Effect of friction stir processing on corrosion of Al-TiB2 based composite in 3.5 wt.% sodium chloride solution

The microstructure and corrosion behavior of as-cast and friction stir processed in-situ Al-TiB<inf>2</inf> based composite in 3.5 wt.% sodium chloride solution were investigated. The microstructure was characterized using X-ray diffractometry, scanning electron microscopy and electron backscattered diffraction technique while corrosion behavior was evaluated using linear/cyclic potentiodynamic, electrochemical impedance spectroscopy and ASTM-G67 tests. The composite contains sub-micron TiB<inf>2</inf> particles in an aluminum matrix with both blocky and fine clusters of Al<inf>3</inf>Ti agglomerated around TiB<inf>2</inf> and displays a low uniform corrosion rate. It is also resistant to pitting as substantiated by the absence of a positive loop in cyclic potentiodynamic tests. This is due to the non-conductive nature of TiB<inf>2</inf> particles and a controlled amount of blocky Al<inf>3</inf>Ti phase. However, both friction stir processed and as-cast composites are susceptible to inter-granular corrosion where Al<inf>3</inf>Ti and TiB<inf>2</inf> at grain boundaries provide initiation sites for corrosion. Electrochemical impedance study attributes this to the adverse effect of Al<inf>3</inf>Ti and TiB<inf>2</inf> on the protective oxide surface film, which increases with immersion time.