Metal finishing Q&As since 1989

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How can I get good adhesion of organic coatings to anodized surfaces

2004

We have trouble obtaining adequate MIL-PRF-23377 [on DLA] (i.e., epoxy-polyamide) primer adhesion on MIL-A-8625 / MIL-PRF-8625 [on DLA] Type 2 Class 2 ù Black and Type 3 Class 2 ù Black coatings. The anodized surfaces are wiped down with MIL-T-81772 Type 2 thinner (i.e., a mixture of MEK, MIK, and propylene glycol methyl ether) just prior to the priming process. The Type 2 coating is produced via voltage control. The Type 3 coating is produced at constant current density in a sulfuric acid/oxalic acid solution. Both coatings are sealed in a nickel acetate solution. Since the parts have some areas masked off during the priming process, the anodize coatings must be sealed prior to the priming process. We cannot control the elapsed time between the anodizing process and the MIL-PRF-23377 priming process.

1. What surface pretreatment process on anodized coatings will provide a good base for an epoxy-polyamide primer? Do we simply need mechanical abrasion on the surface?
2. Do zinc chromate primers stick to anodized coatings better than epoxy-polyamide primers?
3. Should we replace the solvent wipe down of the parts with a mild detergent wipe down? If so, will the mild detergent significantly attack the anodize coating and/or unseal, to some extent, the anodized coating? I'm worried about attacking the anodize coating at the interface of the masked and unmasked areas of the part.
4. Would the following process work?

• Anodize
• Dye, skip sealing process
• Mask
• Coat with MIL-PRF-23377 primer
• Cure primer coating at room temperature
• Unmask
• Seal in low temperature nickel acetate solution

5. Is anyone aware of any studies that correlate the anodized coating sealing method, i.e., hot water, sodium dichromate, low temperature nickel acetate, medium temperature nickel acetate, high temperature nickel acetate etc., to the degree of organic coating adhesion achieved?

Jim Hovelsrud
Process Engineer - Indianapolis, Indiana, USA

You never have good paint adhesion after nickel acetate. Dichromate sealing is not acceptable for black anodizing. You can use D.I water sealing with phosphate additives. It works good. I do not advise you to work without sealing. Even some masking materials are suitable for unsealed anodizing you will have then problem with black dying.

Ilya Ostrovsky
Germany/Israel
2004