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The influence of N, W, Mo and Cu addition, alone or combined, on the active and passive corrosion behaviour of type 17Cr-13Ni stainless steels was investigated in de-aerated 2 N H 2 SO 4 solutions. Potentiodynamic polarization curves and current decay curves were determined in this medium. Surface characterization was examined by X-ray photo-electron spectroscopy (XPS).

Three austenitic stainless steels (a Fe-17Cr-13Ni-0.15N alloy, an ultra pure Fe-18Cr-10Ni alloy and a type 304 stainless steel) were used in this study in order to investigate the effect of nitrogen on the electrochemical behaviour and on the nature of the passive film formed on the steels in acidic solution (0.5 M H2SO4). The presence of nitrogen in the alloy does not apparently have a significant influence on the response of the material to anodic polarization in the acid electrolyte. Surface analysis of the alloys by XPS after passivation shows that nitrogen is enriched at the surface by anodic segregation during dissolution and passivation of the nitrogen-bearing alloys. After passivation of the alloys three chemical states of nitrogen are detected in the N 1s spectrum. The peak at low binding energy (397.7 ± 0.1 eV) corresponds to nitrogen bonded essentially to chromium under the form of a nitride which is incorporated in the passive film. The N 1s peaks observed at higher binding energies (400.2 ± 0.1 and 402 ± 0.1 eV) correspond to nitrogen species located on the surface of the passivated alloys, which are produced by reaction of the alloy surface with the solution.

The influence of sulphide ions on the corrosion resistance of some special stainless steels (Z2 CNDU 25-25, Z2 NCDU 25-20, Z2 CNDU 25-6, Z1 CN 25-20, Zl CD 29-4) in 40 wt % H 3 PO 4 with 330 ppm SO 4 2− and 1000 ppm Cl − at 80 °C has been investigated using electrochemical (polarisation curves) and spectrometric (SIMS) techniques. The corrosive effect of sulphide ions is confirmed: they increase the anodic current and shift the corrosion potential to more negative values. The presence in the solution of sufficient sulphide ions prevents the establishment of passivity. Critical sulphide ion concentrations above which the alloys 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 S 2− ; 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 order of decreasing dissolution rate, the five steels are ranked as follows: 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 oxysulphides. Contrary to nickel, the presence of chromium and molybdenum is essential to increase the corrosion resistance of stainless steels in phosphoric acid with sulphides as impurities.