Finer grain size of TA15 by laser directed energy deposition: contradictory role against NaCl and H2SO4 corrosion

The corrosion behavior of laser directed energy deposition (LDED)-fabricated TA15 alloy was evaluated in both 3.5% NaCl and H2SO4 (pH = 0) solutions, with wrought TA15 as control group. In both solutions, TA15 alloys develop a TiO2-rich surface layer with certain amounts of Al2O3, ZrO2 and MoO3. Due to active dissolution of the surface passive film, the H2SO4 solution causes more severe corrosion damage to the TA15 alloy than the NaCl solution. Compared with its wrought counterpart, LDED TA15 alloy exhibits slightly lower corrosion resistance in NaCl solution, and a much higher corrosion resistance in H2SO4 solution. The rapid heating and cooling during LDED will greatly affect microstructure and eventually corrosion resistance. While both alloys show α/α’ phase, the LDED sample has fine basketweave grain and thus much smaller grain size. The higher grain boundary density and high ratio of HAGBs in the LDED sample are not beneficial to the passive film stability maintenance in NaCl solution, but favorable for the quick formation of a stable passive film with lower defect concentrations in active H2SO4 solution.