Independent of thermal loading scenarios, the waste packages at the proposed repository at Yucca Mountain, Nevada will be exposed to environmental conditions of temperatures below 100 C with a range of possible relative humidities. Under high relative humidities, the formation of thin water films on the metal surfaces will occur. Previous work at ambient conditions has shown that on susceptible metals, the water-film-covered metal has an enhanced corrosion rate relative to the dry oxidation of the metal. The transition relative humidity where oxidation changes from dry oxidation to aqueous electrochemical corrosion is called the critical relative humidity. Determination of this transition is important in estimating the performance of the candidate materials (especially corrosion allowance materials), because of the marked increase in degradation rate above the critical relative humidity, typically 10-10{sup 2} times This activity is concerned with short term measurements of corrosion and oxidation rates using a high sensitivity microbalance. The experimental work will determine the critical humidity level for candidate metals under a variety of conditions. From the literature, the critical humidity level typically are around 70% relative humidity, but this level is affected by the metal, gas composition, temperature, and the presence of contaminants on the surface, particularly contaminants that are deliquescent. A series of experiments will bracket a range of humidity, temperature, metal alloy, and surface condition.