Hydrogen embrittlement occurs when metals become brittle, and eventually may fracture, because of the introduction and diffusion of hydrogen into the metal. Generally, this happens by accident and must be planned against for critical applications.
Microscopic image of hydrogen embrittlement that caused an oil pipeline to burst near Leipzig, Germany, in 1993. via martin-moesser.de
Hydrogen embrittlement occurs when metals become brittle, and eventually may fracture, because of the introduction and diffusion of hydrogen into the metal. Generally, this happens by accident and must be planned against for critical applications.
How Does it Happen?
Hydrogen embrittlement is produced through a simple process in which the hydrogen is first introduced to the surface of the metal, and then is allowed to diffuse into the metal’s internal structure. This process tends to increase at high temperatures, because the solubility of hydrogen increases with increasing temperatures. Once the hydrogen atoms have diffused into the metal, they recombine to form molecular hydrogen gas, which collect in voids and cause stress in the material. Once enough hydrogen has collected in the metal, the material can experience reduced ductility and tensile strength, eventually forming cracks in a related process known as hydrogen induced cracking (HIC).
Generally, high strength steel is the most susceptible to hydrogen embrittlement and processes that can cause more hydrogen embrittlement include cathodic protection, electroplating, and arc welding. However, arc welding is a special case because the hydrogen in that case is released from moisture typically contained in the electrode coatings. Specialty low-hydrogen electrodes are used for applications in which hydrogen embrittlement must be minimized.
Hydrogen Embrittlement Prevention
There are several ways to prevent hydrogen embrittlement and all of them work by minimizing hydrogen’s ability to contact the metal surface. Fabrication methods to avoid include embrittling procedures like acid pickling as well as contact with sulfur or phosphate. Additionally, hydrogen embrittlement can be reversed in materials that have not begun to experience hydrogen induced cracking. The process, known as baking, consists of removing the source of hydrogen, which causes the hydrogen in the material to diffuse out under heat treatment.
In related processes, exposure to hydrogen can lead to the production of different gases within a material, causing stress and eventual failure. For example, when steel is exposed to hydrogen under high enough temperatures, the hydrogen atoms will decarburize the steel and form methane gas in the material, which then gathers in voids and at grain boundaries. This process generally generates a higher pressure, and therefore a higher stress, within the metal. Alternatively, steam embrittlement occurs when copper alloys containing oxygen are exposed to hydrogen at high temperatures, the hydrogen reacts to form steam which collects at high pressures along the grain boundaries. Under such high pressures, the grain boundaries are wrenched apart, causing the bulk material to fail rapidly.
Hydrogen embrittlement and its related embrittlement mechanisms are difficult to detect during routine inspections, therefore it is imperative that they be planned for during the manufacturing process especially when using highly susceptible alloys. It is fortuitous that hydrogen embrittlement prevention methods have been in use for decades, and are not too technically difficult to execute.
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