Whilst there can be many reasons for not getting a perfect result; remember even the profesionals get it wrong on ocassions.  Below is a list of general questions that are being asked by the both Professional and DIY anodisers when discussing various anodising faults.

Below is a table with some of the potential problems that may occur during the aluminium anodising process and its subsequent colouring, together with its typical cause and potential remedy.




Fault Possible reason Remedy

Decrease in colour strenght / depth

  • Dye Bath contamination
  • Dye used up
  • Improve rinsing
  • Add dye &/or extend dying time
  • Replace bath

Colour depth change in same batch

  • Inconsistence in film thickness
  • Increase air agitation

Colour Depth differences

  • Irregular anodising conditions
  • Different alloys
  • Keep anodising conditions constant
  • Dye only same alloy

Dark edges

  • Irregular current leading to heavy buildup of the film
  • Reduce current density
  • Increase air agitation

Patchy spots

  • Electrolyte carry over to dye bath
  • Oily
  • Local overheating when mechanical polishing
  • Gas bubbles on anodize pores
  • Improve rinsing
  • Strip and re-anodize
  • Increase air agitation

Dark dye spots

  • Un-dissolved dye particles
  • Boil up & agitate dye bath
  • Clarify dye bath by filtration

Dull / Chalky surface (removable by wiping)

  • Inadequate film thickness / too soft
  • Reduce anodize temp/acid conc./time
  • Increase air agitation

Surface pitting & soft coating

  • Parts too close to the cathode
  • Increase tank size or place the parts further away


 some additional information

Bath Make-up

For all baths with the exception of aluminum dyebaths, fill tanks 2/3 full of water. Add the required amount of products slowly with mild agitation. Add water to the operating level and mix again.

USE PERSONAL PROTECTIVE EQUIPMENT  goggles-gloves-overall the chemicals and process is CORROSIVE

Corrosive sq NEVER... add water into acid as this will cause excess heat generation and spattering which will lead to serious personal injuries.

For the make-up of aluminium dyebaths, fill tanks with 1/3 of water and increase to the dyeing temperature. Add the measured amount of dyes into the water whilst stirring for at least 10 minutes. Add water to make to the final volume and continue stirring until the dye solution is fully homogenious. Check the pH of the solution and adjust to the correct pH, if necessary.

Water Quality

Whenever possible, de-ionized water should be used to make up all baths and for rinsing operations. If de-ionized water is not available, clear tap water with low hardness can potentially be used but take care. Special attention should be taken to ensure that the water should be free from chlorides, phosphates, silicates and iron. Hard water and inferior water quality does cause a lot of problems in a metal finishing operation. These may include amongs others the following simptoms; reduction of dye uptake from a fresh anodic coating, excessive bleeding in the sealant solution, bath turbidity, corrosion pitting due to chloride ions, etc.

Rinsing & Water break test

The rinse following each operation is an important step in each finishing sequence. If not rinsed thoroughly, acid residues clinging on the dyed parts are likely to cause spotty dyeings. Over carry residue also contaminate the dye bath which leads to inactivation of the dye bath and may result in break down of the dye structure. De-ionized water should be used for make-ups and rinsing after each bath operation. The parts should stay in each rinsing bath for at least 2 minutes to ensure adequate washing of the parts.

The water break test is probably one of the most important steps in any metal surface operation. To pass the test, water should be sheet off the part rather than beads off when withdrawing from the rinsing bath. Water beading means there is still an oil residue present on the aluminium surface.

pH control

A pH meter is used to check the pH values of the working solution. It is advisable to use pH meters that can self regulatingu upon temperature variation for large production applications for hobbyist/DIYer pH strips may be used whilst not perfect they will give you an adequate indication on the pH level of the bath.  Acetic acid should be added to lower the pH of the dye solution. Sodium hydroxide should be added to raise the pH of the dye solution. Sodium acetate can also be used as a pH buffer of the dyebaths. The pH adjustment of hot sealant solution can also be done in the same way.

Activation of anodized parts before dyeing

Corrosive sq This process is corrosive goggles-gloves-overall  USE PERSONAL PROTECTIVE EQUIPMENT

Fresh anodized aluminum parts should preferably be dyed without unnecessary delay but we all know that may not be possible at times. If parts dry excessively, one may find that the adsorptive capacity for the dyes diminishes. Re-activation of the anodic pores can be done by treating the parts in 10% of diluted nitric acid  for about 30 seconds followed by an adequate rinse. 

Stripping of coloured anodic coating

 Corrosive sq These proceses are corrosive goggles-gloves-overall

 Stripping of colours before sealing

If the dyed aluminium parts are found to be off-shade prior to sealing, they can normally be stripped or lightened in depth by treatment in dilute nitric acid or sulfuric acid. These acids cause no serious damage to the anodic coating, which can be re-dyed after the treatment note however that this needs to be kept to a minimum. However, there are some dyes that can not be stripped or lightened in this way. In some cases the only option is that the anodic coating has to be stripped completely in order to remove the colours on the parts. Trial and error should be used to confirm the strip ability of each dye color on the anodic coating.

Stripping of colours after sealing

If the aluminium colour fault is one that cannot be corrected by stripping and re-dyeing, or is only detected after the sealing process, then the coating as well as the dyeing will have to be removed and reprocessed again. The easiest way to remove the anodic coating is by immersion in a 10 % caustic soda solution at 60oC. However, the underlying base metal is also attacked in this process, which may require a new pre-treatment to regain the original surface structure.

REMEMBER this should be used as a last resort as thread and size tolerances will change.....


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