热浸镀锌铝镁无铬钝化钢板黑变原因分析

Cause analysis of blackening of hot-dip Zn–Al–Mg coated steel plate with chromium-free passivation

摘要

[目的] 探究热浸镀锌铝镁钝化钢板在湿热环境下表面黑变的机理，为其性能优化提供依据。[方法] 选取硅烷钝化与环氧树脂钝化两类典型无铬钝化锌铝镁钢板，检测了其耐指纹性、耐高温性、耐有机溶剂性、耐盐雾性等性能；采用 X 射线衍射(XRD)和扫描电子显微镜(SEM)分析了湿热环境中钝化板黑变前后组织结构变化。[结果] 两种钝化板的耐指纹性、耐高温性、耐溶剂性、耐碱性等性能均较好，但膜层结构均存在不足：环氧树脂钝化膜表面分布微孔且厚度不均；硅烷钝化膜厚度不均。在湿热环境下，钝化膜的保护作用逐渐减弱，令镀层暴露并腐蚀生成以氧化锌为主的疏松产物。该产物增强了钝化板表面对可见光的吸收和漫反射，最终导致钝化板在视觉上发暗发黑。[结论] 钝化膜的微观结构缺陷是湿热环境下镀层腐蚀与表观黑变的主要原因。本文的研究结果为通过改善钝化膜均匀性与致密性以防控黑变提供了明确方向。

Abstract

[Objective] To study the mechanism of surface blackening of passivated hot-dip galvanized Zn–Al–Mg steel plates in a damp-heat environment, and to provide a basis for performance optimization. [Method] Two typical chromium-free passivated Zn–Al–Mg steel plates, one with silane passivation and the other with epoxy passivation, were selected. Their properties including fingerprint resistance, high-temperature resistance, organic solvent resistance, and salt spray resistance were tested. The changes in the structure and composition of the passivated plates before and after blackening in a damp-heat environment were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). [Result] Both passivated plates exhibited good fingerprint resistance, high-temperature resistance, solvent resistance, and alkali resistance, but both passivation films had structural deficiencies: the epoxy passivation film had micropores distributed on its surface and uneven thickness; the silane passivation film also showed uneven thickness. Under damp-heat conditions, the protective function of the passivation films gradually deteriorated, exposing the coating to corrosion and forming loose corrosion products mainly composed of zinc oxide. These products enhanced the absorption and diffuse reflection of visible light on the surface of the passivated plates, ultimately leading to a visual darkening and blackening of the plates. [Conclusion] The microstructural defects of the passivation films are the primary cause of coating corrosion and surface blackening in a damp-heat environment. The results of this study provide a clear direction for preventing blackening by improving the uniformity and compactness of the passivation films.

关键词

热浸镀 / 锌铝镁合金 / 无铬钝化 / 黑变 / 湿热环境

Key words

hot-dip coating / zinc–aluminum–magnesium alloy / chromium-free passivation / blackening / damp-heat environment

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