11.2 Polythionic Acid Stress Corrosion Cracking (PASCC)

11.2 Polythionic Acid Stress Corrosion Cracking (PASCC)

Polythionic Acid Stress Corrosion Cracking (PASCC) is one of the most insidious degradation mechanisms in refining hydroprocessing assets. Governed by API RP 571 Section 3.52 and industry standard NACE SP0170, PASCC results in rapid, catastrophic intergranular failure of sensitized stainless steel and nickel alloy components. What makes PASCC distinctive is that cracking usually occurs during shutdowns, startups, or other periods when air and moisture reach sulfide-scaled surfaces, rather than during normal hot operation.

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Description of Damage & The Two-Stage Degradation Cycle

PASCC is an intergranular stress corrosion cracking (IGSCC) mechanism that requires a distinct two-stage chronological sequence:

+-----------------------------------------------------------------------------------------+
|                    STAGE 1: HIGH-TEMPERATURE SENSITIZATION (ONLINE)                     |
|                                                                                         |
|  Temperature: 750 °F - 1500 °F (400 °C - 816 °C)                                        |
|  Mechanism:   Carbon diffuses rapidly to grain boundaries.                              |
|               Chromium carbides (Cr23C6) precipitate along grain boundaries.            |
|               Adjacent grain boundary matrix is depleted of Cr to < 12%.                |
|               Concurrent formation of iron sulfide (FeS) scale from sour hydrocarbons.  |
+-----------------------------------------------------------------------------------------+
                                             |
                                             v
+-----------------------------------------------------------------------------------------+
|                  STAGE 2: ACID FORMATION & CRACK PROPAGATION (OFFLINE)                  |
|                                                                                         |
|  Environment: Unit shuts down, cools below water dew point (< 212 °F / 100 °C).         |
|  Reaction:    FeS + O2 (Air) + H2O (Moisture) -> H2SxO6 (Polythionic Acid) + Fe2O3      |
|  Failure:     Polythionic acid rapidly dissolves Cr-depleted grain boundaries.          |
|               Residual tensile stresses drive catastrophic intergranular cracking.      |
|               Full through-wall failure can occur in hours to days.                     |
+-----------------------------------------------------------------------------------------+

1. The Sensitization Phenomenon (Online Service)

In standard 300-series austenitic stainless steels (such as Type 304 and Type 316), carbon is held in solid solution at elevated fabrication temperatures. However, when these alloys operate within or cool slowly through the sensitization temperature window of 750 °F to 1500 °F (400 °C to 816 °C), carbon atoms diffuse rapidly to the grain boundaries. There, carbon combines with matrix chromium to form chromium carbides—predominantly $Cr_{23}C_6$.

Because chromium diffuses through the metallic crystal lattice hundreds of times more slowly than interstitial carbon, the chromium required to form $Cr_{23}C_6$ is drawn almost entirely from the microscopic zone immediately adjacent to the grain boundary. Consequently, the chromium concentration in this grain boundary envelope drops from its bulk value of $16\%\text{ to }18\%$ down to below 12% by weight. In metallurgical engineering, $12\%\text{ Cr}$ is the absolute thermodynamic threshold required to maintain a protective, passive chromic oxide ($Cr_2O_3$) barrier. These depleted boundaries are rendered chemically "active" and defenseless against acidic attack.

2. Acid Generation During Outages (Downtime Service)

During normal desulfurization or hydroprocessing operation, sulfur in the hydrocarbon feed reacts with steel and alloy surfaces to produce an adherent iron sulfide ($FeS$) corrosion scale. As long as the unit remains hot and pressurized with hydrogen, no liquid water can exist. When the unit is shut down for turnaround and cooled below the water dew point, ambient air (oxygen, $O_2$) and liquid water (moisture, condensation, or steam-out water) enter the equipment.

The iron sulfide scale reacts spontaneously with oxygen and liquid moisture to synthesize a family of polythionic sulfur acids ($H_2S_xO_6$, where $x = 3, 4, \text{ or } 5$):

$8\text{FeS} + 11\text{O}_2 + 2\text{H}_2\text{O} \longrightarrow 2\text{H}_2\text{S}_4\text{O}_6 + 4\text{Fe}_2\text{O}_3$

These polythionic acids (trithionic, tetrathionic, and pentathionic acids) are weak organic sulfur acids, but they possess extreme specificity: they aggressively attack the unpassivated, chromium-depleted grain boundaries. Under the influence of residual tensile stresses from welding or cold forming, intergranular cracks propagate through the component wall within hours to several days of air exposure.

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Affected Materials & Vulnerability Hierarchy

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Critical Factors Driving Cracking

1. Degree of Sensitization: Driven by the alloy's carbon content and the time-at-temperature within the 750 °F to 1500 °F (400 °C to 816 °C) window. Higher carbon grades (e.g., 304H) sensitize faster and more severely than low-carbon 'L' grades.
2. Presence of Sulfide Scale: An iron sulfide ($FeS$) or metal sulfide scale layer deposited during sour hydrocarbon service is the mandatory chemical precursor for acid generation.
3. Coexistence of Oxygen and Liquid Water: Oxygen ($O_2$) and liquid water ($H_2O$) must both be present simultaneously. If equipment is opened to dry air (zero moisture) or purged with deaerated water (zero oxygen), polythionic acid cannot form.
4. Tensile Stress: Sustained tensile stresses are required. Residual stresses from welding (which approach base metal yield strength in un-PWHT joints) and cold tube bending are more than sufficient to drive cracking without any applied mechanical or internal pressure stress.

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Microscopic Morphology & Appearance of Damage

PASCC displays a distinct failure morphology that differentiates it sharply from chloride SCC:

• Microscopic Path: Cracking is strictly intergranular (IG), propagating exclusively along the prior austenitic grain boundaries where chromium carbide precipitation occurred. The crack path follows a classic "sugar-cube" or rock-candy crystalline profile.
• Branching Signature: Highly branched crack networks traversing three-dimensional grain boundary junctions.
• Fracture Characteristics: Complete absence of macroscopic ductility, necking, or uniform wall thinning. Components fracture in a brittle manner under minor mechanical impact or residual stress.
• Deposits & Residues: Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) reveal significant sulfur and iron oxide deposits inside the crack fissures.

   CHLORIDE SCC (Section 3.17)              POLYTHIONIC ACID SCC (Section 3.52)
+-----------------------------------+     +-----------------------------------+
| Transgranular (TG) Cleavage       |     | Intergranular (IG) Grain Boundary |
| Cuts directly through grains      |     | Follows prior austenite boundaries|
| 'Lightning-bolt' / feathery       |     | 'Rock-candy' / crystalline path   |
| Occurs during ONLINE operation    |     | Occurs during OFFLINE shutdowns   |
| Driven by Chlorides + O2          |     | Driven by FeS + Air + Moisture    |
+-----------------------------------+     +-----------------------------------+

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Affected Refining Units & Equipment

PASCC is prevalent in high-temperature refining units processing sour (sulfur-bearing) feeds:

• Hydroprocessing Units (Hydrotreaters & Hydrocrackers):
  • Reactor feed/effluent heat exchangers.
  • Reactor effluent air coolers (REAC) and inlet headers.
  • High-pressure separator piping and internal vessel cladding ($321\text{ or }347\text{ SS}$ weld overlay).
  • Hydroprocessing furnace tubes and transfer lines.
• Catalytic Reforming Units: Furnace tubes, cross-over piping, and effluent heat exchangers where trace sulfur is present.
• Fluid Catalytic Cracking (FCC) Units: Fractionator overhead circuits, light cycle oil piping, and slurry pumparound exchangers.
• Delayed Coker Units: Furnace tubes and hot vapor piping operating in the sensitization range.

Prevention, Mitigation & NACE SP0170 Protocols

Because the damage occurs during shutdowns, turnaround procedures must strictly follow NACE SP0170 (Protection of Austenitic Stainless Steels and Other Austenitic Alloys from PTA SCC during Shutdown of Refinery Equipment).

1. NACE SP0170 Shutdown Operational Protocols

When shutting down hydroprocessing units containing sensitized stainless steel, the NACE SP0170 protection methods are applied before equipment cools below the water dew point:

1. Nitrogen Purging & Blanketing:
   • Maintain an inert, dry nitrogen purge or blanket inside the equipment during cooldown, downtime, and maintenance so oxygen cannot reach the sulfide scale.
   • Eliminating oxygen prevents the oxidation of iron sulfide to polythionic acid.
2. Alkaline Neutralization Wash (Soda Ash Wash):
   • Circulate or spray an aqueous alkaline solution containing 2 wt% sodium carbonate ($Na_2CO_3$, soda ash) and an alkaline-stable surfactant / wetting agent.
   • Soda ash neutralizes any acidic species that form and leaves an alkaline film on internal surfaces that prevents acid attack if air subsequently enters.
   • Critical Exam Rule: Sodium hydroxide ($NaOH$, caustic) must NEVER be used for neutralization because caustic residues can induce catastrophic Caustic Stress Corrosion Cracking (API RP 571 Section 3.15) during subsequent unit startup!
3. Dehumidified Dry Air Purging:
   • If personnel must enter the vessel for internal inspection without a soda ash wash, purge with dry, dehumidified air whose dew point is low enough that no liquid water can condense on the equipment.
   • Keeping relative humidity near zero ensures that moisture cannot condense to create the liquid water phase necessary to synthesize polythionic acid.

2. Metallurgical Controls & Thermal Stabilization

Long-term engineering prevention relies on specifying stabilized alloys and thermal stabilization treatments:

• Specify Chemically Stabilized Grades:
  • Type 321 Stainless Steel: Chemically stabilized with Titanium ($Ti \ge 5 \times \%C$).
  • Type 347 Stainless Steel: Chemically stabilized with Niobium / Columbium ($Nb \ge 8 \times \%C\text{ or }10 \times \%C$).
  • In these alloys, titanium or niobium has a far higher thermodynamic affinity for carbon than chromium does, preferentially tying up carbon as $TiC$ or $NbC$.
• Thermal Stabilization Heat Treatment (Mandatory for Severe Service):
  • Standard fabrication welding can still cause local chromium carbide precipitation in stabilized grades. To achieve complete protection, specify a post-fabrication thermal stabilization heat treatment at 1600 °F to 1650 °F (870 °C to 900 °C) for a minimum holding time of 4 hours.
  • At this specific temperature, chromium carbides dissolve while highly stable titanium or niobium carbides precipitate, leaving all matrix chromium uniformly distributed in solid solution to maintain passivity.
• Low Carbon 'L' Grades (304L, 316L): Limit carbon to $\le 0.03\%$. Effective for short thermal exposures and lower temperatures, but will sensitize if exposed to temperatures $> 750\text{ °F}$ for extended operational periods.

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Inspection & Non-Destructive Examination (NDE)