氢促进不锈钢点蚀形核机制的SPM原位研究

Pitting is so dangerous and destructive for its unpredictable nature and rapid propagation that may cause tremendous economic losses and damages. The generation and the absorption of hydrogen are unavoidable in many practical industrial processes. It has been found that hydrogen increases the pitting corrosion rate.In the 6%FeCl3 solution, the pitting was observed at the specimen precharged with hydrogen within 6 days, while not at the specimen without hydrogen for more than 30 days.In spite of vast studies on hydrogen-induced pitting corrosion, the effect of hydrogen on pitting is still not understood completely, because of the lack of direct evidence from the aspect of micro-scale.That might be due to the reason that the vast majority of hydrogen preferentially gathers in various defect positions, i.e. hydrogen traps, such as grain boundaries and vacancies, so the passive film becomes active and damaged in local area, which acts as the initiation site of pitting nucleation. .In this work, in situ Scanning probe microscopy (SPM) measurements will be applied to investigate the influence of hydrogen on the pitting corrosion in stainless steel in an effort to understand the mechanism of hydrogen-promoted pitting nucleation.The relationship between surface potential and the microstructure, and changes in local potential and type of the passive film induced by hydrogen aggregation will be studied using scanning Kelvin probe force microscopy (SKPFM) and current sensing atomic force microscopy(CSAFM).In addition,continuous changes of topography of hydrogen aggregation sites, and the process and distribution of pitting nucleation in stainless steel will be observed by electrochemical atomic force microscopy (ECAFM) to reveal the relationship between the occurrence of pitting nucleation and hydrogen aggregation.

点蚀是不锈钢的主要失效形式之一，而氢能极大地促进不锈钢的点蚀。不锈钢在生产、氢气氛保护热处理及使用过程中经常会有氢的进入。2507双相不锈钢在6%的FeCl3溶液中浸泡一个月也不发生点蚀，但若含有微量的氢则很快就会发生点蚀。虽然提出了一些解释，但氢促进不锈钢点蚀形核的机理还不是很清楚，更缺乏微观尺度的直接证据。氢可能在材料缺陷处富集，导致局部电学性能发生变化，从而诱发点蚀。本课题拟用扫描探针显微镜（扫描开尔文探针力显微镜、电流成像原子力显微镜和磁力显微镜等）在微观尺度原位研究材料表面电位与微结构的关系，氢富集导致不锈钢局部电位的变化、钝化膜局部半导体类型的变化；利用电化学原子力显微镜研究氢富集局部的形貌变化过程，观察点蚀的形核过程和分布，对比扫描开尔文探针显微镜研究结果，揭示点蚀形核的发生与氢富集的关系及影响点蚀的原因，从而弄清氢促进点蚀形核的微观机理。

不锈钢表面因容易形成致密的钝化膜而使其具有高抗腐蚀能力，然而点蚀对不锈钢具有很强的破坏性，成为不锈钢的主要腐蚀失效形式之一。尤其是在不锈钢中含有氢的情况下，点蚀更容易发生。不锈钢在生产、氢气氛保护热处理及使用中会导致氢的进入，这就不可避免地导致不锈钢点蚀的加剧。虽然进行了大量的研究，但氢促进点蚀形核的机理尚不清楚，更缺乏微观尺度的直接证据。. 本课题运用扫描开尔文探针力显微镜(SKPFM)和电流成像原子力显微镜(CSAFM)在微观尺度原位观察了氢导致不锈钢表面结构性质的变化，包括表面电位分布、钝化膜半导体导电性和类型和微结构变化，同时原位研究了点蚀形核的发生的位置与氢导致微结构变化的内在关系。研究结果表明，充氢后铁素体的功函数高于奥氏体的功函数，这与充氢前的实验结果是相反的，这是由于氢在奥氏体中具有高的溶解性和低端扩散性。氢富集在双相钢的缺陷处，呈现在SKPFM图像上的低电位区，即点蚀形核点。充氢导致了奥氏体发生了马氏体相变，同时在铁素体中出现了针状氢化物，并且点蚀形核点的位置与微结构的变化密切相关。为了进一步研究，在6%的FeCl3溶液中连续观察了充氢的双相钢样品发生点蚀的过程，点蚀大多发生在奥氏体/铁素体和奥氏体相界面上，少数发生在铁素体内，这与SKPFM的观察结果一致。充氢后的CSAFM研究表明，充氢极大提高了钝化膜的导电性，并且充氢导致钝化膜的半导体类型发生了反转，由p型转为n型。我们知道，n型半导体钝化膜易于发生点蚀。本课题研究中，我们首次运用I-V曲线判断钝化膜的半导体类型，结果得到了XPS测量结果的支持，这一研究结果用传统方法是无法获得的。. 通过本课题的研究，弄清了氢促进点蚀形核的微观机理，即氢容易在不锈钢的缺陷处富集，导致局部钝化膜半导体的导电性和类型发生了变化直至钝化膜破裂，从而诱发点蚀，为不锈钢点蚀行为的预测提供了实验和理论依据，以便于采取有效的措施对点蚀的发生进行控制，为耐点蚀材料的设计提供实验依据。