Hydrogen Damage

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:

2H⁺ + 2e^- → 2H_Adsorbed (acidic waters)
2H₂O + 2e^- → 2H_Adsorbed + 2OH^- (neutral waters)

Normally, the adsorbed hydrogen at the surface recombines to form hydrogen gas:

2H_Adsorbed → H₂

However, recombination poisons such as sulfide (S^2-), 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.

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 are higher.

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.