What Is Hydrogen Induced Cracking (HIC)?
Hydrogen induced cracking (HIC) is a form of material degradation that occurs when atomic hydrogen diffuses into steel and recombines at internal discontinuities (inclusions, laminations, or segregation bands) to form molecular hydrogen gas. The gas pressure at these trap sites creates internal blisters and stepwise cracks that propagate parallel to the rolling direction. HIC does not require external stress; it is driven entirely by hydrogen charging from the process environment, making it distinct from stress corrosion cracking (SCC).
HIC Mechanism
| Stage | Description |
|---|---|
| 1. Hydrogen generation | H2S in the process fluid reacts with the steel surface, producing atomic hydrogen (H) as a cathodic reaction by-product |
| 2. Hydrogen absorption | Atomic hydrogen diffuses into the steel lattice (H2S and cyanides act as recombination poisons, increasing absorption) |
| 3. Hydrogen trapping | Hydrogen atoms migrate to planar discontinuities; MnS inclusions, oxide inclusions, banded microstructures, and segregation zones |
| 4. Molecular recombination | Atomic hydrogen recombines to H2 gas at the trap site; the gas cannot escape and builds pressure |
| 5. Blister and crack formation | Internal pressure creates blisters (visible on the surface) or stepwise cracks linking adjacent trap sites through the wall thickness |
Susceptible Materials
HIC primarily affects carbon and low-alloy steels used in sour service (H2S-containing environments):
- Carbon steel pipes: API 5L, ASTM A106, A333, A516, A537
- Low-alloy steels: A387 Gr.11/22 (less common but possible in high-severity sour service)
- Plate steel: pressure vessel plates with centerline segregation are particularly vulnerable
Austenitic stainless steels and nickel alloys are immune to HIC due to their face-centered cubic (FCC) crystal structure, which has very low hydrogen diffusivity.
NACE TM0284 Testing
NACE TM0284 (now AMPP TM21452) is the standard test method for evaluating HIC resistance. Test specimens are immersed in a sour solution (NACE Solution A: 5% NaCl + 0.5% acetic acid, saturated with H2S) for 96 hours at 25°C. After exposure, specimens are sectioned and evaluated for:
| Parameter | Definition | Typical Acceptance Criteria |
|---|---|---|
| CLR (Crack Length Ratio) | Total crack length / specimen width x 100% | CLR ≤ 15% |
| CTR (Crack Thickness Ratio) | Total crack thickness / specimen thickness x 100% | CTR ≤ 5% |
| CSR (Crack Sensitivity Ratio) | Sum of (crack length x crack thickness) / (specimen width x thickness) x 100% | CSR ≤ 2% |
HIC-Resistant Steel Requirements
Producing HIC-resistant steel requires controlled steelmaking:
| Requirement | Purpose |
|---|---|
| Low sulfur content (S ≤ 0.002%) | Reduces MnS inclusions that act as hydrogen trap sites |
| Calcium treatment | Modifies remaining sulfide inclusions from elongated MnS to spherical CaS (less harmful) |
| Low phosphorus (P ≤ 0.015%) | Reduces centerline segregation in continuously cast slabs |
| Controlled carbon equivalent (CE ≤ 0.43) | Limits hard microstructures that trap hydrogen |
| Normalized or TMCP rolling | Produces uniform, fine-grained microstructure free of banding |
| No centerline segregation | Soft reduction during continuous casting eliminates the primary crack initiation zone |
NACE MR0175/ISO 15156 Compliance
All carbon and low-alloy steel piping for sour service (H2S partial pressure above 0.3 kPa per NACE MR0175/ISO 15156) must be tested to NACE TM0284 and meet project acceptance criteria. Hardness limits (typically 248 HV10 max) apply to base metal, weld, and HAZ to prevent sulfide stress cracking (SSC). For piping inspection, UT scanning per ASTM A578 Level C detects pre-existing laminations that could act as HIC initiation sites.
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