Degradation research of protective coating on AZ91D Mg alloy components via simulated contamination

AbstractAccelerated environmental (hygrothermal) exposure experiments are performed on organic paints coated on commercial die-cast AZ91D Mg alloys to investigate the effects of contamination on blistering. Specifically, artificial human perspiration spray is used to contaminate the substrate surface. Blistering occurred only for paints that are spread on surfaces with the perspiration present. More blisters gradually form at longer test times, and the volume of blistering increases. Scanning electron microscopy indicates that blistering is initiated by contamination and/or substrate corrosion at the interface of the organic paints and the substrate. Blistering is characterized for two samples exposed to the hygrothermal environments for various times, and is found to be initially empty in the early stages. Hydrophilic chloride contaminants from the perspiration lead to in situ adhesion loss. Simultaneously, the paints volume expands, and the associated compressive stress causes it to bulge. After long-term test exposure, chloride anions corrode the substrate under the films, and MgO, Mg(OH)2, and Mg2(OH)3Cl corrosion products fill the blisters. Finally, a model of blistering evolution is discussed