Stress corrosion cracking (SCC) and hydrogen induced cracking (HIC): causes and prevention

Stress corrosion cracking (SCC) occurs when three conditions meet: a susceptible material, a tensile stress (often residual, near welds) and a specific environment (chlorides, caustic, amines, carbonates). Hydrogen induced cracking (HIC) and its SOHIC variant affect carbon steel in wet H2S service. Both families are prevented by removing at least one condition: a suitable material, stress relief, a coating barrier and targeted inspection.

These mechanisms are feared because they crack steel without visible thinning: a vessel can pass a thickness survey and leak shortly after. This guide is written for reliability, inspection and maintenance leads in Quebec and Ontario plants who need to recognize these forms, tell them apart from other types of industrial corrosion and choose a realistic prevention strategy.

What is stress corrosion cracking?

SCC is progressive cracking caused by the combined action of tensile stress and a corrosive environment, on a material that is susceptible to that specific environment. No single condition is enough: an austenitic stainless steel without chlorides does not crack, and neither does the same steel without tensile stress.

The cracks are fine, often branched, and grow perpendicular to the main stress. They can be transgranular or intergranular depending on the material and environment. The stress rarely comes from operating pressure alone: residual stresses from welding, forming or bolting are often enough. That is why weld heat-affected zones are the first places to watch.

SCC differs from pitting corrosion, although a pit can act as a crack initiation site by concentrating stress.

What are the most common forms of SCC in plants?

Four forms keep showing up in process plants, each tied to a material and an environment. API 571 describes these damage mechanisms and their drivers; the table sums up the recognized trends.

For caustic SCC, NACE SP0403 covers prevention practices, and API RP 945 addresses amine stress corrosion cracking. These documents tie the risk to concentration, temperature and the stress-relief condition of welds: the site specification sets the applicable limits.

How does hydrogen crack steel (HIC and SOHIC)?

In wet H2S service, corrosion releases atomic hydrogen that enters the steel instead of escaping as gas. It recombines at inclusions and internal flaws, builds local pressure and forms blisters or stepwise cracks: that is hydrogen induced cracking.

• HIC. Cracks parallel to the surface, linked in steps, driven by steel cleanliness. No applied stress is needed. NACE TM0284 is used to evaluate a steel's resistance to HIC.
• SOHIC. Stacked arrays of small HIC cracks aligned through the wall under stress, typically near non-stress-relieved welds.
• SSC (sulfide stress cracking). Brittle failure of hard steels or overly hard weld zones. NACE MR0175 / ISO 15156 governs material selection for H2S service.

HIC develops mostly in drums, columns and sour water piping. It grows quietly, which is why an inspection program matters.

Wet H2S damage is not limited to refineries. Carbon steel equipment that holds water with dissolved hydrogen sulfide, such as some wastewater, biogas or process water systems, can be exposed. The first step is to confirm the presence of free water and H2S, then check whether the steel and the welds were specified for that service.

What factors raise the cracking risk?

Risk rises with tensile stress, the concentration of the aggressive species and, for most forms, temperature. Local conditions matter more than the process average.

• Local concentration. Evaporation under wet insulation, dead legs, crevices: chlorides or caustic build up there. See our corrosion under insulation guide.
• Non-stress-relieved welds. Residual stresses drive caustic and amine SCC and SOHIC.
• Hardness. Hard welds or zones are more prone to SSC.
• Thermal cycles and shutdowns. Start-up and shutdown phases change the chemistry and can concentrate contaminants.
• Process changes. A new feed, higher H2S content or a new chemical can make a material unsuitable.

How are SCC and HIC detected?

They are found with targeted non-destructive testing, because thickness gauging does not see them. The method depends on the mechanism, the material and access.

• Wet fluorescent magnetic particle testing (WFMT). The reference method for surface-breaking cracks in carbon steel, after surface preparation.
• Dye penetrant testing. For non-magnetic austenitic stainless steels.
• Advanced ultrasonics (phased array, TOFD). To size HIC and SOHIC through the wall and track their growth.
• Visual examination under insulation. After targeted removal where moisture is likely.

Intervals and scope come from inspection codes: our article on API 510 covers pressure vessels, and the one on API 570 covers piping. Assessing a crack once found calls for a fitness-for-service evaluation, not a visual judgment.

How do you prevent cracking?

SCC and HIC are prevented by removing at least one of the required conditions. The levers combine, and the most effective one depends on the mechanism.

• Material. Duplex stainless or nickel alloys against chloride SCC, HIC-resistant steels qualified to NACE TM0284, materials compliant with NACE MR0175 / ISO 15156 in H2S service.
• Stress. Post-weld heat treatment against caustic SCC, amine SCC and SOHIC; weld hardness control.
• Environment. Control of chlorides and moisture, inhibitors, operating conditions kept within the design window.
• Coating. A barrier between metal and environment, especially on the outside.
• Inspection. A risk-based program with methods matched to the mechanism.

Where none of these levers can be applied quickly, the inspection interval is usually tightened until the next shutdown allows a lasting fix. A strategic advisory diagnosis helps rank these levers by equipment, shutdown window and budget.

What role do coatings play against SCC?

A coating acts on the environment condition: it keeps chlorides and water off the metal. It is the most common measure against external chloride SCC on insulated stainless steel, where NACE SP0198 recommends coating stainless steel before insulating it.

On hot equipment the system must handle the temperature: see our article on high-temperature coatings. Inside equipment, the logic matches process vessel corrosion protection. Our protective coatings page presents the systems, and coating inspection verifies continuity, since a breached coating concentrates attack at the flaw. On pipelines, external SCC is tied to coating condition and field joints.

Frequently asked questions

Can SCC occur without wall loss?

Yes. That is its main feature: the crack grows through the wall without measurable thinning. Thickness gauging alone is therefore not enough; a crack detection method is needed.

What is the difference between HIC and SSC?

HIC is internal cracking caused by hydrogen recombining at inclusions, with no applied stress. SSC is brittle failure under stress, mostly in hard steels and hard welds.

Is a coating enough to remove the risk?

No. It removes the environment condition as long as it stays intact. Material choice, stress relief and inspection remain necessary, especially where the coating can be damaged.

Should stainless steel be coated under insulation?

In most cases, yes. NACE SP0198 recommends coating insulated austenitic stainless steel within the temperature range where chloride SCC is possible; the site specification sets the range and the system.

Suspect SCC or HIC on equipment, or planning a shutdown with insulated stainless? Our strategic advisory service builds the diagnosis and coating strategy, which our crews and qualified subcontractors then carry out, from Bécancour to Ontario. Send photos, drawings and service conditions through the request a quote form: pricing on consultation, reply within 24 business hours.