THE INFLUENCE OF ALLOYING ELEMENTS ON THE CORROSION-RESISTANCE OF STAINLESS-STEELS IN PHOSPHORIC-ACID MEDIUM - POLLUTED BY SULFIDE IONS

The influence of sulphide ions on the corrosion resistance of some spe cial stainless steels (Z2 CNDU 25-25, Z2 NCDU 25-20, Z2 CNDU 25-6, Z1 CN 25-20, Z1 CD 29-4) in 40 wt % H3PO4 with 330 ppm SO42- and 1000 ppm Cl- at 80 degrees C has been investigated using electrochemical (pola risation curves) and spectrometric (SIMS) techniques. The corrosive ef fect of sulphide ions is confirmed: they increase the anodic current a nd shift the corrosion potential to more negative values. The presence in the solution of sufficient sulphide ions prevents the establishmen t of passivity. Critical sulphide ion concentrations above which the a lloys Z2 CNDU 25-6, Z2 CNDU 25-25 and Z1 CD 29-4 are no longer able to passivate, are respectively, 1, 5 and 15 ppm S2-; so, the Z1 CD 29-4 alloy with high chromium content (29% Cr) and without nickel has good behaviour in such polluted phosphoric acid. In the passive state, in o rder of decreasing dissolution rate, the five steels are ranked as fol lows: Z1 CN 25-20, Z2 CNDU 25-6, Z2 NCDU 25-20, Z1 CD 29-4 and Z2 CNDU 25-25; sulphur is present in the films probably as sulphides or oxysu lphides. Contrary to nickel, the presence of chromium and molybdenum i s essential to increase the corrosion resistance of stainless steels i n phosphoric acid with sulphides as impurities.