热基锌铝镁镀层钢切边初始腐蚀机理和保护性能研究

Initial Corrosion Mechanism and Protection Performance of Hot-Based Zn-Al-Mg Alloy Coating Steel Cut Edge

以低铝低镁型Zn1.5Al1.1Mg热基锌铝镁镀层板为研究对象,采用SEM、EDS、XRD等方式,明确了低铝低镁锌铝镁镀层板其切边的初始腐蚀机制,并通过SEM、XRD对浸泡实验和循环腐蚀实验后的试样其切边处的腐蚀产物种类和形貌进行分析,研究锌铝镁镀层对其切边处的保护性能。结果表明:锌铝镁镀层其切边是由纯Zn相、Zn-MgZn_(2)二元共晶、Zn-MgZn_(2)-Al三元共晶3种组织构成;含有MgZn_(2)相的三元共晶和二元共晶在锌铝镁切边的腐蚀初期通常作为腐蚀萌生点,进而扩展到整个镀层;在锌铝镁镀层切边的长期腐蚀过程中,Zn元素起到主要保护作用,游离的Zn^(2+)通过形成碱式碳酸锌[Zn_(5)(OH)_(6)(CO_(3))_(2)]和碱式氯化锌[Zn_(5)(OH)_8Cl_(2)·H_(2)O]在镀层切边处堆积,阻碍腐蚀介质的纵向扩展,当其转变为多孔氧化锌时,镀层保护失效,切边受到破坏。

The low-aluminum and low-magnesium Zn1.5Al1.1Mg hot-based Zn-Al-Mg alloy coated plate was employed as research object,the initial corrosion mechanism of the cut edges of low aluminum,low magnesium Zn-Al-Mg alloy coated plates was clarified by SEM,EDS,XRD and other methods.Moreover,through SEM and XRD method,the types and morphology of corrosion products at the cut edges of the specimens after immersion and cyclic corrosion experiments were analyzed for evaluating the protective performance of Zn-Al-Mg alloy coatings on their cut edges.Results showed that the cut edge of the Zn-Al-Mg alloy coating was composed of three microstructures:pure Zn phase,Zn-MgZn_(2) binary eutectic and Zn-MgZn_(2)-Al ternary eutectic.In general,the ternary eutectic and binary eutectic containing MgZn_(2) phase served as corrosion initiation points during the initial corrosion stage of Zn-Al-Mg alloy cut edges,and then extended to the entire coating.During the long-term corrosion process of cutting edges in Zn-Al-Mg alloy coatings,the Zn element played a major protective role.Specifically,free Zn^(2+)accumulated at the cutting edge of the coating by forming basic zinc carbonate[Zn_(5)(OH)_6(CO_(3))_(2)]and basic zinc chloride[Zn_(5)(OH)_8C_(l2)·H_(2)O],hindering the longitudinal expansion of the corrosive medium.Nevertheless,when these precipitates transformed into porous ZnO,the protective capabilities of the coating failed,and the cut edge was damaged.