Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

What is Hydrogen Embrittlement? - Causes, Effects and Prevention

   

Hydrogen Embrittlement occurs when metals become brittle as a result of the introduction and diffusion of hydrogen into the material. The degree of embrittlement is influenced both by the amount of hydrogen absorbed and the microstructure of the material. Microstructures which bestow high strength, often monitored by hardness level, or having specific distributions of grain boundary particles or inclusions, can result in increased susceptibility to embrittlement. The phenomenon usually becomes significant when it leads to cracking. This happens when sufficient stress is applied to a hydrogen-embrittled object. Such stress states can be caused both by the presence of residual stresses, associated fabrication operations such as forming and welding, and applied service stresses. The severity of hydrogen embrittlement is a function of temperature: most metals are relatively immune to hydrogen embrittlement, above approximately 150°C.

Hydrogen is normally only able to enter metals in the form of atoms or hydrogen ions. Thus, gaseous hydrogen is not absorbed by metals at ambient temperatures, as it is in molecular form, in which pairs of atoms are tightly bound together. However, as the temperature rises, the molecules tend to dissociate into individual atoms allowing absorption at temperatures which, for example, are associated with petroleum refining or heat treatment procedures. Higher rates of absorption are experienced in molten material and this means that casting and welding operations can provide particular opportunities for the entry of hydrogen into metallic materials. Hydrogen ions are also produced by reactions associated with processes such as corrosion, electroplating and cathodic protection. Consequently, there is ample opportunity for the entry of hydrogen into metallic components.

Cracking associated with hydrogen embrittlement has been given a variety of names depending on the situations in which it occurs. Commonly used terms include:

Cold Cracking and Delayed Cracking

These terms are often associated with hydrogen cracks that can form during cooling of the weld metal and workpieces after welding of steels.

Hydrogen-induced Cracking (HIC) or Hydrogen Pressure-induced Cracking (HPIC)

Apart from its general meaning, this is a jargon term referring to a specific morphology of cracking occurring in steel pipelines and vessels which absorb hydrogen during service.

Hydrogen-induced Stress Cracking (HISC)

This expression was originally applied to service cracking experienced in duplex stainless steels but is now used more widely.

Environmentally-assisted Cracking (EAC)

Refers to cracking that can occur due to interaction between the component and the surrounding service environment. Hydrogen is only one of the agents which can be responsible for this type of cracking.

Disbonding

Apart from its general meaning, its jargon use relates to the spalling of internal, weld-deposited cladding in vessels used for processing with high temperature hydrogenous gases.

Stress Corrosion Cracking (SCC)

Some specific mechanisms of this phenomenon are related to interaction with hydrogen.

Sulphide Stress Cracking (SSC)

Corrosion in environments containing hydrogen sulphide can cause hydrogen absorption and cracking.

The specific crystal structure of metals is important, as it affects the rate at which hydrogen can diffuse and deformation mechanisms. On this basis, ferritic steel has been considered more susceptible to hydrogen embrittlement than alloys with different crystal structures, such as austenitic stainless steels, nickel alloys and aluminium alloys. However, it is apparent that hydrogen can embrittle most engineering alloys, to some extent. Whether this is a practical problem depends on how the application affects microstructure and the availability of hydrogen.

When it does occur, hydrogen embrittlement can cause reduced ductility and a lessening of load-bearing capacity, which can lead to cracking and brittle failures, below the anticipated proof or yield strength of the susceptible materials.

How to Prevent Hydrogen Embrittlement

Hydrogen embrittlement can be prevented by minimising contact between the metal and any sources of atomic hydrogen. For example, for service in gaseous hydrogen, carbon steel can be restricted to temperatures below approximately 200°C. In potentially corrosive service, environmental conditions should be controlled so that hydrogen ions are not generated by reactions on the metal surface. In practical terms, this means that the metal should either not be subjected to conditions causing corrosion or be protected from such environments, e.g. by the application of coatings. The electrochemical conditions of processes involving acid pickling, or those conferring cathodic protection, should be controlled so that hydrogen is not liberated at the component surface. For heat treatment in furnace atmospheres containing hydrogen, hydrogen can be allowed to escape, before low temperatures are reached. During welding operations, depending on the welding process employed, some absorption of hydrogen might be inevitable. Thus, careful control of welding conditions for hardenable steels can be necessary, in order to limit hydrogen absorption, avoid the formation of excessively hard microstructures or to allow hydrogen to escape before critical low temperatures are experienced by the workpiece.

If significant levels of hydrogen are likely to be absorbed during a particular processing operation, embrittlement problems can be avoided by using a thermal exposure, sometimes known as a ‘baking’ procedure, which allows hydrogen to escape before exposure to critically low temperatures. In some types of equipment, shutdown procedures are employed in which cooling rates are controlled to enable hydrogen levels to reach suitably low values, before low temperatures are reached.

Another method for preventing the problem of embrittlement is through materials selection, i.e. using materials that are less vulnerable to hydrogen embrittlement. For example, the ISO 15156 Standard prescribes hardness limits for materials which will not be subject to SSC in hydrogen sulphide environments.

Hydrogen Embrittlement Testing at TWI

TWI has many years of experience assisting industry with resisting the effects of hydrogen on materials. Extensive research work has defined welding procedures to prevent hydrogen cracking in steel weldments. The emphasis in hydrogen-related work has now shifted to include ways in which environmental exposures create embrittlement and cracking due to hydrogen. Laboratory facilities address issues relating to: sour service in oil and gas production; performance in other corrosive environments; effects of cathodic protection on the behaviour of subsea dissimilar joints and the influences of both high pressure and high temperature hydrogen on materials which will provide equipment for the ‘hydrogen economy’.

For more information please email:


contactus@twi.co.uk