Anodising

• Increased resistance to corrosion, including salt water.
• Increased resistance to abrasion and scratching, easier to clean and maintain.
• Lower coefficient of friction: Suitable for use with polymer bearings for sliding systems.
• Enhanced surface finish, removal of machining patterns.
• Longer life, colours unaffected by ultraviolet light.

ISO 7599 Decorative anodising: up to 25 µm.
ISO 10074 hard anodising: up to 100 µm.

Yes, slightly, as it is a conversion process approximately half of the thickness is ingress and only half growth.
Holes can be blanked to avoid any change in diameter.

Yes, as the coating is converted into insulative aluminium oxide, the surface of the material will no longer conduct electricity.
To maintain electrical conductivity a chromate conversion coating can be applied (Surtec 650), or holes/threads blanked.

To anodise a component, it must first be supported on a jig, and have a point electrical contact. Anodising uses acid and electrolytic passivation to increase the oxide layer. Adding a threaded hole is best practice as this can provide a secure mechanical and electrical connection point.
As it is a multi-stage dipping process, parts with intricate internal features can trap air or fluid. This can normally be overcome but considering it in your design is ideal.

A metal part is submerged into an acid electrolyte bath (sulphuric acid or chromic acid may be used). An electrical current is passed through the part and electrolytic solution, to encourage the build-up of an oxide film. When anodising the part is fully submerged so almost the entire surface is coated. There will always be an tiny area of that is uncoated, as the process requires at least one conductive point on the part surface (typically this is hidden in a thread or hole).  The most common method is sulphuric acid anodising. A cleaned part is placed into the acid and a positive charge is applied. Plates are also immersed into the acid with a negative charge applied. The aluminium part combines with the negatively charged oxygen ions to create aluminium oxide. It is an electrolytic passivation process.
Anodised or passivated metals include aluminium, titanium and stainless steel, but anodised aluminium is the most common due to its affordability, ease of machining and low density.

Protolabs offers this process for aluminium alloys only.

There are a few quick tests that can be performed to check if a part is anodised.

• Visual inspection: even clear anodising has a subtly different colour or sheen. The pickling process (a pre-preparation stage) will also remove most surface optical effects and tool marks, resulting in a more uniform finish.
• Electrical conductivity: using an ohmmeter or continuity test, to check for electrical conductivity and resistance. Anodised parts are non-conductive and have high surface resistance.
• Hardness: using a ball indent or instrumented indentation test, force is applied to create an indentation (mark) the softer the material the deeper this mark is, anodised aluminium has a harder surface.
• Corrosion resistance: anodised parts will perform much better in an elevated temperature saltwater test.

Anodising is generally chosen for 1 of 2 reasons as a decorative coating or as a protective coating. If you require an aesthetically pleasing appearance, anodising can be considered, due to its silver colour and because it can be dyed. A minimum thickness of 5 to 25 micrometres is recommended, or if you wish to dye it afterwards 15 to 25 microns is suggested. Protolabs parts that are anodised for decorative purposes meet the ISO 7599 standard.
If you require a protective coating for wear and corrosion resistance, hard anodised parts are a good option. The thicker the coating the higher the protection. Protolabs parts that are hard anodised meet the ISO 10074 standard.

Aluminium naturally creates its own oxide layer when exposed to the atmosphere, referred to as passivation. When steel or iron oxidise the material expands, so rust tends to flake off allowing further corrosion. When aluminium oxides, it contracts, effectively shrink wrapping the material in its own self-healing corrosive barrier.

However, this does not mean that the metal is safe from corrosion, the layer is thin and can be worn away or effected by other materials. Anodising uses acidic processes to create an even thicker protective barrier around the part, giving it extra protection. This means that anodised and especially hard anodised aluminium benefits from higher corrosion resistance, increased abrasion resistance and it’s colour is unaffected by ultraviolet light.