Anodising is an electrochemical process that converts the metal surface by a controlled growth or aluminium oxide (bauxite) at predetermined current densities. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodised aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from ageing and wear.
Passivation is the process of making a material “passive”, usually by the deposition of a layer of oxide on its surface. In air, passivation affects the properties of almost all metals–notable examples being aluminium, zinc and titanium. In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide that is a few nanometres thick.
Pure aluminium naturally forms a tough resistant oxide, almost immediately, that protects it from further oxidation in most environments. Aluminium alloys, however, offer little protection against corrosion.
The rack plating process involves locating the items to be coated on an electrically conductive jig / rack. The rack is then submerged into a series of tanks containing the appropriate chemical solutions, to commence the plating process. This is a more costly option than barrel platings but may be more suitable for large or complex parts.