Effects of Al, Si and Mo on passivation characteristics of Fe-10Cr alloys

A study has been made on the effects of Al, Si and Mo on anodic polarization characteristics of Fe-10Cr alloys in 0.05-1.0 kmol.m(-3) H2SO4 and NaCl aqueous solutions. Potential decay curves have also been measured in order to evaluate the stability of the passive films formed on Fe-10Cr alloys containing Al, Si and Mo. The analysis of the chemical composition of the passive film has been carried out by AES and XPS. The addition of Mo was very useful to decrease the critical passivation current density of Fe-10Cr alloys which contained Al and Si. The passive current density decreased with addition of 2 mass%Mo to an Fe-10Cr-3Si alloy, while it increased in an Fe-10Cr-3Al alloy. Fitting potential was moved toward the noble direction by the addition of Mo, except for the case of an Fe-10Cr-3Al-3Si-2Mo alloy. It seems that Laves phase (Fe2Mo) was precipitated in this alloy, and Mo-depleted zone was formed locally. The activation time was increased by the addition of Al, Si and Mo to Fe-10Cr alloys. Two step potential decay curves were obtained in Si-containing alloys, and this behavior of the decay curve was shown more clearly in an Fe-10Cr-3Si-2Mo alloy. The results of AES and XPS analysis indicated that Cr and Al concentrated in the passive film of an Fe-10Cr-Al alloy. In the case of an Fe-10Cr-Si alloy, Si concentrated in the surface region of the film and Cr did in the internal region of the film. The confirmation was not possible for the enrichment of Mo in the passive film.