Hydrogen Induced Cracking (HIC) is a critical issue affecting metals used in harsh industrial environments, particularly in the oil and gas sector where wet hydrogen sulphide (H₂S) is present. This form of internal cracking can severely compromise the integrity and performance of equipment such as valves, pipelines, and pressure vessels. In this article, we explain what HIC is, how it develops, the role of hydrogen and steel microstructure, and the most effective detection, prevention, and testing methods. Whether you’re a valve manufacturer, engineer, or asset owner, understanding HIC is essential to improving safety, extending equipment life, and complying with industry standards.

Source: MDPI

 

What Causes Hydrogen Induced Cracking?

To understand hydrogen induced cracking (HIC), we must dig into how hydrogen atoms behave, how they get into steel (especially under H₂S or “sour” conditions), and how that ultimately leads to internal pressure that cracks the metal.

Role of Hydrogen Atoms and Hydrogen Sulfide (H₂S)

Hydrogen (in atomic form) is the key “actor” in HIC. Here’s how it typically enters and behaves:

• In environments containing hydrogen sulfide (H₂S), it reacts at the metal surface to produce atomic hydrogen (H atoms) which can then be absorbed into steel.

• H₂S acts as a recombination poison: it impedes hydrogen atoms from recombining into molecular hydrogen (H₂) too early at the surface, increasing the likelihood that atoms diffuse inward rather than escaping.

• In wet, acidic or sour media, cathodic reactions reduce H⁺ ions to atomic hydrogen at the steel surface; these atoms may then enter the metal lattice.

• Once inside, hydrogen seeks defects, inclusions or microvoids, where it can accumulate or become trapped.

Because of these factors, steel in H₂S‑rich (“sour”) environments is particularly vulnerable to hydrogen uptake and hence to HIC.

 

Mechanisms of Hydrogen Diffusion and Absorption in Steel

After hydrogen atoms are generated at the surface, several concurrent processes govern their movement and fate within the steel:

• Diffusion through lattice / interstitial sites
  Hydrogen atoms are extremely small and can move through interstitial positions (gaps between atoms) in the steel lattice. The diffusion rate depends on temperature, the crystal structure (ferrite, martensite etc.), and local stress fields.

• Trapping
  Hydrogen atoms may be captured by microstructural features such as inclusions, dislocations, carbides or voids. When trapped, they are less mobile.

  • Some traps are reversible (hydrogen can escape under certain conditions)

  • Others are irreversible (hydrogen stays “locked in”)

• Recombination and precipitation of molecular hydrogen (H₂)
  In certain trapped sites, hydrogen atoms may recombine to form molecular hydrogen (H₂). This occurs especially when local hydrogen concentrations rise, and the sites provide a nucleation zone.

• Movement toward defects and stress concentration zones
  Because cracks or defects provide more “free volume,” hydrogen preferentially migrates there. Under the influence of stress (residual or applied), stress gradients can attract hydrogen (“stress‑driven diffusion”).

Thus, absorption and diffusion are not simple bulk motion — they’re strongly influenced by microstructure, traps, and stress fields.

 

Internal Hydrogen Pressure Theory — Overview

One of the most widely accepted models to explain HIC progression is the Internal Hydrogen Pressure Theory (sometimes called hydrogen gas pressure model). Here’s how it works, in simplified form:

• Hydrogen accumulates in trap sites or microvoids
  As hydrogen diffuses and becomes trapped, sites inside the metal gradually fill with atomic hydrogen.

• Recombination to H₂ and volume expansion
  When two hydrogen atoms meet in a void, they can recombine into a hydrogen molecule (H₂). The formation of molecular hydrogen leads to local volume expansion and internal pressure.

• Pressure-induced crack initiation
  That local pressure exerts tensile stress on the surrounding matrix. When this pressure exceeds a critical threshold, it can nucleate microcracks or blister-like separations along weak planes or interfaces.

• Crack propagation and coalescence
  The microcracks can grow, connect and coalesce. Depending on applied or residual stress, they may propagate further. Over time, this can lead to significant internal cracking—often parallel to the surface.

• Stepwise (laminar) cracking
  In many steels, the cracks develop in a “stepwise” manner—laminar cracks that follow planes of weakness (e.g. aligned inclusions or bands) inside the steel.

It’s worth noting: while the internal pressure model captures many observed features of HIC, it is not the sole mechanism. Other contributing mechanisms—such as hydrogen‑enhanced localized plasticity (HELP) or hydrogen‑induced decohesion (HEDE)—may also play roles, especially near crack tips or under stress.

 

How Does HIC Develop in Metals?

Hydrogen induced cracking (HIC) doesn’t appear out of nowhere — it evolves through a sequence of processes inside the metal. First comes crack initiation, then propagation. Along the way, microstructural features like inclusions or impurities act as enablers or accelerators. Let’s walk through those steps.

Crack Initiation and Propagation Process

The emergence and growth of cracks in HIC involves several stages:

Initiation
Hydrogen atoms accumulate at favourable locations — such as inclusions, voids, grain boundaries, or zones of high stress concentration. Over time, the local pressure from recombined hydrogen (H₂) or embrittlement effects causes micro‑voids to develop and microcracks to nucleate.
In sour environments, repeated hydrogen ingress and trapping accelerate this process.

Propagation
Once initiated, cracks do not remain static. They propagate, typically in a stepwise or laminar fashion, following planes of weakness (e.g. aligned inclusions or banding). Under tensile stress (residual or applied), these microcracks may coalesce and grow deeper.

Hydrogen gas pressure in trapped cavities continues to drive crack growth. Also, mechanisms such as hydrogen enhanced localisation of plasticity (HELP) or hydrogen induced decohesion may assist in crack advance, particularly near crack tips.

As cracks link and grow, they can migrate through the interior, sometimes parallel to the surface, and eventually become critical defects.

Transitioning now from the macroscopic crack evolution to the finer details, let’s look at how the metal’s microstructure plays into this.

 

Typical Microstructural Features Affected by HIC

Not all steels behave equally under hydrogen attack. The details of microstructure strongly influence HIC susceptibility and the crack path:

Grain structure / boundaries
Grain boundaries can act both as diffusion pathways and as preferential crack paths (intergranular fracture). In some cases, cracks may propagate along grain boundaries, especially if those boundaries are weakened by hydrogen or segregated solutes.

Microconstituents (ferrite, pearlite, bainite, martensite, etc.)
The relative amounts of phases matter. For instance, lath bainite has been shown to be more sensitive to HIC than ferrite or granular bainite.
Also, zones of hard microconstituents (e.g. martensite–austenite islands) may generate internal stress mismatches and act as local stress‑concentrators.

Segregation / banding
If alloying elements or impurities get segregated during processing (leading to bands of different composition or hardness), these alternate zones become weak planes where cracks may prefer to propagate.

Precipitates / carbides / carbonitrides
Fine precipitates or particles can influence hydrogen trapping behaviour; depending on their nature and distribution, they may either act as traps or facilitate crack paths when poorly bonded to the matrix.

Thus the microstructure serves as both the “terrain” through which hydrogen moves and the network of potential weak points cracks can exploit.

This brings us to one of the most critical enablers: inclusions and impurities.

 

Influence of Inclusions and Impurities on HIC

Inclusions and impurity particles — often non‑metallic oxides, sulfides, nitrides — are notorious contributors to HIC. Their role is multifaceted:

• Inclusions often serve as hydrogen trapping sites. Hydrogen atoms migrating through the lattice can become locked near or within inclusions, raising local concentration and favouring recombination into H₂.

• Poorly bonded inclusion–matrix interfaces or mismatches in thermal expansion can produce microresidual stresses around inclusions. These local stress fields exacerbate susceptibility to crack initiation.

• The shape, size, and orientation of inclusions matter. Elongated sulfide inclusions, for example, aligned parallel to rolling direction, are more dangerous because they facilitate crack advance in that orientation.

• Inclusions reduce the “cleanness” of steel, and higher inclusion content or larger inclusion area has been correlated with greater susceptibility to HIC.

• Some inclusions (e.g. oxides versus nitrides) may behave differently in their trapping capacity or crack facilitation; research shows certain oxide inclusions are more sensitive to crack growth.

In sum: inclusions act as both nucleation centres for cracks and hydrogen concentration amplifiers.

 

Types and Characteristics of Hydrogen Induced Cracking

Hydrogen Induced Cracking (HIC) can manifest in different ways, depending on environment, material, stress, and microstructure. This section explores the influence of a wet H₂S environment, the characteristic stepwise (laminar) crack patterns, and how HIC differs from other hydrogen‑related failures.

Wet Hydrogen Sulfide (H₂S) Environment Effects

A “sour” or wet H₂S environment is one of the most notorious promoters of HIC. Inside such environments:

• Hydrogen atoms are more easily generated at the metal surface because H₂S facilitates the cathodic reaction and blocks surface recombination of H atoms, driving more hydrogen into the metal.

• The “recombination poison” effect of sulfur means fewer hydrogen atoms recombine to escape the surface; more remain available to diffuse inward.

• Continuous ingress of hydrogen in this environment can lead to accumulation of molecular hydrogen (H₂) in interior traps, building internal pressure that encourages cracks.

• Corrosion reactions and scale formation in H₂S media can also create local defects and stress raisers that assist crack initiation.

So, when steel components are exposed to wet H₂S (e.g. in oil & gas pipelines, sour‐service valves), the risk and speed of HIC are elevated.

 

Stepwise or Laminar Cracking Patterns

One of the distinguishing visual indicators of HIC is the stepwise, or laminar, crack morphology. Cracks tend to propagate in internal layers, often parallel to the original surface, forming a “stacked” appearance. Some features:

• Cracks often follow planes of weakness — bands of inclusions, segregated zones, or layers with higher hydrogen concentration.

• The crack propagation is generally incremental, advancing in small bursts or steps rather than as one continuous crack front.

• Because these cracks are mostly internal and often parallel to the surface, the external geometry may remain intact until cracks coalesce or reach critical size.

• When examined microscopically, one sees flat crack faces, minimal plastic deformation, and sometimes evidence of hydrogen pressure effects (e.g. microvoid coalescence).

The laminar pattern is thus a hallmark of hydrogen induced cracking in low‐ to medium strength steels in sour service.

 

Differences Between HIC and Other Hydrogen Embrittlement Failures

While HIC is a type of hydrogen damage, it is distinct in behaviour, environment, and morphology from other forms of hydrogen embrittlement or hydrogen‐assisted cracking. Some critical differentiators:

 

Detection and Testing Methods for HIC

Detecting hydrogen induced cracking early can prevent catastrophic failure. In practice, a combination of non‑destructive testing (NDT) methods and standardised evaluation protocols is used to find and assess HIC damage. Below we explore common inspection techniques, then turn to industry standards that guide testing and evaluation.

Common Inspection Techniques (Ultrasonic Testing, NDT Methods)

Ultrasonic and other NDT techniques are frontline tools for revealing internal cracks, laminations, blisters and connected HIC defects. The trick is to select methods capable of resolving fine planar cracks hidden inside steel.
Here are some of the more commonly used approaches:

Phased Array Ultrasonic Testing (PAUT / PA‑UT)
This technique uses electronically steered beams (angle sweeps) to inspect for laminar cracks, linking between defects, and crack geometry. It offers good imaging and sizing capabilities for HIC / SOHIC detection.
For example, by sweeping from –30° to +30°, the inspector can distinguish whether defects are isolated inclusions or are interconnected cracks.

Shear Wave / Compression Wave Ultrasonic Testing
Shear waves are often used to detect planar discontinuities parallel to the surface, which is typical for HIC cracks. Compression (longitudinal) waves may help gauge depth or remaining ligament.

Time‑of‑Flight Diffraction (TOFD)
TOFD has good crack‐tip resolution and can support more accurate sizing of cracks. It is sometimes used in conjunction with PAUT or as a verification method.

Raster / Scanning Techniques with S‑scans
A preliminary 0° raster (straight beam) scan is often used to locate areas of lamination, which are then further examined with angled scans.

Other NDT Methods (Supporting / Complementary)

• Magnetic particle inspection (MPI) — applicable only for surface or near‐surface cracks

• Dye penetrant testing — limited to surface breaks

• Radiography / X‑ray — less effective for thin planar cracks parallel to surfaces

• Eddy current / electromagnetic methods — can detect near surface flaws, but limited for deeper internal cracks

In practice, a layered inspection strategy is used: start with broader scans, then focus on suspect zones with higher resolution methods.

Transitioning from how we detect HIC to how we standardise, let’s look at established test protocols.

 

Industry Standards for HIC Testing and Evaluation

To ensure consistency and reliability in evaluating materials’ resistance to HIC, several standards and test methods are widely adopted:

• NACE TM0284 (latest revisions, e.g. 2016)
  Perhaps the most central standard for HIC testing in sour (H₂S) environments, this provides procedures for exposing unstressed specimens to H₂S‐saturated test solutions, then qualifying and measuring the degree of cracking.
  The test method covers pipes, plate steels, fittings and flanges.
  NACE TM0284 is explicitly intended to evaluate resistance to hydrogen induced cracking, and is not a catch‑all test for other sour service issues like sulfide stress corrosion cracking (SSC) or general corrosion.

• NACE MR0175 / ISO 15156
  These standards specify material requirements for use in H₂S-containing environments (oil & gas sour service). While not purely HIC detection standards, compliance with MR0175 / ISO 15156 ensures material selection and treatment criteria that help mitigate HIC risks.

• ASTM & Other Standards for Hydrogen Embrittlement
  For broader hydrogen embrittlement testing (beyond just HIC), there exist standards such as ASTM F519 (mechanical hydrogen embrittlement evaluation), F1624 (incremental step load) and others. These help assess susceptibility under stress or plating conditions.

• Evaluation Criteria & Reporting
  After tests, cracks are typically graded by crack length, crack area ratio (percentage of cracked zone), and crack severity classes (e.g. mild, moderate, severe). These metrics feed into fitness‐for‐service (FFS) assessments. (Standards often give guidelines or reference values.)
  The standardisation of specimen geometry, test duration, H₂S concentration, pH control, degreasing procedures, and sample selection is critical to reproducibility.

In sum, detection and inspection techniques (especially ultrasound) provide the practical “eyes” to find hidden damage, while standards like NACE TM0284 ensure the consistency and credibility of material evaluation.

 

Preventing and Mitigating Hydrogen Induced Cracking

Preventing hydrogen induced cracking (HIC) is far more economical than repairing failures later. A layered strategy combining material choice, protective barriers, and process controls gives the best chance of success. Below, I walk you through those strategies.

Material Selection and Alloy Considerations

Your first line of defence is choosing steels or alloys that are less susceptible to hydrogen absorption, diffusion or trap‑assisted cracking. Some guiding principles and tactics:

• Prefer steels with low levels of impurity elements such as sulfur and phosphorus, which tend to act as hydrogen traps or weaken interfaces. (These impurities facilitate hydrogen accumulation and crack initiation.)

• Use microalloying and controlled chemistry to introduce beneficial traps or localised heterogeneity that can buffer hydrogen rather than concentrate it—as recent research suggests that chemical heterogeneity can improve resistance.

• Avoid very high‑strength steels when possible. As strength increases, toughness and resistance to hydrogen effects often decline.

• Apply post‑processing heat treatments (baking / de‑embrittlement annealing) to drive out residual hydrogen after fabrication or prior to service. Low hydrogen annealing (e.g. ~200 °C) is a commonly used approach.

• Design with safety margins, avoiding stress concentrations, sharp corners or geometry transitions that encourage crack initiation.

By selecting more forgiving alloys and cleaning up the chemistry and microstructure, you reduce the “opportunity” for hydrogen to degrade your metal.

 

Protective Coatings and Surface Treatments

Even with the best alloy inside, the surface is the first barrier against hydrogen ingress. Appropriate coatings and surface treatments can greatly reduce the risk.

• Barrier coatings (metals, claddings, thermal spray overlays)
  For example, aluminium spray or thermal-spray aluminium (TSA) coatings have been shown to act as dual barriers—limiting both corrosion and hydrogen uptake—so that neither sulphide stress corrosion nor HIC is observed.
  Dense metallic claddings (e.g. Ni alloys) or overlay techniques (e.g. weld overlay, thermal spray) can obstruct hydrogen diffusion paths.

• Conversion coatings / passivation
  Applying oxide, phosphate, chromate or other conversion layers can help control surface reactivity and reduce hydrogen generation at the interface.

• Surface polishing / smoothing
  Removing surface defects, smoothing roughness, and eliminating sharp protrusions reduces local stress raisers and lessens hydrogen entry points.

• Inhibitors / hydrogen scavengers
  In some cases, chemical inhibitors or sacrificial barrier additives can discourage hydrogen formation at the surface.

These surface treatments should complement, not replace, good alloy design. Also, coatings must be defect‑free and maintained, since cracks or delaminations in coatings can defeat the barrier.

 

Process Controls and Environmental Management

Even the best materials and coatings can be undermined by poor process control. Managing the environment and operations is critical to limiting hydrogen exposure and ingress.

• Control hydrogen sources in service
  Reduce H₂S concentration, acidity, and free hydrogen ion production in the environment. In sour service, keeping conditions mild and reducing corrosion activity can cut hydrogen generation.
  Use chemical treatment, pH control, and corrosion inhibitors as needed.

• Limit exposure during fabrication / welding

  • Use low‑hydrogen electrodes and consumables, and ensure filler metals are dry and properly stored.

  • Preheat and keep interpass temperatures consistent to allow hydrogen to diffuse out.

  • Post‑weld baking or stress relief to remove residual hydrogen.

  • Control cooling rates so hydrogen does not become trapped prematurely.

• Cathodic protection and electrochemical control
  While cathodic protection helps mitigate corrosion, unbalanced or overly aggressive CP can drive hydrogen evolution. CP systems need to be configured to avoid excess hydrogen generation on the protected steel.

• Monitoring, inspection and feedback loops
  Regular inspection and detection (from earlier section) allow you to catch early HIC signs and feed the lessons back into process improvements.

• Operational strategy
  Where possible, avoid or minimise the periods of sour gas exposure, or schedule time to “degas” systems under benign conditions. Also, ensure that shutdown/startup cycles do not exacerbate hydrogen ingress (e.g. by rapid cooling).

• Environmental control / gas composition
  For systems handling multi‑gas streams (e.g. H₂S + CO₂), manage aggressiveness by controlling partial pressures, water content, injection of inhibitors, or buffering agents.

 

HIC in Valve Manufacturing and Industry Implications

Hydrogen induced cracking (HIC) isn’t just a theoretical problem — in the valve sector, it can directly threaten performance, reliability, and safety. Below, let’s look at how HIC impacts valves, and then review some real‑world examples from industry.

Impact of HIC on Valve Performance and Safety

Valves are critical components: when HIC strikes, several adverse consequences may follow:

• Loss of sealing integrity
  Internal cracks can lead to leakage paths. Even if the external body appears intact, microscopic cracks may open under pressure or cyclic loads, compromising tightness.

• Reduced mechanical strength & brittleness
  As internal cracks propagate or coalesce, the remaining cross‑section loses its load capacity. The valve becomes more brittle, less able to absorb transient loads, vibration or shock.

• Unexpected failure under modest loads
  Because HIC cracks often develop internally and parallel to surfaces, failure can occur suddenly under load without obvious warning signs on the outside.

• Accelerated fatigue / cyclic damage
  Existing cracks can act as stress concentrators under flow pulsations or pressure cycling, accelerating fatigue crack growth and shortening service life.

• Safety, environmental and financial risks
  A valve failure in a sour (H₂S) environment can lead to hazardous leaks, safety incidents, downtime, and costly repairs. The reputational risk for manufacturers or operators is also high.

For valve manufacturers, designing for HIC resistance is thus not optional — it’s essential for product durability and customer confidence.

 

Conclusion

Hydrogen Induced Cracking (HIC) poses a serious threat to the safety, performance, and longevity of valves and other pressure-containing equipment, especially in sour service environments with hydrogen sulphide (H₂S). Understanding how HIC develops, identifying early warning signs through non-destructive testing, and applying effective prevention strategies — such as proper material selection, protective coatings, and process control — are essential steps for valve manufacturers and operators. By addressing HIC proactively, companies can reduce failure risks, meet industry standards, and improve reliability in demanding applications.