In recent years, many unplanned hazard failures of oil-filled electrical transformers have been reported due to presence of corrosion in form of semi-conductor Copper Sulphide (Cu2S) which result in formation Corrosive Sulphur Deposition (CSD) on the significant internal components. Formation of CSD leads to a continuous current path lead to overheating fault and deterioration of insulation system which can turn towards transformer failure. The purpose of this study is to establish an efficient maintenance plan of corrosion based on relevant Condition Monitoring (CM) to reduce the transformer failures. In this paper, many investigations have been reviewed to get sufficient data to understand and describe currently applied CM for diagnose the corrosion inside the transformers. The problem addressed is: is the currently applied CM relevant for early and definitely detection of the corrosion to reduce the probability of failures? The major result is the described gaps between the currently applied CM and selection of relevant CM suggested for providing definitely indication of corrosion. The currently CM implies two techniques; oil analysis based only on evaluation the quality of the oil against corrosion and electrical testing based on the variation of electrical properties of the transformer which can be occurred due to corrosion process as well as other faults. Conclusion of this review is a need to develop a relevant CM of corrosion for establishing a dynamic cost-effective Condition Based Maintenance (CBM) to prevent unplanned hazard failures at early stage which can reduce the negative impacts such as industries economic loss and hazard effect on manpower life as well as environment.