The passivation of FeCrRu alloys in acidic solutions

Abstract

Electrochemical and surface analytical techniques have been used to study the spontaneous passivation of Fe40CrRu alloys in 0.5 M solutions of sulphuric and hydrochloric acids. Passivation is achieved much more readily in alloys containing 0.2 w/o Ru than in alloys containing 0.1 w/o Ru, and for both these alloys passivation is more rapid in 0.5 M H 2SO 4 than in 0.5 M HCl. The increase in the rate of hydrogen evolution and the simultaneous lowering of the rate of anodic dissolution caused by ruthenium addition has been directly demonstrated for Fe40Cr-0.1Ru in 0.5 M H 2SO 4. These processes combine to produce a positive shift in the potential of the alloy, and the conventional passivation process in stainless steels then occurs at the passivation potential of Fe40Cr. Auger analysis shows that ruthenium is only a minor constituent of the passive surfaces of alloys containing 0.2 w/o Ru. Higher ruthenium concentrations (and evidence for a ruthenium redistribution process) are found on the passive surfaces of alloys containing 0.1 w/o Ru, where larger amounts of dissolution occur before the onset of passivation. A qualitative model to explain these observations is suggested. This presumes that the rate of surface diffusion of ruthenium atoms during selective dissolution is an important factor in determining the geometry of the ruthenium distribution, and hence its efficiency in promoting spontaneous passivation.