Coating systems for corrosion mitigation of structural steel have been widely used in aggressive humid environments. Novel materials continue to become available and interest lies in identifying improved material performance for bridge applications. However, long-term maintenance free service life criteria remain a challenge. As a part of research to assess novel coatings, Thermal Diffusion Galvanizing (TDG) coating was investigated with and without various recommended topcoats for both short and long-term exposure. The samples were exposed in outdoor test condition for up to 45 months. To assess the corrosion damage due to exposure, the coating was evaluated by visual inspection, coating thickness, pull-off strength measurement, optical microscopic evaluation and XRD analysis. The 45-month exposed samples were also evaluated by electrochemical techniques to identify electrochemical parameters of the coated TDG after extended outdoor exposure. The results obtained to date generally indicate good performance of TDG in outdoor settings. Although the developed zinc oxide provided indication of protection regardless of the type of topcoat tested, the application of topcoat is important for enhanced durability as corrosion of the TDG alloy layer can be possible.