Comparison of the thermal stability of magnesium phosphate (newberyite) coating with conventional zinc phosphate (hopeite) coating

This article presents a detail comparison of the thermal stability of the new magnesium phosphate (newberyite – MgHPO₄·3H₂O) coating with a conventional coating of zinc phosphate (hopeite – Zn₃(PO₄)₂·4H₂O). It was confirmed that dehydration of zinc phosphate (hopeite) occurs gradually (dehydration start temperature: 115 °C). The start of magnesium phosphate (newberyite) dehydration is indeed shifted to somewhat higher temperatures (about 125 °C) but the dehydration has an intense jump character. When using magnesium phosphate (newberyite) coating for further surface treatment at higher temperatures, dehydration of the coating can result in reduction of the adhesion between the phosphate/primer coatings. Under these conditions, it is recommended to use a coating of conventional zinc phosphate (hopeite) or manganese phosphate (hurealite).