Corrosion Behavior of Binary Titanium Aluminide Intermetallics

The corrosion behavior of arc-melted binary titanium aluminide intermetallics TiAl, Ti3Al, and TiAl3 in aqueous sodium sulfate (Na2SO4) and sodium chloride (NaCl) solutions was measured and compared to that of pure Ti and Al. Effects of electrolyte composition (e.g., sulfate [0.25 M SO42-], chloride [0.1 to 1.0 M Cl-], and pH [3 to 10]) were examined. Anodic polarization of titanium aluminides in aqueous SO42− solutions was similar (showing passive behavior), but no pitting or pitting potential (Epit) was observed. In aqueous NaCl, however, titanium aluminides were susceptible to pitting, and Epit decreased with increasing Al content (i.e., Ti3Al had the highest Epit and, therefore, a greater resistance to pitting, followed by TiAl and TiAl3). For TiAl, Epit was slightly dependent upon pH or Cl− concentration. Pit morphology and Epit values were quite different for TiAl compared to Ti3Al. TiAl showed numerous small pits, whereas Ti3Al exhibited fewer but larger and deeper pits. The larger pit density for TiAl was associated with Al-rich interdendrite regions. One interesting feature of the anodic polarization curves for Ti3Al was a small anodic peak frequently observed at ~ 1.4 VSCE to 1.8 VSCE. Results suggested this peak was associated with pit initiation, since pitting initiated concurrently with the peak or immediately afterward.