MECHANISM OF ACCELERATION OF ELECTRODIC DISSOLUTION OF METALS DURING YIELDING UNDER STRESS

The effect of strain, in particular the effect of varying the rate of its application, both in the elastic and the plastic region, on the anodic dissolution kinetics of several metals has been studied. It has been found in the case of iron in acid solution that the over-all effect of elastic strain is small (i.e., the shift of corrosion potential is about +0.1 mV/1000 kg cm -2) and is mostly due to the change in the rate of hydrogen evolution. The effect of yielding on the rate of the electrodic dissolution of metals (Fe, Mo, Ni, and Cu) has been found to be much more pronounced than that of the elastic deformation. The dissolution rate undergoes a marked rise at the beginning of the plastic region, when straining is carried out at a constant rate, and then shows an asymptotic tendency with increasing strain. The asymptotic values of the anodic current dissolution (c.d.) reached at high values of strain increase linearly with the rate of straining. The slope of this linear relationship increases with increasing anodic potential. Metals which develop high-index planes (h.i.p.) as surface steps upon slipping [e.g., Fe - (112) and (123), Mo - (112)], should give much larger mechano-electrochemical effect in comparison with those which develop only low-index planes (l.i.p.) [e.g., Ni and Cu - (111)], and this should depend on the rate of straining. Some effect found in the l.i.p.-developing metals could be ascribed to increased dissolution rate at slip edges and to increasing surface roughness upon slipping. The rate of metal dissolution on h.i.p. is 50 times larger than that at l.i.p., while the rate of the dissolution at slip edges is about 10 times larger than that at the unstrained metal.