Relationship between hydrogen absorptionand corrosion behavior of Ti-6A1-4V alloy immersed in acidic fluoride solutions

The relationship between hydrogen absorption and the corrosion behavior of Ti-6A1-4V alloy has been examined by immersing the alloy in acidulated phosphate fluoride (APF) solution with various concentrations. Upon immersion in 0.1 mass% APF solution, no hydrogen absorption is observed despite the occurrence of general corrosion. In 0.2% APF solution, slight hydrogen absorption occurs soon after immersion, and then the surface of the specimen becomes uniformly covered with a crystalline corrosion product (Na₃AlF₆), thereby inhibiting further hydrogen absorption. In APF solutions with concentrations higher than 0.4%, the amount of absorbed hydrogen increases with increasing concentration of APF solution and immersion time. The type and morphology of corrosion products sensitively depend on the concentration of APF solution. Upon immersion in 0.4% APF solution, crystalline and massive Na₃AlF ₆ are observed on the surface of the specimen. In 1.0% and 1.5% APF solutions, Na₃AlF₆ with a foldlike shape and granular Na₃AlF₆ and Na₃TiF₆ are observed. In 2.0% APF solution, only granularNa₃TiF₆is observed. The corrosion potentials and anodic polarization curves are analogous in all APF solutions except the 0.2% and 0.4% APF solutions. In contrast, the current density of cathodic polarization curves increases with the concentration of APF solution. The results of the present study suggest that the corrosion products play an important role in the hydrogen absorption behavior of Ti-6A1-4V alloy.