Hydrogen embrittlement fractures occur when a metal absorbs hydrogen from an external source. There are numerous potential sources of hydrogen in both the manufacturing process and service environment. These include moist corrosion, arc welding with damp electrodes, acid pickling or cleaning solutions, and electroplating baths containing hydrochloric acid. In order for a hydrogen embrittlement fracture to occur, a part which has absorbed hydrogen must be subjected to tensile stress. Within a relatively short period of time (usually 48 hours or less) from the first application of this stress, fracture will occur. The mechanism by which hydrogen embrittlement fracture occurs is relatively simple. Individual hydrogen atoms, which even by atomic standards are extremely small, diffuse into the metal at the grain boundaries, which are inherent to metallic microstructure. When the part is stressed as occurs, for example when a bolt is tightened, the microscopic gaps between the grains widen slightly. When this occurs, the hydrogen atoms become mobile, moving along the grain boundaries, and when two atoms meet they combine to form a hydrogen molecule. The amount of volume that a single hydrogen molecule occupies is many times greater than that of two individual hydrogen atoms. This increased volume results in pressure between adjacent grains which literally “pushes” the grains apart, resulting in fracture.
Hydrogen embrittlement typically occurs in relatively high strength materials with a hardness of Rockwell HRC 32 or greater. A frequently encountered example might be a plated high strength bolt which has absorbed hydrogen in the electroplating process.
Hydrogen embrittlement fractures are very similar in appearance to intergrannular fractures resulting from other causes. Specific microscopic features, however, differentiate this failure mode when the fracture is examined using a scanning electron microscope. Identification of these features by an experienced materials engineer is critical to an accurate finding of Hydrogen Embrittlement in the course of a failure analysis.