12.1 Amine Stress Corrosion Cracking

Amine Stress Corrosion Cracking Overview — API RP 571 Section 3.3

1. Fundamental Definition and Phenomenological Nature

Amine Stress Corrosion Cracking (Amine SCC) is a form of alkaline stress corrosion cracking (ASCC) characterized by the initiation and propagation of intergranular surface-breaking cracks in carbon and low-alloy steels. Degradation occurs under the combined influence of residual or applied tensile stress and exposure to aqueous alkanolamine solutions used for acid gas removal (gas sweetening and sulfur recovery).

In refinery and petrochemical operations, amine units remove hydrogen sulfide ($H_2S$) and carbon dioxide ($CO_2$) from gaseous and liquid hydrocarbon streams. While the amine solution itself is alkaline, un-stress-relieved carbon steel weldments exposed to these basic chemical environments undergo localized electrochemical dissolution along grain boundaries, leading to catastrophic brittle-like structural failure if left unmitigated.

                          THE TRIAD OF AMINE STRESS CORROSION CRACKING

                                  Susceptible Metallurgy
                             (Carbon Steel & Low-Alloy Steels;
                              Non-PWHT Weld Metal & HAZ)
                                         ▲
                                        / \
                                       /   \
                                      /     \
                                     /       \
                                    /  AMINE  \
                                   /    SCC    \
                                  /             \
      Tensile Stress ◄───────────────────────────► Chemical Environment
 (Residual Welding Stresses,                  (Aqueous Alkanolamines: MEA, DEA,
  Cold Forming, Applied Loads;                 DGA, MDEA, DIPA; Lean Amine > Rich;
  Often Exceeding Base Yield)                  Operating or Ambient Temperatures)

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Alkanolamine Chemistry and Solvent Aggressiveness

Alkanolamines are organic compounds containing at least one hydroxyl group ($-OH$) and one amino group ($-NH_2$, $-NHR$, or $-NR_2$). The hydroxyl group reduces vapor pressure and increases aqueous solubility, while the amino group provides the necessary alkalinity to absorb acid gas constituents through reversible acid-base neutralization reactions.

1. Acid Gas Absorption Reactions

When sour process gas contacts an aqueous amine solution in an absorber tower, the basic amine neutralizes the acidic components:

• Hydrogen Sulfide Neutralization: $R\text{-}NH_2 + H_2S \rightleftharpoons R\text{-}NH_3^+ + HS^-$
• Carbon Dioxide Absorption (Carbamate Formation - Primary & Secondary Amines): $2R\text{-}NH_2 + CO_2 \rightleftharpoons R\text{-}NH_3^+ + R\text{-}NH\text{-}COO^-$
• Carbon Dioxide Hydrolysis (Tertiary Amines): $R_3N + H_2O + CO_2 \rightleftharpoons R_3NH^+ + HCO_3^-$

2. Relative Solvent Aggressiveness Ranking

All alkanolamine solutions can induce Amine SCC in non-stress-relieved carbon steel. However, field experience and empirical laboratory testing demonstrate distinct differences in solvent aggressiveness:

[!IMPORTANT] Exam Rule: Although MEA is the most aggressive amine and MDEA is the least aggressive, API RP 571 and API RP 945 explicitly emphasize that no amine formulation is immune to cracking. Non-stress-relieved carbon steel will crack in all commercial alkanolamine systems, including formulated and specialty solvent blends containing piperazine or physical solvent activators (e.g., sulfolane).

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Lean Amine vs. Rich Amine Cracking Kinetics

A critical distinction tested on the API 571 examination is the operational behavior of lean amine versus rich amine streams.

                               LEAN VS. RICH AMINE COMPARISON

               LEAN AMINE STREAM                          RICH AMINE STREAM
  (Stripper Bottoms / Absorber Return)           (Absorber Bottoms / Stripper Feed)
  ┌─────────────────────────────────────┐        ┌─────────────────────────────────────┐
  │ - Higher Cracking Frequency         │        │ - Lower Cracking Frequency          │
  │ - Stripped of H2S and CO2           │        │ - Saturated with Dissolved H2S/CO2  │
  │ - Lacks Protective FeS Scale        │        │ - Protective FeS Scale Maintained   │
  │ - Higher Operating Temperatures     │        │ - Lower Operating Temperatures      │
  │   (230 °F to 260 °F / 110-127 °C)   │        │   (100 °F to 140 °F / 38-60 °C)     │
  │ - Direct Active Metal Dissolution   │        │ - Acid Gas Corrosion Risk (Uniform) │
  └─────────────────────────────────────┘        └─────────────────────────────────────┘

1. The Role of Iron Sulfide ($FeS$) Passivation

In rich amine service (amine containing high dissolved concentrations of $H_2S$), the free sulfide ions react immediately with carbon steel surfaces to precipitate a dense, adherent iron sulfide ($FeS$, mackinawite) passivating film: $Fe + H_2S \rightarrow FeS + H_2$ This continuous iron sulfide layer isolates the bare metal substrate from the alkaline electrolyte, preventing localized anodic dissolution at grain boundaries and significantly reducing the incidence of stress corrosion cracking. (Note: Rich amine systems can suffer general or localized thinning if high fluid shear or $CO_2$ flash strips the protective scale, as covered in Section 3.2, but cracking itself is suppressed).

2. Vulnerability of Lean Amine Circuits

In lean amine service, the acid gas has been stripped out in the regenerator column. With negligible $H_2S$ remaining in solution, the metal cannot maintain an iron sulfide protective film. Instead, the wetted steel surface is exposed directly to the alkaline amine solvent and trace degradation products (heat-stable amine salts, bicine, glycolic acid, and amine oxides). Anodic dissolution occurs selectively along grain boundary slip steps, leading to a markedly higher frequency of cracking in lean amine piping, pumps, and regenerator bottoms compared to rich amine circuits.

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Temperature Thresholds, Residual Stress & API RP 945

1. The Myth of the "Safe Operating Temperature"

Historically, early refining specifications assumed that Amine SCC only occurred at elevated temperatures (e.g., above 140 °F or 150 °F / 60 °C to 65 °C). This assumption led to catastrophic failures when un-stress-relieved carbon steel piping operating at ambient temperatures (70 °F to 100 °F / 21 °C to 38 °C) suffered through-wall cracking within months of startup.

API RP 571 and API RP 945 conclusively document that Amine SCC can occur at ambient operating temperatures in non-PWHT carbon steel. While elevated operating temperatures accelerate cracking kinetics and shorten the incubation time, temperature alone cannot prevent cracking in non-stress-relieved components.

2. The Driving Force: Residual Welding Tensile Stress

Amine SCC is mechanically driven by tensile stress. In pressurized refinery equipment, internal operating pressure generates hoop stress; however, the primary driver for Amine SCC is residual tensile stress from welding and cold fabrication:

• As a weld puddle solidifies and cools, thermal contraction is restrained by the adjacent cold parent metal, generating localized residual tensile stresses that regularly reach or exceed the room-temperature yield strength of the steel (40,000 to 70,000+ psi / 275 to 480+ MPa).
• Unannealed cold-formed pipe bends, rolled plate sections, and cold-flanged nozzle necks possess high residual outer-fiber tensile stresses, rendering them highly vulnerable even in the absence of internal operating pressure.

3. API RP 945: The Industry Standard for Amine Service

To prevent catastrophic environmental cracking, the refining industry relies upon API Recommended Practice 945: Avoiding Environmental Cracking in Amine Units. Key recommendations of API RP 571 and API RP 945 include:

1. PWHT for Lean Amine Service:
   • RP 571 states that PWHT is now commonly recommended for all lean amine systems (excluding fresh amine) at all operating temperatures, regardless of amine type, except where the steel is completely clad or overlaid so the welds are not exposed.
   • Some refiners also PWHT rich amine equipment, for amine SCC resistance, wet H2S (SSC and SOHIC) resistance, or both.
   • RP 945 gives guidance on PWHT for the various amine services.
2. Recommended Heat Treatment Parameters:
   • Minimum holding temperature: 1175 °F ± 25 °F (635 °C ± 15 °C).
   • Minimum soak time: One hour minimum, or one hour per inch of thickness, whichever is greater.
   • This thermal treatment reduces residual welding stresses to safe, low levels (typically below 10,000 to 15,000 psi / 70 to 100 MPa) and tempers hard weld microstructures.
3. External Attachment Welds:
   • RP 571 applies the same PWHT recommendation to repair welds and to all internal and external attachment welds (pipe support shoes, lifting lugs, insulation support rings, structural clips, and ladder/platform brackets) welded directly to the pressure boundary must be post-weld heat-treated.
   • External fillet welds generate localized residual tensile stress fields that penetrate completely through the vessel or pipe wall to the internal wetted surface, initiating Amine SCC on the inside diameter directly opposite the external attachment.

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Affected Materials and Metallurgical Susceptibility

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Morphology and Physical Appearance of Damage

                    INTERGRANULAR AMINE SCC CRACK MORPHOLOGY

      Internal Wetted Surface (Alkaline Amine Solution Exposure)
      ══════════════════════════╤═════════════════════════════════
                                │ ◄── Crack initiates at ID surface
                                │
                                ├───┐ ◄── Branched intergranular path
                                │   │
               Ferrite /        │   └───┐
               Pearlite         │       │
               Grains           ├───────┘
                                │
                                ├───┐
                                │   │
                                ▼   ▼
                 Crack tips follow grain boundaries;
                 Filled with dense black oxide/sulfide scale.

1. Macroscopic Characteristics

• Surface Origin: Cracks always initiate on the internal wetted process surface and propagate outward through the wall thickness.
• Weld Orientation: Cracks are predominantly localized to the heat-affected zone (HAZ) and adjacent base metal, running parallel to the weld seam along the high-residual-stress contour of the weld toe.
• Transverse Weld Cracks: In highly restrained joints, cracks can also develop transversely across the weld metal itself, perpendicular to the weld axis.
• Scale Characteristics: Crack fissures are typically tight and filled with dark oxide or iron sulfide corrosion products (magnetite $Fe_3O_4$ or mackinawite $FeS$).
• Cracking Arrays: Damage often manifests as parallel arrays or "spider-web" networks of fine surface cracks that coalesce into macroscopic fissures.

2. Microscopic Characteristics

• Intergranular Crack Path: Under high-magnification metallographic examination (optical microscope or SEM), Amine SCC cracks propagate strictly along the prior austenite and ferrite grain boundaries (intergranular).
• Branching Morphology: Cracks exhibit extensive microscopic branching, with secondary fissures deviating from the primary crack trunk.
• Absence of Plastic Deformation: The surrounding base metal shows zero macroscopic necking, thinning, or plastic elongation, producing a classic brittle fracture appearance.

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Affected Process Units and Equipment

Amine treating systems operate across nearly every modern refining and petrochemical complex. Critical equipment assets prone to Amine SCC include:

1. Amine Absorber / Contactor Columns: High-pressure towers treating fuel gas, LPG, or hydrogen; cracking concentrates around un-stress-relieved nozzle necks, internal tray support rings, and bottom shell courses.
2. Amine Regenerators / Strippers: Low-pressure columns operating at elevated reboiler return temperatures (230 °F to 260 °F / 110 °C to 127 °C); high incidence of cracking in column bottom heads, stripper trays, and vapor return lines.
3. Rich/Lean Amine Heat Exchangers: Shell-and-tube exchangers subject to thermal cycling and high operating stresses; cracking occurs in carbon steel shells, channel heads, and tubesheets.
4. Lean Amine Coolers and Surge Tanks: Vessels and piping operating at intermediate temperatures, where historical operators frequently omitted PWHT.
5. Interconnecting Carbon Steel Piping: Piping welds, field repairs, instrument bridles, tie-in connections, and small-bore branch connections lacking post-weld stress relief.

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Inspection, NDE and Turnaround Practices

               RECOMMENDED NDE STRATEGY FOR AMINE SCC DETECTION

        Internal Surface Inspection           External Volumetric Inspection
       (During Planned Turnarounds)           (On-Stream / Non-Intrusive)
  ┌────────────────────────────────────┐ ┌────────────────────────────────────┐
  │ 1. Surface Preparation:            │ │ 1. Phased Array Ultrasonic (PAUT): │
  │    Grit blasting to white metal    │ │    Sectorial scanning of weld HAZ; │
  │    (SSPC-SP 5 / NACE No. 1).       │ │    Accurate depth & length sizing. │
  │                                    │ │                                    │
  │ 2. Primary Method: WFMT            │ │ 2. Time-of-Flight Diffraction:     │
  │    Wet Fluorescent Magnetic        │ │    (TOFD) Excellent tip-diffraction│
  │    Particle Testing with AC Yoke.  │ │    characterization of tight cracks│
  │                                    │ │                                    │
  │ 3. ACFM / High-Frequency ECT:      │ │ 3. Angle Beam Shear Wave UT:       │
  │    For tight cracks or claddings.  │ │    A-scan verification of flaw ID. │
  └────────────────────────────────────┘ └────────────────────────────────────┘

1. Internal Surface Inspection (Turnaround)

• Surface Preparation: Because Amine SCC cracks are tight and filled with tenacious iron oxide/sulfide scale, thorough surface cleaning is critical. Surfaces must be abrasive grit blasted to white metal finish (SSPC-SP 5 / NACE No. 1) prior to inspection.
• Wet Fluorescent Magnetic Particle Testing (WFMT):
  • The gold standard and primary NDE method for detecting Amine SCC on internal carbon steel surfaces.
  • Must be performed using an electromagnetic AC yoke. Alternating current creates a skin-effect magnetic field that concentrates magnetic flux precisely at the surface, providing maximum sensitivity for tight surface-breaking cracks.
  • DC yokes produce deep, subsurface magnetic flux that fails to generate adequate magnetic leakage fields across tight, shallow cracks.
• Alternating Current Field Measurement (ACFM): Highly effective for crack screening and depth estimation without requiring absolute white-metal grit blasting, and suitable for inspecting stainless steel weld overlays.

2. External Non-Intrusive Volumetric Inspection (On-Stream)

• Phased Array Ultrasonic Testing (PAUT): The primary external method for detecting, imaging, and sizing Amine SCC from the pipe or vessel exterior while equipment remains in service. Sectorial scans focused on the weld HAZ detect ID-connected crack indications.
• Time-of-Flight Diffraction (TOFD): Provides exceptional through-wall sizing accuracy by detecting ultrasonic acoustic waves diffracted from the upper and lower tips of the crack.
• Standard Angle Beam Shear Wave (UT): Effective for screening weldments using 45°, 60°, and 70° wedges to locate ID-connected planar reflectors.

3. Operational Safeguards During Turnaround Decontamination

Improper turnaround procedures can inadvertently induce or accelerate Amine SCC:

• Steam-Out Procedures: Steaming out non-PWHT carbon steel vessels containing residual amine exposes stressed steel to elevated temperatures (212 °F to 300 °F / 100 °C to 149 °C), rapidly accelerating cracking kinetics.
• Water Washing: RP 571 recommends thoroughly water-washing non-PWHT carbon steel before welding, heat treatment, or steam-out to remove amine residues.