硫酸根离子对铁在弱碱性溶液中阳极溶解和钝化行为的影响

采用浸泡实验, 电化学测试和表面分析技术研究了硫酸根离子浓度对铁在稀碳酸氢钠溶液中开路状态和阳极极化行为的影响. 在无硫酸根离子及含有少量硫酸根离子的碳酸氢钠溶液中, 铁的开路电位约为(-0.225±0.005) V, 并呈现钝化状态, 其电化学阻抗很大, 腐蚀速率较低. 在含有较高浓度硫酸根离子的碳酸氢钠溶液中, 铁的开路电位为(-0.790±0.010) V并呈现活性溶解状态, 其电化学阻抗较小, 腐蚀速率较高, 同时阳极极化曲线上能观察到活化-钝化转变现象. 由于铁在含有较高浓度硫酸根离子的碳酸氢钠溶液中处于活化状态, 阳极极化曲线上存在数个电流峰. 足够高的硫酸根离子浓度会导致铁表面预先形成或转变而成的氧化膜失效. 相比于自然曝氧状态, 在除氧条件下较低的硫酸根离子浓度即可引起铁在碳酸氢钠溶液中由钝态向活性溶解态的转变.

Effects of Sulfate Ions on Anodic Dissolution and Passivity of Iron in Slightly Alkaline Solutions

The effects of sulfate concentration on the open circuit state and anodic polarization behavior of iron in dilute bicarbonate solutions were investigated using immersion tests, electrochemical measurements, and surface analysis techniques. In the absence of sulfate or in the presence of a low concentration of sulfate in dilute bicarbonate solutions, iron was in a passive state, with a corrosion potential of (-0.225±0.005) V. A high electrochemical impedance and low corrosion rate were obtained. No obvious active-passive transition was observed in the anodic polarization curves. In the presence of a high concentration of sulfate in dilute bicarbonate solutions, iron was in an active dissolution state, with a corrosion potential of (-0.790±0.010) V. A low electrochemical impedance, high corrosion rate, and typical active-passive transition in anodic polarization curves were observed and related to the sulfate concentration. In the presence of a high concentration of sulfate, the anodic polarization curves showed current peaks as a result of iron activation by sulfate ions. Sulfate ions of sufficiently high concentration in solutions degraded previously formed oxide layers on iron or transformed oxide layers in bicarbonate solutions. The transition of the open circuit state from passivity to active dissolution occurs at a lower sulfate concentration in a deaerated solution than in an aerated solution.