高耐蚀锌铝镁镀层研究现状

从锌铝镁镀层的熔池界面反应、镀层组织、表面和切边腐蚀机理、腐蚀产物类型变化等方面, 对高耐蚀锌铝镁镀层的研究进展进行了详细分析. 根据Al成分含量的不同, 将商用及实验室锌铝镁镀层分为"低铝"、"中铝"和"高铝"锌铝镁三种类型: 不同类型的锌铝镁镀层的金属间化合物层生长动力学存在差异, 为了控制镀层厚度, 应合理控制浸镀时间、温度与熔池成分; 凝固组织也存在差异, "低铝"与"中铝"会析出Al或Zn初晶、Zn/MgZn₂二元共晶组织、Zn/MgZn₂/Al三元共晶组织, "高铝"会产生富Al枝晶、枝晶间富Zn相、Mg₂Si相、MgZn₂相, 不产生共晶组织; 发生表面腐蚀时, "低铝"与"中铝"中MgZn₂相先电离, 并生成碱性锌盐、双层氢氧化物等致密的腐蚀产物, 抑制腐蚀; 发生切边腐蚀时, 锌铝镁会出现自修复现象, 在切边钢基或镀层破损处形成碱性锌盐, 保护基体. 

Research status of high corrosion-resistant Zn-Al-Mg coating

Zn-Al-Mg alloy coating, the most promising protective steel coating of the 21st century, is widely employed in construction, automotive, and other fields, due to its high surface and edge corrosion. In recent years, with the increasing demand for Zn-Al-Mg coating, a series of basic studies on Zn-Al-Mg coating materials has been carried out by foreign scholars, making significant progress and achievements. Simultaneously, the gap in the galvanizing industry between domestic and international has been expanding year by year. In order to gradually reduce gradually this gap with foreign countries, it is necessary to summarize and review the research achievements of foreign researchers. In this paper, the research progress into high corrosion resistant Zn-Al-Mg hot dip coatings was reviewed from the perspective of interfacial reactions in pots, coating structures, corrosion mechanisms of surface and cut edges, as well as corrosion product types of Zn-Al-Mg coatings. According to the range of Al content, laboratory and commercial Zn-Al-Mg coatings are divided into three types: "low-aluminum, " "medium-aluminum, " and "high-aluminum" coatings. There are differences in these coatings, including growth kinetics in the intermetallic compound layers of the different types of coating. In order to control the thickness of the coating, reasonable immersion time and temperature should be controlled. There are also differences in the solidification structures of the three types. Primary Al or Zn crystal, Zn/MgZn₂ binary eutectics, and Zn/MgZn₂/Al ternary eutectics would form in "low-aluminum" and "medium-aluminum, " while Al-rich dendrites, an intergranular Zn-rich phase, a Mg₂Si phase, and a MgZn₂ phase would occur with "high-aluminum" coatings. During surface corrosion in "low-aluminum" or "medium-aluminum, " the MgZn₂ phase is ionized first, giving rise to a dense corrosion product to inhibit corrosion, such as basic zinc salt (BZS) or layered double hydroxide (LDH). Meanwhile, in the cut edge, a self-healing phenomenon occurs; the proposed explanation in this paper for this is Mg-containing corrosion product flowing or pH changing. However, there are some disputed aspects that need further study. In the hot dipping process, the intermetallic compound thickness should be controlled by the interfacial reaction at the steel/liquid melts through changing the molten bath temperature and holding time. The influence of Mg₂Zn₁₁ phase and MgZn₂ on the corrosion resistance of Zn-Al-Mg coating is also controversial, so that the microstructure of Zn-Al-Mg coating needs further investigation for corrosion. Furthermore, a kinetic model of the corrosion process should be established to discover the controlling factors in the corrosion reaction, so that the life of the coating can be extended.