CORROSION BEHAVIOR OF BINARY TITANIUM ALUMINIDE INTERMETALLICS

The corrosion behavior of arc-melted binary titanium aluminide interme tallics TiAl, Ti2Al, 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 [0.2 5 M SO42-], chloride [0.1 to 1.0 M Cl-], and pH [3 to 10]) were examin ed. Anodic polarization of titanium aluminides in aqueous SO42- soluti ons was similar (showing passive behavior), but no pitting or pitting potential (E(pit)) was observed. In aqueous NaCl, however, titanium al uminides were susceptible to pitting, and E(pit) decreased with increa sing Al content (i.e., Ti3Al had the highest E(pit) and, therefore, a greater resistance to pitting, followed by TiAl and TiAl3). For TiAl E (pit) was slightly dependent upon pH or Cl- concentration. Pit morphol ogy and E(pit) values were quite different for TiAl compared to Ti3Al. TiAl showed numerous small pits, whereas Ti3Al exhibited fewer but la rger and deeper pits. The larger pit density for TiAl was associated w ith Al-rich interdendrite regions, One interesting feature of the anod ic polarization curves for Ti3Al was a small anodic peak frequently ob served at similar to 1.4 V-SCE to 1.8 V-SCE. Results suggested this pe ak was associated with pit initiation, since pitting initiated concurr ently with the peak or immediately afterward.