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Hydrogen is ubiquitous in the earth’s crust and atmosphere and, as such, it is important to protect metals susceptible to hydrogen damage from exposure if they are being operated in such environments. There are four basic types of hydrogen damage, and these are solid solution hardening, internal defect generation, embrittlement due to hydride, and embrittlement due to hydrogen.
Hydrogen can diffuse into metals and alloys from several sources during processing and subsequent service. These sources include the dissociation of moisture during casting and welding, thermal decomposition of gases, and pickling and plating operations. Hydrogen can also be generated from cathodic reactions during corrosion in service and from cathodic protection measures by sacrificial anodes and impressed current.
Ferritic and Martensitic Steels
The effects of hydrogen are well known in ferritic and martensitic steels, where it can diffuse to suitable sites in the microstructure and develop local internal pressure resulting in the characteristic form of hydrogen embrittlement.
Low Carbon Steels
In low carbon steels, which have inherent ductility, hydrogen may not give rise to cracking but will cause blisters to develop at inclusions. This can lead to delamination in-plate due to the directional nature of the inclusions.
Hydrogen Sulphide Environments
Steels for sour gas service, where the environment contains wet hydrogen sulfide, must have very low sulfur levels or have been treated with additions to control the shape of the inclusions during deoxidation to minimize the danger of hydrogen embrittlement and blistering.
Failure is time-dependent and occurs at low rates of strain as the load-bearing cross-section is reduced during slow crack growth in the embrittled region. Susceptibility for embrittlement is higher in alloys with higher yield strengths, i.e. those that are cold-worked, age-hardened or in their martensitic form. The sites at which hydrogen is trapped include the original austenite grain boundaries and the interfaces between the matrix and non-metallic inclusions, for example, manganese sulphides. These then result in both intergranular cracking (with separation at the prior austenite boundaries) and transgranular cracking (flaking or quasi-cleavage) which is associated with the inclusions.
Hydrogen can assist in the propagation of corrosion fatigue cracks and can also cause sulphide stress corrosion cracking in ferritic and martensitic steels, including the stainless grades.
Detecting Hydrogen Damage
Detecting hydrogen damage in components is important for monitoring the state of any equipment made of a metal or alloy that might be susceptible to hydrogen damage. The following methods can be used to adequately quantify and measure hydrogen damage:
- Ultrasonic Echo Attenuation
- Amplitude-based backscatter
- Velocity ratio (of shear-to-longitudinal wave velocity)
- Creeping wave velocity
- Advanced Ultrasonic Backscatter Techniques (AUBT)
- Pitch-catch mode shear wave velocity
- Time-of-flight diffraction (TOFD)
- In-situ metallography
Addressing Hydrogen Damage
The first and foremost method for preventing hydrogen damage is the obvious option of preventing direct contact between a metal and the hydrogen-containing agent. Controlling the environment during operations such as casting and melting will allow for the exposure to hydrogen to be moderated.
Other than preventing exposure, it is also possible to give the metal or alloy a metallurgical treatment, which would serve to reduce the susceptibility of the material to damage caused by hydrogen, chemical means or otherwise.
Sources and Further Reading
This article was updated on 25th June, 2019.