Common Defects: Stains

Stains can be found on clear and color work. A useful strategy is to determine whether it is a surface stain (can be cleaned), imbedded in the coating (gone after CrO3/PO4 stripping), or on the aluminum surface (still seen after stripping). Determining this will narrow your focus. The shape and location of the stain can also be valuable information. There may be a correlation with how the part was racked.

On Surface

Often associated with rinsing downstream of anodize, particularly post seal. Make sure your rinse tanks are clean and being used.

In Coating

If the stain is in the coating, it may be metal related. Reprocess the part and see if it comes back. Sometimes thinner and softer coatings will mitigate this kind of metal problem. If the stain is in a black dyed coating, e.g. reddish stain near edges, the dye may be coming to the end of its useful life. Often this is associated with aluminum content in the dye. Running the dye pH too low may exacerbate this problem.

Under Coating

If under the coating the focus should be on the pre-treatment. The problem may be scale not being removed before etch (try de-smutting before etching). It could be etch transfer staining (lower the etch activity and speed up the transfer). The stain may be due to poor storage conditions of the metal before it arrived on your floor (often the shape of the stain is a dead giveaway).

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