12.2 Carbonate Stress Corrosion Cracking & Ammonia SCC

Carbonate Stress Corrosion Cracking (ACSCC) — API RP 571 Section 3.12

1. Definition and Distinction from Other Alkaline Cracking Modes

Carbonate Stress Corrosion Cracking, also known as Alkaline Carbonate Stress Corrosion Cracking (ACSCC), is an environmental cracking mechanism that affects carbon steel and low-alloy steels exposed to alkaline sour water environments containing dissolved carbon dioxide ($CO_2$), ammonia ($NH_3$), and hydrogen sulfide ($H_2S$).

Candidates must maintain a strict technical distinction between ACSCC and other alkaline cracking mechanisms:

• Amine SCC (Section 3.3): Requires the presence of alkanolamine chemical solvents (MEA, DEA, MDEA, etc.) used in acid gas treating units.
• Caustic SCC (Section 3.15): Driven by concentrated sodium hydroxide ($NaOH$) or potassium hydroxide ($KOH$) solutions added for neutralization or chemical treating.
• Carbonate SCC (Section 3.12): Occurs in sour process water where no amine and no caustic are present. The alkaline pH and cracking driving force arise naturally from process chemistry—specifically the dissolution and interaction of process-generated ammonia, carbon dioxide, and hydrogen sulfide.

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Environmental Drivers and Water Chemistry of ACSCC

ACSCC occurs primarily in the overhead and gas recovery sections of Fluid Catalytic Cracking (FCC) units, as well as in sour water strippers (SWS) and delayed coker gas concentration plants. The damage mechanism is governed by a delicate chemical equilibrium between dissolved gases in the condensed aqueous phase.

                    FCC PROCESS WATER CHEMISTRY EQUILIBRIUM FOR ACSCC

       Process Vapor: CO2 (Combustion / Cracking)  +  NH3 (Feed Nitrogen Hydrocracking)
                                         │
                                         ▼ Condensed Aqueous Sour Water Phase
       Dissolution Reactions:
       1. CO2 + H2O ◄───► H2CO3 ◄───► H+ + HCO3- (Bicarbonate Ion)
       2. NH3 + H2O ◄───► NH4+ + OH- (Generates Alkalinity: Drives pH to 8.0 - 10.0)
                                         │
                                         ▼ Elevated Alkaline pH Shifts Equilibrium
       3. HCO3- + OH- ◄───► CO3(2-) + H2O (Free Carbonate Ion Generation)
                                         │
                                         ▼ Carbonate Above ~100 ppmw (pH-Dependent)
                       ALKALINE CARBONATE STRESS CORROSION CRACKING
                                         │
                 ┌───────────────────────┴───────────────────────┐
                 ▼                                               ▼
       Absence of Cyanides:                            Presence of Cyanides (HCN):
       Cracking moderated by thin                      Cyanide reacts with iron sulfide:
       protective FeS surface film                     FeS + 6CN- → [Fe(CN)6](4-) + S(2-)
       (Cracking occurs in non-PWHT welds)             Scale stripped → Severe Accelerated Cracking

1. The Carbonate Ion Equilibrium

In FCC fractionator overheads, $CO_2$ is generated by catalyst regeneration flue gas blow-by or cracking reactions, while $NH_3$ is produced by the thermal breakdown of nitrogen-containing compounds in the heavy gas oil feed:

1. Carbon dioxide dissolves into aqueous water condensate to form carbonic acid ($H_2CO_3$), which dissociates into hydrogen ions and bicarbonate ions ($HCO_3^-$).
2. Ammonia gas dissolves readily in water, forming ammonium ($NH_4^+$) and hydroxide ($OH^-$) ions, driving the solution pH upward into the alkaline range (pH 8.0 to 10.0).
3. Under these alkaline conditions, hydroxide ions deprotonate bicarbonate, shifting the equilibrium toward the formation of free carbonate ions ($CO_3^{2-}$): $HCO_3^- + OH^- \rightleftharpoons CO_3^{2-} + H_2O$

2. Critical Chemical Thresholds

• Carbonate Concentration: RP 571 notes that carbonate levels above about 100 ppmw have been suggested as capable of causing ACSCC, depending on pH, although a precise threshold is difficult to define. Likelihood increases with carbonate concentration.
• Solution pH: RP 571 states that ACSCC does not occur below pH 7; susceptibility spans roughly pH 7.5 to 11, most failures have occurred between pH 8 and 10, and the likelihood generally increases as pH increases.
• The Synergistic Role of Cyanides ($HCN$): Hydrogen cyanide is generated in FCC cracking reactors from feed-bound nitrogen. In the aqueous overhead condensate, cyanide acts as a severe cracking promoter. Cyanide ions react with the protective iron sulfide ($FeS$) film on the steel surface, forming soluble ferrocyanide complexes ($[Fe(CN)_6]^{4-}$): $FeS + 6CN^- \rightarrow [Fe(CN)_6]^{4-} + S^{2-}$ By actively dissolving and stripping the passivating iron sulfide scale, cyanides expose bare metal at high-stress weld toes to aggressive carbonate cracking.

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Critical Factors, Morphology and Mitigation of ACSCC

1. Mechanical Stress and Post-Weld Heat Treatment

Like Amine SCC, ACSCC is driven by residual tensile stresses resulting from welding and cold fabrication. Un-stress-relieved welds containing residual stresses approaching the yield point of the base material are exceptionally prone to cracking. Operational applied stresses (internal pressure, thermal expansion) contribute, but residual stress is the necessary condition.

2. Morphology of ACSCC

• Macroscopic Appearance: Cracks initiate on the internal process-wetted surface and propagate outward. They concentrate predominantly in the heat-affected zone (HAZ) of carbon steel welds, running parallel to the weld seam; RP 571 notes the cracks are usually within about 2 in. (50 mm) of the weld.
• Spider-Web Arrays: Away from the immediate weld toe, cracking can develop in adjacent base metal as interconnected, multi-directional "spider-web" or craze-cracking patterns.
• Microscopic Path: The crack path is strictly intergranular (following ferrite grain boundaries) and exhibits extensive branching. Cracks are filled with black iron oxide and iron carbonate corrosion products.

3. Prevention and Mitigation Strategies for ACSCC

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Ammonia Stress Corrosion Cracking — API RP 571 Section 3.4

Ammonia Stress Corrosion Cracking manifests in two completely distinct metallurgical and industrial operational environments:

1. Copper-Zinc Alloys (Brasses): Cracking under exposure to moist ammonia, air, and moisture (historically termed "season cracking").
2. Carbon Steels: Cracking in pressurized storage and transport equipment handling anhydrous ammonia ($NH_3$).

                    THE DUAL DOMAINS OF AMMONIA STRESS CORROSION CRACKING

               COPPER-ZINC ALLOYS (BRASSES)                     CARBON STEEL
             "Season Cracking" in Moist NH3             Anhydrous Ammonia Equipment
  ┌───────────────────────────────────────────────┐ ┌───────────────────────────────────────────────┐
  │ - Affected: Yellow Brass (>15% Zn),           │ │ - Affected: Carbon steel storage spheres,     │
  │   Admiralty Brass, Aluminum Brass.            │ │   transport bullets, piping, nurse tanks.     │
  │ - Resistant: 70/30 Cu-Ni, Pure Copper.        │ │ - Environment: Liquid/vapor anhydrous NH3     │
  │ - Environment: Trace NH3 + Moisture + Air/O2. │ │   contaminated with trace Oxygen (>1-5 ppm).  │
  │ - Mechanism: Cupric-ammonium complex ion      │ │ - Mechanism: Cathodic depolarization and film │
  │   formation: [Cu(NH3)4](2+).                  │ │   rupture under residual welding tensile stress.│
  │ - Morphology: Intergranular or transgranular. │ │ - Morphology: Fine intergranular weld cracks. │
  │ - Mitigation: Stress relief annealing;        │ │ - Mandatory Mitigation: PWHT + addition of    │
  │   upgrade to 70/30 Cu-Ni or titanium.         │ │   minimum 0.2 wt% Water (H2O) inhibitor.      │
  └───────────────────────────────────────────────┘ └───────────────────────────────────────────────┘

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Ammonia SCC in Copper-Zinc Alloys (Season Cracking)

1. Historical Background and Metallurgy

The phenomenon of "season cracking" was first identified during the 19th-century British military campaigns in India. During the warm, humid monsoon season, deep-drawn brass rifle cartridge cases stored near stables cracked spontaneously along internal residual stress contours due to trace ammonia vapor off-gassing from horse urine.

In modern refining and chemical plants, copper alloys are widely deployed in cooling water heat exchanger tubing, surface condensers, and overhead product coolers.

2. Metallurgical Susceptibility Hierarchy

The susceptibility of copper alloys to ammonia cracking depends directly on zinc content:

• Yellow Brass (Cu-30Zn): Contains $\ge 15\%\text{ to }30\%\text{ Zn}$; highest susceptibility to rapid cracking.
• Admiralty Brass (UNS C44300 / Cu-28Zn-1Sn-0.04As): Formulated with tin and arsenic to prevent dealloying (dezincification), but remains highly vulnerable to ammonia SCC in refinery condensers.
• Aluminum Brass (UNS C68700 / Cu-22Zn-2Al-0.04As): Vulnerable to ammonia SCC in the presence of air and moisture.
• Copper-Nickel Alloys (90/10 Cu-Ni [UNS C70600] and 70/30 Cu-Ni [UNS C71500]): Contain zero zinc; highly resistant to ammonia SCC. 70/30 Cu-Ni is virtually immune under typical refining operating conditions.
• Pure Copper: Essentially immune to ammonia SCC under practical operating limits.

3. Chemical Mechanism and Environmental Requirements

Cracking of brass requires the simultaneous presence of four components:

1. Ammonia or Ammonium Ions: Concentrations as low as a few parts per million (ppm) are sufficient to trigger cracking.
2. Moisture / Liquid Water: Necessary to form an aqueous electrolyte film on the metal surface.
3. Oxygen or Oxidizers: Dissolved oxygen is mandatory. Oxygen oxidizes cuprous ($Cu^+$) ions to cupric ($Cu^{2+}$), which reacts with ammonia to synthesize the aggressive complex ion tetraamminecopper(II), $[Cu(NH_3)_4]^{2+}$: $2Cu + 8NH_3 + O_2 + 2H_2O \rightarrow 2[Cu(NH_3)_4]^{2+} + 4OH^-$ This soluble complex dissolves copper preferentially at grain boundaries or slip planes.
4. Tensile Stress: Residual stresses from tube rolling into tubesheets, cold drawing, or U-tube bending.

4. Morphology in Copper Alloys

• Cracking can be intergranular (predominant in low-stress conditions or annealed microstructures) or transgranular (predominant in cold-worked alloys or Admiralty brass exposed to aggressive conditions).
• Cracks show extensive branching and are filled with dark, copper-rich oxide products.

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Anhydrous Ammonia SCC of Carbon Steel

1. Process Environments and Critical Oxygen Contamination

Anhydrous ammonia (ammonia containing negligible water) is stored and transported as a pressurized liquefied gas at ambient temperatures or refrigerated at $-28\text{ °F }(-33\text{ °C})$. In chemical plants, fertilizer production, and refinery SCR/SNCR NOx control systems, carbon steel is the standard construction material for tanks, spheres, railcars, and piping.

Carbon steel handles pure, air-free anhydrous ammonia without degradation. However, if the ammonia is contaminated with trace quantities of air or oxygen ($O_2 > 1\text{ to }5\text{ ppm}$), severe, brittle-like stress corrosion cracking occurs in un-stress-relieved weldments.

2. The 0.2 wt% Water Inhibition Mandate

The most critical engineering fact concerning anhydrous ammonia cracking on the API 571 exam is the inhibitive role of water:

• Water Addition: Adding a minimum of 0.2 wt% water (2,000 ppmw) to anhydrous ammonia completely inhibits stress corrosion cracking of carbon steel under normal ambient handling.
• Mechanism of Inhibition: Dissolved water promotes rapid passivation of the steel surface by forming an adherent, protective iron oxide/hydroxide film that halts localized anodic dissolution at the crack tip, even in the presence of oxygen contamination.
• Refrigerated Storage Exemption / Caution: Atmospheric, refrigerated storage tanks operating at $-28\text{ °F }(-33\text{ °C})$ experience an extremely low cracking incidence due to reduced electrochemical kinetics. However, high-stress welds in ambient-temperature pressure spheres and transport bullets require both PWHT and water inhibition.

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Inspection and NDE Methods for ACSCC and Ammonia SCC

                     INSPECTION METHODS SUMMARY

  Damage Mode          Primary Internal Method         Primary External Method
  ────────────────────────────────────────────────────────────────────────────
  Carbonate SCC        Wet Fluorescent Magnetic        Phased Array Ultrasonic (PAUT)
  (Carbon Steel)       Particle Testing (WFMT)         Time-of-Flight Diffraction (TOFD)
                       Using AC Electromagnetic Yoke   Angle Beam Shear Wave UT
  ────────────────────────────────────────────────────────────────────────────
  Ammonia SCC          Internal Visual (VT) /          Phased Array Ultrasonic (PAUT)
  (Carbon Steel)       WFMT on Vessel Welds            Acoustic Emission Testing (AET)
  ────────────────────────────────────────────────────────────────────────────
  Ammonia SCC          Eddy Current Testing (ECT)      N/A (Exchanger Tubing —
  (Copper Alloys)      Internal Bobbin Probe           Requires Internal Bore Access)

1. NDE for Carbonate SCC (Carbon Steel)

• WFMT (AC Yoke): The primary surface inspection technique for internal vessel and piping welds during shutdowns. Requires white-metal abrasive blasting (SSPC-SP 5).
• PAUT and TOFD: Volumetric ultrasonic examination from the vessel OD allows non-intrusive detection and sizing of ID-connected HAZ cracks while equipment is on-stream.

2. NDE for Ammonia SCC in Heat Exchanger Tubes (Copper Alloys)

• Eddy Current Testing (ECT): The standard non-destructive technique for inspecting non-ferromagnetic Admiralty and aluminum brass tubes from the inside diameter using a bobbin probe. ECT reliably identifies both circumferential and longitudinal cracking, wall thinning, and localized pitting.