Corrosion behaviour of duplex stainless steels containing minor ruthenium additions in reducing acid media

Abstract

The dissolution behaviour, as well as the time, potential and concentration dependence of HSO ⁴₋ SO ⁴_2t- and Cl⁻ accumulations measured by an in situ radiotracer method on surface oxide-layers of duplex stainless steels containing various amounts of ruthenium as additive are presented and discussed. Several independent techniques, such as mass loss tests, potentiodynamic responses, radiotracer and ICP methods, were used to characterize the complex features of the passivation phenomena of steels modified with ruthenium. The experimental results reveal that the interaction of bisulfate/sulfate ions with passive oxide layer is stronger than those of chloride ions on the stainless steels studied. Both the extent and the strong character of bisulfate/sulfate accumulation are more likely related to the redistribution of the main alloying components (Cr, Ni, Mo) as well as the Ru in the surface oxide-film formed on steels passivated spontaneously in dilute HCl and H₂SO₄. It is found that the ruthenium additions to the base duplex stainless steel significantly increase the corrosion resistance in reducing acid environments. There is evidence of anodic inhibition and this seems to be responsible for the observed increased corrosion resistance of the duplex stainless steels with small ruthenium additions.