锌铝镁镀层和纯锌镀层在典型大气环境中初期腐蚀行为研究

Study on Initial Corrosion Behavior of Zn-Al-Mg Coating and Pure Zinc Coating in Typical Atmospheric Environment

目的增加对不同大气环境中锌铝镁镀层钢腐蚀行为的了解,研究中国地区不同气候条件下1a内锌铝镁镀层钢腐蚀产物的组成和腐蚀速率。方法采用SEM、GDS、XRD研究了纯锌(GI)镀层材料和锌铝镁合金(ZM)镀层材料的结构,对比研究了GI镀层和ZM镀层在吐鲁番、江津、青岛以及万宁4个大气试验站的腐蚀试验。结果万宁GI的腐蚀速率为ZM的4.56倍,江津GI的腐蚀速率为ZM的3.76倍,青岛GI的腐蚀速率为ZM的2.84倍,吐鲁番GI的腐蚀速率为ZM的2.42倍,对于GI镀层和ZM镀层,按腐蚀速率从大到小的顺序依次为万宁、青岛、江津、吐鲁番。江津和万宁GI镀层的自腐蚀电流密度较小,万宁ZM镀层的自腐蚀电流密度最小。锌的大气腐蚀速率主要受相对湿度、氯离子含量、SO_(2)含量等影响,主要通过影响腐蚀产物组成来影响锌的腐蚀速率。腐蚀产物的保护性受其化学组成、导电性、黏附性、致密性、溶解性、厚度、形态和亲水性等因素影响,不同的锈层结构对阴极氧还原的抑制作用不一样,高氯环境对GI的保护性腐蚀产物Zn_(5)(CO_(3))_(2)(OH)_(6)有较大的破坏性,在高碱性环境下,容易使腐蚀产物转变成疏松导电的ZnO。在高氯环境中,由于镀层中MgZn_(2)的阳极溶解,释放出可与OH反应的Mg^(2+)以形成氢氧化镁(Mg(OH)_(2))。在ZM表面用Mg(OH)_(2)代替氧化锌被认为可以降低阴极氧还原反应(ORR),具有缓冲阴极位置的pH值升高的作用,能使碱式碳酸锌变得稳定,显著提升耐腐蚀性。结论ZM镀层材料在高盐高湿环境下能形成稳定的腐蚀产物,降低阴极氧还原速率,在典型大气环境下具有广阔的应用前景。

The work aims to improve the understanding of corrosion behavior of Zn-Al-Mg coated steel in different atmospheric environments,and to study the composition and corrosion rate of corrosion products of Zn-Al-Mg coated steel in China area under different climatic conditions within 1 year.The structures of the GI coating and the ZM coating were studied by SEM,GDS and XRD,and the corrosion tests of the GI coating and the ZM coating in Turpan,Jiangjin,Qingdao and Wanning,were compared.There were no peaks of Mg and Al elements in the GI coating,but there were obvious Mg and Al elements in the ZM coating,which were evenly distributed in the coating.The surface of GI plate was a pure zinc coating,and there were a lot of binary eutectic phases composed of Zn and MgZn_(2) on ZM surfaces.The corrosion rate of Wanning GI was 4.56 times that of ZM,Jiangjin GI was 3.76 times that of ZM,Qingdao GI was 2.84 times that of ZM,and Turpan GI was ZM.The traditional electrochemical test adopted the solution system,which was equivalent to the corrosion of galvanized sheet in solution.The metal corrosion rate basically did not change with the thickness of the liquid film,and the diffusion of oxygen in it was slow,which became a control link of the test.In this paper,the flowing electrolyte solution was used for the test to increase the diffusion coefficient of oxygen in the electrolyte.For the GI coating,the self-corrosion current density in Jiangjin and Wanning was low,and for the ZM coating,the self-corrosion current density in Wanning,Hainan was the lowest.The atmospheric corrosion rate of zinc was mainly affected by relative humidity,chloride ion content,SO_(2) content,etc.,and the corrosion rate of zinc was mainly affected by the composition of corrosion products.The protection of corrosion products depended on its chemical composition,conductivity,adhesion,compactness,solubility,thickness,morphology and hydrophilicity.Different rust layers had different inhibition effects on cathodic oxygen reduction.Zn_(5)(CO_(3))_(2)(OH)_(6) had a good inhibition effect on oxygen reduction.The overall current density of the ZM coating was smaller than that of the GI coating,which was consistent with the atmospheric corrosion law.ZM material had obvious advantages in Wanning area with high salt and humidity,but less advantages in Turpan area with relatively mild corrosion environment.This was because different environments would affect the structure and properties of the corrosion products,and the structure and properties of the products would affect their corrosion resistance.The high chlorine environment had great damage to the protective corrosion of GI Zn_(5)(CO_(3))_(2)(OH)_(6),and it was easy to transform the corrosion products into loose and conductive ZnO in the high alkaline environment.In the high chlorine environment,Mg^(2+) ions which could react with OH were released due to anodic dissolution of MgZn_(2) in the coating to form magnesium hydroxide(Mg(OH)_(2)).Replacing zinc oxide with Mg(OH)_(2) on ZM surface was considered to reduce cathodic oxygen reduction reaction(ORR),buffer the increase of pH value at cathode position,stabilize basic zinc carbonate,and significantly improve corrosion resistance.The ZM coating material can form stable corrosion products and reduce cathodic oxygen reduction rate in a high-salt and high-humidity environment,and has a broad application prospect in a typical atmospheric environment.