Cracking in a Lean/Rich Amine Exchanger: Amine SCC After a 'Correct' PWHT

Most probable mechanism: amine stress corrosion cracking (wet H2S/amine SCC of carbon steel).

The four diagnostic points line up:

1. Lean amine at 250°F is the classic amine SCC condition. Amine cracking is associated with lean (low-H2S-loading) amine service, particularly above about 140°F; high amine concentration (45 wt% MDEA is high) and the presence of H2S in the lean solvent raise susceptibility. Cracking was found on the lean side, not the rich side, which fits this mechanism and argues against a simple wet-H2S/SSC explanation driven by high H2S partial pressure.
2. Transverse, branching cracks at weld toes are SCC morphology - environmentally assisted, stress-corrosion, following residual stress at a weld toe rather than the straight, non-branching, surface-parallel planes of HIC.
3. Dark corrosion product in the crack faces confirms the crack was wetted and chemically active over time, not a manufacturing defect.
4. No cracking in the regenerator overhead at 210°F - the overhead is a different chemistry regime (CO2/H2S in condensed water, where carbonate SCC would be the concern), and lack of cracking there, while the hotter lean amine exchanger cracked, points at the lean amine/high-temperature combination rather than at generic sour water.

The impingement zone at the rich inlet is where the richest, coolest amine contacts the hot lean shell surface, producing local turbulence, concentration and thermal gradients - a plausible site for the first cracking.

A hardness cap is the correct control for sulfide stress cracking, and PWHT does reduce residual stress - both are legitimate, and the documented 1200°F PWHT at 160–175 HBW already satisfies them. Yet the exchanger still cracked. The lesson is that amine SCC is not reliably controlled by hardness alone, and hardness/PWHT cannot be the whole integrity argument.

Additional controls that are technically justified:

• Chemistry: drive lean amine H2S (and heat-stable salt content) down. Lean amine at 0.4 wt% H2S is high enough to matter; better regeneration and contaminant control reduce the corrodent. Monitor and control amine concentration - a 45 wt% MDEA solution is aggressive and dilution within design limits lowers severity.
• Temperature: reduce lean amine temperature at the exchanger. Even a 20–30°F reduction moves the surface further from the amine SCC temperature threshold.
• Design/detailing: eliminate crevices and stress concentrators at the impingement zone; use smooth, full-penetration welds; consider an impingement plate or flow distribution change so hot lean amine is not locally splashed at a weld toe.
• Material: for new or replaced components in the susceptible zone, solid 300-series stainless (e.g., 304L/316L) or a corrosion-resistant cladding/weld overlay is the recognized upgrade, with the caveat that the alloy must be qualified for the amine environment and PWHT/thermal treatment needs.
• Corrosion control general: amine reclaiming/filtration to remove heat-stable salts and degradation products that raise corrosivity.

A higher PWHT temperature and a tighter hardness cap may be adopted as part of the fix, but they do not address the chemistry, temperature and detail that created the cracking.

• Do not repair only the visible indications. Cracks are tight, branching, and likely longer than the MT indication. Use wet fluorescent MT over 100% of accessible internal welds and HAZs on both sides, then size every indication with UT shear wave/TOFD or a surface technique matched to the crack geometry; record depth, length, and whether it reaches the pressure-containing ligament.
• Repair by excavation and qualified repair welding, with post-repair WFMT and hardness verification; where depth or density is excessive, replace the section or shell course. Full-component replacement should be evaluated if the cracking is widespread, since weld repair of SCC in the same environment tends to recur.
• Consider upgrade at repair - stainless or clad material in the impingement zone, or a redesigned inlet to remove the two-phase impingement that seeded the cracking.
• Emphasize process side integrity checks: verify amine chemistry (lean H2S loading, heat-stable salts, concentration) and the exchanger temperature profile before returning to service, so the environment is improved rather than only the metal.
• Set the re-inspection interval from the observed crack depth and the service time, with a shorter interval for the impingement zone, and add periodic external checks (UT for wall loss, since amine systems also suffer corrosion) plus monitoring of lean amine H2S and iron content as a leading indicator of corrosivity.