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).