Q. I understand that iron and poor current can cause a pink coating on boric-sulfuric anodized (BSA) parts. We currently process parts for Boeing, and the pink coating is a cause for rejection. We tested the BSA solution for copper and iron, and the copper level is at 11.64 ppm and the iron level is at 11 ppm. What levels of iron are permissible before reaching the pink-colored coating? Are there any other things we could be missing? E.M.
A. I am not familiar with the “pink coating” phenomenon using the boric-sulfuric process. I don’t think the problem is related to the dissolved copper and iron in the bath, however. The Boeing spec (BAC5632) has a limit for copper in the bath of 237 ppm; there is no mention of a limit for iron. In a “clear anodize” bath, any coloring of the product during the anodizing cycle is usually related to the alloy and/or temper of the parts being anodized. However, this is usually evident only with thicker coatings than the BSA process calls for. You don’t mention the coating thickness you are getting, nor do you mention the alloy being anodized. I am not familiar with which alloy series tends to produce pink coatings, but I don’t think this is your problem.
I also don’t know what you mean by “poor current.” If the parts are anodized in accordance with the Boeing spec, i.e., 15 volts for 20 to 25 min, the current density will normally be in the range of 3.5 to 4 asf, as long as there is some dissolved aluminum in the bath. A brand new bath with no aluminum (not recommended) will give a current density of 4 to 6 asf. If this is consistent with what you are getting, then the current is fine. There may be a separate problem with racking of the parts that might cause a lack of current flow to some parts and not others. This would result in the parts with poor electrical contact having slightly less coating than the parts with good electrical contact.
Sometimes the best way to correct the problem is to start over. This may correct the problem, but it may not tell you what was actually causing the problem. I would recommend the following course of action if you feel that the troubleshooting approach is not working:
I would appreciate knowing if this causes the problem to go away.