Relationship Between Conventional Pitting and Protection Potentials and a New, Unique Pitting Potential

A literature review was performed to identify test methods that have been used to examine pitting susceptibility of stainless steels and nickel-base alloys in chloride (Cl^–) containing environments. Several techniques were identified and a critical discussion of the electrochemical methods is provided with special attention given to the causes of variations observed in the pitting, or breakdown, potential (E_pit) and the protection, or repassivation, potential (E_prot). Experiments were performed for type 317L SS and alloy G3 in solutions containing high levels of Cl^–. The test solutions were designed to simulate environments present in flue gas desulfurization systems. Cyclic potentiodynamic polarization (CPP) experiments were performed to determine how E_pit and E_prot were affected by scan rate. Constant potential-time experiments were performed for up to 78 days to determine how incubation time was influenced by potential. Modified ASTM F-746 tests were performed to establish the protection potential as a function of prior pitting history. Analysis of the data suggests that there exists a unique pitting potential (E_u), defined by the stochastic models for pit initiation, that equals the most active value of E_pit (long incubation times) and that equals the most noble value of E_prot (measured following only minimal pit growth), i.e., E_u = E_pit = E_prot.