304不锈钢在NaCl水溶液中亚稳蚀孔转变为稳态蚀孔的机理研究

In the project,a novel method will be developed to remove the cover of both metastable and stable pits. After pitting corrosion test, the sample of 304 stainless steel with pits sealed by cover is put in pure water,and then the sample surface can be impacted by ultrasonic wave from a powerful ultrasonic transducer.When the cover broken down the pits inner solution released into the water.The composition and concentration of species containning in the pits inner solution can be determined by a chemistry analysis procedure.The effects of pit cover on metastable pits repassivation will be investidated by means of potentiostatic measurements metastable and stable pits growth current curves with or without impact of ultrosonic wave,by the way the mechanism of dilute lead repassivation can be verified.Based on the above data, and combined with characteristics of metastable pits propgation,the critical transition conditions for metastable pitting to stable pitting corrosion can be got through statistical analysis and processing. The electrochemical natures will be measured in the simulated solutions containning as the same species,concentrations as the pits inner solution.A new model shoud be established finally for metastable and stable pits propgation,transition from metastable pitting to stable pitting corrosion.The results of the projiect is of significance to promote development of corrosion science and apllications of stainless steel in marine areas.

项目采用自行设计的亚稳、稳态蚀孔口盖去除技术，获得封闭蚀孔内腐蚀反应活性粒子的浓度变化规律，研究亚稳蚀孔钝化、向稳态蚀孔转变的机理；在恒电位极化条件下亚稳、稳态蚀孔生长电流曲线测量时全程或适时叠加大功率超声波，实证研究亚稳蚀孔的“稀释钝化”机理和蚀孔口盖（金属残余盖板、盐膜、或氢氧化物等腐蚀产物）对亚稳蚀孔和稳态蚀孔生长过程的影响；在以上研究结果的基础上，积累数据，结合亚稳蚀孔生长的几何特征，运用统计学方法提炼出亚稳蚀孔向稳态蚀孔转变的判据（临界条件）；模拟蚀孔内部环境，溶液的成分和浓度、pH值等，研究304不锈钢点蚀过程中的电化学特性，建立亚稳、稳态蚀孔生长和亚稳蚀孔向稳态蚀孔转变的机理。项目预期目标如能实现将推动不锈钢点蚀研究的发展，并为不锈钢点蚀防护方法研究和不锈钢在海洋大气、海水环境的应用研究提供基础。

项目深入研究了304不锈钢亚稳、稳态蚀孔生长和亚稳蚀孔向稳态蚀孔转变的机理。通过恒电位极化、紫外分光法测定了稳态蚀孔内Fe2+离子浓度；通过电化学实验、浸泡实验研究了304不锈钢在蚀孔模拟液中的腐蚀行为，建立了金属离子浓度与304不锈钢腐蚀行为的对应规律，得到影响亚稳蚀孔向稳态蚀孔转变的关键离子浓度，此外分析了蚀孔内主要金属离子对不锈钢点蚀阴极反应的影响；通过施加搅拌研究了液相传质过程对亚稳蚀孔形核、转变及稳态蚀孔生长的影响；此外利用示踪法为蚀孔外Fe3+的存在提供实证，系统的讨论了Fe3+在蚀孔早期生长过程中所起的作用，完善了蚀孔生长机理。主要结论如下：.以304不锈钢点蚀为例，稳态蚀孔内亚铁离子及其衍生物浓度可以达到3.29 M，孔内金属溶解与金属离子浓度，密切相关，当腐蚀产物中Fe2+浓度低于1.5M时，孔内金属腐蚀溶解速度在020A/cm2范围内，金属可以维持钝化状态，当孔内Fe2+浓度高于1.5M时，金属腐蚀速度与金属离子浓度呈线性关系，孔内金属难以钝化，蚀孔可以持续生长。Cr、Ni等元素可以提高不锈钢的耐蚀性，但是稳态蚀孔内的Cr3+、Ni2+将更有利于点蚀的生长。针对蚀孔内外腐蚀环境的差异设计了双舱电解池实验研究蚀孔内外的相互作用，结果表明NaCl溶液和FeCl3溶液中的不锈钢耦接后，两侧的金属腐蚀速度均有明显增加，增大NaCl溶液和FeCl3溶液中试样面积比时，FeCl3溶液中不锈钢的腐蚀速度可以增大到单一溶液中的2.2倍。通过施加搅拌改变点蚀处液相传质过程，搅拌速度增加，不锈钢表面亚稳态点蚀萌生数量减少，亚稳态蚀孔寿命降低、点蚀孕育期增加，稳态蚀孔生长速度降低，稳态蚀孔宽深比降低。显色法实验表明亚稳态蚀孔周围铁离子浓度可以维持0.3 M或者更高的浓度，并且证明了蚀孔周围存在Fe3+，Fe3+一方面是蚀孔周围pH值降低，另一方面Fe3+可以在电极表面发生还原反应促使蚀孔生长。在304不锈钢点蚀生长过程中特别是亚稳态点蚀阶段，孔外阴极反应可能有吸氧反应、Fe(Ⅲ)还原反应、析氢反应，三种阴极反应优先度依次降低。