Attack of Phosphoric Acid on Metals

Iron and steel are readily attacked by both dilute and concentrated orthophosphoric acid. The dilute acid will form insoluble surface films of acid iron phosphates which act as protective layers and prevent further oxidation (Section 12.7). 

In more concentrated form, phosphoric acid is used for pickling and cleaning of metals. In conjunction with sulphuric or chromic acids it is used in the electropolishing of steel and aluminium (Section 12.7). 

Phosphoric acid is not as corrosive as other common mineral acids. The corrosive effects of 5 N acids on mild steel at 20°C, for example, lie in the order  

While Fe, Zn, Mn, A1 and Sn are readily attacked to some degree by phosphoric acid, Ni, Cu, Cd, Sb and Ti are fairly resistant, and metals such as Ag, Zr, W, Ta, Pt, Mo and Au are unattacked, at any rate at normal temperatures. Bismuth forms a protective layer of BiP04 which prevents further attack. At 150-200°C, the order of corrosion resistance to M H3PO4 has been given as 

Attack on the metal may in some cases involve the formation of acid-resistant films of insoluble metal phosphates (e.g. Fe, Zn, Mn) and perhaps metal oxides. 

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