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

Abstract

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.

Examination of Mechanisms for Liquid-Air-Interface Corrosion of Steel in High Level Radioactive Waste Simulants

“…Therefore, a chloridebased LAI corrosion susceptible solution was employed for the study of nitrite concentration change in the meniscus because it required a much higher concentration of nitrite as inhibitor compared to the nitrate-based solution. 9 A relatively strong nitrite peak was obtained in this case. Approach A was used for this test.…”

“…For example, the microelectrode experiments in a previous work did not differentiate the LPR and EIS behaviors in the meniscus and bulk solutions. 8,9 Therefore, a direct determination of the solution concentration gradient of nitrate and nitrite as illustrated in Figure 1a would be more convincing to support the salt concentration cell theory.…”

“…9,33,34 The increase in meniscus nitrate to nitrite concentration ratio may occur if nitrite was more involved in the reduction reaction. To examine this hypothesis, the variation of meniscus NO 3 − to NO 2 − concentration ratio was studied during LAI corrosion in 3.58 M NaNO 3 + 0.35 M NaCl + 4 M NaNO 2 at pH 11.…”

“…This hypothesis is consistent with the laboratory observation that LAI corrosion originates at the very top of the meniscus because the greatest access of CO 2 to hydroxide ions would be at the top of the meniscus. 9 However, there are uncertainties about this hydroxide depletion theory. For example, LAI corrosion still initiated under CO 2 -free conditions, i.e.…”

“…Mild steels are susceptible to corrosion at the Liquid-Air Interface (LAI) in high-level liquid nuclear waste and simulants, [1][2][3][4][5][6][7][8][9] which contain inhibiting sodium nitrite and sodium hydroxide, and highly concentrated aggressive sodium nitrate. Although LAI corrosion has not yet been reported to cause any leaks in service for the nuclear waste storage tanks in Hanford, WA or Savannah River, SC, a long-term LAI corrosion event could potentially penetrate through the tank wall.…”