We have seen some dark spots on anodized aluminum handrail. Our anodizer said that the corrosion is in the pipe material prior to anodizing but it is only evident after the anodizing process. Would you agree? B.B.
If it is corrosion that was on the part prior to anodizing it is very possible that it won’t come out depending on the severity of the pitting. It is also possible that very light corrosion on the mill finish parts may not be evident prior to anodizing, but will be seen after anodizing. Anodizing can highlight this condition. Usually though, this condition would not be severe enough to cause pitting in the part. Actually, pitting from corrosion can never be removed, only hidden, perhaps, by the caustic etching process. The only way the pitting might be removed would be by mechanical means, such as polishing or brush sanding.
You make reference to “dark spots” on the anodized parts. If the pit itself is “dark” it very well could be the result of electrolytic “corrosion” or pitting caused in one of the process tanks and would be the responsibility of the anodizer.