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Should I include supplemental Chromate treatment with Electrodeposited Coatings of Zinc on Steel?

February 4, 2015

Q. Hello, I am a mechanical engineer and was recently asked to review our process for pretreatment and painting of a steel electrical enclosure to be installed on a marine vessel. The enclosure will house a PLC. My question is regarding the application of TT-P-645B, Zinc Molybdate Alkyd Primer. We are having the steel enclosure electrodeposited with zinc per ASTM B633, Type II (Yellow Chromate Treatment). Then it is to be primed with TT-P-645B, Sherwin-Williams N42Y100 and painted with Sherwin-Williams Gray Enamel N45A100. Is it possible to skip either the Yellow Chromate or the TT-P-645 Molybdate primer? Does one adversely affect the other? It is my understanding that the Molybdate primer replaced the Chromate treatment to eliminate hexavalent chromium, is that true?

Thanks in advance for any insight you may have.

David Olszewski
Mechanical Engineer - Sparks Glencoe

February 6, 2015

If the Standard required Chromate should be done, I have done it before painting without any problem.

Carlos Carrera, Aviation Chemical Processes Eng.
- Miami, Florida

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