Most aluminium wheels are clear coated for corrosion resistance and aesthetic appearance. Unprotected aluminium wheels quickly corrode and pit when exposed to road salt and excessive moisture. If the corrosion continues unchecked for too long, the cosmetic damage may be too great to reverse. Generally, several coats are applied to aluminium wheels to guarantee a suitable corrosion resistance. After each coat the wheels are left inside an air electric furnace for drying at 170 ± 5°C for 1 hour. From the heating curve in Fig. 3, it is observed that it takes approximately 20 minutes to heat the wheels from room temperature to 145°C. Due to slow heating, the coating treatment effect experienced by the wheels during the heating stage is not negligible. Then, the wheels are maintained for 35 minutes in a range of temperature between 145 and 170°C. The temperature and time used in the present work for powder coating activate the diffusion mechanism of the solute atoms, such as Mg and Si, leading to the precipitation of dissolved elements and the coarsening of existing precipitates, i.e. the bake hardening effect. The influence of powder coating cycles on the hardness of T6 heat treated wheels is shown in Fig. 22. The hardness increases progressively after each
coating cycles of about 3%. The average hardness of wheels after machining is around 92
HB, while after 3 coating cycles the hardness increases up to 98 HB.