What Does
Hydrogen Blistering (HB) Mean?
Hydrogen blistering is the growth of subsurface cavities in metal, bulged like blisters, as a result of significant internal pressure increase due to hydrogen molecular gas formation close to the surface.
Corrosionpedia Explains Hydrogen Blistering (HB)
Hydrogen blistering occurs because of the diffusion of atomic hydrogen inside metal. Atomic hydrogen is formed by the hydrogen ions accepting electrons from a cathodic surface or environment. Atomic hydrogen so formed is the only chemical form of hydrogen capable of entering steel’s microstructure by diffusion penetration, as hydrogen gas in molecular form is not able to be diffused. Hydrogen damage to metals and alloys due to blistering as well and hydrogen embrittlement deep within, are only produced by initial atomic hydrogen diffusion.
When two hydrogen atoms combine to form a molecule of hydrogen gas outside the metal surface, the risk of hydrogen diffusion is reduced. However, sometimes these hydrogen atoms do not readily combine to create a hydrogen gas molecule, and instead, they diffuse directly into the metal surface. Certain impurities inside the metal like sulfides, poisonous cyanides and arsenic chemicals help hydrogen to readily diffuse into steel. It is this diffusion that can force hydrogen damage of metals such as blistering and cracking.
Hydrogen blistering results in considerable degradation of crucial mechanical strength and other properties of materials, such as alloys, due to diffused hydrogen. The metal and alloys also lose ductility as well as load capacity.
Hydrogen atoms combine chemically to form hydrogen gas molecules in metallic structures. As these gas molecules are formed inside the metal structure, an increase in pressure occurs. When the high pressure is located close to surface, blistering reduces this pressure. When it occurs away from the surface, the metal will develop cracks. It can lead to eventual fatigue failure.
Steel with hydrogen damage, like blisters and cracks, can fail when subjected to a sustained low-level load, or a cyclical load. While a steel specimen may pass initial tests, it may fail after some time, even at low stress levels. Since this damage can’t be reversed by annealing process, the loss of mechanical strength is permanent. This is also called hydrogen stress failure.