Hydrogen embrittlement phenomenon

The most important problem associated with the use of pure palladium membranes is the hydrogen embrittlement phenomenon. When the temperature is below 300°C and the pressure below 2.0 MPa, the ]S-hydride phase may nucleate from the... 

The most important problem associated with the use of pure palladium membranes is the hydrogen embrittlement phenomenon. When the temperature is below 300 °C and the pressure below 2.0 MPa, the 3-hydride phase may nucleate from the a-phase, resulting in severe lattice strains (see Figure 13.3), so that a pure palladium membrane becomes brittle after a few cycles of oc (3 transitions. Such a problem can be overcome by using Pd-alloy containing another metal, such as silver. The palladium alloys have a reduced critical temperature for the oc-P phase transition. Pd-Ag membranes can operate in hydrogen atmosphere at temperatures below 300 °C without observing... 

Glass that has been under stress for a period of time may fracture suddenly. Such delayed fracture is not common in metals (except in cases of hydrogen embrittlement of steels) but sometimes does occur in polymers. It is often called static fatigue. The phenomenon is sensitive to temperature and prior abrasion of the surface. Most important, it is very sensitive to environment. Cracking is much more rapid with exposure to water than if the glass is kept dry (Figure 15.11) because water breaks the Si-O-Si bonds by the reaction — Si-O-Si—H H2O -> Si-OH + HO-Si. 

What is the mechanism of this phenomenon Very early during investigations of this field, it was realized that metals become embrittled because at some stage of their career, their surface was the scene of a hydrogen-evolution reaction either because the metal was deliberately used as an electron-source electrode in a substance-producing cell or because parts of the metal became electron-source areas in a corrosion process. In fact, the phenomenon has come to be known as hydrogen embrittlement. 

This model is considered to be useful to improve the knowledge of the role played by the factor of hydrogen accumulation in prospective rupture sites by stress-assisted diffusion, one of the key items in hydrogen embrittlement, a very dangerous phenomenon that frequently accompanies structural metals and alloys in service. 

Ferrite is essentially pure iron with a body-centered cubic crystal stracture. Ferrite forms from austenite at about 1675°F (915°C), as the austenite cools from a normalizing heat treatment. Because ferrite does not contain enough carbon to permit the formation of martensite, it is not hardenable by heat treatment. The most common truly ferritic steel is Type 405 SS, a ferritic stainless steel. The generic term ferritic steel often refers to carbon or low-alloy steels that contain other phases in addition to ferrite. Such steels are usually hardenable by heat treatment. Ferritic steels become brittle at low temperatures. This phenomenon is reversible, that is, the steels regain their former toughness after being warmed up. Ferritic steels are also susceptible to hydrogen embrittlement. 

The exact mechanism of stress cracking corrosion is unknown but it is thought to be related to hydrogen embrittlement and to be another phenomenon whose intensity is due to the effect being localized. It was seen in the earlier discussion that in many corrosion situations the cathodic part of the process is hydrogen... 

Hydrogen embrittlement is a phenomenon whereby hydrogen is absorbed in the metal (diffuses), exerts local stresses, and leads to embrit-tiement of material, such as high strength steels. 

This phenomenon is generally encountered in high strength steels of Rockwell hardness above 22 in a sour oil field environment. It is a special case of hydrogen stress cracking, also called hydrogen embrittlement. Factors responsible for sulfide stress corrosion cracking (SCC) are ... 

It is postulated that specific ions are absorbed and interact with strained bonds at the surface of the crack tip, thus reducing the bond strength, and permitting continued brittle fi acture. This theory has been supported by observations in SCC. By chemisorption of the environmental species on the crack tip, the local fracture stress of the metal lattice is reduced. The theory has been applied to hydrogen embrittlement and liquid metal embrittlement. The adsorption phenomenon may be used to interpret the crack propagation mechanism of alloys which fail by hydrogen embrittlement, such as aluminum alloy 7075. 

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