Sulfates in the Passive Iron Layer

Abstract

Abstract Anodic polarization data were obtained on iron in 20 N H2SO4 at 25 C (77 F). Along the polarization curve, the potential was held constant at 20 mv increments and the current was recorded at the end of 60 minutes. Minimum currents, the average of four experiments, were noted at 260, 520, 810, 1060, 1320 and 1600 mv noble potential with reference to the saturated calomel electrode (S.C.E.). The various chemical compositions were identified with reproducible X-ray diffraction data. A technique involving an ethyl alcohol rinse permitted the X-ray study of the passive surface. The passive iron layer was identified as FeSO4.H2O, Fe2(SO4)3 H2O,FeSO4.4H2O and as FeSO4.7H2O at 260, 520, 810 and 1320 mv noble to S.C.E., respectively. The structures were reported to be responsible for the steady-state minimum currents. The X-ray diffraction values of the specimens held at 1060 and 1600 mv (S.C.E.) were not identified. The anodic polarization curve was discussed in terms of specific electrochemical events.