Glue Adherence to Black Anodize

Question

We’re having a problem with glue not adhering to anodized parts made from extruded 6063-T6 aluminum and finished with heavy etch and black anodize per MIL-A-8625, Type II, Class 1. The problem parts come in batches, something like a 25–50% fallout rate, when we normally have about 0.5–1% fallout. The first time we researched the problem, we focused on our anodizer as the possible source. Nothing we found there stuck out, so we have been looking at the environment the part sees between the anodizer and our gluing operation. Right now, all parts are cleaned by hand with Isopropyl alcohol and a paper towel. It has worked for thousands of parts until now.

Can you make any other suggestions as to how we should prep an anodized surface, or if there is something we might be overlooking at the anodizer, or is there is something they might not be telling us about their process? J.W.

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Answer

It is important to know exactly how the parts are being sealed.  So first of all, we need to know what type of sealing process is being used on your parts. Is it a mid-temp nickel acetate, mid-temp non-nickel, nickel fluoride, or hot deionized water? Also, it would be helpful to know the temperature, concentration, pH and length of time of sealing.

Chances are that your anodizer is using a mid-temperature nickel acetate seal—probably the most common sealing method, especially in job shop anodizing. Mid-temp nickel seals are proprietary products manufactured to provide good seal quality in a relatively short period of time (typically 10 min) and without smut. Most proprietary nickel sealing products I am familiar with have a certain amount of surfactants (wetting agents) in them to help reduce smut and improve appearance. However, surfactants, by their very nature of being used to reduce surface tension, may also cause some loss of adhesion for glue, paint, etc.  My recommendation for bonding surfaces or paint adhesion would be to seal with deionized water only at 205–207°F for 10–15 min. The only additive in the sealing bath would be about 1 lb of sodium or ammonium acetate per 100 gal of bath to help buffer the pH. Bath pH should be 5.7–5.8. If your dyed parts are sealed in this manner, you may likely experience better and more consistent glue adhesion.