Anodizing on cast aluminium gives different shades of grey. As the thickness increases the darkness increases. Automobile applications are usually anodized to 10-15 microns thickness which is known as clear anodizing. Marine applications are anodized upto 40 microns which are called as Hard anodizing.
Certain applications require corrosion resistance to chemicals. For such components, alodine coating offers a better corrossion resistance by hardening the surface. Aircraft parts are usually chromated. The process is done mostly after machining process. Sometimes, chromating is done before powder coating to act as a primer for the coating process.