Samples of mild steel AISI-SAE 1018, 1045 and 1080 were electrochemically phosphatised, both potentiodynamically and potentiostatically, in a concentrated H3PO4 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 FePO4·2H2O(c) films were studied using the potential step technique. It was found that the mechanism governing the formation of FePO4·2H2O(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 H3PO4 adsorption; j d(t), the current density involved during electrodissolution of iron of the steel electrode in the zones which were not covered yet by the FePO4·2H2O(c) film; j g(t), the current density contribution due to 2D nucleation and growth of an insoluble FePO4·2H2O(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 FePO4·2H2O(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 FePO4·2H2O(c) film begins to form and that this was also a function of the carbon contents.