Electrocrystallization mechanism of iron phosphate coatings onto mild steel electrode surfaces

摘要

Samples of mild steel AISI-SAE 1018, 1045 and 1080 were electrochemically phosphatised, both potentiodynamically and potentiostatically, in a concentrated H_3PO_4 aqueous solution. The conversion coating layers formed were characterised by means of scanning electron microscopy and energy-dispersive x-ray analysis. The kinetics and mechanisms of the electrocrystallization process of FePO_4·2H_2O_((c)) films were studied using the potential step technique. It was found that the mechanism governing the formation of FePO_4·2H_2O_((c)) anodic film was common for all the steel samples, namely, the overall electrodic process comprises four different simultaneous contributions to the overall current (j(t)). The individual contributions were: j_(ad)(t), the current density contribution due to H_3PO_4 adsorption; jd(t), the current density involved during electrodissolution of iron of the steel electrode in the zones which were not covered yet by the FePO_4·2H_2O_((c)) film; jg(t), the current density contribution due to 2D nucleation and growth of an insoluble FePO_4·2H_2O_((c)) conducting adlayer; and j_f(t), the current due to the growth of the passive layer induced by electrodissolution of iron metal and transport of the resulting Fe(II) ions through the FePO_4·2H_2O_((c)) film. However, each stage contributed to the overall process in a different manner, depending upon the carbon contents. The results from voltammetry indicated that it is possible to establish the potential at which the FePO_4·2H_2O_((c)) film begins to form and that this was also a function of the carbon contents.