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Coating of graphene and graphene/polymer composites on metals improves the corrosion resistance of metal substrates. On other hand, graphene embedded inside metal (especially Mg) matrices increases or decreases corrosion, is a crucial factor and must be explored. In present study, electrochemical behaviors of magnesium alloys (AZ31 and AZ61) and their composites reinforced with graphene nanoplatelets (GNPs) were carried out in 3.5% NaCl solution by polarization method. The surface morphology of composites before and after corrosion tests were analyzed using scanning electron microscopy. Experimental results revealed that presence of graphene nanoplatelets in different matrices decrease corrosion resistance of composites. This may be attributed to presence of graphene nanoplatelets which activates the corrosion of magnesium/alloys due to the occurrence of galvanic corrosion and this effect increases with increasing graphene nanoplatelets content. Further, an appropriate model describing the corrosion mechanism was proposed. Keywords: Metal matrix composites, Electro-chemical properties, Corrosion