Characterization and prediction of carbon steel corrosion in diluted seawater containing pentaborate

This study addresses the influence of Na₂B₁₀O₁₆, which may be used for criticality control of fuel debris in the Fukushima Daiichi Nuclear Power Station, on the corrosion behavior of carbon steel in diluted artificial seawater. The corrosion forms of carbon steel were categorized as uniform corrosion, localized corrosion, and passivity based on the balance between the dilution ratio of artificial seawater and the concentration of Na₂B₁₀O₁₆. The changes in corrosion forms were arranged on a water quality region map. Passivity was maintained by adding 3.7 × 10⁻² M or more of Na₂B₁₀O₁₆ to artificial seawater with a dilution ratio of 100-fold or more. The criticality control of the fuel debris and corrosion mitigation of the carbon steel components may be achieved simultaneously in the water quality. The prediction of the corrosion form of carbon steel was attempted by the extended Larson–Skold Index (LSI) = ([Cl⁻] + 2[SO₄²⁻])/([HCO₃⁻] + 2[B₁₀O₁₆²⁻]). However, because the passivating action of B₁₀O₁₆²⁻ was remarkably stronger than that of HCO₃⁻, the prediction was difficult under the simple addition of equivalent concentrations. The localized corrosion of carbon steel under the addition of Na₂B₁₀O₁₆ preferentially occurred from the crevices of the test specimens, as was the case in stainless steel.