439M铁素体不锈钢点蚀诱发机理

 439M铁素体不锈钢广泛应用于汽车排气系统消声器中，其主要失效形式是点蚀破坏。采用点腐蚀试验、去钝化起始pH值试验、微区电化学试验，并结合扫描电镜及能谱分析，观察分析了点腐蚀和夹杂物的形貌，研究了点蚀诱发时夹杂物的变化情况。研究结果表明，439M不锈钢的点蚀主要在复合氧化物夹杂处诱发，而TiN夹杂物未诱发点蚀；夹杂物诱发点蚀时，复合氧化物夹杂的CaO优先溶解。最后，建立了在氯离子环境下439M不锈钢中复合氧化物夹杂诱发点蚀的模型。

Mechanism of induced pitting corrosion in 439M ferritic stainless steel

439M ferritic stainless steel is widely used in automobile muffler of exhaust system and the pitting corrosion damage is the main failure form. The pitting corrosion test， the initiation pH of depassivation and micro-electrochemical test were applied to study the morphology of the pitting corrosion and inclusions combined with the analysis ofscanning electron microscopy (SEM) and energy dispersive spectroscopy(EDS). Also the variation of inclusions was researched at the start of pitting corrosion occurrence. The results indicated that the pitting corrosion was found to be initiated at the interface between the round oxide inclusion complex and the matrix， while little pitting corrosion occurred arround the TiN inclusion. The composite oxide inclusions containing Ca-oxides part can be preferential dissolved when inclusions induced pitting corrosion. Finally， the model of the initiation process of the composite oxide inclusion-induced pitting corrosion in 439M ferritic stainless steel exposed to chloride ions environments was established.