EFFECT OF ALLOYING ELEMENTS ON CORROSION BEHAVIOR OF CR IN HCL SOLUTIONS

The corrosion behaviour of chromium-base alloys in HCl solutions with a concentration of more than 10% was investigated by measurements of the polarization curves, the change in corrosion potential with time and the average corrosion rate in the temperature range between room temperature and the boiling point. The corrosion resistance of chromium-base alloys was improved by adding noble metals such as palladium and ruthenium. From the immersion test it was shown that the chromium-base alloys containing more than 0.05 mass% palladium and more than 0.2 mass% ruthenium revealed high corrosion resistance. The addition of molybdenum also improved the corrosion resistance, but its effect was smaller than that of the additions of palladium and ruthenium, even if about 10% molybdenum was added to the alloys. The additions of palladium and ruthenium to chromium increased the cathodic current, and decreased the anodic dissolution current as the Cr3+ or Cr2+ ion in the active state. Accordingly the corrosion potential of the alloys shifted to the noble direction and the alloys were easily passivated. The effect of the palladium and ruthenium additions on the corrosion behaviour of chromium is therefore caused mainly by cathodic depolarization.