What do defects look like on anodised aluminium parts? What can cause them? And how can you be sure that your anodising processor is producing parts of the required quality?
Pitting on the Surface
A typical defect caused by a contamination in the anodising bath or on the part itself is a pitting on the surface. The pitting is a local breakdown of the oxide layer. The contamination could be a trapped machining oil, a polishing compound, or even a smut from a poor pre-treatment etch. In order to prevent such a defect, make sure that the part has gone through the proper pre-clean stages and the deox stage before it enters the anodising tank.
Powdering or Chalky Patches
Powdering or Chalky Patches: These patches appear as a white, powdery surface that can easily be wiped off. These patches are formed when the bath temperature has risen above the normal processing temperature of 18-22°C (for Sulphuric Acid Anodising). The oxide is dissolved faster than it is formed, resulting in a soft, friable surface. Check the processor’s bath temperature records for the affected run.
Blotchy or Uneven Colour
Uneven colour after dyeing most likely is a pre-treatment problem. Most anodizing defects are caused by pre-treatment inadequacies. Uneven etch, too short rinse times between etch and anodizing, etc. cause the dye to penetrate unevenly into the oxidized surface. Parts made of different alloy grades within the same batch can also display the same kind of problem as different alloy grades have different surface finishes and thus affect anodizing/oxidizing in different ways.
Streaking or Tide Marks
These form as Streaks or Tide Marks down a part and are caused by the part having been left on a poor angle in the tank. As the electrolyte sits against the surface of the part it forms into Streaks or Tide Marks as it is an area of Stagnant Electrolyte on the surface of the part. Movement of the solution will cure this problem.
Burn Marks at Edges
Burn marks on edges usually indicate too high current density, especially at sharp edges. This issue has to be addressed in terms of current density for the specific part geometry.
If you want Anodising, https://www.poeton.co.uk/surface-treatments/anodising/ is a good place to start.
Rarely are anodising defects totally random, and give an immediate insight as to where things have gone amiss during the anodising process.
