Hard Anodising Thickness and Tolerance Limits
Hard anodising builds a tough oxide layer on aluminium, but the coating changes part dimensions in ways that catch engineers out if they have not planned for it.
Typical Thickness Ranges
There are typically thickness ranges for hard anodising of aluminium parts, and for general purposes, a coating thickness of 50 µm is typically used. Thicker coatings, that are to increase wear resistance, take longer to process, and can have a negative effect on fatigue strength of parts that are to be subjected to cyclic loading. Therefore, the thickness of the anodic coating must be specified based on the requirements of the end use application.
How Growth Splits Between Inward and Outward
Of the total coating thickness roughly half will penetrate into the metal to form oxide within the metal, and the other half will form oxide on the surface of the metal, i.e. above the surface of the part being anodised. The oxide formed on the surface of a shaft for example will add approximately 25 µm to the surface of the shaft, therefore if a bore is turned into a shaft the bore will tighten, and the shaft will grow.
Alloy Grade Affects What Is Achievable
High-copper content alloys (e.g. 2024, 7075) can only achieve thin, poor quality coatings. Other alloys with high copper content (e.g. 2024, 2618A) can produce a coating up to 50µm but this will become powdery and drop in hardness above this thickness. Pure grades (e.g. 6082, 5083) can produce thicker, harder coatings.
Tolerance Windows Processors Can Hold
Thickness is controlled within a tolerance of ± 10 μm on hard anodised components. However, some processors can guarantee as low as ± 5 μm on very simple flat and cylindrical components. In general, however, it is very difficult to control the coating on the internal surfaces of recesses, sharp corners etc. These tend to coat to the thinner end of the required thickness.
Specifying Coating Allowance on Drawings
Drawing Notes – Coating Allowance – Machine Dimensions in advance of anodising. e.g. bore of 20.00 mm diameter would require machining to 20.05 mm to allow for anodic coating build-up (25 µm per surface). Separate values for pre-coat and post-coat dimensions should be noted on the drawing with a general note including stating the hard anodising specification, e.g. hard anodise to 50 µm ± 10 µm per ISO 10074.
Mask Threads and Tight-Tolerance Features
Any threaded hole or feature with less than 50µm fit clearance should be masked before anodising. It only takes a thin layer of anodic oxide to cause problems with components such as screws, snap fits, etc. within threaded holes. The feature(s) in question should be plugged/capped prior to the part being anodised.
A little planning ahead at the drawing stage can avoid very expensive rework when parts arrive in an unacceptable condition.