Optimization of Hopeite Coating Formation on AZ91D Magnesium Alloy by Chemical Conversion: The Role of Treatment Time

Hopeite (Zn3(PO4)2·4H2O) coatings, obtained via zinc phosphate chemical conversion (ZPCC), have gained increasing attention in the biomedical field. Such coatings are particularly appealing owing to their combined capability of excellent corrosion resistance and biocompatibility. This study investigates the influence of processing time on the formation of hopeite coatings on AZ91D magnesium alloy by zinc phosphate chemical conversion (ZPCC). X-ray diffraction (XRD) analysis confirmed that the coatings primarily consisted of hopeite, with trace amounts of phosphophyllite. The maximum coating mass and thickness were achieved after a 30 min treatment at 55 °C. Electrochemical measurements demonstrated superior corrosion resistance for coatings formed within 15–30 min. Additionally, coating self-adhesion tests revealed strong bonding between the coating and the substrate.