Corrosion Coating

Introduction
The first point to emphasise is that the Resistance of aluminium and aluminium alloys to normal, ambient environmental corrosion is excellent. Its natural corrosion resistance is one reason why aluminium alloys find such diverse applications, from beverage cans, via architectural uses to aircraft construction. The chief source of the protection to corrosion, comes from the adherent, self-healing oxide film which is always present in an ambient air atmosphere.
Although aluminium is reactive, the oxide film that forms on it is inert and protects it from further oxidation. The oxide film is about 2.5nm thick on fresh rolled sheet and it builds up over several years to some 10-20nm (1nm = 10-9 m) in thickness.

There are two important points to remember about corrosion:
1. It can cost money
2. Component may no longer be able to perform.

Corrosion Types
There are many types of corrosion.Common varieties which may be met in an industrial and/or component service situations are:
(a) General corrosion
(b) Crevice corrosion
(c) Poultice corrosion
(d) Fretting corrosion
(e) Stress corrosion
(f) Galvanic corrosion
(g) Pitting corrosion
(h) Intergranular corrosion and exfoliation

Corrosion Protection

In a number of electrolytes, e.g. dilute sulphuric, chromic, oxalic and phosphoric acids, the reaction products on aluminium anodes are sparingly soluble and strongly adherent.
In anodising, the anodic oxide film consists of two layers,an inner, dense and thin “barrier”, layer and an outer porous film often with a cellular structure.
The outer layer is formed by the electrolyte from the inner layer, the dimension of which is determined by the applied anodic voltage. As formed, the outer layer is both hydrous and hydroxylated and largely amorphous. On immersing in boiling deionised
water for some 30 minutes it crystallises to the stable mineral boehmite, Al2O3.H2O.
This is termed “sealing”. In practice, various anodising procedures are used to produce specific surface properties. For example, anodising in dilute, cold, acid produces hard, durable surfaces. More concentrated acid gives thicker, more uniform
films appropriate for dyeing, before sealing.

Chemical Conversion Coatings

Oxidation of the aluminium surface can easily be achieved by the use of chemical oxidants without an externally impressed current. Such processes are, however,electrochemical in nature, involving local cell actions. They are known as Chemical
Conversion Coatings, With chromic acid.
In this case fluorides are added to the chromic acid to remove the initial Al2O3 and allow the local cell action to proceed. With phosphoric acid this anodic product is an aluminium phosphate.
To a large extent conversion coatings are now competitors of anodising, as they provide cost-effective substrates for modern organic finishes.

Other methods of Protection
Attack by alkalis, even in the form of soap, can be severe. Inhibitors, such as sodium silicate, chromates and dichromates are used [6] as conversion coatings to protect aluminium alloys used in the structure of, for example, washing machines.
Sheet products may be clad, for example with pure aluminium, which reduces susceptibility to corrosion.