Volume 5, Issue 8 (September 2008)
Crevice Corrosion of Grade-12 Titanium
The crevice corrosion of ASTM grade-12 titanium (Ti-12; 0.8 wt % Ni, 0.3 wt % Mo) has been studied in neutral 0.27 mol/L NaCl at temperatures up to 120°C using a galvanic coupling technique. The results were compared to those of similar experiments conducted on ASTM grade-2 titanium (Ti-2) specimens containing various amounts of iron impurity. The microstructural properties of the Ti-12 were determined by standard metallographic techniques and transmission electron microscopy. Penetration depth profiles were determined using metallographic and image analysis techniques. The redistribution of alloying elements (Ni, Mo) and impurities (Fe), and the location of absorbed hydrogen, were determined by secondary ion mass spectrometry (SIMS) imaging. The rate and extent of crevice propagation were significantly suppressed on Ti-12 compared to Ti-2, especially at higher temperature (120°C). Up to 97 % of the total amount of crevice propagation was driven by proton reduction inside the creviced area rather than by oxygen reduction outside the crevice. SIMS imaging shows that Ni accumulates on the corroding surface, probably in the form of residual Ti2Ni particles. Proton reduction is catalyzed on these particles, leading to their hydriding and an increase in the relative area of internal cathodes compared to available anodic surface area. This self-induced “cathodic modification” effect leads to repassivation of the corroded site before extensive damage can be sustained.