Effect of ultrasonic agitation during the activation process on the microstructure and corrosion resistance of electroless Ni-W-P coatings on AZ91D magnesium alloy.

Two pretreatment method, including the normal activation and the ultrasonic agitation in acidic bath, were used during the pretreatment of AZ91D substrate, for which the electroless Ni-W-P coating was prepared. The microstructure, bonding strength and corrosion resistance were compared. The effect of different activation condition on the microstructure and corrosion resistance was investigated and discussed. Results demonstrate that ultrasonic agitation has negligible effect on the microstructure, chemical composition, but improves the bonding strength and corrosion resistance of Ni-W-P coating. The corrosion resistance of Ni-W-P coating via ultrasonic pretreatment was less dependent on the deposition time. Ultrasonic agitation facilitates the deposition of Ni on the β phase and the eutectic phase, and improves the compactness at Mg/Ni interface by decreasing the inclusion of MgF 2 , which are critical for improving the quality of Ni-W-P coating. • Ultrasonic agitation (UA) was applied on AZ91D to prepare Ni-W-P coating. • Chemical composition and structure were not affected by UA. • The corrosion resistance and bonding strength were improved by UA. • The corrosion resistance of coating by UA was independent on deposition time. • UA improves the corrosion resistance by removing the activation products. [ABSTRACT FROM AUTHOR]