Letter 1070

Corrosion on inside of parts where outside is zinc phosphated

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I understand that to prepare steel or cast iron for painting, a thorough solution is phosphating. Can someone tell me what kind of corrosion resistance I should expect on the interior surface? The (cast iron part) interior will remain unpainted and the exterior will be painted. Assume the zinc phosphating procedure is in accordance with specification TT-C-490 [link is to spec at TechStreet] and has an average weight of 300 mg/sq. ft. (150-500 mg/sq.ft. required per spec).

thanks

C. A. Sanchez
self employed

Dear Mr. Sanchez:

Don't expect to receive much corrosion protection from the zinc phosphate coating itself (less then 24 hours NSS). Zinc phosphate will compliment the performance of the painted portion of your part but provide little protection on its own.

Consider electrocoating the piece to provide interior protection.

Edward Koneczny
- Birmingham, Michigan

Zinc phosphate applied as per TT-C-490 "C" rev or prior does enhance the metal protection. As long as the coating is thick enough to form full crystals, you will gain protection.

Otherwise you will only increase your primer binder.

Unfortunately, the government has split the thicknesses and created what I call an unformed crystal coating [TT-C-490 "D"] and a heavy plating [DOD-P-16232 [link is to spec at TechStreet]F]. The "heavy" coatings in 16232 are listed to .0004" thick. However, after some 20+ years using the stuff applied by plating contractors, it is even better when up to .002" thick. All this assumes a primer and then a topcoat will be applied. (Chromate sealer can be left out if you use an epoxy or poly- type primer.) Now, we have been writing about steel as the substrate in my case.

N. Lassario

During degreasing with conventional alkalies like soda ash +sodium meta silicate+ tsp +acid slurry combination sometime gives very poor results . Can any one help me by suggesting some better combination.

praveen verma
fine chem - India

Here in the U.S.A., very few shops attempt to formulate their own cleaners anymore. To do really good work at development of cleaners is a full-time job. It is just so much more practical to buy proprietary cleaners than to suffer poor cleaning day after day trying to learn what the suppliers learned 25 years ago. Contact some of the major suppliers and you may find that the cost of pre-formulated cleaners is not prohibitive.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

My question is:
In tne Zinc Phosphating process, what is the best relation PO4/Zn and how much of Cu and NO3?

ALFREDO BERRIO
- Columbia