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Hydrogen Blistering

The schematic diagram below shows hydrogen diffusion, say, in a tank containingan acid electrolyte. Hydrogen evolution occurs because of a corrosion reaction orcathodic protection. Most of the hydrogen atoms combine to form hydrogenmolecule at the external surface.

Ha + H c 2H o H2

What is hydrogen blisteering?Hydrogen Blistering (HB) refers to the formation of subsurface planar cavities, called hydrogen blisters, in a metalresulting from excessive internal hydrogen pressure. Growth of near-surface blisters in low-strength metals usuallyresults in surface bulges.

Mechanisms

What causes hydrogen blistering? Hydrogen ions are reduced t ohydrogen atoms that adsorb on the steel surface.Some of the hydrogen atoms will diffuse through the steel and accumulate at hydrogen traps, typically voids aroundinclusions.

When hydrogen atoms meet in a trap andcombine, they form hydrogen gas (H 2)molecules in the trap. The accumulationhydrogen gas inside the extremely small cavitywill lead to the buildup of excessiveinternal hydrogen pressure. At certain times,this internal hydrogen pressure will becomesufficient to cause the steel to blister.

Blisters occur usually in low strength steels(

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induced crack, HIC) and usually H 2S.Promoted by cyanides

corrosion:

H+ + e H

surface. Cracks parallelto the surface, when

defects are deep.Use of adequatematerials (anti-HICsteel).

Chemical treatment ofcorrosive medium withwater, polysulfides orinhibitors

.

Hydrogenembrittlement

High strength steel inenvironments same asabove

Same as aboveSevere loss of ductilityat low strain rates and

delayed fracture

Same as hydrogen blistering

Nearly atmospherictemperature by rapidcooling of highstrength equipmentoperating at conditionssame as in hydrogenattack.

Same ashydrogen attack

Severe loss of ductility

at low strain rates andhydrogen assisted crackgrowth

Use of adequate

materials.Hydrogen degassing.

In summary, hydrogen attack occurs in these stepsFormation of hydrogen ions (H +)Reduction of hydrogen ions to H atomsCombination of hydrogen atoms to form hydrogen moleculeEscape of hydrogen molecules from the surface (H 2)Diffusion of some H atoms to inside of the surfaceAccumulation of hydrogen atoms in voids to form molecular hydrogenExertion of significant pressure on the walls of the tankBlistering followed by rupture depending on the steel in use

PreventionUse steels metallurgically modified to minimize voids and inclusions to preventhydrogen gas formationRemove poisons like arsenic and cyanide compounds and phosphorous containingions. 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.