Anodising on Aluminium Sheet and Plate

Anodising on Aluminium Sheet and Plate

Aluminium sheet and plate are typically different to extrusions and castings and have their own idiosyncrasies with anodizing that need to be considered when specifying.

Choose the Right Alloy Grade

The aluminium alloy grade chosen for anodizing has a greater effect on the resulting anodized finish than most people realise. Generally the 1xxx and 5xxx series of aluminium alloys produce the best anodized finishes – clear and consistent. The 2xxx and 7xxx series of aluminium alloys contain copper and/or zinc and can produce darker and patchy anodized finishes. When specifying a finish on aluminium, it is crucial to verify the aluminium alloy designation and temper(s) before specifying a finish.

Account for Mill Finish Variation

Sheet and plate stock arrives at the mill with a rolled surface which can have a directional quality to it. This surface can show through on a decorative or clear anodize finish giving two apparently identical panels from different coils of stock a different appearance. For jobs where appearance is critical it is best to use stock from the same mill and temper.

Watch for Thickness Variation Across Large Panels

The differences in thickness of a sheet or plate through its length can cause variations in anodizing time and hence coating colour and thickness. By specifying the thickness of your raw material and perhaps discussing the racking of your material through the anodizing bath with your anodizer, these problems can be minimised.

Factor in Dimensional Growth

The anodic layer grows into the surface of the aluminium partially and outward from the surface of the aluminium partially. The outward growth of the anodic layer needs to be allowed for in design, and typically is grown in from the surface. For close tolerance flat parts, it is better to design in the growth of the anodic layer from the start rather than trying to put it in after the part has been anodized.

Match Bath Type to the Application

Sulphuric acid anodising (often just referred to as anodising) is typically used for general Aluminium sheet and plate applications, producing a coating in the 5 to 25-micron range. For higher wear applications such as sliding parts, Hard anodising at lower temperatures can produce coatings up to 50 to 100 microns.

By getting the required details right before processing for anodising, you can save loads of time and money and a lot of headaches as well, especially for flat products.