Occurrence of Hydrogen Embrittlement and how to prevent it
Hydrogen embrittlement is a commonly occurred problem that has been experienced for several years. Recently even new and unexpected type of hydrogen embrittlement problem is seen in aerospace industry.
This type of corrosion occurs when an essentially hydrogen free material is mechanically evaluated in gaseous hydrogen. It is well agreed among investigators that molecular hydrogen should dissociate to atomic hydrogen to cause embrittlement. Both adsorption and absorption usually occur. Hydrogen should diffuse through the lattice for embrittlement to occur during testing in gaseous hydrogen, hydrogen embrittlement should then be considered similar to internal reversible hydrogen embrittlement.
Hydrogen embrittlement has been experienced at different levels of gas pressures, temperatures and mechanical tests. Embrittlement seems to be the most severe at room temperature. Purity of gas and test strain rate has an essential role in determining the level of embrittlement.
Even the high strength structural materials for example steels are found to be susceptible to hydrogen embrittlement. But Nickel alloys are resistant to internal reversible hydrogen embrittlement. This variation in sensitivity may be connected to some hidden surface properties of nickel alloys.
Vigorousness of embrittlement has been found to vary depending on alloy form and annealing temperarue. Inconel bar 718 performance in hydrogen embrittlement environment is evaluated. The bar and forgings annealed at lower temperature are more severely embrittled as compare to same forms annealed at higher temperature. The minimum affected structure is one which is fine grained with a uniform dispersion of the precipitate. The most embrittled structure is one which is widely grained with intergranular precipitates.
However these microstructural effects may be suitable for Inconel 718, other vigorously embrittled nickel base alloys do not comprise niobium for example Hastelloy X. It is unlikely that hydrogen environment embrittlement can be featured exclusively to precipitated phases. The contribution of grain size and boundaries is also unknown.
Hydrogen gas pressure: Various materials evaluated in a single test at hydrogen pressure of 70 MN/m2 at room temperature. Notched tensile strength and smooth and reduction of area were considered as the cause of embrittlement. It was found that magnitude of corrosion was directly proportional to the square root of the hydrogen gas pressure.
Hydroben environment embrittlement occurs at gas pressure considerably below atmospheric pressure. Fatigue crack growth rates of Nickel 200 at room temperature increased by a range of 1 micro-N/m2 to 20 kN/m2.
Hydrogen gas composition: The effect of gas purity is noticeably described by the crack extension. Crack growth in a stressed precracked sheet sample of H11 steel could be begun by the introduction of pure hydrogen. It was found that crack propagation rates were not affected by an atmosphere of hydrogen comprising oxygen below 200 ppm at a total gas pressure of 0.1 MN/m2. Although at higher gas pressures, even lower magnitudes of oxygen contaminants prevent embrittlement in gaseous hydrogen. This preventive effect of oxygen is associated to the preferential adsorption of the oxygen at newly developed cracks.
By using specific nickel base super alloys it is possible to prevent hydrogen embrillement in applications. You should consult with our team for your application requirements.