SURFACE-TREATMENT OF RUSTED STEEL WITH PHOSPHORIC-ACID SOLUTIONS - A STUDY USING PHYSICOCHEMICAL METHODS

Although many papers have been published about the use of phosphoric acid as a pretreatment for rusted steel surfaces, the main mechanisms involved in its action are not yet well understood. In the present paper the authors advance a further contribution towards a knowledge of those mechanisms through the results of experimental work in which they have used a variety of physicochemical methods including: energy dispersive spectroscopy associated with scanning electron microscopy (EDS/SEM), X-ray diffraction (XDR), infrared spectroscopy (IR/FTIR) and Mossbauer spectroscopy. The results obtained show that the nature of the 'conversion layers' formed depends strongly on the concentration of the phosphoric acid solutions used and on the prior exposure time of the steel surfaces. Mechanisms are proposed for their formation.