Properties of the anodic oxide coating (Topic: 14008)

The anodic oxide coating consists of two layers, the porous thick outer layer growing on an inner layer which is thin and dense, and called the barrier layer. This layer is very thin, usually between 0,1 and 2,0% of the total film, and its thickness depends on the composition of the electrolyte and the operating conditions. The outer layer is porous due to the attack from the electrolyte. See figure below. It is important to know that the anodic oxide layer is an integrated part of the metal, and flaking or delaminating will not occur which could happen with lacquer coatings or plated coatings.

Structure of a typical cell of an anodic oxide coating showing basic units which determine its size. The cell size is twice the cell wall thickness plus the pore diameter [28]

The attack of the electrolyte - and therefore the porosity - is dependent of the following parameters:

1. Type and concentration of the electrolyte

2. Temperature and agitation of the electrolyte

3. Time in the anodising tank (see also next point)

4. Current density

The porosity tells us something about the quality of the anodic oxide layer. High porosity gives a poor quality, and low porosity gives a good quality of the layer. However, it is the pores in the anodic oxide layer that make it possible to colour the surface layer, and in addition anodising improves the following properties of aluminium:

1. Response to colouring

2. Corrosion resistance

3. Wear resistance

4. Surface hardness

5. Surface appearance

6. Electrical resistance

7. Fire protection

Response to colouring

Due to the porous nature of the bulk of the anodic film, it is possible to deposit colouring media in the pores. The colour will hence be incorporated in the layer, and not lay upon the oxide layer as with an organic coating. Many colours are available as absorbed dyes, but only a limited number are suitable for exterior exposure. Electrolytic colouring is used to impart bronze shades and black. These are suitable for exterior use. At present it is not possible to produce a pure white anodic film.

Corrosion resistance

The oxide layer built in the anodising process will improve the corrosion resistance of aluminium. A low porosity oxide film will have a good resistance against e.g. pitting corrosion, galvanic corrosion, and general corrosion. Accelerated corrosion tests like salt spray test have showed that the chloride ions in the salt spray solution have little effect on the oxide layer. Increasing oxide thickness improves the corrosion resistance. Pitting- and galvanic corrosion are the most usual corrosion types on anodised aluminium, and are usually confined to applications where thin, poorly sealed anodic films have been applied or incorrect assembly practices have been used after anodising.

Wear resistance

Optimising the anodising parameters wear resistance (abrasion resistance) will be improved. Especially hard anodising will increase the abrasion properties, but also normal architectural anodising will give better wear resistance.

Surface hardness

Normal architectural anodising increases the surface hardness from a value between 60 and 130 VPN to a value between 200 and 350 VPN.

Surface appearance

The appearance of the anodised product can be further enhanced by surface texturing prior to anodising. Such treatment may be mechanical (scratch brushing, linishing or polishing), chemical or electrochemical polishing. Combinations of these methods are often employed.

Electrical resistance

The anodic oxide layer has high electrical resistance. Anodised aluminium is suitable for electric components, e.g. transformers and capacitors.

Fire protection

Anodic oxide layers give a minor increase in the fire protection of aluminium constructions. The melting point of the oxide surface increases from approximately 650°C to approximately 2000°C.