Metal finishing Q&As since 1989

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Effect of sulfur oixide on MIL-C-5541, class 3 conversion coating

Our company manufactures electro-mechanical connectors.

One of our products is made with aluminum alloy 6061-T6 and coated with MIL-C-5541 chem film, class 3.

How does sulfur dioxide in the air affect aluminum parts coated with MIL-C-5541 conversion coating, class 3?

Concentration of sulfur dioxide in the air is unknown.

Laban Young
QA Engineer - Irvine, California, USA
April 1, 2008

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Hi, Laban. I probably don't understand the context. You can't design for unknown and untested exposure conditions. But you can have your parts tested in a sulfur dioxide accelerated salt spray chamber if that helps. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 2, 2008

SO₂ + H₂O = H₂SO₃, while not the same as H₂SO₄, it is an acid and will attack the chem film and the aluminum, especially the alloy ingredients like copper.

James Watts
- Navarre, Florida
April 4, 2008

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April 7, 2008

Mil-C-5541 describes chromate conversion coatings for aluminum. An accelerated corrosion test that uses Sulfur Dioxide as one of the corrosive agents is the Kesternich Test ( DIN 50018, ISO 3231 or ASTM ASTM G87 ). In my personal (somewhat) limited experience, chromate conversion coatings do not improve the corrosion resistance of zinc coatings in this test. I would assume that the reason for this is that the sulfur dioxide is a strong enough reducing agent to reduce the hexavalent chromium to the trivalent form, which is non-inhibitive, and also strong enough to prevent the oxidation of the trivalent chromium. (I would be interested to know if others have seen this same phenomenon.) I'm just passing along what I understand the chemistry to be. You can draw your own conclusions.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.