Factors influencing the transition from metastable to stable pitting in single-crystal beryllium

The pitting potential (E-pit) of single-crystal beryllium in 0.01 M sodium chloride decreased with crystallographic orientation in the order (0001) > (10 (1) over bar0) > (11 (2) over bar0). Orientations which were associated with the lowest E-pit, (10 (1) over bar0) and (11 (2) over bar0), were characterized by crystallographically oriented parallel plates of unattacked Be on the pit interior and square pit walls. While pit geometries in the (0001) surface appeared to be random, corrosion propagation was often in the [10 (1) over bar0] and [(1) over bar2 (1) over bar0] families of directions. Potentiostatic current/time data revealed that metastable pitting was characterized by two types of transients: (i) transients with a lifetime <1 s, similar in shape to the current/time data observed in the repassivation process, and (ii) transients whose lifetimes lasted 5-140 s and were associated with physical damage on the sample surface. The nucleation frequency of repassivation events was directly proportional to passive current density and inversely proportional to applied anodic potential. Although the frequency and magnitude of damage events increased with increasing applied anodic potential for any given orientation, the total accumulated damage was greatest for orientations with the highest E-pit. Prepassivation resulted in a decrease in the frequency of repassivation events but had only minimal effect on the frequency and magnitude of the damage events. The results are discussed in terms of physical bulk metal properties as well as critical pitting environments (i.e., ohmic and mass transport models). (C) 2000 The Electrochemical Society. S0013-4651(00)03-066-4. All rights reserved.