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Aluminum electronics enclosures mounted on steel in a moist marine atmosphere

Our company plans to build an outdoor-use electronics enclosure which will consist of a 15 gauge aluminum box with both slot and screw connection to a mild steel bezel and support. The bezel has a Zinc Chromate coating which will directly touch the aluminum. The support is not chromated and the screws in use will be M3 size stainless (unknown composition though I expect something close to 18-8). Exposure to moist marine air is anticipated.

Should I anticipate reaction between the chromate and aluminum? Will it be reactive enough to create passivation at the metal-to-metal junctions? Will it cause adhesion of the surfaces?

Basically the same questions for the screws. Will I corrode the aluminum at its contact with the screws? Would a chrome-plate steel screw be more passive than the stainless?

Thanks for your help.

Wesley Miller
someone found out I studied metallurgy 100 years and a career as a programmer ago. - Charlotte, North Carolina, USA
May 4, 2010

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First of two simultaneous responses --

The chromate on the steel will retard the tendency towards galvanic corrosion in the presence of "moisture" between the steel and the aluminum. Eventually that galvanic corrosion will start. You may want to do some salt spray testing. What do you mean by "adhesion of the surfaces"?

Stainless screws will be better because the copper-nickel-chrome of "chrome-plate" will break off the sharp threads and start corroding. Hard metals like chrome and nickel, when plated on sharp threads will always break off during threading into nuts or sheet metal.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

June 25, 2010

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Second of two simultaneous responses -- June 27, 2010

Aluminium box
Mild steel Bezel with a Zinc Chromate coating
Mild steel Support with no coating
Stainless steel screws (not sure of the grade)
Exposure to moist marine air

Why so many galvanic couples?

Select materials for assemblies to avoid galvanic corrosion.

T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note: The good doctor offers a fascinating blog, "Advancement in Science" )