Application Notes

High Temperature Hydrogen Attack (HTHA)

High temperature hydrogen attack (HTHA) is an irreversible deterioration of the mechanical properties of steel due to the reaction of hydrogen with carbon at temperatures in excess of 200°C.

Atomic hydrogen may react with dissolved carbon (particularly with cementite) in steel to form methane (CH4) inside the material. This chemical reaction causes the decarburisation of the steel which then results in deterioration of the mechanical properties of the steel, particularly its strength. The methane molecules are too large to diffuse out of the steel. Thus, they are trapped in and exert pressure on the grain boundaries. These voids, the methane bubbles, grow through microscale creep deformation due to diffusion at the surface and grain boundaries or dislocations. Cracks result. The accumulation of methane may also cause blistering. High temperatures and pressure accelerate HTHA. However, some chemical additives in the steel can help to reduce this problem. Cr, Mo, V, Nb and Ti appear to decrease the tendency for steel to crack.

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