孔蚀发展过程动力学分析

根据物质守恒、电荷守恒关系和电化学动力学,对球形、柱形蚀孔且其孔内存在和不存在沉积层,共四种典型状态下蚀孔的发展过程进行动力学分析,得到孔蚀电流、蚀孔深度和孔径随时间的变化关系。结果表明:孔蚀电流随时间发展共有四种特征函数,t~(1/2)、t、t~2和t~*ln(t),每一函数均对应特定的蚀孔发展状态.此四种函数构成一般蚀孔电流随时间关系的基集合,线性组合后得到普遍性孔蚀发展动力学方程。详细讨论了孔蚀发展与诸影响因素的关系,孔内沉积层存在与否及性能如何对蚀孔发展有显著影响,对球形孔的影响大于对柱形蚀孔。进而分析缓蚀剂抑制孔蚀发展的可能途径和效果,确定了孔蚀发展缓蚀剂须具备的条件;模型的正确性得到三方面结果的证实:实测304不锈钢在NaCl介质中孔蚀发展过程的电流关系,由建立的动力学方程给予了很好的解释。模型得到的孔蚀电流增长的最大方式是时间二次方,最小方式是平方根,与大量的文献结果吻合。孔蚀深度随时间仅有两种变化关系,幂函数和指数函数,与孔蚀深度的统计研究结果一致。

KINETIC ANALYSIS ON PITTING GROWTH

On the basis of law of conservation of mass and charge and electrochemical kinetics of corrosion, a kinetic analysis has been performed on self-propagation process of pits under four typical conditions, i.e. hemispherical and cylindrical pits in the presence and absence of precipitation layer inside the pit. The time dependence of pitting current, depth and radius for the four kinds of pit was obtained. The summarized results demonstrate that the dependence of pitting current on time comply with four functions of time, that are t1/2, t, t2 and t*ln(t), and each of them corresponds to a specific pattern of pitting growth. Furthermore, a general equation describing pitting current increasing with time has been put forward by linearly combining these extracted time functions. The resistance of participation layer plays an important role in pitting growth more obviously on hemispherical pits than on cylindrical pits. The possible ways to retard pitting growth by inhibitor and the necessary requirements for inhibitor were discussed. The kinetic equation established was validated from three aspects. The variation of pitting current with time for 304 stainless steel in NaCl solution under potential control can be explained well according to the equation. Second, the maximum increase of pitting current is the square of time and the minimum is square root. This result is confirmed by a great amount of previous studies. Finally, pit depth varies with time only as a function of power or logarithm. This is consistent with statistical analysis of pit depth data.