HIC vs SSC: Key Differences and NACE Test Standards Explained
If you work with steel plates in sour service, you have probably seen the terms HIC and SSC used side by side — and often confused with each other.
They are not the same thing. They are two different failure modes caused by the same root problem: hydrogen entering the steel in a wet H₂S environment. They require different tests, different acceptance criteria, and different material controls.
This guide explains the difference between HIC and SSC, the standards that govern them (NACE TM0284, NACE TM0177, NACE MR0175, NACE MR0103), and how to specify them correctly in a technical specification.
1. What Is HIC (Hydrogen-Induced Cracking)?
HIC stands for Hydrogen-Induced Cracking. It occurs when hydrogen atoms, generated by wet H₂S corrosion, diffuse into the steel and accumulate at internal defects — inclusions, segregation bands, or laminations.
These hydrogen atoms combine into molecular H₂, which cannot diffuse out. The trapped gas builds up internal pressure, eventually forming stepwise cracks inside the steel — with no external stress required.
Key characteristics of HIC
- No external stress needed — the damage is driven entirely by internal pressure
- Driven by steel cleanliness — sulfur, phosphorus, inclusions, and segregation are the main risk factors
- Tested by NACE TM0284 — 96-hour immersion test, no applied stress
- Evaluated by three ratios: CLR (Crack Length Ratio), CTR (Crack Thickness Ratio), CSR (Crack Sensitivity Ratio)
💡 Plain-language: HIC is like internal pressure building up in a sealed container — nothing is pulling or pushing from outside. The steel cracks from the inside out.
2. What Is SSC (Sulfide Stress Cracking)?
SSC stands for Sulfide Stress Cracking. It occurs when hydrogen enters the steel and the material is under tensile stress — either from welding residual stress, operating load, or both.
The combination of hydrogen and stress leads to brittle cracking, often in the weld heat-affected zone (HAZ) where hardness is highest.
Key characteristics of SSC
- Requires tensile stress — without stress, SSC does not occur
- Driven by hardness — typically limited to ≤22 HRC for carbon and low-alloy steels
- Tested by NACE TM0177 — 720-hour test with constant tensile stress (usually 80% yield strength)
- Evaluated by: whether fracture occurs, time to fracture, and critical stress
💡 Plain-language: SSC is like a brittle material under tension — add hydrogen and it snaps. The harder the steel, the higher the risk.
3. HIC vs SSC: Key Differences at a Glance
Here is a direct comparison for engineers and procurement teams who need to specify the right test:
4. Why Passing HIC Does Not Mean Passing SSC
This is one of the most common misunderstandings in sour service material selection. HIC and SSC have different weak points:
HIC depends on cleanliness
Even if the hardness is acceptable, a steel with high sulfur, high phosphorus, many inclusions, or severe segregation can still fail HIC.
SSC depends on hardness
Even a very "clean" steel can fail SSC if the weld heat-affected zone has hardness above the limit — typically 22 HRC.
⚠️ Practical consequence: A steel plate may pass HIC but fail SSC, or pass SSC but fail HIC. The two tests must be performed and accepted separately. Neither can substitute for the other.
5. Test Standards Explained: TM0284 vs TM0177
NACE TM0284 — HIC Test
Standard title: "Evaluation of pipeline and pressure vessel steels for resistance to hydrogen-induced cracking"
- Specimen: Unstressed, freely immersed
- Solution A: 5.0% NaCl + 0.5% CH₃COOH, saturated with H₂S, initial pH ~3.0
- Duration: 96 hours
- Evaluation: CLR, CTR, CSR
- Acceptance: Specified in the technical specification — common values are CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%; stricter values are CLR ≤ 10%, CTR ≤ 3%, CSR ≤ 1%
NACE TM0177 — SSC Test
Standard title: "Laboratory testing of metals for resistance to sulfide stress cracking and stress corrosion cracking in H₂S environments"
- Specimen: Under constant tensile stress (typically 80% yield strength; MR0103 requires 85%)
- Loading methods: A (tensile), B (three-point bending), C (C-ring), D (double cantilever beam)
- Solution A: 5.0% NaCl + 0.5% CH₃COOH, saturated with H₂S, initial pH 2.6~2.8
- Duration: 720 hours (30 days)
- Evaluation: Fracture or not, time to fracture, critical stress
- Acceptance: No fracture after 720 hours = pass
6. How to Specify HIC Testing in a Technical Specification
If you are writing a technical specification for SA-516 Gr.70 or similar pressure vessel steel, here is a recommended wording:
Steel plate SA-516 Gr.70: HIC shall be tested and accepted in accordance with NACE TM0284 (current edition), using Solution A, with acceptance criteria: CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%.
For stricter requirements: CLR ≤ 10%, CTR ≤ 3%, CSR ≤ 1%.
Key points to include
- Test method: NACE TM0284 (current edition)
- Acceptance criteria: specific CLR/CTR/CSR limits
- Solution type: usually Solution A
- Specimen condition: e.g., after simulated PWHT
- Specimen dimensions and quantity
7. NACE MR0175 vs MR0103: Which Applies to Your Project?
Two NACE material standards govern sour service selection, and they are not interchangeable:
⚠️ Do not mix them up: Using an upstream standard downstream may be overly conservative and increase cost. Using a downstream standard upstream may result in insufficient material selection and risk of failure.
Two different qualification philosophies
- MR0175 (upstream) — Environmental restriction method: A material is acceptable only if its actual service environment (H₂S partial pressure, Cl⁻ concentration, temperature, pH) is within the boundaries allowed by the standard.
- MR0103 (downstream) — Binary determination method: A material is acceptable if it meets the specified metallurgical and manufacturing requirements (composition, hardness, welding controls). No further environmental assessment is needed.
8. How the Standards Fit Together
Before we go further, it helps to see how the material standards, failure modes, and test standards relate to each other:
9. Why PWHT Matters for SSC (But Not for HIC)
Post-Weld Heat Treatment (PWHT) is a common requirement in sour service specifications. But its effect is not universal:
- For SSC: PWHT reduces hardness in the weld and HAZ, and relieves residual stress — both critical for preventing SSC. Highly effective.
- For HIC: PWHT has very limited effect, because HIC damage occurs inside the base metal and is not driven by welding stress.
💡 Key takeaway: PWHT is a tool for SSC control, not for HIC. If your specification requires HIC resistance, PWHT alone will not solve it — you need clean steel with low S, P, and inclusion content.
10. Dual-Resistant: What It Means and What It Doesn't
"Dual-resistant" is a common industry shorthand. It means exactly one thing:
Dual-Resistant = HIC resistant + SSC resistant
A dual-resistant steel plate must pass both the NACE TM0284 HIC test and the NACE TM0177 SSC test. Neither can be omitted.
⚠️ Common mistakes:
❌ Dual-resistant ≠ SSC only
❌ Dual-resistant ≠ sulfide resistant + chloride resistant
✅ Dual-resistant = HIC + SSC, both must pass
Plain-language analogy: Dual-resistant is a combo meal — burger + cola. SSC alone is just the cola. You cannot call the cola a combo meal.
11. Full Mapping: HIC, SSC, Cl-SCC vs Standards
For carbon steel plates in sour service, Cl-SCC is less of a concern than HIC and SSC. It becomes critical mainly for stainless steels and high-alloy materials in chloride-rich environments. The table below maps all three failure modes against selection standards and test standards:
12. Quick Reference
13. Final Thoughts
HIC and SSC are two different failure modes with the same root cause. Understanding the difference is critical for specifying the right steel, the right tests, and the right acceptance criteria:
- HIC = no stress required, driven by steel cleanliness, tested by NACE TM0284
- SSC = tensile stress required, driven by hardness, tested by NACE TM0177
- Passing one does not mean passing the other — both must be tested and accepted separately
- MR0175 (upstream) covers SSC, Cl-SCC, and HIC — uses the environmental restriction method
- MR0103 (downstream) covers SSC only — uses the binary determination method
- PWHT helps SSC but has very limited effect on HIC
- Dual-resistant means both HIC and SSC must pass
If you are specifying SA-516 Gr.70 or another pressure vessel steel for sour service, make sure your technical specification clearly states:
- Which NACE material standard applies (MR0175 or MR0103)
- Whether HIC testing is required, and if so, the acceptance criteria (CLR/CTR/CSR limits)
- Whether SSC testing is required, and the test conditions
- Whether PWHT is required, and for what thickness
- Which laboratory will perform the tests (ISO 17025 accredited by CNAS or equivalent)
💡 Our view: HIC and SSC are frequently confused, and this confusion leads to specification errors in sour service projects. The safest approach is to specify both tests clearly and confirm the applicable NACE material standard before ordering steel plates.
📚 Further Reading & Standards
- 🔗 Boiler & Pressure Vessel Steel Plates – Product Page
- 🔗 Q345R Steel Plate – Product Page
- 🔗 SA516 Grade 70 – Performance Page
- 📄 NACE MR0175 / ISO 15156, NACE MR0103 / ISO 17945, NACE TM0284, NACE TM0177, NACE SP0472
Need Sour Service Steel Plates with Certified HIC / SSC Testing?
CEHE STEEL supplies SA516 Gr.70 and other pressure vessel steel plates for sour service, with full MTC documentation, third-party inspection support, and HIC / SSC testing from CNAS-accredited ISO 17025 laboratories.