Revisiting the cracking of chemical conversion coating on magnesium alloys

Abstract Cracks formed on chromate-free conversion coating (CFCC) have long been regarded as the result of dehydration. In this work, multiple forms of defects inside the phosphate-vanadium (P-V) coating and phosphate-calcium-vanadium (P-Ca-V) coating were observed and analyzed by TEM. Results show that the conversion coatings are rich with defects near the metal/coating interface. The “vena contracta” phenomenon of cracks, the trench found at the root of crack, and the isolated pores found inside the coating indicate that the cracks originate from the metal/coating interface. An entrapped hydrogen-induced coating cracking mechanism was proposed. Solutions towards governing the hydrogen-induced cracking were proposed.