Hydrogen embrittlement
Under the hydrogen embrittlement is meant the change in brittleness that is caused by the penetration and incorporation of hydrogen into a metal lattice. This sequence is similar to corrosion fatigue of the material - in consequence it comes to hydrogen-induced cracking, which in particular the use of susceptible materials is limited for hydrogen storage.
Effect
Hydrogen embrittlement occurs when on the metal surface - (for example when etching ) either by hydrogen corrosion, or in another reaction in the metal processing, is involved in the hydrogen -, atomic hydrogen is formed which diffuses more rapidly into the material as he assembles at the surface of the material non-diffusible to H2 molecules. A portion of the hydrogen is thereby incorporated into the metal matrix, and it can, as in the case of titanium, a metal hydride formed. In other cases, the hydrogen stored preferably from at defects or grain boundaries. The result in both cases is an embrittlement of the metal.
For sufficiently large Zugeigen and / or load voltages there is the risk of a delayed brittle fracture. One speaks of a delayed brittle fracture, because the damage takes time and breaks the material due to the Gleitblockierungen almost without deformation. This effect is similar to a stress corrosion cracking and limit the use of metals for hydrogen storage.
Process
By certain chemical reactions, atomic hydrogen formed penetrates into the texture of metallic material, where it recombines to lattice defects back to molecular hydrogen and remains there. The pressure increase thus leads to internal stresses and related to embrittlement of the material, without causing an increase in strength occurs. The end result finally cracks that propagate from the inside out. The stress corrosion cracking of the resulting during the corrosion process, hydrogen diffuses to the crack tip and then accelerated the crack speed.
Hydrogen embrittlement of steel
Steel and titanium are often affected by embrittlement if they were a long time in contact with hydrogen. In the steels, however, the austenitic stainless steels (eg stainless steels) are an exception. These are largely insensitive to hydrogen embrittlement and are among the standard materials of hydrogen technology. High-strength steels with high martensite content and yield strengths greater about 800 MPa are particularly at risk of being damaged by the hydrogen embrittlement.
The hydrogen embrittlement occurs in particular during welding and during the electroplating galvanizing of high strength steels (eg screws from strength class 10.9 and later). Hydrogen is formed on the cathodically connected steel and diffuses into the steel. Thus, the screw releases the hydrogen again, they must immediately several hours of heat treatment at 200-300 ° C ( Wasserstoffarmglühen, also known as tempering or annealing ) are subjected. Because hydrogen has a high rate of diffusion, even at low temperatures, it is possible to drive out at temperatures of up to 200 ° C and the hydrogen in the steel, without changing it metallurgically.
Hydrogen embrittlement of copper
Under the hydrogen embrittlement is meant the formation of cracks and voids in the oxygen-containing copper grades, such as the E- Cu58 and Cu57 - E (DIN 1787), when in contact with hydrogen-containing gases. These copper grades are mainly used in electrical engineering due to their high electrical conductivity. They are prepared not under exclusion of oxygen. In the molten metal up to 0.09% can solve (m / m) oxygen and it is used to this, small amounts of copper (I ) oxide ( Cu2O ).
When heated above 500 ° C, for example at the autogenous welding or brazing, the hydrogen diffuses in the gas burner, the metal surface and reduces the copper ( I) oxide according to the reaction:
The water vapor breaks the structure, because the copper ( I) oxide is present as a thin network of Cu - Cu2O eutectic at the grain boundaries. This phenomenon is also called hydrogen disease.