Phosphating of Iron and Steel

Iron is readily attacked by orthophosphoric acid over a wide range of temperature and concentration. This attack involves the initial formation of sparingly soluble dihydrogen phosphate (12.32). As the pH rises in the vicinity of the metal snrface, insoluble monohydrogen phosphate is precipitated (12.33), and eventually tribasic iron phosphate may also form (12.34). 

Iron-phosphating processes are used on radiator panels, refrigerators and other items not subject to severe external exposure (0.2 0.8 g/m ). Such coatings have superior paint-adhesion properties 

When the coating compound incorporates some of the metal being treated (as above), it is often referred to as a conversion coating. 

These simple treatments are not always suitable, however, and other cations such as Zn + or Mn + are added to the phosphoric acid. 

Zinc-phosphating processes are used, for example, on car bodies, which require maximiim protection with thicker coatings (0.5 - 4.5 g/m ). 

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