Effect of anodic passivation on the corrosion behaviour of Fe-Mn-Al steels in 3.5%NaCl

Abstract

This chapter presents as study in which the effects of anodic passivation in 30% HNO3 on the structure of the passive film and on the corrosion behavior of three Fe-Mn-Al steels in 3.5% NaCl solution were studied by electrochemical measurements, X-ray diffraction, and scanning electron microscopy. In general, the corrosion resistance of the Fe-30Mn-4.5Al-4Cr (in wt-%) alloy, characterized by the polarization curves, was found to be superior to that of Fe-24Mn-5.7Al and Fe-23Mn-8Al, as passivated in 30%HNO3 or in the unpassivated condition. Polarization measurements and XRD analysis indicated that the rather good passivation of Fe-30Mn-4Al-4Cr steel was due to the presence of Al and Cr. The high passivity of Al and Cr induced the formation of a passive film that was more stable than that formed in the steels without Cr. The passive film on this steel mainly consisted of the oxides of Al, Cr, Mn, and Fe. Green Rust was observed as the main product of anodic dissolution of the unpassivated Fe-24Mn-5.7Al and Fe-23Mn-8Al alloys. Prolonged anodic ageing in the anodic passive regime in 30%HNO3 induced a thick, protective, and stable passive film that enhanced the corrosion resistance in 3.5%NaCl. The highest corrosion resistance of Fe-30Mn-4.5Al-4Cr can be attributed to the enrichment of AI2O3 and Cr2O3 and the depletion of Fe and Mn oxides in the passive film. Hence, anodic passivation ageing of passive films in an oxidizing electrolytic solution can be suggested as a surface modification method for improving the corrosion resistance of Fe-Mn-Al and Fe-Mn-Al-Cr type steels.