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Corrosion on Clear Coated Die-Cast A380 Aluminum Parts for Gym Equipment

Hello

I am an engineer with a gym equipment manufacturer in the US. We have A380 aluminum parts that are die cast, machined to 30 micro inches and clear coated with a lacquer and polyurethane finish (what is commonly known as PU oil) in China. After 6 months of service in a gym environment (high humidity, sweat, intermittent wipe down), I have noticed corrosion (dark gray splotches and spider webs) on the part. When I scrape the surface with a razor, I get a chalky white powder as if the clear was flaking off the part. Does anybody have an idea on what is happening and what could be done to remedy existing parts in the field? Also, what would be the preferred method (and product(s)) for putting a clear finish on these parts?

Nick Kostrikin
Manufacturer - Fresno, California, USA
December 15, 2008

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January 2, 2009

The splotches and spider webs sound like corrosion under the paint. Without seeing a picture, I'd wager the spider webs are filiform corrosion, which can present itself as "trails" under the aluminum.

Is the powder from corroding aluminum or from the paint itself? This could lead you to a pretreatment or paint problem.

Either way, I'd investigate both pretreatment and paint curing. Are they doing anything besides the polishing before painting (e.g., washing, solvent wiping, etc.)? You should get some data on both how they're pretreating the aluminum and how they're curing the paint.

Christian Restifo
- Pittsburgh, Pennsylvania

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I think it is a bad new for you, white powder is generally aluminum oxide and it grows under the coating layer and the adherence between aluminium and coating become bad.

I think you have an attack from salt skin (I am sorry but I do not know the word in English).

Generally, I do not recommend to apply clear coating onto polished aluminium because the adherence is bad (no hole or scratch to hook, buffing compound onto polished part, bad degreasing...). If you want to apply clear coating, I recommend to have a brushed finish (to increase adherence).

I think the best way for aluminium gym equipment protection is to do a clear anodizing onto these parts. Hard and easy to clean. To have a good aesthetic effect, prefer a etch cleaning before anodizing.

Hope help to you,

Nicolas Duthil
- Bordeaux, France
January 30, 2009