Localized Corrosion of Stainless Steels in High Temperature Potash Brine
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Potash brines are highly corrosive at the elevated temperatures found in potash processing. Cyclic potentiodynamic polarization and exposure tests have been carried out with a wide range of stainless steels used in the potash industry. Hastelloy C276 (a nickel alloy) showed excellent resistance to localized corrosion in high chloride concentration solutions over wide range of temperatures. A wide range of lower cost alloys with less resistance to localized corrosion have been investigated to determine their corrosion behavior at 22 and 90°C in saturated potash brine. The alloys studied included: austenitic, duplex and cast stainless steel alloys. The results of the electrochemical polarization and exposure tests are compared to the predictions of Localized Corrosion Resistance Index [LCRIwt % = %Cr + 3.3 (%Mo) + 16 (%N)]. The electrochemical polarization results show that LCRI can roughly predict the resistance of stainless steel alloys (in the same group i.e. austenitic, duplex) to initiation of localized corrosion. The average pit depth and the five deepest pits have been measured and compared to LCRI for the tested alloys. The deepest pits (the cause of metal failure) found in the exposure tests show that the LCRI cannot predict the propagation of the pits. Austenitic stainless steels are very sensitive to initiation of pits at the edges. The pits on the edges are deep and wide. Residual stresses adjacent to the edges produce stress corrosion cracking at the bottom of each pit which accelerates corrosion. Comparing austenitic and duplex stainless steels to identical LCRIs shows a better resistance of duplex stainless steels to localized corrosion. Duplex stainless steels consist of ferrite-austenite phases with more chromium and molybdenum in the ferrite than in the austenite phase. It was found that the austenite-ferrite interface is prone to the initiation of localized attack, but the attack propagates preferentially in the ferrite phase. The immunity of the austenite phase is related to the higher nitrogen concentration in that phase.