The purpose of the present study is to assess microstructure on the mechanism and corrosion resistance of the most widely used ?+? titanium alloy, Ti-6Al-4V after gas tungsten arc and friction stir welding processes. For this, the corrosion behavior of Ti-6Al-4V alloy was studied in deaerated 5% HCl at room temperature. Firstly, the corrosion behavior of common microstructures of Ti-6Al-4V alloy including fully lamellar, fully equiaxed and bimodal were studied in 0.9% NaCl and 5% HCl solutions. Fully equiaxed, fully lamellar and bimodal microstructures were obtained through thermomechanical and heat treatment. The results of polarization potentiodynamic experiments showed that all microstructures exhibit spontaneous passivity in NaCl solution and active passive transition in HCl solution. Despite there weren’t significant differences between corrosion rates of polarization tests, but fully lamellar microstructure exhibited the worst corrosion resistance according to the immersion tests. These results were confirmed according to Scanning Electron Microscopy (SEM) observations of immersion samples in 5M HCl solution in 35 ?C. Results showed that the volume fraction of the alpha and beta phases, microstructure and distribution of components and alloying elements distribution between alpha and beta phases are the most important factors influencing the corrosion behavior of Ti-6Al-4V alloy. Furthermore, the alpha/beta interfaces were determined as preferred corrosion attacked sites. The corrosion behavior of welded samples was also investigated in deaerated 5% HCl solution. The potentiodynamic polarization experiments showed that the GTA weldment exhibited higher corrosion rate in comparison to the base metal. This bahavior was attributed to the acicular microstructure, porosity and large beta grains. The presence of secondary alpha in transformed beta regions improves the corrosion resistance heat affected zone. Friction stir welding was performed with the peak temperatures both above and below the ? transus. All samples exhibited active/passive transitions in deaerated 5% HCl at room temperature. The results indicated that the ? FS processed samples exhibited superior corrosion behavior compared to the base metal and ?/? FS processed samples. SEM analysis of the corroded surfaces after immersion indicated that the ? phase corroded preferentially in the base metal and transition zone, which had primarily fully lamellar microstructures. However, the stir zone of the ?/? FS processed sample corroded more severely than the ? FS sample as a result of preferential attack of the ? phase. This was rationalized on the basis of a difference in partitioning of the alloying elements, which controls the composition of the ? and ? phases.
