Electroplating and metal finishing

Anodising

Anodosing involves turning the surface of an aluminium product, item or object into a thicker layer of aluminium oxide. Oxides can form naturally but this method of electroplating uses a controlled environment to form the thicker finish. As a standard, aluminium Anodosing produces a silver finish, but during the oxidation process, dyes can be added to change the colour of the coating that forms.

In general, the benefits of Anodosing include:

Increased Durability & Robustness
Increased Corrosion Resistance
Increased Electrical insulation or non-conductive
Increased aesthetic and decorative appeal
Increased stain resistance

Anodising is a complex process that electrically converts the aluminium surface and sub-surface to a columnar structure of aluminium oxide. The porous structure can then be sealed with various chemistries to add additional properties.

Anodised coatings are typically sealed to improve their corrosion protection that is filling or hydrating the pores in the columns, using either hot water, chromates or acetates.

The most dense coatings are produced from pure aluminium alloys. The presence of alloying elements like copper and silicon produces macro-porosity in the coating, reducing its corrosion protection properties.

Sulphuric acid anodising is typically 10 – 15µm thick. It is used for its anti-corrosion properties, when additional durability is required or when colouring by dying is required. Currently we can offer a black colour anodising on small articles only, but this is planned to be expanded in the near future.

Anodising takes place at the metal interface, so that the oxide grows out of the surface, converted from the metal. Since the oxide has twice the volume of its parent metal, the coating grows 50% out from the original surface and 50% into the original surface. In consequence, the increase in the size of the anodised part is half that of the coating thickness, a factor to be taken into account when considering size tolerance.

As the coating grows, the electrolyte and electrical current must continue to gain access to the oxide metal interface, producing more oxide. Since aluminium oxide is an insulator, this could be a problem, but the coating structure forms a matrix of porous (sub-micron size) columns, from the surface to the interface, and oxide conversion continues at the base of the columns. As the coating thickness builds, and the electrical conductivity to the interface decreases, the rate of coating growth falls exponentially, so that the process cannot continue indefinitely.