Effects of Pretreatment on the Adhesive Strength and Corrosion Resistance of Chromium - Free ZnAl Coating Prepared on the Surface of NdFeB Permanent Magnet

Abstract

In order to promote the application of low - cost and high - performance chromium - free ZnAl coating in the protection of NdFeB permanent magnet, the effects of different pretreatment processes (alkali washing, pickling, sandblasting, sandblasting + pickling) on the adhesive strength and corrosion resistance of chromium - free ZnAl coating prepared on the surface of NdFeB permanent magnet were studied using SEM and TR300 profilometer. Results showed that both sandblasting and sandblasting + pickling (5 s) could significantly increase the adhesive strength between the chromium - free ZnAl coating and the NdFeB substrate; the ZnAl coating prepared after an alkali washing had the strongest corrosion resistance to the ammonium nitrate immersion (up to 230 min). The pretreatment processes of sandblasting and sandblasting + acid pickling (5 s) could increase the surface roughness of the substrate and enlarge the contact area of ZnAl coating solution with the substrate surface to enhance the adhesive strength, but the corrosion resistance of the coating was far lower than that of the coating treated with alkali washing due to the damage to the substrate surface caused by the high - speed impact of sand particles during sandblasting. After the substrate was pretreated with pickling (50 s), the corrosion resistance of the coating was greatly reduced. Therefore, as a low - damage and environmental friendly pretreatment process, alkali washing was the best pretreatment for ZnAl coating prepared on sintered NdFeB surface.

Keywords

• chromium - free znal coating|ndfeb permanent magnet|pretreatment|adhesive strength|corrosion resistance