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 cha racterized by crystallographically oriented parallel plates of unattacked B e on the pit interior and square pit walls. While pit geometries in the (00 01) 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 char acterized by two types of transients: (i) transients with a lifetime <1 s, similar in shape to the current/time data observed in the repassivation pro cess, and (ii) transients whose lifetimes lasted 5-140 s and were associate d with physical damage on the sample surface. The nucleation frequency of r epassivation events was directly proportional to passive current density an d inversely proportional to applied anodic potential. Although the frequenc y and magnitude of damage events increased with increasing applied anodic p otential for any given orientation, the total accumulated damage was greate st for orientations with the highest E-pit. Prepassivation resulted in a de crease in the frequency of repassivation events but had only minimal effect on the frequency and magnitude of the damage events. The results are discu ssed in terms of physical bulk metal properties as well as critical pitting environments (i.e., ohmic and mass transport models). (C) 2000 The Electro chemical Society. S0013-4651(00)03-066-4. All rights reserved.