A failure associated with steel corrosion was identified in early 2011 in a bridge external post-tensioned tendon, approximately eight years after construction. Large voids in the grout and pockets of non-homogeneous material were identified. The non-homogeneous grout was characterized by high moisture content, and in most cases, the chloride content was lower than conservative threshold values. The non-homogeneous grout also had high pH and high content of sulfates (approximately in the range of 10,000-ppm). As a result, there was an interest in the study of possible corrosion development in repaired systems in which the affected tendons have been re-grouted with dissimilar grouts. The presence of two distinct grouting materials, manifested by the existing/simulated base grout and a newly introduced repair grout, provided the dissimilar grout condition studied. Corrosion activity was monitored in un-stressed mock up assemblies, in sections retrieved from the failed tendon, and in samples immersed in simulated pore solution. Corrosion activity was monitored through macrocell current, linear polarization resistance, open circuit potential, potentiodynamic scans, and electrochemical impedance spectroscopy. Samples in simulated pore solutions were studied at various pH levels and constituent concentrations. All samples were repaired or built with commercially available grouts. After analysis, no evidence of corrosion development was found when both existing and repair grout were free of material deficiencies. Corrosion activity was noted in the presence of nonhomogeneous grout and an increase in rate was observed due to macrocell coupling with sections containing normal grout. Results suggest that early exposure to sulfate to hydroxyl ion ratio as low 0.35 may prevent steel passivation and result in early high corrosion rates. Otherwise, sulfate to hydroxyl ratios as high as three may not be sufficient to initiate corrosion after formation of passive layer.