Hydrodynamic Effects during Electrochemical and Exposure Pitting Tests with Passive Films

Electrochemical pitting scans (⁠dE/dt=60 and 1200 mV/h) and exposure tests have been conducted with nickel in an electrochemical flow cell to determine the effects of solution flow on initiation, propagation, and repassivation processes. The flat nickel specimens were exposed to 0.5 wt% NaCl solution in a rectangular duct at Reynolds numbers ranging from 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 or exposure tests, and it is concluded that the pitting initiation of clean surfaces is not affected by the solution flow rate.

There was a marked decrease in the values of the protection potential (⁠E_p⁠) with decreasing flow rate during the pitting scans at the high scan rate at which the pits were much less developed. This suggests that propagation of the small pits is more difficult at high flow rates, which was borne out by the results of the exposure tests in which the pits formed at the high flow rates were smaller than those obtained under laminar and stagnant conditions. The E_p values were not affected by flow at the low scan rate at which larger pits were formed. The E_p value 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 ∼25 mV more active than those determined during the 60 mV/h pitting scans. Potentiostatic tests at such potentials revealed that breakdown with the formation of small pits could be achieved ∼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.