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Bituminous paint as an isolator between aluminum and steel

(2005)

I work for a commercial curtainwall installer and we have a condition where we have aluminum angles (painted with a bituminous paint) sitting on steel shims (with a red oxide finish).
The bituminous paint happens to have aluminum flake pigment (to retain the aluminum color). The consultant on this project is concerned that the aluminum flakes in the paint will cause a galvanic reaction with the steel and the cured concrete (both of which contact the pained aluminum angle).
Will the flake pigments cause a galvanic reaction?

Chris Anderson
cladding technologies - Corona, California, USA


(2005)

I wouldn't worry about it. Aluminum flake impedes moisture permeation and blocks or reflects IR, UV and visible light. The aluminum has a very thin oxide film and doesn't cause (any significant) electrical conductivity or galvanic behavior. Can't say about your paint, but aluminum-pigmented roofing paint per Federal spec. TT-C-498 [link is to free spec at Defense Logistics Agency, dla.mil] must be able to pass a 168-hour salt spray test on steel Also, the red oxide paint probably contains corrosion inhibitors.

Bituminous painting is a standard practice for aluminum embedding in concrete. With aluminum-pigmented paint, I would expect fresh concrete to have a slight caustic effect diminishing over time as its surface becomes more neutral.

Ken Vlach
- Goleta, California
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