不锈钢中MnS夹杂相局域溶解的第一性原理研究

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	不锈钢中MnS夹杂相局域溶解的第一性原理研究
	王宇佳
学位类型	博士
导师	马秀良 ; 胡培君
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	不锈钢点蚀第一性原理计算 Mns Mncr2o4 Stainless Steel Pitting Corrosion First-principles Calculation Mns Mncr2o4
摘要	"不锈钢的点蚀是材料科学中的一个经典问题。前人的研究已经证实点蚀形核的第一步是MnS夹杂相的溶解，我们最近的透射电子显微学实验已确定MnS夹杂相的最初溶解位置。根据这些实验结果，本文利用第一性原理计算方法并结合电化学原理，对不锈钢中MnS夹杂相的溶解位置、溶解形态、溶解机理等问题进行了研究。通过透射电镜实验，我们已发现MnS夹杂相优先在MnCr2O4纳米八面体颗粒的周围发生溶解。本文利用第一性原理计算准确的算出了MnCr2O4的标准生成焓，并且估算出其标准熵，进而计算出MnCr2O4的电极电位，发现它的电极电位比MnS高很多，当MnCr2O4与MnS接触在一起时，就会发生接触腐蚀，导致MnS的溶解被加速。实验还发现不同的MnCr2O4纳米颗粒周围MnS的溶解速度不同。根据电中性原理， MnS阳极溶解反应速度的不同是由于MnCr2O4表面的阴极反应速度差异所造成的。实验证实：MnCr2O4颗粒大多是以八面体形态存在，其外表面一般为（111）面。MnCr2O4的（111）面可能由金属原子表面和氧原子表面构成。第一性原理计算表明：在金属原子表面上，阴极反应可以很容易的进行；在氧原子表面上，阴极反应不能进行。对于实验中观察到的一些不规则的MnCr2O4纳米颗粒，我们计算了（100）和（110）表面上的阴极反应，发现表面金属原子的比例越高，O2分子裂解反应的势垒越低。 MnS的溶解机理一直是不锈钢点蚀问题中的一个重要问题。本文利用第一性原理分子动力学以及过渡态搜索方法研究了MnS表面单原子台阶角落处一个Mn离子的溶解过程，结果发现Mn离子溶解的过程中存在两个过渡态，势垒分别为0.87eV和1.00eV，这表明MnS的溶解是比较困难的，需要靠MnCr2O4这样的物质对其进行催化或者进行电位极化才能促使它发生较快的溶解。
其他摘要	" Pitting corrosion of stainless steels is one of the classical problems in materials science. Previous studies have evidenced that the first step of the pitting nucleation is the dissolution of MnS inclusions. The recent TEM experiments by our group have determined the initial dissolution site of MnS inclusions. Based on these experiments, this thesis discussed the dissolution site, pattern, and mechanism of MnS inclusions in stainless steel with the help of first-principles calculations and electrochemical principles. The TEM experiments showed that MnS inclusions dissolve preferentially around MnCr2O4 nano-octahedra. This thesis calculated the standard enthalpy of formation of MnCr2O4 by first-principles calculations and estimated its standard entropy, thus calculated the electrode potential of MnCr2O4. It is found that the electrode potential of MnCr2O4 is much higher than MnS. So, contact corrosion would occur and the dissolution of MnS will be accelerated when MnCr2O4 and MnS contact with each other. It was found that the dissolution speeds are different for MnS around different MnCr2O4. According to the principle of charge conversation, the speed difference of MnS anodic dissolution is determined by that of cathodic reaction (oxygen reduction reaction, ORR for short) on different MnCr2O4 surfaces. The surfaces of MnCr2O4 nano-octahedra are in the index of (111). There are two types of (111) surfaces: the one terminated by metal atoms and the other one by oxygen atoms. First-principles calculations show that ORR could occur readily on metal-terminated surfaces, but could not occur on oxygen-terminated surfaces. For the irregular MnCr2O4 nano-particles which are observed in experiments, the ORR on (100) and (110) surface is also considered. It is found that as the percentage of surface metal atom increases, the barrier of O2 dissociation reaction decreases. The dissolution mechanism of MnS is an important problem in the pitting corrosion of stainless steel. In this thesis, the dissolution process of a Mn ion at the corner of the mono-atomic step on MnS surface is studied by first-principles molecular dynamics and transition state search. It is found that these exist two transition states whose barriers are 0.87 and 1.00eV. It means that the dissolution of MnS is difficult at room temperature. The existence of some catalyst such as MnCr2O4 or the electrochemical polarization could make MnS dissolve quickly.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64469
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	王宇佳. 不锈钢中MnS夹杂相局域溶解的第一性原理研究[D]. 北京. 中国科学院金属研究所,2012.