[Lewis]

Hello,

I would like to know how to remove the black residue, that remains on the surface of carbon steel after rust removal by treatment with citric acid. Household-typical means and minimal abrasive action would be preferred, although the surface is pitted under the rusted areas.

Thanks,
Lewis

[hollytara]

If it is carbon steel, the black may be residual carbon - the citric acid would dissolve the metal but leave behind the carbon. 

The carbon might be removable with hydrogen peroxide - you could either use hydrogen peroxide from the pharmacy, or use something like Clorox2, which has hydrogen peroxide mixed with surfactants. 

If you test it on a small area, usually if the hydrogen peroxide is working you will see bubbles forming.