Hydrogen Blistering

The schematic diagram below shows hydrogen diffusion, say, in a tank containing an acid electrolyte. Hydrogen evolution occurs because of a corrosion reaction or cathodic protection. Most of the hydrogen atoms combine to form hydrogen molecule at the external surface.

H^a + H^c 2H^o H₂

The combination of some hydrogen atoms to form molecules is prevented by some poisons like As, Bi and Sb, and the atomic hydrogen diffuses into the metal. They diffuse in to the small voids, and surface heterogenities present in the metal. In the void they combine to form molecular hydrogen.

Unfortunately, the molecular hydrogen does not diffuse, the concentration and pressure of hydrogen gas with a very small void increases enormously, the equilibrium pressure of molecular hydrogen in contact with atomic hydrogen is in the order of several hundred thousands atmospheres, sufficient to rupture and form blisters and cause rupture of the walls. Generally low strength steels are susceptible to hydrogen blistering.