Effect of Temperature on the Pitting of Type 304 Stainless Steel in Bromide and Chloride Solutions Containing Nitrate Ion

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Pitting behavior of type 304 stainless steel has been investigated in LiBr and NaCl solutions containing nitrate ion through measurements of pitting potential (E′pit) as a function of temperature, and pitting temperature (T′pit) as a function of potential. In the absence of nitrate ion, E′pit in NaCl solution became less noble with temperature while E′pit in LiBr solution was less sensitive to temperature. A similar relation was found between T′pit and potential. The inhibitive effect of nitrate ion against pitting was only apparent below 298K in LiBr solution and below 323K in NaCl solution. Above these temperatures, E′pit was not improved significantly by the addition of nitrate. Nitrate ion inhibited pitting effectively when E′pit ranged over ca. 400mV (vs. Ag/AgCl/2mol kg-1 KCl). Actually, T′pit decreased with potential up to ca. 400mV and then turned to increase in NaCl+NaNO3 solution. In LiBr+LiNO3 solution, T′pit decreased with potential up to ca. 600mV and increased slightly above it. It was estimated that LiBr solution became extremely aggressive above ca. 400mV canceling out the inhibitive action of nitrate at nobler potentials.
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