THERMODYNAMIC ASPECTS OF THE CREVICE CORROSION OF IRON IN CHROMATE CHLORIDE SOLUTIONS

Potential-pH data are reported for the crevice corrosion of iron in 23 different chromate/chloride solutions. In all solutions where there was crevice corrosion, the steady-state electrode potential of iron with the crevice was -0.600 to 0.650 V(Ag/AgCl); and the crevice pH was between 4.5 and 6.0. These values correspond to the region of the Fe/H2O equilibrium diagram where iron undergoes corrosion as Fe2+ and where hydrogen evolution is thermodynamically possible. For the CrO42-/Cl- solutions where crevice corrosion was inhibited, the potential-pH values correspond to the domain of passivation by Fe2O3 + Cr2O3. Surfaces which pit when open do not necessarily undergo crevice corrosion when shielded. Electrode potentials for the initiation of crevice corrosion are more negative than the critical pitting potential for open iron surfaces in the same solution, and the electrode potential in the active crevice becomes more negative with time. The resulting steady-state potential of -0.630 ± 0.025 V(Ag/AgCl) is in good agreement with experimental values for scaled-up models of crevices or pits and with the theoretical value calculated previously from concentrated solution thermodynamics.