The LSV-KOH test was also used, a recent methodology proposal for analyzing these alloys. Seven specimens were solution annealed at 1100 °C for 60 min and heat-treated at 850 and 950 °C for 5, 20, and 60 min to promote the formation of different fractions of the sigma phase. The sigma phase volume fraction present in these specimens was determined by quantitative metallography. The DL-EPR analyses in all three conditions proved to be limited in quantifying the sigma phase, especially in specimens heat-treated at 950 °C. On the other hand, from the analyses with the LSV-KOH technique, it was possible to construct a reference curve between the sigma volume fractions and the electrochemical response of the LSV-KOH test (charge density), obtaining a linear regression coefficient of 0.97. This curve can then be applied to quantify the sigma phase. For comparison, the best regression coefficient obtained for DL-EPR was 0.53. Accompanied by the quantitative analysis, a qualitative analysis of the sigma phase can be made from a deeper analysis of the LSV-KOH voltammograms. In these analyses, it was possible to infer whether the sigma phase morphology is lamellar or in blocks. From the results obtained, it was possible to show that the LSV-KOH test is more precise in quantifying the sigma phase and allows a morphological analysis.