Wire and Arc based Additive Manufacturing (WAAM) processes are novel technologies that are seen as promising candidates for fabricating complex 3-Dimensional large scale components. These processes offer avenues to locally tailor metallurgy of the melt pool and thus produce functionally and compositionally graded components for improved performance in extreme conditions. This thesis explores in-situ alloying of commercial Manganese Aluminium Bronze (MAB) with copper rich Cu3Si filler wire during deposition, as a microstructure control strategy for designing components that are resistant against selective phase corrosion. Evolution of microstructure during WAAM processing of MAB and during in-situ alloying with Cu3Si wire is investigated using optical and electron microscopy, x-ray diffraction and fluorescence. The microstructure of the as-deposited MAB bead is observed to change from continuous connected dual phase to cellular single phase upon controlled addition of Cu3Si in a MAB melt pool. Corrosion tests showed reduced severity of surface attack and an improvement in the rate of corrosion by 3 times for the new alloy. Thus this technique can be used in industries to improve corrosion resistance as an energy efficient alternative of conventional high temperature aging treatments. Additionally, the possibilities of improving mechanical behaviour of the new alloy (cellular single phase) are explored using the principles of grain refinement in the context of WAAM.