A nuclear waste cooling water system, constructed from an austenitic stainless steel, is known to be susceptible to localized corrosion because of the combined presence of low concentrations of chloride (Cl−) ions in the water (<10 mg/L) and oxidants produced by water radiolysis. Corrosion propagates in the system as pitting corrosion, and inspections of accessible components indicated that large pit cavities could form. To investigate possible inhibition options, an artificial pit, termed the wire-electrode artificial pit, was developed and shown to be capable of sustaining corrosion under representative conditions. Tests carried out using this artificial pit to investigate the effect of sodium nitrate (NaNO3) added to the bulk water are reported here. The results showed that nitrate (NO3−) was capable of affecting inhibition at 60°C, although large concentrations and long times were required, e.g., inhibition took up to 20 days at 60,000 mg/L NO3−. Complimentary polarization tests were also carried out in a range of artificial pit solutions as a function of nitrate concentration to aid interpretation of the wire-electrode tests. The existence of a threshold molar NO3−/Cl− ratio for passivation to occur was identified but not clearly defined.