An assessment of the crack tip potential of β-titanium alloys during hydrogen environmentally assisted crack propagation based on crack tip and passive surface electrochemical measurements

We have previously proposed a scenario for intergranular hydrogen-assisted crack initiation of metastable β-titanium alloys exposed to aqueous chloride solution. This scenario requires the loss of potentiostatic control due to ohmic drop in solution such that the crack tip potential falls to a potential where hydrogen production is viable regardless of the applied potential. It is of interest to examine the crack tip potential during crack growth to discern whether hydrogen production is viable during crack growth as well. Crack tip potential excursions were examined by analyzing crack tip and passive film currents. The magnitude of the galvanic currents between filmed surfaces and the bare crack tip were estimated using a model compact tension specimen, which comprises a fixture with a similar geometry to an actual compact tension specimen. These experiments indicate that capacitive charging and discharging occurs on the passive surfaces surrounding a depassivated crack tip. High speed measurements of crack tip anodic current during potentiostatic crack growth indicate that coupling of the crack tip to the counter electrode is incomplete. Unaccounted charge suggests that a significant amount of current is supplied by the bare surface itself and that the crack tip approaches its bare surface open circuit potential during crack growth. Thus, hydrogen production was found to be viable during crack growth even though the applied potential was more positive than the reversible potential for hydrogen production.