Laboratory tests were made which showed that the corrosion rate of steel is greater under a blue filter than under a red filter in the presence of sunlight, provided that a thin film of water covers the metal surfaces. Outdoor tests of four-months duration verified the laboratory results. Steel specimens were exposed under molded glass filters which transmitted radiation within specific spectral regions. The results indicate that ultraviolet light activates the metal surface so that corrosion is initiated sooner or progresses at a faster rate than it would in its absence. Infrared light affects the metal surface so that subsequent exposure to ultra-violet light does not produce the same increase in corrosion.