Hydrogen embrittlement

Hydrogen embrittlement (or hydrogen grooving) is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen. Hydrogen cracking can pose an engineering problem especially in the context of a hydrogen economy. However, commercially workable and safe technology exists globally in the hydrogen industry, which produces some 50 million metric tons per year.

Hydrogen embrittlement is also used to describe the formation of zircaloy hydride. This use of the term in this context is common in the nuclear industry.

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Process

The mechanism begins with hydrogen atoms diffusing through the metal. When these hydrogen atoms re-combine in minuscule voids of the metal matrix to hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength, up to where it can crack open, in which case it would be called Hydrogen Induced Cracking (HIC). High-strength and low-alloy steels, aluminium, and titanium alloys are most susceptible.

Hydrogen embrittlement can happen during various manufacturing operations or operational use, anywhere where the metal comes in contact with atomic or molecular hydrogen. Processes which can lead to this include cathodic protection, phosphating, pickling, and electroplating. A special case is arc welding, in which the hydrogen is released from moisture (for example in the coating of the welding electrodes; to minimize this, special low-hydrogen electrodes are used for welding high-strength steels). Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals (notably hydrogen sulfide in sulphide stress cracking, or SSC, a process of importance for the oil and gas industries).

Counteractions

If the metal has not yet started to crack, the condition can be reversed by removing the hydrogen source and causing the hydrogen within the metal to diffuse out - possibly at elevated temperatures. Susceptible alloys, after chemical or electrochemical treatments where hydrogen is produced, are often subjected to heat treatment in order to remove absorbed hydrogen.

In the case of welding, often pre- and post-heating the metal is applied to allow the hydrogen to diffuse out before it can cause any damage. This is specifically done with high-strength steels and low alloy steels such as the chrome/molybdenum/vanadium alloys. Due to the time needed to re-combine hydrogen atoms to the harmful hydrogen molecules, hydrogen cracking due to welding can occur over 24 hours after the welding operation is completed.

Hydrogen attack

If steel is exposed to hydrogen at high temperatures, hydrogen will diffuse into the alloy and combine with carbon to form tiny pockets of methane at internal surfaces like grain boundaries and voids. This methane does not diffuse out of the metal, and collects in the voids at high pressure and initiates cracks in the steel. This process is known as hydrogen attack and leads to decarburization of the steel and loss of strength.

Test

There is an ASTM standard for testing of embrittlement due to Hydrogen Gas [1]- F1459-06 Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE). Another ASTM standard exists for quantitatively testing for the Hydrogen Embrittlement threshold stress for the onset of Hydrogen-Induced Cracking due to platings and coatings from Internal Hydrogen Embrittlement (IHE) and Environmental Hydrogen Embrittlement (EHE) [2] - F1624-06 Standard Test Method for Meaurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique. References: ASTM STP 543,"Hydrogen Embrittlement Testing" [3]and ASTM STP 962,"Hydrogen Embrittlement: Prevention and Control." [4]

• NACE TM0284-2003 (NACE International) Resistance to Hydrogen-Induced Cracking
• ISO 11114-4:2005 (ISO)Test methods for selecting metallic materials resistant to hydrogen embrittlement [5].
• ASTM F1940-07a- Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners
• ASTM F519-06e2-Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments

See also

• Hydrogen damage
• Hydrogen piping
• Nascent hydrogen