I've just cleaned a bunch of pieces of new black pipe by soaking them in a hot TSP solution. They don't feel oily, and the lettering is gone, but they are still vaguely black in color. I have read many times that giving steel items a light etch in phosphoric acid will leave a thin plating of iron phosphate which is a good strike coat for painting. Since there are phosphate ions aplenty in TSP, might I have just gotten the desirable iron phosphate coating without having to find a bunch of phosphoric acid?
Second question: unfortunately, these parts aren't going to be painted, they're going to be galvanized. The galvanizing shop is real leery of items fabricated from black pipe because of the coating they get - that's the reason for the TSP dip in the first place. I'm considering a light etch in some dilute HCl (muriatic i.e. hydrochloric acid) which should completely remove any doubt on the galvanizing shop's mind. Question: can I neutralize HCl with TSP? I know that TSP in solution is basic. I'm just wondering what happens when you mix TSP with HCl - obviously, you'd get Na+ ions, H+ ions, PO3-- ions, and Cl- ions. Sort of like a mix of phosphoric and hydrochloric acids, except for the sodium. I can't figure it out, my college chem days are long over.
Final question: assuming #2 does NOT work, and further assuming it's bad news to dump TSP into the sewer, is there any easy cheap way to neutralize the TSP and make it less environmentally harmful? My neighbor, no dummy, suggests using it as fertilizer. Does that make any sense?
Grant Erwin Kirkland, Washington