High corrosion resistance of electroless Ni-P with
chromium-free conversion pre-treatments on AZ91D magnesium alloy
chromium-free conversion pre-treatments on AZ91D magnesium alloy
(Key Laboratory of Automobile Materials of Ministry of Education,
College of Materials Science and Engineering, Jilin University, Changchun 130025, China)
College of Materials Science and Engineering, Jilin University, Changchun 130025, China)
Abstract: Phosphate-manganese, tannic acid and vanadium conversion coatings were proposed as an effective pre-treatment layer between electroless Ni-P coating and AZ91D magnesium alloy substrate to replace the traditional chromate plus HF pre-treatment. The electrochemical results show that the chrome-free coatings plus electroless Ni-P coating on the magnesium alloy has the lowest corrosion current density and most positive corrosion potential compared with chromate plus electroless Ni-P coating on the magnesium alloy. These proposed pre-treatment layers on the substrate reduce the corrosion of magnesium during plating process, and reduce the potential difference between the matrix and the second phase. Thus, an electroless Ni-P coating with fine crystalline and dense structure was obtained, with preferential phosphorus content, low porosity, good corrosion-resistance and strengthened adhesion than the chromate plus electroless Ni-P.
Key words: magnesium alloy; corrosion resistance; electroless; chromium-free pre-treatment