Hydrogen embrittlement

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Hydrogen induced cracking ( English hydrogen-induced cracks , HIC )

Under the hydrogen embrittlement is meant the change in brittleness caused by the penetration and incorporation of hydrogen into a metal mesh is caused. This consequence of corrosion is similar to material fatigue - the result is hydrogen-related cracking, which in particular limits the use of susceptible materials for hydrogen storage .

effect

Hydrogen embrittlement occurs when atomic hydrogen is produced on the metal surface - either through hydrogen corrosion or in another chemical reaction in metal processing in which hydrogen is involved (e.g. pickling ) - which diffuses into the material faster than it combines on the material surface to form non-diffusible H 2 molecules. Some of the hydrogen is stored in the metal lattice and, as in the case of titanium , a metal hydride can be formed. In other cases, the hydrogen is preferentially deposited on defects or grain boundaries. In both cases the result is an embrittlement of the metal.

Scanning electron microscope image of hydrogen-related damage to an ultra-high-strength steel material with the material number 1.4614, with brittle and ductile fractures

If the tensile and / or load stresses are sufficiently high, there is a risk of delayed brittle fracture . One speaks of a delayed brittle fracture because the damage takes time and the material breaks with almost no deformation due to the sliding blockages. This effect is similar to stress corrosion cracking and limits the use of metals for hydrogen storage .

process

Atomic hydrogen formed by certain chemical reactions penetrates the structure of metallic materials, where it recombines to molecular hydrogen at lattice defects and remains there. The associated increase in pressure leads to internal stresses and embrittlement of the material without increasing the strength. Ultimately, the end result is cracks that spread from the inside out. In stress corrosion cracking, the hydrogen produced during the corrosion process diffuses to the crack tip and accelerates the crack speed there.

Hydrogen embrittlement in steel

Steel and titanium are often affected by embrittlement if they have been in contact with hydrogen for a long time. In terms of steels, however, austenitic steels (e.g. CrNi steels) are an exception. These are largely insensitive to hydrogen embrittlement and are among the standard materials in hydrogen technology. High-strength steels with a high martensite content and a yield point greater than approx. 800 MPa (including, for example, screws from strength class 10.9 and higher) are particularly at risk from hydrogen-related damage.

Possible causes for hydrogen-related damage can be both

  • production-related, d. H. for example through the generation of hydrogen during galvanic deposition (e.g. galvanizing or in pickling processes), as well as during welding ,
  • or operational, d. H. for example by hydrogen corrosion ,

be. During galvanic deposition, hydrogen is formed on the cathodically connected steel and diffuses into the steel. In the event of corrosion, metal dissolution, especially in the case of a lack of oxygen, can form elemental hydrogen .

In order for components to release the hydrogen again, a heat treatment of several hours at approx. 200-300 ° C ( low hydrogen annealing , also called tempering or tempering ) must be carried out immediately after being exposed to hydrogen . Since hydrogen has a high diffusion speed even at low temperatures, it is possible to drive the hydrogen out of the steel at temperatures of up to 200 ° C without metallurgical changes. Common test standards are DIN 50969-1 and -2 for safeguarding production processes against production-related hydrogen embrittlement, as well as DIN 50969-3 for safeguarding against subsequent, operational influencing variables.

Hydrogen disease (hydrogen embrittlement) in copper

Hydrogen disease is the chemical reaction of oxygen bound as copper (I) oxide in types of copper containing oxygen, such as CuETP, to form copper and water. Hydrogen disease is often mistakenly referred to as hydrogen embrittlement. The two mechanisms differ from one another. In the case of copper, oxygen must be present in the material in the form of copper (I) oxide in order to react with atomic hydrogen, which diffuses noticeably into the copper material from around 500 degrees Celsius. If no oxygen is available, as is the case with the oxygen-free types CuOF, CuOFE and others, or if this is bound by the addition of phosphorus, as is the case with CuPHC, for example, hydrogen disease cannot occur. As a further requirement, the hydrogen must be in atomic form and not as a gas, so it must be reduced. In the case of oxygen-containing copper types such as CuETP with up to 400 ppm oxygen, the hydrogen disease can lead to cracks and cavities. These types of copper are mainly used in electrical engineering due to their high electrical conductivity . They are not produced under the exclusion of oxygen . Up to 0.09% (m / m) oxygen can dissolve in the molten metal and small amounts of copper (I) oxide (Cu 2 O) are formed.

When heated above 500 ° C, for example during autogenous welding or soldering with a reducing flame, the hydrogen of the burner gas, which has been reduced to atomic hydrogen, diffuses into the metal surface and reduces the copper (I) oxide according to the following reaction:

The water vapor bursts the structure because the copper (I) oxide in cast materials is a thin network of Cu-Cu 2 O eutectic on the grain boundaries . This phenomenon is called hydrogen sickness.

See also

literature

Individual evidence

  1. ↑ The materials experts in Friedrichshafen. In: Material testing, advice and services. STZ Werkstoffe Corrosion and Corrosion Protection, accessed on December 19, 2017 .
  2. ↑ Avoidance of production-related hydrogen-induced brittle fractures in high-strength steel components - Part 1: Preventive measures. Retrieved December 19, 2017 .
  3. ↑ Avoidance of production-related hydrogen-induced brittle fractures in high-strength steel components - Part 2: Tests. Retrieved December 19, 2017 .
  4. ↑ Avoidance of production-related hydrogen-induced brittle fractures in high-strength steel components - Part 3: Subsequent operational influences and extended tests. Beuth Verlag, accessed December 19, 2017 .
  5. ↑ Oxygen solubility in copper melts
  6. ↑ The use of acetylene / oxygen burners is prohibited because of the formation of explosive copper (I) acetylides.