Hydrogen Induced Cracking (HIC) or hydrogen embrittlement is a brittle mechanical
fracture caused by penetration and diffusion of atomic hydrogen into the crystal
structure of an alloy. It occurs in corrosive environment under constant tensile
stress, similar to stress corrosion cracking (SCC)
; however, cathodic protection initiates or enhances HIC but suppresses or stops
SCC. The cracks are usually non-branching and fast growing, and are more often transgranular
(through the grains) rather than intergranular (through the grain boundaries).
HIC occurs in high strength steels when atomic hydrogen dissolves in the crystal
lattice of the metal rather than forming H2 gas. In the oilfield, the
presence of H2S gas often leads to sulfide stress
cracking (SSC), which is a special case of hydrogen induced stress cracking.
Hydrogen may enter a metal surface by the cathodic reduction of hydrogen or water:
Normally, the adsorbed hydrogen at the surface recombines to form hydrogen gas:
However, recombination poisons such as sulfide (S2-), prevent hydrogen
gas from forming and the adsorbed hydrogen moves through the metal, thereby weakening
it. Hydrogen sulfide (H2S) is especially aggressive in promoting hydrogen
damage because it provides not only the sulfide poison, but hydrogen ions (H+
) as well.
- 2H+ + 2e- → 2HAdsorbed (acidic waters)
- 2H2O + 2e- → 2HAdsorbed + 2OH-
Sulfide stress cracking (SSC) occurs in high-strength drill pipes, casing, tubing,
and sucker rods. Like stress corrosion cracking (SCC), cracking may not occur below
a threshold stress, however, increasing strength and applied stress, increasing
H2S concentrations and increasing acidity (decreasing pH) increase SSC susceptibility.
As opposed to SCC, decreasing temperature also increases SSC susceptibility. Time
to failure is minimum at room temperature. The ramification is that, steels become
most susceptible to SSC near the surface where the highest strength is required
to carry the weight of the string. Increasing the wall thickness of the tubular
can reduce the applied stress thus allowing the use of lower strength steels, but
strength must be balanced against the applied load at the top of the joint due to
increasing weight. High strength casing may be used deeper in the well where temperatures
In SCC, failure initiates at the crevices on the metal surface, usually in the pits.
Thus, SCC susceptibility of steels is related to its susceptibility to pitting.
Whereas SSC generally initiates at impurity inclusions in the metal, hence it is
dependent on the hydrogen absorption characteristics of the metal.
Microstructure of steel also influences the SSC susceptibility. Quenched and tempered
steels have better SSC resistance than normalized and normalized and tempered steels.
Acceptable hardness limits for many alloys in sour service are described in the
National Association of Corrosion Engineers (NACE) Specification MR-01-75. For SSC
resistance, the hardness of carbon and low alloy steels must be maintained below
22 Rockwell Hardness C (HRC). Tubulars based on AISI 4100 series low-alloy steels
are acceptable up to HRC 26. Higher alloyed steels may have higher hardness levels.
A special case of hydrogen damage is known as hydrogen blistering. Hydrogen blistering
occurs when hydrogen atoms diffuse into the steel, and hydrogen gas nucleates at
internal defects and inclusions, forming voids which eventually generate enough
pressure to locally rupture the metal.
Hydrogen blistering is occasionally observed in the oilfield in sour systems.