Sometimes after using the caustic etch to strip the anodic coating off parts that are being reworked, we have noticed blotchy areas on the parts. These areas feel raised to the touch. If we go longer in the etch, we still have a hard time removing the blotchy areas and the rest of the part becomes over-etched and rough. We believe that these raised areas are due to an old coating that is not being removed by the etch. Why does this happen and what do you recommend as a remedy? It seems to happen most often on parts with heavier coatings and in corners. P.M.
This does sound like uneven removal of the old anodic coating. It happens sometimes particularly in recessed areas and on parts with a fairly heavy coating—perhaps one or more mil in thickness. There are a number of factors that can exacerbate this such as uneven temperature in the etch tank, bath concentration on the low side, lack of vigorous bath agitation. In a way, it demonstrates how tough the anodic coating can be once it is properly sealed. There are a number of things that can be done to help this situation besides making sure that the items just mentioned are at their optimum. Two important factors come to mind in particular. One is to soak the parts to be stripped for up to one hour in the deoxidizer tank. A 50/50 mix of nitric acid and water will give you a 35% nitric bath which is pretty good as a deox/desmut and will also break the seal on the parts to be stripped. Most of the proprietary deoxidizers work well for breaking the seal and softening the coating, also. Make sure the conditions in the caustic etch bath are at optimum before you strip the rework parts. Another, and perhaps the best, way to do this is to use a non-etch stripper. There are at least one or two of these products offered by the proprietary chemical suppliers that work well. The idea of a non-etch stripper is that it removes only the anodic coating and does not attack the base metal.