Anodic polarization behaviour of cast iron alloys in deaerated concentrated H3PO4 solutions containing Cl- and F- ions

The anodic behaviour of four cast iron alloys containing up to 16.7% Ni, in deaerated 60 wt% H3PO4 with and without 5 x 10(-3) M F-, Cl- ions and 1:1 Cl-/F- mixture was studied by the potentiostatic technique. Values of E-corr of the alloys are markedly influenced by their composition. The anodic behaviour in the active region is controlled by Fe in the alloys and the dissolution reaction is characterized by Tafel slopes, b(a), between 64 and 88 mV (decade)(-1). A two-electron transfer mechanism for the anodic dissolution is proposed. Passivation of the alloys is due to the formation of oxide layers including Fe2O3 and/or Fe3O4. Both critical and passive c.d. (I-cc and I-P) are markedly increased in the presence of Cl- ions, but the presence of F- ions inhibit the active dissolution of the alloys. The Tafel slope for oxygen evolution reaction (o.e.r.) in the transpassive region, b(O) is 240 +/- 25 mV. In the proposed mechanism for the o.e.r., the rate determining step is an electron transfer reaction and possible interpretation of the high Tafel slopes is given based on the dual barrier model.