Letter 24046

Brass/aluminum and aluminum/galvanized steel compatibility

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Please advise on compatibility for anodized aluminum female threaded onto brass. Also, please advise on compatibility for anodized aluminum flanged joint with galvanized steel employing 316SS bolts/nuts.

Dean Ratzlaff
engineering company - Bellingham, Washington

The short answer is that dissimilar materials are always galvanically incompatible.

How incompatible depends on the environmental conditions, the separation of the two metals on the sea-water series, the criticality of corrosion, and the exact details of the connection.

Anyone who has moved a washing machine, and consequently had to unscrew aluminum hose fittings from a faucet, will know the answer to your first situation: completely incompatible! Unless you consider removal with the aid of a hacksaw acceptable :-)

The second situation doesn't sound nearly as bad and could be perfectly satisfactory in certain situations. I see galvanized steel coupled to anodized aluminum in all kinds of non-critical outdoor situations like walkways, stadium seats and rails, etc. The stainless screws could cause some localized pitting, but it may not be a real problem at all.

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