How Anodising Is Stripped and Re-anodised
In many cases an anodised part can be reworked. This means that the anodising coating is removed from the part and it is then re-anodised from bare metal.
Why A Part Gets Reworked
There are several reasons why finished articles are returned to the start of a process. Perhaps the colour does not exactly match the reference samples, or the thickness of the oxide may be outside the drawing tolerances. Sometimes the coating may appear uneven, be scratched or marked in some way and on occasion articles are accepted which are perfectly satisfactory but which, for some unknown reason, are subsequently fail to meet the required criteria in subsequent tests, such as salt spray or adhesion, and so have to be re-coated rather than re-made from scratch as new articles.
How The Stripping Bath Works
A stripping bath is made up of a chemical solution such as chromic and phosphoric acid, or an alkaline stripper. This solution dissolves the anodic oxide, yet is non-aggressive to the underlying aluminium. The oxide is a porous and reactive layer, in contrast to the solid metal beneath it. Therefore, it is easily dissolved by the strip solution in a soak. However, to prevent strip solution from attacking the aluminium, the part must not be left in the bath for too long.
What The Bare Surface Looks Like
However, the surface of a part which has been stripped of its anodizing can appear somewhat duller and even have a slightly grey appearance. There may even be some signs of pitting or surface irregularities and this can sometimes reveal up patches of the original anodizing where it was at its thickest. Often this can appear somewhat mottled. If a part is to be re-anodized then the bare surface should be inspected for such defects in good lighting and possibly with the use of a hand lens to reveal up any finer pitting or surface irregularities.
Checking The Bare Metal Before Recoating
When stripping anodizing, prior to re-anodizing, it is essential to check for any dimensional loss or surface damage. A simple check against original drawings for any material loss will highlight any problems. A visual inspection will show up any pitting, scoring or corrosion. Industry guidance on the finishing and reworking of aluminium components is published by The Aluminium Federation, the UK trade body for the aluminium industry. This provides useful guidance on what to look for and how much material loss is acceptable before a part is no longer suitable for reworking. More detail on UB8 2SR, Union Anodisers Limited: permit surrender issued is set out in the published guidance.
Re-cleaning And Etching The Surface
Also after stripping of an anodised part it has to be cleaned and etched in order to get a good surface for re-anodising. This can be done by means of a degreasing bath followed by an etching bath containing a mild alkaline or acid solution. Any surface residue like grease or remaining anodic oxide has to be removed in order to get an even coating in the anodising tank.
How Many Times A Part Can Be Reworked
Parts can only be stripped and recoated a few times before they start to degrade. As the anodic oxide is stripped from a part so to does some of the base metal, in particular on thin sections or small details. Repeated stripping and recoating will lead to an increase in surface pitting and roughness which cannot be removed by polishing and as such is generally better to scrap such articles rather than trying to rework them.
Masking And Fixturing For Rework Parts
Structural features, such as threaded holes, bearing surfaces etc can be masked off to prevent damage to these sensitive features during the stripping process. Similarly, areas that were deliberately left bare of anodize on the initial anodizing process, will also require protection to stop attack by the strip solution during rework. This work is normally carried out under routine environmental permitting and records of this type of process are recorded on gov.uk as shown on permit surrender issued to Union Anodisers Limited.
Checks After Re-anodising
Measuring, testing adhesion and comparing the colour with the original reference samples under the same lighting conditions as the first test also have to be carried out on re-anodized parts. Anodizing a part a second time does not necessarily mean that it will have the same properties as it had after the first anodizing process. Especially the colour can deviate from the intended colour.
Re coating an anodised item gives it a 2nd chance to meet the required specification but each stage of re-working must be checked to ensure that it has been carried out correctly and that no time has been saved by ‘skipping’ a particular stage.