A new phosphate pretreatment process for adhesive bonding of magnesium AZ31 sheets

An economic, high efficiency sodium dihydric phosphate surface pretreatment process (SDPT) was developed to improve the adhesive bond performance of 2 mm thick hot-rolled wrought magnesium AZ31 sheets. A phosphating solution with sodium dihydric phosphate (NaH₂PO₄), additive sodium fluoride (NaF), and accelerator sodium molybdate (Na₂MoO₄), sodium tungstate (Na₂WO₄) and potassium nitrate (KNO₃) were developed to pretreat the magnesium alloys. The content of sodium dihydric phosphate in the phosphating solution was strictly controlled to insure it supplied sufficient acid radical HPO₄ ^2? to phosphate magnesium AZ31 alloy. Furthermore, a suitable H⁺ content to keep the pH values in the range of 5–6 for a phosphating solution was necessary. With this SDPT pretreatment process, a coating consisting of the magnesium phosphate, magnesia (MgO), magnesium hydroxide (Mg(OH)₂), magnesium fluoride (MgF₂) and a minor amount of molybdenum oxide(MoO₃) was formed on the surface of the magnesium AZ31. While the SDPT pretreated adhesive-bonded joints had better initial bond strength than phosphate–permanganate process pretreated joints, the corrosion resistance of SDPT pretreated joints was slightly inferior.