Hydrogen Embrittlement and Environmental Cracking

Molecular hydrogen (H2) does not penetrate steel readily at ambient conditions. The danger is atomic hydrogen (H), which is produced by corrosion reactions, electrochemical processes, and chemical environments at the metal surface. Nascent atomic hydrogen is small enough to diffuse rapidly through the body-centred cubic iron lattice, travelling to grain boundaries, inclusions, and crack tips where it causes damage.

• Acid pickling: the standard industrial process for removing mill scale from steel before coating. Every mole of iron dissolved generates two moles of atomic hydrogen at the surface. Insufficient baking after pickling leaves hydrogen trapped in the steel.
• Electroplating: chromium, zinc, and cadmium plating all evolve hydrogen at the cathode (the workpiece). High-strength fasteners plated without post-plate baking are a known failure population.
• Welding: moisture in electrode coatings or on the base metal dissociates in the arc and introduces hydrogen into the weld metal and heat-affected zone. Delayed cold cracking (also called hydrogen cold cracking) can appear hours to days after welding.
• Cathodic protection: at moderate negative potentials the cathodic reaction is oxygen reduction (harmless). At overprotected potentials, hydrogen evolution dominates, generating atomic hydrogen at the surface of the protected structure.
• Sour service: wet H2S environments. The bisulfide anion adsorbs on steel, inhibits recombination of atomic hydrogen into H2 gas, and forces more hydrogen to enter the metal rather than evolve harmlessly.

Once inside, hydrogen diffuses preferentially to regions of high triaxial tensile stress, the stress state found at the tip of a sharp crack or notch. Hydrogen therefore concentrates at exactly the locations where fracture initiates.

No single mechanism explains HE across all alloy-environment combinations, and the mechanistic debate continues. Three models have the strongest experimental support:

1. Hydrogen-enhanced decohesion (HEDE)
   Hydrogen accumulates at grain boundaries and raises the local concentration enough to reduce the cohesive energy , the force needed to separate two atomic planes. Crack propagation follows grain boundaries with very low plastic work, giving the intergranular fracture morphology characteristic of HE in high-strength steel.
2. Hydrogen-enhanced localised plasticity (HELP)
   Hydrogen in solution reduces dislocation-dislocation interaction energies, allowing dislocations to move more easily. Plasticity becomes extremely localised at the crack tip in an atom-thick band rather than spreading over a volume. The macroscopic result looks brittle even though microscopic plasticity is actually enhanced.
3. Adsorption-induced dislocation emission (AIDE)
   Hydrogen adsorbed at the crack tip lowers the energy barrier for dislocation nucleation and emission. This drives rapid crack growth with some ductile character visible at very high magnification (small dimples), unlike the completely cleavage-like HEDE fracture. Common in lower-strength steels and aluminium alloys.

Sour oil and gas service, hydrocarbons produced with significant hydrogen sulfide content, is one of the most aggressive hydrogen-charging environments in industry. H2S is mildly acidic in water and generates atomic hydrogen by corrosion. Additionally, the bisulfide ion (HS-) adsorbs on steel surfaces and poisons the recombination reaction: without this poisoning, two atomic H atoms recombine to form H2 gas and escape. With sulfide present, atomic H diffuses into the steel instead.

NACE MR0175/ISO 15156 is the controlling standard for sour-service materials selection. Its key requirements for carbon and low-alloy steels include a maximum hardness of 22 HRC (approximately 250 HV), restrictions on microstructure and heat treatment, and in some cases mandatory slow-strain-rate or constant-load testing to demonstrate resistance under the specific H2S partial pressure anticipated in service. The hardness limit exists because hardness tracks yield strength, which tracks HE susceptibility.

In a failure investigation of sour-service cracking, the first question is always whether the failed component met the standard at the time of manufacture. Field-portable Vickers hardness testers can measure in situ. Exceedance of the 22 HRC limit , often caused by repair welding without proper post-weld heat treatment, or by using an incorrect grade , is one of the most commonly documented root causes of SSC failures.

Cathodic protection (CP) polarises a metal structure to a sufficiently negative potential that anodic dissolution is suppressed. When the applied potential is too negative, however, the cathode reaction shifts from oxygen reduction to hydrogen evolution, and the protected steel itself becomes a site of atomic hydrogen generation.

High-strength steel components associated with CP-protected structures are the vulnerable population. Anchor bolts on offshore platforms, prestressed tendons on piers, rock bolts in mine workings, and spring pins in subsea assemblies have all failed by HE from overprotective CP. The problem is most acute when current distribution is uneven and some zones receive potentials far more negative than the design target.

Both HE and SCC produce brittle fractures in steels that would normally fail in a ductile manner, and both often produce intergranular fracture. Distinguishing them matters because the remediation is different: HE points to a hydrogen source to be eliminated, while SCC points to a specific environment-material incompatibility.

• Fracture initiation site: HE in high-strength steel commonly initiates internally, below the surface, where hydrogen has diffused and concentrated. SCC typically starts at the free surface in contact with the corrosive environment. A subsurface initiation point on a fracture surface with no corresponding external corrosion is a strong indicator of HE.
• Grain-boundary morphology: in HEDE-driven HE the intergranular facets are typically clean and smooth with little evidence of corrosion product. In SCC, corrosion product is often visible on the grain faces, and the crack path may show secondary branching consistent with the specific corrosive medium.
• Slow-strain-rate test: testing a specimen in the suspected environment at a controlled slow strain rate and comparing elongation and reduction-in-area to values in air. A significant reduction in air-to-environment ductility confirms environmental susceptibility. Charging the specimen cathodically during the test can replicate HE conditions in the laboratory.
• Hydrogen content measurement: vacuum hot-extraction at 300 to 400 degrees C releases diffusible hydrogen from steel specimens. Values above 1 to 2 ppm (weight) in carbon or low-alloy steel are considered elevated and consistent with service hydrogen charging.

Liquid-metal embrittlement (LME) is mechanistically related but involves no hydrogen. When a solid metal in contact with a liquid metal of limited solid solubility is subjected to tensile stress, catastrophic intergranular fracture can occur. Engineering-relevant LME pairs include copper in contact with liquid mercury, aluminium alloys in contact with liquid gallium, and steel in contact with liquid zinc (encountered in hot-dip galvanising of high-strength steels). The fractographic appearance can resemble HE; diagnosis requires identifying the liquid-metal source and confirming the alloy-temperature conditions.