I have treated some surface rust on zincalume roof sheets with phosphoric acid and the rusty area went blackish, but the surrounds turned to a white powdery appearance, which I now understand could be zinc phosphate. Also some steel angle surface rust that I treated with phosphoric acid, I then sprayed with a zinc spray like galmet and got the same white powder. Is there a chance that I have stripped the protective zinc from the roof sheet or angle, or is the zinc coating on these things way too thick for the phosphoric acid to have consumed all the zinc. Also does the zinc phosphate cause any problems for the durability of the zinc coating on steel? Thank you.
322400cookie-checkPhosphoric Acid to treat rust on Zincalume Sheet – has left white powder. Is this OK?
You haven’t stripped the zinc off – there’s too much to remove with what you’ve done. The problem is purely cosmetic, and zinc phosphate won’t cause any problems. The trouble with cleaning rust off zincalume is that the zinc is more reactive than the iron – hence the reaction you observe