Aluminum rusts itself a protective coating (aluminum oxide). The self-made coating usually prevents further rust. However in more aggressive environments (salt water, nicks, rubbing, oxygen...) that ultra-thin self-made coating gets penetrated. This allows more protective coating to grow. And more. And more. Pretty soon, the excess white powdery protective coating is creating a habitat for moisture, which accelerates the production of more rust.
The usual solution is to anodize the whole part, which gives a slightly thicker and much harder anodic aluminum oxide coating. And the parts are pretty.
Another solution is to paint the part. However, any tiny surface defect in the coating (paint) allows the exposed aluminum itself to oxidize (rust), which isn't a problem until moisture works it's way under the paint. There will always be surface defects (nicks, wear points, fittings....). Another problem painting aluminum is painting over rust and not knowing it. Paint will stick to aluminum much better than to aluminum oxide. Its really easy to accidentally hide structural corrosion behind pretty paint. So chemically etching away all the invisible rust, then immediately painting in a moisture starved environment, produces a great paint job, but at that point anodizing gives a much more durable finish.
More to your question, mineral oil or WD-40 (NASA's Water Displacement experimental formula attempt #40) provide a great self-repairing moisture and oxygen barrier (rust prevention). Synthetic oil not so good, since it tends to bead up. WD-40 was designed to leave no long-term residue (oil) and needs to be re-applied more often. Mineral oil (natural or petroleum based oils) traps dust and is consequently much more problematic on sensitive electronics (the oil/dusty layer often has unpredictable electric conductive properties, confusing circuit boards). Super thick oils (grease) make for the most long lasting protection.
Another aluminum rust problem: stainless steel fasteners. Different metals have different relationships with their electrons. Consequently, any mixing of metals allows for Galvanic corrosion. The galvanic difference between aluminum and stainless steel turns this bimetallic connection into a battery in the presence of ionic moisture, particularly salty moisture. This electrical corrosion is accelerated in air/oxygen starved environments (more "battery" working surface). So once moisture gets under a bolt, you can't see it, but the metal underneath is being corroded away. Oil or grease to the rescue! Any non-conductive (dielectric) grease helps keep the moisture (electrolyte) off the metals, preventing galvanic corrosion. Silicone grease is great for nearly everything (except arcing electrical connections).
Caution with conductive greases (metal powder added intending to boost conductivity). The particular metal added to the grease could cause more of a galvanic miss-match between now these three or more types of metals.
Mark Stafford
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Posted by: mstafford@natca.net
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