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The microstructural, mechanical and galvanic corrosion behaviors of a friction stir welded (FSWed) butt joint of dissimilar 5052-H32 aluminum and DP 1200 steel sheet alloys are investigated. At an offset of 0.5 mm towards the DP steel base metal (BM), on the advancing side (AS) of the welding direction (WD), the microstructural characterization reveals a distinct but complex stir zone (SZ) made up of 3 layers. There is (i) the top layer made up an aluminum matrix with randomly scattered steel fragments which act as reinforcements but reduce the corrosion resistance of the region,(ii) a middle layer having a mixed lamellar structure of Fe-Al solid solutions and intermetallic compounds (IMCs). They increase the hardness of the SZ but are brittle enough to reduce the weld joint strength due to their thickness, and (iii) the bottom region which is made of deformed steel. When compared with the DP steel BM, the deformed …