Letter 35065

Electrically Insulating a Cast Metal Part [Oregon] 

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I am looking for some kind of coating or finish capable of electrically insulating a cast metal part. The base metal needs to possess high strength properties, be relatively inexpensive, and able to be cast, so most likely aluminum. The finishing coat needs to be scratch resistant, meaning anodization is less than ideal. Furthermore the part in question functions in constant contact with pure hydrogen and water and must withstand corrosion. Final constraint is one face of this part acts as a mounting surface requiring high flatness so finishing coat must be applied in uniform thickness.

Any potential solution is greatly appreciated. Thank you.

Anthony Flores
fuel cells - Bend, OR, USA

I followed you up to the point where you said anodization is less than ideal, Anthony. It sounds exactly ideal :-)

Ted Mooney, P.E.  finishing.com Brick, NJ

  
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Ted, above, is right, depending on the voltage. Aluminum Castings have a lot of silicon on the surface, thin wall castings have even more silicon on the surface. You do not anodize silicon and it is conductive. If the anodizer is 100% successful in removing the silicon from the surface, you anodize only aluminum, then depending on contact pressure and voltage, you may be "non-conductive". I would not bet on it. Top it off with a heavy coating of powder paint. The powder paint will not be any rougher than the casting was to start.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

Thank you for the insights. However, I am concerned standard anodize will be susceptible to scratching or other types of wear. Complete electrical insulation of this part must be 100%. Hard anodize is an option I am exploring. The other requirements must also not be overlooked. Can anyone comment on whether a hard anodized aluminum part, possibly with a powdercoat finish, resist corrosion when exposed to air, water, or hydrogen?

Anthony Flores
- Bend, OR, USA

Hardcoat will work, however, Bob's comment about the Si content of castings is quite appropriate. Yes, a fluoride etch will remove Si, however, in particular high end applications I've used the approach of covering up the Si and all other alloying elements with either ion-vapor deposited aluminum (i.e. IVADIZE process) or the denser aluminum plating (i.e. Alumiplate). Both deposit 100% pure aluminum and thus provide perfect basis for subsequent anodizing - corrosion resistance of pure, unalloyed aluminum is several orders of magnitude better than "dirty" (i.e. alloyed) aluminum.

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

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