Downhole heat exchangers (DHEs) are commonly used in the utilization of low-temperature geothermal resources. The lifetime of DHEs is frequently determined by corrosion failure at the air-water interface. Paraffin or other hydrocarbons are sometimes put down the well to inhibit corrosion and appear to help, although little data is available. Experiments described here show that hydro-carbon floating on the air-water interface prevents formation of a thin aqueous film on the metal above the bulk fluid and that it is this thin film which promotes corrosion. Oxygen access is also critical. Even with the use of paraffin, the corrosion rate in the bulk solution remains unacceptably high in the presence of air; whereas, corrosion rates drop to low levels without the use of paraffin, provided air is rigorously excluded. Thorough sealing of the well head should eliminate or greatly reduce corrosion problems at the air-water interface