Critical Holes Out of Tolerance

The aerospace detail parts we send out for anodizing are often coated with a chemical film (Alodine) to aid in the application of masking. When the Alodine is dissolved before anodizing, the precise diameters of holes are often eroded oversize, which will scrap the part in many cases. What is the best way to specify on a drawing or purchase order that such erosion cannot be tolerated? Should a certain masking technique be specified? Are there certain processes that should be forbidden?

Q. The aerospace detail parts we send out for anodizing are often coated with a chemical film (Alodine) to aid in the application of masking. When the Alodine is dissolved before anodizing, the precise diameters of holes are often eroded oversize, which will scrap the part in many cases. What is the best way to specify on a drawing or purchase order that such erosion cannot be tolerated? Should a certain masking technique be specified? Are there certain processes that should be forbidden? C.S.

A. You don’t say if the holes in question are to be anodized or left in the chem film condition, but both conditions can be addressed.

Could it be possible that the parts are being alkaline-etched in order to remove the chem film? It is not necessary to alkaline-etch the parts simply to remove the film. This can be done in a strong acid or in the anodizing process itself. I don’t recommend that the chem film coating be removed in the anodizing bath, however. It’s not very desirable to have chrome in the anodizing tank unless, of course, it is a chromic acid (Type I) anodizing bath. You also don’t indicate what type of anodizing is being done. If it is Type I, the chem film will be removed during anodizing. If it is Type II or Type III anodizing, I would recommend using the deoxidizing bath to remove the chem film before anodizing. None of the above processes should prove deleterious to the hole tolerances.

If an alkaline etching step must be involved, for reasons I am not aware of, the holes should be plugged during the etching cycle and subsequent rinsing steps. The plugs can be pulled out prior to deoxidizing if the holes are to be anodized. At that point, the hole tolerances should be unaffected and the parts will be ready for anodizing in the Type I, II or III bath.

If the holes need to be left in the chem film condition with no anodic coating, the plugs can be left in until after anodizing.

Once the processing details are worked out, the anodizer can be specifically included in the purchasing or contract documents. Also include information about the results required.