Development of Liquid-Air-Interface Corrosion of Steel in Nitrate Solutions

Liquid-air interface (LAI) corrosion is an increasing concern in the assessment of the deterioration of the integrity of underground steel storage tanks used for high-level liquid radioactive wastes. This work is aimed at expanding the current knowledge of this form of corrosion by probing its nature to provide insights for further mechanistic studies. LAI corrosion initiated in the form of random pits at the very top of the LAI meniscus under potentiostatic polarization, which is not where attack occurs as the result of the classical oxygen concentration cell mechanism. LAI corrosion pits grew upward to form hemispherical solution drops above the meniscus and gas bubbles collected on corrosion products in the meniscus. The effects of deaeration, the presence of a crevice former, and pre-passivation of the steel on LAI corrosion were also studied. LAI corrosion occurred in nitrite solutions containing sulfate or chloride instead of nitrate. No correlation in the passive current density or pitting potential with LAI corrosion susceptibility was found in solutions with varying nitrate and nitrite concentrations. Electrochemical measurements with a carbon steel microelectrode did not indicate that an aggressive meniscus solution develops to initiate LAI corrosion.