|4. Forms of Corrosion|
4.7 Hydrogen Blistering [3/3]
Mechanism of Hydrogen Blistering
Atomic hydrogen is the only species capable of penetrating steel, the molecular form of hydrogen is not capable of diffusion. Hydrogen blistering and hydrogen embrittlement are only produced by atomic hydrogen.
Diffusion of atomic hydrogen
The hydrogen ion H+ accepts electrons from the cathode and becomes hydrogen atom
2H+ + 2e H2
Sometimes hydrogen atoms do not combine together to form hydrogen gas, instead, they diffuse straight away in a a metal surface. Certain impurities (poisons) like sulfides, cyanides and arsenic compounds prevent the formation of molecular hydrogen and help hydrogen to diffuse into steel. It is the diffusion of hydrogen atoms that can can induce hydrogen damage such as blistering and hydrogen induced cracking of steel.
Figure below shows a case of high temperature hydrogen attack on a cooling tower heat exchanger tube.
The photographs below shows a specimen from a downhole tubing showing blistering of epoxy coating.
In summary, hydrogen attack occurs in these steps
Formation of hydrogen ions (H+)
Reduction of hydrogen ions to H atoms
Combination of hydrogen atoms to form hydrogen molecule
Escape of hydrogen molecules from the surface (H2)
Diffusion of some H atoms to inside of the surface
Accumulation of hydrogen atoms in voids to form molecular hydrogen
Exertion of significant pressure on the walls of the tank
Blistering followed by rupture depending on the steel in use
Use steels metallurgically modified to minimize voids and inclusions to prevent hydrogen gas formation
Remove poisons like arsenic and cyanide compounds and phosphorous containing ions. The poisons retard the formation of hydrogen molecules.
Use coatings impervious to hydrogen diffusion.
Use nickel based alloys because of their low hydrogen diffusion rates.
Use inhibitors in closed systems because they prevent hydrogen attacks.