The electrolytic oxidation of aluminum is known as anodizing, by electrolyzing suitably prepared aluminum in an acid solution (typically sulfuric or chromic acid), the surface layer of the aluminum can be converted to a coating with protective, decorative or functional properties.
The coating formed is analogous to a honeycomb and will accept dyes of different colors, anodized aluminum is generally sealed after processing, this sealing seals the porous structure of the aluminum oxide and improves corrosion resistance, sealing can be carried out in a variety of different solutions, including hot water, low temperature nickel sealing and sealing with dichromate solution.
The substrate composition and condition (alloy type, grade temper etc.), have a significant effect on the anodizing characteristics of the components and the subsequent finish obtained, with different materials requiring different process conditions, consequently it not recommended to mix alloy types during the same process run, alloys must be strictly segregated and identified before processing.
The alloy type also has a bearing on the post anodizing appearance, with the final film color dependent upon a number of interacting factors, including, alloy composition and form of supply i.e. cast or wrought , surface finish, pretreatment prior to anodizing type of anodic film (sulfuric, hard or chromic) and anodic film thickness.
When specifying anodizing on drawings or orders it is recommended that wherever possible national, international or defense standards are used to provide the processing instructions to your supplier, typically the following information should also be provided;
- Specification reference
- Base material composition and condition
- Significant surfaces and areas where measurements of coating thickness are to be made.
- Any areas that must not be used as contact points for jigging/wiring
- Any special inspection requirements
- Any special packing requirements