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topic 54827

Very hard dark grey corrosion on cast iron!

April 16, 2010

Q. Ok, I work for a performance auto shop. We do porting and polishing of intake manifolds, turbos, exhaust manifolds, etc. We use a 120-grit aluminum oxide for blasting media at about 100 psi or a little over for our rust removal on cast iron.

I have been noticing parts of the manifolds, usually in the same areas, have this dark grey layer under the oxidation. sometime it comes off with some patience. other times I can hit the same square inch of area for over ten minutes and it may only reduce it slightly. I thought it was some sort of slag from the casting process but have blasted items clean of the grey crap and re inspected after a year of road use here in MN and its reappeared! Any help on what this advanced stage of oxidation is called and the best method to attack and remove it would be very helpful!

Matt Homes
Porter and Coater - Hopkins, Minnesota

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