Influence of sulphide ions on corrosion resistance of special austenitic stainless steels in phosphoric acid

Abstract

The influence of sulphide ions on the corrosion resistance of some special stainless steels in 40 wt-% H₃PO₄ with 330 ppm SO²⁻ ₄ and 1000 ppm Cl⁻ at 80°C has been investigated using electrochemical (polarisation curves) and spectroscopic (infrared, SIMS) techniques. The behaviour of ZI NCDU 25–20 and ZI CNDU 25–25 commonly used in the phosphoric acid industry is compared with that of ZI CN 25–20 to estimate the role of the alloying elements. The corrosive effect of sulphide ions is confirmed: they shift the corrosion potential to more negative values and increase the anodic current in the active and passive states. In order of increasing corrosion rate the three steels are ranked as follows: ZI CNDU 25–25, ZI NCDU 25–20, ZI CN 25–20. The corrosion products consist largely of magnetite in a more or less oxidised state with chromium and nickel substituted for iron. The film formed on the ZI CNDU 25–25 alloy is more protective because of the high chromium content of the mixed oxide and the effect of molybdenum. The film retards the diffusion of corrosive ions through to the metal andfacilitates the development of passivity. However, the presence of a sufficiently high concentration of sulphides in solution prevents the establishment of passivity.