Cast iron Black residue

Alan8n · Jan 30, 2012

I know the topic of "black oxide" after using a rust converter / electrolysis has been covered before ...and there even are conflicting opinions on it....but...
I have been stripping paint from some cast iron parts using a wire wheel / angle grinder and after the paint comes off there there seems to be a black layer that I need to get thru before the iron becomes shiny. On some pieces it takes quite an effort to get thru the black layer.
I've also played around with cleaning the cast to shiny and letting it sit there for some time to see what happens to it. While I don't see any red rust type of coating, the cast will definetly get dull and turn dark (black ?). I aasume that is a form of oxidation. In some articles I read that it is a stable form of oxidation as opposed to the red oxidation which is not.
Question is...can you prime over this black layer or does it all have to be shiny clean before primering ??
Any opinions welcome...Thanks...Alan

showcrop · Jan 30, 2012

By Wire wheel/angle grinder I will assume that you have a wire wheel in your angle grinder. That wire wheel will make rust shine black. I just did it today. So I took off my wire wheel and put on an abrasive disk and applied it to the shiny black and the rust went blowing away. Now unless you have microscope eyes you want to put the phosphoric acid wash right to it and here and there in the shiny brite steel you will see little black spots where the acid turns the little grains of rust into inert iron phosphate. You should keep it wet for ten minutes then wash it off good and you won't get that white residue that you see so much about on here. You can get the phosphoric at the auto parts store, or get the same stuff for 1/4 the money at a restaurant supply store.

Red Ford · Feb 1, 2012

The black layer you are finding is the slag that formed on the surface of the casting when it was made. It"s pretty tough, but it can be removed using strong chemicals, media blasting, or grinding, sanding, aggressive wire brushing.

Although paint adhesion is potentially a little better without the layer, removal is not essential.

Cast iron Black residue

Alan8n

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showcrop

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Red Ford

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