甲酸浓度对316L不锈钢腐蚀钝化—活化转变行为的影响

Concentration effects of formic acid on the corrosion behavior of 316L stainless steel from passivation to activation

为深入认识316L不锈钢在甲酸溶液中的钝化-活化转变行为,在90℃、质量分数为0~30%的甲酸溶液中对316L不锈钢进行全浸试验和阳极极化曲线测试.研究了甲酸质量分数对316L不锈钢腐蚀速率、腐蚀形貌、开路电位、初始钝化电位、临界电流密度、钝化电流密度和钝化膜破裂电位的影响规律,分析了H^(+)和HCOO^(-)含量对活化区、过渡区和钝化区阳极反应的影响机制.结果表明,316L不锈钢在甲酸溶液中发生非均匀的全面腐蚀.当甲酸质量分数达到30%、腐蚀速率为1.2×10^(-3)mm·a^(-1)时,316L不锈钢就具有明显的钝化−活化转变.随着甲酸质量分数增加,316L不锈钢的初始钝化电位正移、临界电流密度增大、钝化电流密度增大、钝化膜破裂电位负移.甲酸溶液中H^(+)和HCOO^(-)含量的增加,会加速316L不锈钢活性溶解,抑制钝化膜生长,促进钝化-活化转变.

One of the most aggressive organic acids for stainless steel is formic acid.In particular,the corrosion of 316L stainless steel in aqueous formic acid solutions at high temperatures is directly related to its safe operation and production efficiency.To better understand the passivation-activation transition behavior of 316L stainless steel in aqueous formic acid solutions,an investigation was conducted at the formic acid mass fractions of 0.5%,5%,15%,and 30%at 90℃.Laboratory immersion tests with a period of 1200 hours were performed at each formic acid mass fraction to document the corrosion rates and the corrosion morphologies of 316L stainless steel,and electrochemical tests,including open circuit potentials and anodic polarization curves,were conducted in the presence of dissolved oxygen using conventionally divided glass cells with three electrodes.The influences of formic acid mass fraction on corrosion rate,corrosion morphology,open circuit potential,primary passivation potential,critical current density,passive current density,and passive film breakdown potential were analyzed.In addition,the effects of H^(+) and HCOO^(-) ions on anodic reactions occurring in the active region,the active-passive transition region,and the passive region were discussed.Due to the stability of the passive state,the laboratory immersion tests showed that at the formic acid mass fractions of 0.5%,5%,and 15%,316L stainless steel suffered from slight corrosion,and thus no measurable weight losses could be acquired.However,in the 30%aqueous formic acid solution,the corrosion rate of 316L stainless steel reached 1.2×10^(-3)mm·a^(-1),which indicated that 316L stainless steel was in the active state and the passivation-activation transition had occurred.The corrosion of 316L stainless steel in aqueous formic acid solutions is characteristic of non-uniform generalized corrosion.According to the results of electrochemical tests,with an increasing mass fraction of formic acid,the open circuit potentials and the primary passivation potentials became nobler,the critical current densities and the passive current densities increased,and the passive film breakdown potentials shifted to negative values.It is suggested that the passivation-activation transition of 316L stainless steel in aqueous formic acid solutions may be due to the competitive adsorption between HCOO^(-) and OH^(-) ions.Therefore,formic acid mass fraction increased,anodic dissolution accelerated,the formation of passive film was delayed,and corrosion susceptibility increased.In short,the concentration of formic acid significantly influences the corrosion behavior of 316L stainless steel from passivation to activation.