Surface Peeling of Ti-6Al-4V Alloy Specimens during Hydrogen Charging

The behavior of hydride peeling during hydrogen charging in sulfuric acid solution was investigated on the Ti-6Al-4V alloy specimens with different metallographic structures. In addition, an X-ray diffraction analysis was carried out to clarify the hydride formation process near specimen surface.The amount of hydride peeling increased monotonically with increasing hydrogen charging time. In the specimens with coarse acicular α+β structure, the hydride peeled more severely, while in the specimens with granular α+β the amount of peeling was small. When the hydride peeling occurred stably, the surface of the specimens with coarse acicular α+β structure was almost composed of hydride, and the amount of hydride decreased sharply with increasing depth, x, to almost zero at x\fallingdotseq1 mm. In the specimen with granular α+β structure, on the other hand, the surface consisted of a mixture of hydride and α phase, and the amount of hydride decreased gradually with increasing depth. The peeled powder was almost hydride independently of structures. The reason for the larger peeling amount of the specimen with coarse acicular α+β seemed to be that the β phase with a large diffusion coefficient of hydrogen was divided by long and narrow α phases, and the hydrogen diffusion from the surface to the inside of the specimen was obstructed, i.e. hydride was formed only near the specimen surface, and the sharp gradient of internal compressive stress in the thickness direction due to volumetric expansion of hydride resulted in the large amount of surface peeling.