Low-alloy steel components used in sour service environments often fail because hydrogen sulphide (H₂S) attacks the surface of the material long before the bulk structure loses strength.
NACE MR0175 / ISO 15156 was developed to reduce the risk of sulphide stress cracking (SSC), hydrogen-induced cracking (HIC) and related hydrogen-assisted failure mechanisms in oil and gas equipment exposed to H₂S-containing environments.
For engineers, the standard focuses on controlling hardness, surface condition, stress and environmental exposure to prevent surface-driven failure.
NACE MR0175 / ISO 15156 is an international standard that defines which metallic materials can be safely used in oil and gas environments containing hydrogen sulphide (H₂S). The standard exists to reduce the risk of environmentally assisted cracking mechanisms such as sulphide stress cracking and hydrogen-induced cracking. The standard is particularly important for:
For low-alloy steel components, the standard places strict controls on:
Low-alloy steel becomes vulnerable in sour service because H₂S promotes hydrogen ingress into the material, increasing the risk of hydrogen-assisted cracking. In sour environments, corrosion reactions at the steel surface generate atomic hydrogen. Normally, some hydrogen recombines and escapes harmlessly. However, H₂S interferes with this process, allowing hydrogen to diffuse into the steel itself.
Once inside the material, hydrogen accumulates around grain boundaries, areas of tensile stress, surface defects and micro-cracks. This weakens the steel locally and can eventually lead to cracking under load. Importantly, these failures are often surface-initiated rather than bulk-material failures. This is why low-alloy steel components can appear structurally sound externally while cracks continue developing beneath the surface.
NACE MR0175 imposes hardness limits because harder steels are generally more susceptible to sulphide stress cracking (SSC). For many carbon and low-alloy steels, the standard limits hardness to approximately 22 HRC (around 237 HBW). Higher hardness levels are associated with reduced ductility, higher residual stress, increased crack sensitivity and greater hydrogen trapping.
This creates a significant engineering challenge. Increasing hardness may improve wear resistance, but it can also increase susceptibility to hydrogen-assisted cracking in H₂S environments. This trade-off becomes particularly important in applications exposed to both mechanical wear and sour service corrosion. Examples include valve internals, actuator shafts, pump components and flow control equipment.
In many cases, engineers are forced to balance wear resistance against SSC resistance rather than optimising for one property alone.
| Factor | Effect in sour service |
| Higher hardness | Improved wear resistance but increased SSC susceptibility |
| Rough surface finish | Increased local stress concentration and crack initiation risk |
| Residual tensile stress | Accelerated hydrogen-assisted cracking |
| Surface porosity or micro-cracking | Increased pathways for corrosive ingress and hydrogen penetration |
Surface condition is critical because hydrogen ingress and crack initiation begin at the component surface. Even when a low-alloy steel grade complies with NACE MR0175 hardness requirements, poor surface integrity can still accelerate failure.
Surface degradation creates local stress concentrations that promote cracking under cyclic or sustained loading. Common surface-related risk factors can include corrosion pitting, machining marks, wear damage, coating porosity and micro-cracking. This is why protective coatings are so important for component lifespan.
However, many conventional coatings struggle in sour environments because they contain porosity, micro-cracks or binder phases that create pathways for corrosive ingress and hydrogen penetration. The traditional coatings engineers have relied on (hard chrome plating, thermal sprays, etc.) introduce a wide array of limitations:
Advanced surface engineering can significantly improve the performance of low-alloy steel components in sour service environments without requiring full material substitution. Rather than replacing the entire substrate with stainless or nickel alloys, modern surface technologies can enhance wear resistance, erosion resistance, corrosion protection and resistance to hydrogen ingress.
Low-temperature CVD tungsten carbide coatings are one example of this approach. Unlike line-of-sight coatings, the process deposits a dense, pore-free and metallurgically bonded layer uniformly across internal bores, threads, and complex geometries.
This matters because many sour service failures initiate in difficult-to-protect internal regions where conventional coatings struggle. By strengthening surface durability while preserving the underlying low-alloy steel’s compliance and mechanical properties, surface engineering offers a more targeted route to extending component life.
NACE MR0175 is ultimately about controlling surface-driven failure in sour service environments. While low-alloy steels remain essential due to their strength and cost-efficiency, their long-term reliability depends heavily on how the surface behaves under exposure to H₂S, wear and corrosion.
Rather than replacing the bulk material entirely, advanced surface engineering offers a more targeted solution. Hardide’s pore-free CVD tungsten carbide coatings help protect low-alloy steel components against erosion, corrosion and hydrogen-related surface degradation whilst maintaining the benefits of the underlying substrate.
For engineers balancing compliance, durability and cost, improving the surface is increasingly proving more effective than changing the material itself.