Galvanic Corrosion Tin Plated aluminum - Alu?

Q. In our sensor application we currently have a sensor with a 6061 aluminum body that is thread mounted on a mating part of tin plated 6262 aluminum. Between the two parts is an O-ring that prevents medium leakage. In a salt spray test (SWAAT ASTM G85) leakage occurs after a few hundred cycles.
I think this is caused by a classic case of galvanic corrosion on the O-ring sealing surface of the sensor because:
- Tin is compared with aluminum the more noble metal
- Sensor and mating part are electrically coupled at the thread
- Sensor and mating share a common electrolyte (SWAAT solution)
- Sealing surface where leakage occurs is very close to where the parts are electrically coupled
- SWAAT solution contains acid (pH 3.0) and this attacks the natural aluminum oxide layer on the aluminum, rendering the aluminum more susceptible to galvanic corrosion

Now my questions:
- I am correct in assuming galvanic corrosion is the problem here?
- Could chromating (hexavalent chromium free chromating) the aluminum body of the sensor solve the problem?

Thanks in advance for all replies.

A. I think you are correct about what the problem is, Jaap, but I don't think chromating the aluminum instead of anodizing it would help; in fact I think it would make the situation worse. Heavier anodizing, or a better sealing process would probably help.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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November 2005

Q. Dear sir.

I am very glad to reach here to communicate with you about some ASTM questions, thank you!

Okay, I have read some ASTM standards, but I do not know the difference between the ASTM G 85-98 and ASTM G 85-94, especially, the testing time. mainly because I do not have the standard of ASTM G thank you very and best regards!

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A. Hi Oota,

The last two digits are the year of issuance. ASTM G85-98 is at least two revisions out of date, and ASTM G95-94 at least three issues out of date. The current spec is ASTM G85 -19.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Ted is available for instant help
or longer-term assistance.