This research studied the influence of steel surface condition and solution chemistry on the critical inhibitor concentration required for spontaneous passivation of carbon steel in solutions typical of hot potassium carbonate plant (HPC). The inhibitor was added to the solution as V2O5. The critical inhibitor concentration depended on solution composition and on the steel surface condition. An inhibitor concentration of 30 g/l may be required to ensure spontaneous passivation under all conditions. The spontaneous passivation of clean polished carbon steel surfaces required a critical inhibitor concentration of 0.5-1.8 g/l. A minimum level of V5+ is required for inhibition, so that monitoring the V5+ concentration may be crucial to successfully managing corrosion protection in plant.