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Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2004

Painting Aluminum Aircraft

Q. I am building an aluminium aircraft and am using a milspec spray pack Zinc chromate coating for the insides of the structure. Is this type of coating better than the harware store variety cold gal zinc spray packs that do not contain zinc chromate and what is the chromate part of zinc chromate? Also, has Alodine coating proven to provide better protection than zinc chromate that has a sacrificial anode effect? Does your standard zinc chromate coating etch into the surface?

A. Thank you for your e-mail. Please note that zinc chromate primers are very different from Alodine. Alodine is one step in a multi-stage pretreatment process and it does not provide sacrificial protection to the aluminum substrate. Instead, it acts as a passivating inorganic thin coating over which a primer can be applied. In the same vein a zinc chromate primer does not provide sacrificial protection but its corrosion inhibiting properties retard the corrosion process.

The military aerospace industry in the U.S. uses MIL-P-23377 primer followed by MIL-C-83286 polyurethane topcoat as the standard coating system. On the inside unexposed surfaces only the Alodine and MIL-P-23377 are used.

You cannot compare the MIL-spec coatings with those that you purchase at a hardware store. The latter are intended for the consumer market and do not possess the corrosion resistance or durability that you can expect from the specification products.

Finally, the MIL-P-23377 primer does not etch into the substrate but performs as a conventional primer.

Q2. Thank you very much for your assistance. I just want to clear up something if you don't mind me asking. You say that the zinc chromate primer does not have a sacrificial effect to prevent corrosion, but I was under the impression that in wet conditions the moisture caused the zinc chromate and aluminum to act as an anode and cathode. Causing a decay in the primer thus protecting the aluminium.

A2. A zinc-rich primer predominantly contains zinc dust and very little resin. It does not perform as a sacrificial coating in the way you mentioned in your e-mail. Zinc rich primers are liquid coating versions of galvanizing. In the presence of moisture a minute amount of the zinc chromate dissolves and spreads over the aluminum surface to form a passive layer. It is true that over a period of years it is possible that in very humid environments the zinc chromate will be depleted, but this is not due to a sacrificial action but rather to its dissolution of new in the moisture.

I hope this has explained the issue more clearly.

Best wishes,

Ron Joseph

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