Research On Chrome-Free Pre Treatment Of AZ31 Magnesium Alloy And Electroless Ni-W-P Alloy Plating Technology

To realize wider application of AZ31 magnesium alloy in aerospace sector, researches on AZ31 magnesium alloy surface strengthening technology is imperative. By comparing and studying the current researches on the surface strengthening technology for AZ31 magnesium alloy home and abroad, a new chromium-free pre-treatment conversion technology is proposed with the manganese dihydrogen phosphate, phosphoric acid, acetic acid and oxalic acid as the main ingredient of the chromium-free formula. Research on magnesium alloy electroless Ni-P alloy plating technology and electroless Ni-W-P ternary alloy plating technology have been to improve the corrosion resistance and hardness of AZ31 magnesium alloy.The surface micro-structure, cross section microstructure, surface phase structure, ingredients of the surface coating, corrosion resistance, hardness and other properties of the pre-treatment of magnesium alloy, electroless nickel plating and electroless Ni-W-P plating are tested and its mechanical properties, corrosion resistance and abrasion resistance characterized with SEM, XRD, CHI electrochemical working station, microscopic hardness tester and other testing tools.The results indicate: with NiSO4 as the main salt, NaHPO₂ as the reducing agent, Na2WO4 as the main additives and chemical conversion coating technology as the basis, the formula of electroless Ni-W-P alloy plating and the reaction conditions are finally known. The optimized electroless Ni-W-P alloy coating on the magnesium alloy is dense and uniform with good bonding force. The thickness of the coating is about 5μm with 7.4% of W and 3.6% of P, which is a low-phosphorus formula. After the electroless Ni-W-P plating treatment, the surface micro hardness of the magnesium alloy is significantly improved to 580VHN. With the electroless nickel plating treatment on the magnesium alloy, the E_corr is -0.74V and icorr is 60.3 A/cm²; with the electroless Ni-W-P plating treatment on the magnesium alloy, the E_corr is -0.50V and icorr is 15.5 A/cm². The corrosion potential moves obviously towards the positive value; the risk of corrosion significantly curbed down; and the corrosion current reduced. Therefore the conclusion could be made that the electroless Ni-W-P coating protects the magnesium alloy.