The presence of hydrogen at high temperature and pressure can cause embrittlement of carbon steel and alloy steels. Atomic hydrogen is able to permeate into steel and reacts with iron carbides to form methane. The methane accumulates at the grain boundaries and cannot diffuse through the metal resulting in high internal stresses that eventually lead to cracking of the metal at the grain boundary.
Hydrogen attack can be prevented by adding carbide stabilizing elements such as molybdenum, chromium, tungsten, vanadium, titanium and columbium to the steel. Note that addition of non carbide forming elements such as nickel or silicon does not influence the hdrogen attack resistance of steels. 18Cr-8Ni steels are resistant to HIC due to presence of high chromium content.
Another common phenomenon experienced in the oil and gas industry is hydrogen blistering. In this type of corrosion mechanism, atomic hydrogen diffuses into the surface of carbon steel. The atomic hydrogen then collects in the discontinuities of the metal where atomic hydrogen combines to form molecular hydrogen. Once molecular hydrogen is formed it cannot diffuse through the metal. Eventually enough pressure builds up in the void area to cause cracking of the metal.
Hydrogen Induced Cracking (HIC)
When corrosion of steel occurs in an acid solution (e.g. carbonic acid) containing H2S, iron sulphides and hydrogen are formed. The hydrogen is first produced as hydrogen atoms (H) which normally combine rapidly to form hydrogen molecules (H2). The latter is hydrogen gas that escapes to the immediate environment.
The combination of hydrogen atoms at a surface is hindered if an active sulphide film is present. This prolongs the residence time of the hydrogen atoms on the surface and gives opportunity for them to dissolve in the metal. The hydrogen will then diffuse through the metal until it reaches a second surface. This might be the other side of the plate, in which case the atoms will combine. Alternatively, the atom may encounter other surfaces within the plate, e.g. laminations or gross inclusions. These features will 'trap' the hydrogen, there will be no active sulphide film present to prevent combination to form hydrogen gas, and a gas pocket will develop within the metal. The inevitable pressure build-up will cause development of separations at the flaw. The pressure will be relieved by plastic deformation and blistering in low strength steels (up to 550 MPa yield). In high strength steels cracking propagates in a step-wise manner in the through thickness direction.
The critical partial pressure of H2S for hydrogen induced cracking to occur is about 0.1 psi although pipeline damage has been reported at H2S partial pressure as low as 0.05 psia. The susceptibility of carbon steel to HIC can be mitigated by controlling the chemical composition in particular reducing the concentration of manganese sulphide inclusions in the steel. A reduction in inclusions is achieved by lowering the sulphur content of the steel to below 0.002%. Addition of trace elements such as calcium, to the steel to give a residual Ca/S ratio in the range 2 - 4 provides shape control of the manganese sulphide inclusions. Shape control reduces the propensity to HIC because the calcium reacts with the manganese sulphide to form an intermetallic sulphide that is not deformed during the forming of the pipe. Therefore, the spherical shape of the inclusions is retained and they are not rolled into platelets where atomic hydrogen can accumulate forming hydrogen gas leading to cracking problems.
Material degradation due to HlC does not appear to have a lower bound in applied stress with the result that this form of cracking damage can be observed in normally resistant, fine grained ASTM A-516 Grade 60 or 70 and other similar steels. The cracking damage in these steels is very often time dependent and is related to the flux of atomic hydrogen generated by the corrosion reactions at the metallic surfaces. Combining action of atomic hydrogen into hydrogen gas is a cumulative process and builds up of hydrogen gas pressure eventually leads to hydrogen induced cracking (HlC). Steel pipelines and pressurised equipment can operate satisfactorily for up to 8 to 10 years before any cracks are discovered. Hydrogen induced cracking failures of sour gas processing equipment and transmission pipelines have been reported at facilities in Libya, Saudi Arabia and other Middle-Eastern plants.
Stress Oriented Hydrogen Induced Cracking (SOHIC)
Stress orientated hydrogen induced cracking occurs in the area of grain enlargement in weld heat affected zones. SOHlC appears to be related to segregation of micro-alloying elements in the banded steel microstructure. The combination of applied and high residual stress results in work hardening of the stronger bands resulting in SSC.
For stainless steels and non-ferrous materials, resistance against hydrogen embrittlement shall be controlled by specifying that the actual hardness of the material shall be in accordance with NACE MR0175