A Study on Stochastic Models for Localized Corrosion

对于不锈钢等耐腐蚀材料，已知一半以上的腐蚀失效是由局部腐蚀引起的。众所周知，局部腐蚀数据在空间上或时间上表现出广泛的分散，即统计和概率变化。因此，为了更可靠地评估各种结构材料的失效寿命，需要更详细地研究局部腐蚀盾构的统计和概率性质。本研究旨在完善前人提出的点蚀生灭随机模型，并在此模型的基础上，采用蒙特卡罗模拟法模拟实际的蚀坑形成过程。将模拟的点蚀电位分布和产生点蚀的感应时间分布与实验观测的分布进行了比较，以建立模型的置信度。生死随机模型的基本假设如下：1)坑的产生服从Posson随机过程，因此坑的产生时间的分布服从指数概率分布。坑产生率对应的跃迁概率由势函数给出。2)坑核的大小随时间增大，速率定律的比例常数表示为势的指数函数。3)在某一临界尺寸以下，坑核因再钝化而停止生长。坑核的死亡过程也服从泊松过程，因而生命时间的分布规律也服从指数概率规律。4)超过临界尺寸的大坑不会死亡，会继续生长。基于上述模型和假设，蒙特卡罗模拟得到如下结果：1)点蚀电位的分布近似服从正态概率分布。2)点蚀电位中位数与电位扫描速度的平方根成正比。3)深坑产生感应时间的概率分布服从畸变分布。发现上述结果与实验中观察到的结果具有较高的置信度。在此基础上，分析了非晶合金热处理后再结晶导致非晶合金非均质性增加的原因。少

For corrosion resistant materials such as stainless steels, more than half cases of corrosion failure had been known to be caused by localized corrosion. It is well known that data of localized corrosion show wide scatter in space or in time, namely statistical and probabilistic variations. For this reason, statistical and probabilistic nature of localized corrosion shckild be studied in more detail in order to evaluate failure life of materials for various structures with more confidence.This study aims to refine the birth and death stochastic model of pitting corrosion which had been proposed previously and to simulate the actual pit foreation process by using Monte Carlo similation based on the proposed model. Simulated distributions of pitting potential and induction time for pit generation were compared with the observed distributions by the experiments in order to establish the confidence of the proposed model. The basic assumptions for the birth and death stochastic model are as … More follows: 1) Pit generation obeys a Posson stochastic process, so that the distribution of pit senation time follows exponential probability distribution.Transition probability corresponding to the pit generation rate is given by a function of potential. 2) The size of pit nucleus increases with time and the proportional constant of the rate law is expressed as an exponential function of potential. 3) Below a critical size, the growth of pit nucleus will stop due to repassivation. Death process of pit nucleus obeys also Poisson process, so that the distribution law for life time until death follows also exponential probability law. 4) Large pit which grows over a critical size will not die and continue to grow.Based on the above model and assumptions, Monte Carlo simulation provided the following results: 1) The distribution of pitting potential obeys normal probability distribution as a first approximation. 2) Median of pitting potential is proportional to the square root of potential sweep velocity. 3) The probability distribution of induction time for pit generation obeys distorted distribution. The above results were found to simulate the results observed in the experiments with a high confidence. Increase in heterogenuity due to recrystalization of amorphous alloys by heat treatment was analyzed based on the model. Less