Cr、V合金化弹簧钢腐蚀产物的相组成与转化

采用中性盐雾腐蚀实验对不同Cr与V含量的合金弹簧钢进行了24~288 h的腐蚀实验,用光学显微镜(OM)观察腐蚀样品的表面宏观形貌,通过扫描电镜(SEM)观察腐蚀产物(简称锈层)截面情况,用能谱仪(EDS)分析确定了腐蚀产物中Cr、V和Cl含量与分布情况,用X射线衍射(XRD)和Rietveld分析确定了腐蚀产物各锈层相的相对含量。结果表明:当腐蚀时间达到288 h时,钢表面逐渐形成内层(30~50μm)和外层(100~180μm)的两层结构锈层。其中外层主要是由γ-FeOOH组成,很容易剥落;而内层包含α-FeOOH和Fe3O4,结构较致密,与基体结合比较牢固。Cr和V在内层锈层中明显富集,而没有Cl^-,说明在内层锈层区域Cl^-侵入受到阻止;而外层锈层中Cr和V基本没有富集,且含有一定量的Cl^-;通过XRD分析腐蚀的不同阶段和不同部位的锈层成分图谱及相对含量关系,分析了γ-FeOOH形成和γ-FeOOH转化为α-FeOOH的过程。基于上述分析构建了不同相的转化模型。

Phase Composition and Evolution of Corrosion Products of Spring Steels with Cr- and V-addition

The corrosion performance of four spring steels with different contents of Cr and V was assessed via neutral salt spray corrosion tests for 24~288 h, which were then characterized by means of optical microscope, SEM with EDS and XRD, peculiarly the relative amount of phases in the corrosion products was also acquired by Rietveld quantitative phase analysis of the relevant X-ray diffraction patterns. The results show that: after test for 288 h, bi-layered rust scales formed on the steel substrate, which consists of a dark brown inner layer (30~50 μm) and a red-brown outer layer (100~180 μm). The outer layer is mainly composed of γ-FeOOH, and tends to spall off easily, while the inner layer contains α-FeOOH and Fe₃O₄, it is denser and firmly bonding to the matrix. Elements Cr and V are obviously enriched in the inner rust layer, but no Cl^- were detected there, which indicates that the inward migration of Cl^- is effectively prevented from arriving the inner rust layer. However, the outer rust layers contain a certain amount of Cl^- but not Cr and V. The analysis results of the phases composition and their relative content of the rust layers at different corrosion stages comfirmed that the γ-FeOOH phase did form in the initial corrosion stage, and then for a long term the γ-FeOOH phase could gradually transform into α-FeOOH phase. Based on the above analysis, the relevant transformation model was proposed.