A Fourier transform interferometric study of phosphate coatings on iron

A Fourier transform interferometric study of phosphate coatings on iron plates using absorption-reflexion techniques indicated the following succession of chemical events on the surface of Armco iron dependent on time of immersion into the standard zinc phosphating solution: (a) activated formation via a redox reaction of an initial layer of hydrated iron oxide/hydroxides and nitrosyl iron complexes containing isolated PO ₄ ^3– (and possibly NO ₂ ^– ) ions in water solution; (b) chemical transformation of the initial layer into a first, adherent and chemically stable layer containing amorphous hydrated iron phosphate and acid iron phosphates; (c) growth of thicker layers progressively enriched of zinc ions, until only hydrated zinc phosphate crystals are deposited in the outer layer. This complex structure of the coating does not undergo significant chemical changes by heating to normal technological temperatures between 150 and 180° C. Only minor changes in water content are shown by the chemical evolution of the infra-red spectra up to such temperatures. Strong changes of the chemical structure of the coating (producing pyro- and metaphosphates) require temperatures higher than 200° C, and coincide with loss of adhesion of the coating to the iron support.