Quantification of Deleterious Phase Precipitation in a Hyper Duplex Stainless Steel Aged at 700-950 °C Using Optimized Linear Sweep Voltammetry: Effect of KOH Concentration

Abstract

This work investigated the application of the LSV-KOH test to quantify deleterious phases in a UNS S32707 hyper duplex stainless steel (HDSS). HDSS is a metal alloy with excellent corrosion resistance, but it can form deleterious phases (DP) when exposed to high temperatures. These phases can reduce the corrosion resistance and mechanical properties of HDSS by creating chromium-depleted zones in the microstructure. The LSV-KOH test was applied to a UNS S32707 HDSS aged at different temperatures ranging from 700 to 950 °C. The total amount of DP was quantified by quantitative metallography using a scanning electron microscope (SEM). The effect of KOH concentration on the test results was studied and optimized. The optimal KOH concentration found was 6.0 mol L⁻¹. A R² of 0.99 was reached when LSV-KOH response was plotted versus DP%. The LSV-KOH was compared with the DL-EPR test, the results demonstrated that the LSV-KOH test was more sensitive and accurate than the DL-EPR test in detecting and quantifying DP. Further investigation of the specimens’ surfaces after each test demonstrated that LSV-KOH acts directly on deleterious phases, while DL-EPR acts in DP contours. The study demonstrates that the LSV-KOH test has several advantages over the DL-EPR test for evaluating the microstructural transformations of HDSS caused by thermal ageing.