Hydrogen Penetraion into Carbon Steel Caused by Corrosion in Aqueous Solutions

For the purpose of preventing an accident due to the hydrogen embrittlement, we have examined the hydrogen penetration into S25C carbon steel specimens caused by corrosion in aqueous solutions. The specimen weight was measured before and after immersion test, and corrosion rate was calculated from the change in specimen weight. The corrosion rate decreased with increasing of pH in acid solutions of pH2-4. In the solutions of pH4-8, the corrosion rate was approximately constant, and in the solution larger than pH9, the corrosion rate slightly decreased. The corrosion reaction in the solution of pH3 was determined by the anodic reaction of iron dissolution and the cathodic reaction of hydrogen evolution. In the solutions of pH5-9, the reaction was rate-determined by the cathodic reaction, which was a reduction reaction of the O2 gas dissolved in the solutions. For the specimens covered by corrosion products, it was difficult to analyze hydrogen in the carbon steel specimens by using the method of hydrogen extraction by melting at high temperature, because the corrosion products contained large amount of hydrogen. Therefore, we examined a new method to obtain the concentration of hydrogen in the specimen corroded in aqueous solutions. The concentration of hydrogen was measured for the specimens polarized at some electrode potentials, at which the hydrogen evolution reaction was dominant, and the hydrogen concentration at the corrosion potential (Ecorr) was extrapolated from the hydrogen concentration measured at less-noble potentials. In acid solutions a large amount of hydrogen penetrated into the carbon steel specimens, and the hydrogen concentration at Ecorr decreased with increase of the pH. In the neutral and alkaline solutions, the corrosion was determined by the reduction reaction of the O2 gas dissolved in the solutions, and a small amount of hydrogen penetrated into the specimens