The corrosion of aluminum alloy and release of hydrogen in nuclear reactor emergency core coolant: Implications for deflagration and explosion risk

Hydrogen evolution (HE) accompanying the corrosion of aluminum alloy in the sump water formed after a loss-of-coolant accident (LOCA) influences the safety of reactor containments due to the deflagration and explosion risk of the air-hydrogen mixture. In experiments examining the corrosion of Al alloy 6061 in borated solutions simulating the water chemistry of post-LOCA sump water, HE rates were evaluated by analyzing the potentiodynamic cathodic polarization curves from rotating cylinder electrodes, and an empirical formula predicting the HE rates as a function of solution pH was proposed based on the evaluation results. At pH 7, the dominant cathodic reaction during free corrosion was found to be the oxygen reduction reaction and the HE rate was slow; however, the HE reaction became increasingly significant as the solution pH progressively increased to 11. The HE sources on the alloy surface were inferred to be mainly the bulk matrix, which was covered with a less protective hydroxide film in more alkaline solution, as well as the trenches formed by the intense anodic dissolution of the matrix around electrochemically more noble Fe-bearing intermetallic particles. Local alkalization due to preferential oxygen reduction on these intermetallic particles may also contribute to the formation of such trenches and HE; however, this mechanism should have been mostly suppressed by the buffering capacity of the borated solutions.