Atmospheric corrosion of silver from multiple beachfront sites in Florida was examined by coulometric reduction. Exposure was performed at the US Naval Research Laboratory in Key West, Kennedy Space Center, and Daytona Beach. Coupons were exposed for 3-18 months. Silver chloride and sulfide were identified and quantified by a galvanic reduction technique, which separately characterized the corrosion products on the front and back of each coupon. Differences were observed between sites for both chloride and sulfide growth rate. Silver chloride grew fastest at Daytona Beach, but no silver sulfide was observed on any samples at this site. Additionally, corrosion was observed to occur differently on the front and back of the samples. Increasing exposure times showed increasing corrosion film thickness, and exposure at different sites showed different corrosion film thicknesses and compositions. Additionally, the coulometric reduction technique has been examined in-depth to identify and mitigate possible difficulties in results interpretation which arise from thick and/or multi-component films.
Atmospheric corrosion of bare metals is a constantly occurring process that can cause significant damage to metal components and structures. For this reason, it has been extensively studied, especially for steel and aluminum alloys.1-8 Environmental exposure testing is a common method for studying and characterizing the corrosion severity of a location, which allows for comparison with other regions. Much of this data collection is directed toward development of predictive models for corrosion processes, with the goal of estimating corrosion damage based on environmental conditions.8 In addition to modeling, another area of interest in corrosion mitigation research is accelerated corrosion testing, in which corrosion damage due to long-term atmospheric exposure is replicated by a shorter controlled exposure.3 Mass loss is a commonly used metric to characterize the severity of corrosion in these studies2-6,8, however, mass loss alone does not fully characterize the method of attack as it does not address chemistry or surface morphology. For this reason, silver has often been included in atmospheric corrosion testing to serve as an additional indicator of environmental conditions.9-16 Characterization of the corrosion products on silver can give further insight into the environment and which atmospheric species may be participating in the corrosion process on silver and any other exposed metals. Two of the main environmental factors that affect silver corrosion are ozone concentration and UV exposure from sunlight. Chen et al. demonstrated in a salt fog chamber, that without these factors, there is very little corrosion product formed.17 Because many environmental factors, including UV irradiation, affect the sample front differently than the back, utilizing a technique to separately characterize the front and back of the samples is desired.