Critical pitting temperature (CPT) has been defined by Salinas-Bravo and Newman as a temperature at which ilim=icrit where ilim is the diffusion limiting current density as a result of salt precipitation and icrit is the critical (maximum) current density necessary for passivity in pit solution (1). The positive effect of nitrate ion on CPT of 2205 by increasing it towards higher temperatures due to its increasing effect on ilim was presented in detail elsewhere(2). The objective of this work is to validate the theory mentioned above by investigating the effect of nitrate ion addition in chloride containing solution on critical current density of a 2205 duplex stainless steel (DSS 2205). Potentiodynamic polarization tests were conducted on 200 μm diameter DSS 2205 in a 5M HCl solution assuming creates simulated pit environment(3). To identify the effect of nitrate ion addition to the solution containing chloride ion, the critical current density for passivation on a DSS 2205 specimen annealed at 1050°C was measured in 5M HCl and 5M HCl+0.1M NaNO3 solutions. Mentioning that addition of 0.1M NaNO3 to a 0.6M NaCl solution causes an increase of 30°C in CPT of DSS 2205(4).

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