Corrosion Mechanism of Stainless Steel in Nitric Acid Solutions Containing Ruthenium Compounds

Abstract

The effect of ruthenium on corrosion of stainless steel in nitric acid solutions was evaluated through immersion test, electrochemical mesurement, quantitative analysis of RuO₄ or NO_x and corrosion test under gamma-irradiation. Under non-irradiation, Ru³⁺ or RuNO³⁺ was oxidized to RuO₄ in concentrated nitric acid solutions at high temperature, and the corrosion of stainless steel was promoted first through by the reduction of RuO₄ to a lower valent state compound like RuO₂. If the reduction product of RuO₂ was deposited on stainless steel, then the corrosion was further promoted through a catalytic function of the deposit for a reduction of nitric acid to HNO₂. It was found that the promotion of corrosion with ruthenium was extreamly reduced by gamma-irradiation (1×10⁶R/h), due to inhibition of the oxidation of RuO₄ from Ru³⁺ or RuNO³⁺ by formation of nitrous acid resulting from radiolysis of nitric acid solution.