Hydrodynamic Effects During Electrochemical and Exposure Pitting Tests with Passive Metals

Electrochemical pitting scans, dE/dt = 60 mV/h and 1,200 mV/h, and exposure tests have been done with nickel in an electrochemical flow cell to determine the effects of solution flow on the initiation, propagation and repassivation processes. The flat nickel specimens were exposed to 0.5 wt % NaCl solution in a rectangular duct, at Reynolds numbers in the range 420 to 19,600 and under stagnant conditions.

There was no significant effect of flow on the value of the breakdown potential, E_np, in either the electrochemical tests or the exposure tests and it is concluded that the initiation of the pitting of 'clean' surfaces is not affected by the solution flowrate.

There was a marked decrease in the values of the protection potential, E_p, with decreasing flowrate during the pitting scans at the high scan rate where the pits were much less developed. This suggests that the propagation of the small pits is more difficult at high flowrates which was borne out by the results of the exposure tests

where the pits formed at the high flowrates were smaller than those obtained under laminar and stagnant conditions. The E_p values were not affected by flow at the low scan rate where larger pits were formed. E_p for fully developed pits is identified with the potential required for significant hydrogen evolution in acidic-chloride solutions similar to those found in pits.

The E_np values determined in the exposure tests were ca. 25 mV more active than those determined during the 60 mV/h pitting scans Potentiostatic tests at such potentials showed that breakdown with the formation of small pits, could be achieved some 75 mV below the slow scan rate values although gross breakdown with the formation of large pits was not achieved below the slow scan rate value.