Corrosion compatibitity studies on carbon steel and type 304 stainless steel in acid permanganate and organic acid mixtures

Type 304 stainless steel and carbon steel are extensively used as structural materials in the reactor coolant systems of nuclear power plants. The corrosion of these structural materials during normal operation and the subsequent neutron activation of their corrosion products leads to the build-up of radiation on the out-of-core surfaces of the coolant systems. The Dilute Chemical Decontamination (DCD) process is one of the methods used to remove the activity that becomes embedded in the oxide layers formed over these structural materials. Permanganate based oxidising reagents are generally used to dissolve chromium from oxide films containing iron, chromium and nickel on the surfaces of Type 304 stainless steel and other chromium-containing alloys. This study has investigated the behaviour of carbon steel in oxidising permanganate based acidic media. The corrosion/decomposition rate varied with temperature. With permanganate concentrations above 1 center dot 5 mM for permanganic acid and greater than 6 mM for nitric acid and potassium permanganate (NP), passivation of carbon steel surfaces occurred with negligible loss of weight of the carbon steel and insignificant decomposition of the permanganate. The effects of additions of chromate to permanganate on the passivation of carbon steel have also been evaluated. In the case of permanganic acid, the presence of even 50 ppm of chromate was sufficient to protect the carbon steel surface whereas a minimum of 150 ppm of chromate was required for passivation in the NP medium. Electrochemical polarisation experiments were also carried out to support these observations. When the concentration of permanganic acid was above 200 ppm, the open circuit potential of carbon steel in permanganic acid was + 1 center dot 0 V (SCE) and it was observed to be passive whereas at lower concentrations the carbon steel was actively corroding at a potential of -0 center dot 695 V (SCE).