Influence of Grain Boundary Structure and Composition on Hydrogen Absorption into Alpha Titanium Alloys

The oxide film on titanium alloys is the primary barrier preventing hydrogen absorption. Generally, the oxide is rendered permeable to hydrogen by cathodic reduction by either cathodic protection or galvanic coupling to a less noble metal.  

However, using scanning electrochemical microscopy () we have shown that the oxide at grain boundaries is more readily reduced and, hence, more permeable to hydrogen than the oxide on the grain surfaces, potentially allowing hydrogen absorption at potentials achievable without galvanic coupling or cathodic protection. Comparison of and EDX analyses to reactivity maps constructed using, demonstrated that the most reactive sites were Ti_xFe precipitates and Fe-stabilized β-phase in the grain boundaries.

In Pd-alloyed titanium (Grade-7), co-segregation of Pd and Fe to grain boundary locations leads to even more potentially reactive sites. Quantitative analysis of these sites was achieved by fitting probe approach curves to curves simulated with a model based on finite element analysis using COMSOL multiphysics software. Active locations on Grade-7 are not sustainable over long exposure periods in the absence of a coupled anode, whereas the anodic reactivity of the Ti_xFe precipitates means that localized corrosion may be sustainable in grain boundaries on the commercially pure Grade-2 alloy.