No Tank Electrolysis or Anodising?

[David Herbert]

Originally Posted by Grant McCullough

. I believe the phosphoric acid also chemically changes the red rust into black iron oxide, which also protects from corrosion..

Actually the phosphoric acid either removes the rust by dissolving it or if it can't take up any more into solution it changes the remaining rust (Ferrous oxide) which is not chemically stable into ferrous phosphate (which is black), which is chemically stable. It is however still porous and so needs a protective layer of paint.

Most self etching primers like Bondaprimer (sold in the UK) contain phosphoric acid as their active ingredient. Almost any acid will dissolve rust but most will leave an unstable surface that readily combines with oxygen causing almost instant rusting. The key is to leave a non reactive, stable surface.

David