Radioactive liquid waste has been stored in underground carbon steel tanks for nearly 70 years at the Hanford nuclear facility. Vapor space corrosion of the tank walls has emerged as an ongoing challenge to overcome in maintaining the structural integrity of these tanks. The interaction between corrosive and inhibitor species in condensates/supernates on the tank wall above the liquid level and their interaction with vapor phase constituents as the liquid evaporates from the tank wall influences the formation of corrosion products and the corrosion of the carbon steel. An effort is underway to gain an understanding of the mechanism of vapor space corrosion. Localized corrosion in the form of pitting is of particular interest in the vapor space. CPP testing was utilized to determine the susceptibility of the steel in a simulated vapor space environment. The tests also investigated the impact of ammonia gas in the vapor space area on the corrosion of the steel. Vapor space coupon tests were also performed to investigate the evolution of the corrosion products during longer term exposures. These tests were also conducted at vapor space ammonia levels of 0.005 and 0.055 vol.% in air. Ammonia was shown to mitigate vapor space corrosion.