Hydrogen Embrittlement

Constant exposure to hydrogen causes a phenomenon known as hydrogen embrittlement [HE] in many materials. The HE can lead to leakage or catastrophic failures in metal and non-

metallic components. The mechanisms that cause HE effects are not well defined [36]. Factors known to influence the rate and severity of HE include hydrogen concentration, hydrogen pressure, temperature, hydrogen purity, type of impurity, stress level, stress rate, metal composition, metal tensile strength, grain size, microstructure and heat treatment history. Moisture content in the hydrogen gas may lead to metal embrittlement through the acceleration of the formation of fatigue cracks. According to [56, 96], HE can be classified into three categories: (1.) Hydrogen reaction embrittlement

It is a phenomenon in which the hydrogen chemically reacts with a constituent of the metal to form a new microstructural element or phase such as a hydride or gas bubble, e.g. methane gas if combined with carbon, or steam if combined with oxygen. These reactions usually occur at higher temperatures. They result in the formation of blisters or expansions from which cracks may originate to weaken the metal. (2.) Internal hydrogen embrittlement

It means that hydrogen is introduced into the metal during its processing, e.g. chemical reactions with water to form metal oxide and liberate hydrogen. It is a phenomenon that may lead to the structural failure of material that never has been exposed to hydrogen before. Internal cracks are initiated showing a discontinuous growth. Not more than 0.1 - 10 ppm hydrogen on the average is involved. The effect is observed in the temperature range between -100 and +100°C and is most severe near room temperature. (3.) Environmental hydrogen embrittlement

It means that the material was subjected to a hydrogen atmosphere, e.g. storage tanks. Absorbed and/or adsorbed hydrogen modifies the mechanical response of the material without necessarily forming a second phase. The effect occurs when the amount hydrogen that is present, is more than the amount of metal that is present, is more than the amount that is dissolved in the metal. The effect strongly depends on the stress imposed on the metal. It also maximizes at around room temperature.

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