The polarization behaviour of FeCr alloys in acidic sulphate solutions in the active region

Abstract

The polarization behaviour of six Fe Cr alloys was studied in sulphate solution of variou pH, with emphasis on the hydrogen evolution, active dissolution, and evaluation of active corrosion current, in comparison with results on the individual components. The exchange current densities of hydrogen evolution on Fe Cr alloys were found to increase with increasing Cr content up to 30Cr%. The rate of the anodic dissolution of low Cr alloys (< 10%Cr) strongly depended on pH and exhibited a Tafel behaviour with the slope of 40 mV/decade. That of high Cr alloys (> 10%Cr) was independent of pH and the Tafel slope was 60 mV/decade, similar to that of pure Cr, thus there is a threshold Cr content above which the dissolution of the alloy is controlled by the Cr component. The peak current at the active-passive transition decreased with increasing pH or Cr content. It is highly probable that the pre-passive state commences with the formation of chromic hydroxide, which promotes the formation of a ferrous oxide to shift the Flade potential in the negative direction. † Post-doctorate research fellow, present address, Department of Chemistry, Faculty of Science, Cairo University, AR, Egypt.