Correlation between AC magnetic susceptibility and pitting behaviour of sigma-phase containing duplex stainless steel

Abstract

Purpose – The purpose of this paper is to characterize the pitting behaviour of sigma-phase-containing duplex stainless steel and investigate the correlation between magnetic susceptibility and pitting potentials. Design/methodology/approach – Use an alternating current (AC) magnetic susceptometer to trace the change in magnetic susceptibility associated with sigma phase formation and systematic study of the effects of sigma precipitation on pitting parameters as obtained using the anodic potentiodynamic polarization test. Findings – The precipitation of sigma phase impairs the general and pitting corrosion resistance of duplex stainless steel. The pitting potential, the corrosion potential and the AC magnetic susceptibility have good correlations. Unlike the pitting potential and the corrosion potential, the passive current and the corrosion current do not seem to possess any trend with annealing time. Originality/value – The correlation between AC magnetic susceptibility and pitting parameters has not been reported in the literature before.

Short‐term spinodal decomposition – its effects on corrosion behaviour of a duplex stainless steel and feasibility as a strengthening treatment

“…Cyclic polarisation tests were conducted to assess the effects of spinodal decomposition on pitting resistance of 7MoPLUS. As mentioned in Section 2, the corrosion potential (E corr ) and corrosion current density (I corr ) obtained using Tafel extrapolation did not show any trend, similar to what was observed by the present authors when 7MoPLUS was aged in the sigma‐phase formation temperature range (800 °C) . This lack of trend for both E corr and I corr may be associated with the inadequate linearity of the Tafel regions of the polarisation curves, as propounded by Flitt and Schweinsberg .…”

Effect of annealing temperature on the microstructure of hyperduplex stainless steels

“…However, they also compromise the ferrite/austenite balance since they increase the occurrence of intermetallic phases such as sigma phase, chi phase, or chromium nitrides (Sathirachinda, 2011). The precipitation of these intermetallic phases usually occurs in the temperature range 475°C-900°C (Nilsson, 1992;Lo, 2015) and is normally associated with the loss of ductility, toughness and corrosion resistance as they consume a portion of the alloying elements in the ferrite matrix, specially chromium and molybdenum (Padilha, 2012). From all the intermetallic phases, the sigma phase is considered the most detrimental for all duplex alloys because its formation involves the decomposition of ferrite through the eutectoid reaction of ferrite→austenite + sigma, which modifies the final phase balance.…”