Anodising and Conversion Coating

What is anodising?

Anodising is an electrochemical process used to create a durable aluminium oxide layer on metal surfaces. This process is particularly used for aluminium and its alloys. Aluminium is a popular alternative to steel due to it’s light weight and the fact that it can be readily cast and extruded. Anodising improves the metal’s corrosion resistance and durability. When it comes to aluminium alloys, in general the purer the aluminium is, the better it will anodise. Conversely, higher proportions of content of silicon or copper make for a more problematic process with an increased cost.

The anodising process can be controlled to achieve a specific thickness of the oxide layer. Increased thickness improves wear resistance. However, thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance.

Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but more brittle. This makes them less likely to crack and peel from ageing and wear.

Why choose hard anodising?

Hard anodising results in a thicker and more durable oxide layer. This layer is wear and corrosion resistant. The result is perfect for metal parts that need to be durable in harsh environments. Typically hard anodised parts are used in industries like aerospace, automotive and manufacturing.

Anodised parts are also suitable for clean room operations. For example, laboratories, the food industry or medical environments. This is because anodising is chemically inert and does not flake off.

Aluminium oxide also benefits from a higher melting point, in the region of 2050°C. Pure aluminium’s melting point is much lower at 658°C.


Passivation is the process of making a material “passive”. This is usually done 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 which  takes effect almost immediately. This protects it from further oxidation in most environments. However, aluminium alloys 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. These tanks contain the appropriate chemical solutions to start the plating process. This is a more costly option than barrel plating but it may be more suitable for large or complex parts that need extra coverage. Jigging points are usually small, and can be placed in hidden places so they won’t be affect the performance of the component.


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