Hydrogen embritlement occurs with hydrogen occlusion of metals. When considering a corrosion phenomena where steel dissolves with hydrogen out-gassing, the reaction is a steel redox process of oxidation by hydrogen ions.

When this reaction is viewed in detail, it can be divided into the following two reactions.

That is, the electrons (e) emitted by dissolution of steel combines with hydrogen ions (H+) within the solution, then hydrogen gas generation occurs. It is thought, in general, that such redox (Reduction-Oxidation) reaction occurs when metals corrode.

The mechanism of hydrogen gas emission from metal surfaces can be considered as follows. The hydrogen ions (H+) become of atomic hydrogen (H), then combine into molecular hydrogen gas (H2). The atomic state hydrogen will exist as adhered to the metal surface, and a portion of this hydrogen will penetrate into the metal.

The hydrogen entered into metal will diverge under stresses and congregate on lattice defects of the metal, then embrittles the metal. The stress causes the fractures. Especially, the high strength steel can generate hydrogen when corroding, and become embrittled by absorbing the hydrogen. When stresses are applied in this state, the steel will easily fracture.

There are two causes for hydrogen generation from metal surfaces. The first is when steel dissolves in acids, as explained above. When dissolving in solutions, steel generates hydrogen. It typically occurs during pickling and welding processes.

Another is when the subject metal becomes a negative in polarity and an electrical current flows. Typical cases applicable for this are cathodic electrolysis and passivation processes for surface treatments such as electro-plating and electro-cleaning.

The embrittlement fracture occurs only with metals highly susceptible to hydrogen occlusion such as high tensile strength steel. Steels with tensile strength less than 40kg/mm² are hardly affected.

It is said that when compounds such as hydrogen sulfide and arsenic exist in the corrosive environment, atomic state hydrogen bonding into becoming molecular state hydrogen gas. Therefore, the hydrogen concentration at metal surface will increase and causes embrittlement of even lower tensile steels. This phenomenon is also called Sulfide Corrosion Embrittlement, and often causes corrosion caused accidents on hydrogen sulfide containing natural gas/oil drilling, transportation, refining, and storages.

High tensile strength bolts are used for construction of giant structures such as bridges. These bolts, even in neutral air environments, may be subject to fractures due to minute amount of hydrogen in the air. This phenomenon is called "Delayed Fracture of High Tensile Bolts". Also, fractures occurring after a period of time when hydrogen occluded bolts are tightened at less than the rated tensile stress is called "Delayed Fracture".