Pipeline Cracking & Corrosion — SCC, HIC & MIC Fundamentals

1. Stress-corrosion cracking (SCC)

SCC is the growth of cracks under the combined action of a tensile stress and a corrosive environment, in a susceptible material — none of the three alone would do it. External pipeline SCC comes in two forms:

• High-pH SCC — intergranular cracking in a concentrated carbonate-bicarbonate electrolyte (pH ~9–11) under disbonded coating, in a narrow potential window, favoured by temperature (so it clusters downstream of compressor stations).
• Near-neutral-pH SCC — transgranular cracking in dilute ground-water (pH ~6–7, CO₂-rich) with cathodic protection shielded by the coating; associated with a hydrogen contribution to the crack-tip process.

Susceptibility screening (coating type, operating stress > ~60% SMYS, temperature, terrain/drainage, CP effectiveness, age) follows the external-SCC direct-assessment standard NACE/AMPP SP0204. Found SCC is dispositioned by fracture mechanics (an ECA, see the crack-management fundamentals).

2. Sour-service cracking: HIC vs SSC

In wet H₂S (sour) service, atomic hydrogen generated by the corrosion reaction enters the steel and causes several distinct damage modes that are routinely conflated:

• HIC (Hydrogen-Induced Cracking) — hydrogen recombines to molecular H₂ at internal inclusions/laminations (especially elongated MnS), building pressure that forms blisters and stepwise internal cracks. HIC is driven by steel cleanliness and microstructure and needs no applied stress; SOHIC is the stress-oriented variant. Critically, HIC can occur across the range of sour conditions — it is not confined to a particular pH band.
• SSC (Sulfide Stress Cracking) — brittle cracking of hard/high-strength microstructures under applied/residual tensile stress; it is worst at low temperature and is the mechanism whose severity is mapped against the H₂S partial pressure / in-situ pH regions of ISO 15156-2 Figure 1.

The common error: applying ISO 15156-2 Figure 1's pH–pH₂S severity regions to HIC. Figure 1 governs SSC region selection. HIC resistance is instead demonstrated by testing (NACE TM0284 — CLR/CTR/CSR acceptance) and by specifying clean, low-sulfur, shape-controlled (Ca-treated) HIC-resistant steel; it must be considered whenever the service is wet-sour, regardless of where the point falls on the SSC chart. The umbrella materials requirement for sour service is ANSI/NACE MR0175 / ISO 15156.

3. Microbiologically influenced corrosion (MIC)

MIC is localized corrosion accelerated by microbial activity — most notoriously sulfate-reducing bacteria (SRB) that produce H₂S and create aggressive under-deposit micro-environments, plus acid-producing and iron-oxidizing bacteria. It is favoured by stagnant water, low flow (≤ ~3 ft/s), solids/biofilm, and temperatures ~20–60 °C; it shows up as discrete pits under deposits, often at the 6-o'clock position or at dead legs. Screening and monitoring follow NACE/AMPP guidance (microbial testing per NACE TM0194/TM0212); mitigation is pigging, biocide treatment, and water management.

4. Screening & mitigation

All three mechanisms are managed within the integrity-management program as cracking/corrosion threats. The common levers: control the environment (dehydration to stay below the water dew point; biocide/pigging for MIC), control the material (HIC-resistant steel, hardness caps ≤ 22 HRC for SSC per MR0175), control the stress (operating below SCC-susceptible stress, residual-stress relief at welds), and detect early (crack-detection ILI/EMAT for SCC, UT for HIC blisters, coupons/probes and microbial sampling for MIC).

5. References

• ANSI/NACE MR0175 / ISO 15156 (Parts 1–3) — materials for use in H₂S-containing environments; Part 2 Fig 1 governs SSC region selection for carbon/low-alloy steel.
• NACE TM0284 — evaluation of pipeline/pressure-vessel steels for resistance to HIC (CLR/CTR/CSR).
• NACE TM0177 — laboratory testing of metals for SSC/SCC in H₂S environments.
• NACE/AMPP SP0204 — stress-corrosion-cracking direct assessment methodology.
• NACE TM0194 / TM0212 — field monitoring of bacterial growth / detection of MIC in pipelines.