Phosphate coatings on magnesium alloy AM60 part 1: study of the formation and the growth of zinc phosphate films

The corrosion protection by zinc phosphate conversion coating on magnesium alloy AM60 is studied. Three phosphatation solutions containing phosphoric acid, phosphate ions, nitrates and nitrites added with zinc and fluorides were used. Therefore, the present investigation aims to study the role of the phosphating bath components on the phosphating process and to enhance the possibility of obtaining phosphate layers on magnesium AM60 alloy by immersion in the various phosphatation solutions. The morphology and the coating composition on the electrode surface were analysed (SEM, EDX, X-ray diffraction and Raman spectroscopy). The phosphate films formed are mainly composed of tetra-hydrated zinc phosphate, otherwise known as hopeite. A mechanism is proposed to explain the germination and the growth of the phosphate crystals on AM60 alloy.