Atmospheric Corrosion of Silver and Its Relation to Accelerated Testing

Accelerated tests, such as ASTM B117, are commonly used to evaluate the susceptibility of materials and structures to atmospheric corrosion.  However, the results of these tests often do no correlate well with the results of field exposures.  The present work investigates the hypothesis that naturally-generated reactive species present in the environment, such as OH radical and OHCl-, play an important role in corrosion in real atmospheres but are absent from standard laboratory chamber tests.  In this study, the effects of UV light, ozone, humidity, and chloride contamination on the atmospheric corrosion of silver were investigted.  Silver was used because the extent of reaction can be assessed easily through coulometric reduction of the surface films formed.  It was found that neither UV light nor ozone alone generate significant corrosion on bare silver surfaces.  When both are present, however, silver undergoes significant oxidation.  We believe that this effect is due to the creation of atomic oxygen on the silver surface by the photolysis of ozone.  Nanoszied NaCl particles were deposited on silver surfaces using a thermophoretic process.  Under the conditions studied, the presence of salt reduced the amount of corrosion relative to the bare surface.  This effect is thought to be due to the inhibition of atomic oxygen access to the silver surface by the presence of a thin electrolyte layer.