Hydrogen Attack in ammonia plants
In several steps of the ammonia production process, especially in the synthesis section, the pressure shells of reaction vessels as well as the connecting pipes are in contact with hydrogen at elevated pressure and temperature with a potential risk of material deterioration. One the possible risks is Chemical and physical hydrogen attack that can be summarized in the below points:
Chemical Hydrogen Attack.
- Under certain conditions chemical hydrogen attack can occur. Hydrogen diffuses into the steel and reacts with the carbon that is responsible for the strength of the material to form methane, which on account of its higher molecular volume cannot escape.
- The resulting pressure causes cavity growth along the grain boundaries, transforming the steel from a ductile to a brittle state.
- This may finally reach a point where the affected vessel or pipe ruptures, in most cases without any significant prior deformation. This phenomenon was already recognized and principally understood by BOSCH when they developed the first ammonia process.
- The resistance of steel against this sort of attack can be enhanced by alloy components which react with the carbon to form stable carbides (e.g., molybdenum, chromium, tungsten, and others).
- The rate of deterioration of the material depends on the pressure of the trapped methane, the creep rate of the material, and its grain structure.
- Areas highly susceptible to attack are those which have the greatest probability of containing unstable carbides, such as welding seams.
- The type of carbides and their activity are strongly influenced by the quality of post-weld heat treatment (PWHT).
- The risk of attack may exist at quite moderate temperatures (ca. 200 “C) and a hydrogen partial pressure as low as 7 bar.
- Numerous studies, experiments and careful investigations of failures have made it possible to largely prevent hydrogen attack in modern ammonia plants by proper selection of hydrogen-tolerant alloys with the appropriate content of metals that form stable alloys.
- Of great importance in this field was the work of NELSON, who summarized available experimental and operational experience in graphical form.
- These Nelson diagrams give the stability limits for various steels as a function of temperature and hydrogen partial pressure.
- Newer experience gained in industrial applications required several revisions of the original Nelson diagram. For example, 0.25 and 0.5 Mo steels are now regarded as ordinary nonalloyed steels with respect to their hydrogen resistance.
Physical Hydrogen Attack.
- A related phenomenon is physical hydrogen attack, which may happen simultaneously with chemical attack. It occurs when adsorbed molecular hydrogen dissociates at higher temperatures into atomic hydrogen. Which can diffuse through the material structure. Wherever hydrogen atoms recombine to molecules in the material structure (at second-phase particles or material defects such as dislocations) internal stress becomes established within the material.
- The result is a progressive deterioration of the material that lowers its toughness until the affected piece of equipment cracks and ultimately ruptures.
- The phenomenon is also referred to as hydrogen embrittlement.
- It is most likely to occur in welds that not received proper PWHT.
- Holding a weld and the heat-affected zone for a prolonged period at elevated temperature (an operation known as soaking) allows the majority of included hydrogen to diffuse out of the material.
- Rut this may not be sufficient if moisture was present during the original welding operation (for example if wet electrodes or hygroscopic fluxes were used), because traces of atomic hydrogen are formed by thermal decomposition of water under the intense heat of the welding procedure.
- Highly critical in this respect are dissimilar welds, such as those between ferritic and austenitic steels 112.321, where the formation of martensite, which is sensitive to hydrogen attack, may increase the risk of brittle fracture.+
- At higher temperature and partial pressure, hydrogen is always soluble to a minor extent in construction steels.
- For this reason it is advisable not to cool vessels too rapidly when taking them out of service, and to hold them at atmospheric pressure for some hours at 300 “C so that the hydrogen can largely diffuse out (soaking).
- In contrast to the hydrogen attack described above this phenomenon is reversible.