Unwanted lines in anodising

Hello,

Hope someone can help- I work for a company that uses outsourced anodised casing. Its an extruded aluminium cover that is brushed then black anodised.
For a while we have been getting them come in with a few fairly random lines that always run in parallel with each other.
They have a slightly lighter/whiter colouration to them.
The anodiser is saying that it is an extrusion problem (tram lines) we are starting to think otherwise.
If anyone can shed some light I would be most grateful.
Many thanks
James Thompson
electronics - UK

Castings, as a class, are the worst things to anodize, followed by extrusions. Finding parallel lines in an extrusion is quite believable.
You say that they are brushed. Is that parallel to the extrusion line or crosswise to it.
Could the lines be from the brushing?
About the only way that you will be able to find out if it is the anodizer is to split a lot with a very careful inspection for lot uniformity and send the second lot to another anodizer. Then compare the results or similarities of each.
Under etch or more likely overetch has an effect on the product. The actual anodizing tank is the least probable cause (my experience).
James Watts
- Navarre, Florida
2006