Mechanically-Fastened Fiber Reinforced Polymer Composites (MF-FRPs) are currently being used to extend the useful service life of deteriorated bridges. However, the A325 Steel fastener assemblies used to attach the MF-FRP system to the bridges are experiencing noticeable corrosion. Through electrochemical polarization measurements and Tafel analysis, the expected lifetime of the A325 fastener assembly was determined and compared to other similar materials, Military Specification Grade 5 Steel (MTD-STD) and PH 17-4 Stainless Steel (PH 17-4). ASTM B117 salt fog testing was performed on each material fastener assembly to simulate the corrosion that should be experienced by each material. The electrochemical analysis and the ASTM B117 salt fog test confirmed the MIL-STD assembly corroded at a much slower rate compared to either A325 or PH 17-4. It was determined that the useful life of the fastener assembly could be extended from 6.5 year using A325 to 372 years using MIL-STD. Implementation of this engineering materials solution will extend the useful life of the MF-FRP fastener assembly however, a cost benefit analysis determined that continuing to use A325 is still the best option given the desired useful life of the MF-FRP retrofit system is 3 to 5 years.