The thickness of anodising that you should use depends on the part environment and application. The corrosion resistance and long term performance of the anodising depends on two things:

1. Sealing
2. Anodising Thickness

The most important factor in the corrosion resistance of anodising is sealing, not thickness. Even thin coating have good corrosion resistant properties when sealed. The difficulty comes evaluating the quality of sealing without damaging the part, it is important to have a reputable anodising service provider which has implemented rigorous process controls which include periodic salt spray testing. Sealing is important for dye colour retention.