10.3 Sulfide Stress Cracking (SSC)

API 571 Corrosion and Materials Exam

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1 Chapter 1: Exam Structure, Terminology & Corrosion Fundamentals

1.1 API 571 Certification, Exam Structure & RP 571 Framework 1.2 Corrosion Science Fundamentals: Electrochemical Cells, Polarization & Passivity 1.3 Key Process Variables, Morphology Patterns & NDE Terminology

2 Chapter 2: Atmospheric, Water & Soil Corrosion

2.1 Atmospheric Corrosion & Soil Corrosion 2.2 Corrosion Under Insulation (CUI) 2.3 Cooling Water, Boiler Water & Oxygenated Process Water Corrosion

3 Chapter 3: Microbial, Electrochemical & Velocity-Induced Metal Loss

3.1 Microbiologically Influenced Corrosion (MIC) & Brine Corrosion 3.2 Galvanic Corrosion, Concentration Cell Corrosion & Dealloying 3.3 Erosion/Erosion-Corrosion & Cavitation

4 Chapter 4: Refining Sour Water & Overhead Systems Corrosion

4.1 Acidic Sour Water & Alkaline Sour Water (Ammonium Bisulfide) Corrosion 4.2 Ammonium Chloride and Amine Hydrochloride Corrosion 4.3 Flue Gas Dew Point Corrosion & Aqueous Organic Acid Corrosion

5 Chapter 5: Acid Gas, Amine & Mineral Acid Systems

5.1 Amine Corrosion & Carbon Dioxide (CO2) Sweet Corrosion 5.2 Hydrochloric Acid (HCl) Corrosion & Phenol Corrosion 5.3 Caustic Corrosion (Caustic Gouging)

6 Chapter 6: Heavy Oil, Alkylation & Specialty Acid Services

6.1 Naphthenic Acid Corrosion (NAC) 6.2 Sulfuric Acid Corrosion & Phosphoric Acid Corrosion 6.3 Hydrofluoric (HF) Acid Corrosion

7 Chapter 7: High-Temperature Oxidation & Sulfidation

7.1 High-Temperature Oxidation 7.2 Sulfidation in H2-Free Environments 7.3 High-Temperature H2/H2S Corrosion (Couper-Gorman)

8 Chapter 8: High-Temperature Carbon, Nitrogen & Ash Reactions

8.1 Carburization, Decarburization & Metal Dusting 8.2 Nitriding, Fuel Ash Corrosion & Refractory Degradation 8.3 Gaseous Oxygen-Enhanced Ignition and Combustion

9 Chapter 9: High-Temperature Hydrogen Attack & High-Temp Rupture

9.1 High-Temperature Hydrogen Attack (HTHA) Mechanisms & Nelson Curves 9.2 HTHA Inspection, Detection Methods & Mitigation 9.3 Creep, Stress Rupture & Short-Term Overheating

10 Chapter 10: Wet H2S Cracking Mechanisms

10.1 Hydrogen Blistering & Hydrogen-Induced Cracking (HIC) 10.2 Stress-Oriented Hydrogen-Induced Cracking (SOHIC) 10.3 Sulfide Stress Cracking (SSC)

11 Chapter 11: Stainless Steel & Nickel Alloy Stress Corrosion Cracking

11.1 Chloride Stress Corrosion Cracking (Cl- SCC) 11.2 Polythionic Acid Stress Corrosion Cracking (PASCC) 11.3 Caustic Stress Corrosion Cracking (Caustic Embrittlement)

12 Chapter 12: Environmental Cracking in Chemical & Refining Services

12.1 Amine Stress Corrosion Cracking 12.2 Carbonate Stress Corrosion Cracking & Ammonia SCC 12.3 HF Cracking (HSC & HF SCC of Ni Alloys) & Ethanol SCC

13 Chapter 13: Mechanical, Thermal & Corrosion Fatigue

13.1 Mechanical and Vibration-Induced Fatigue 13.2 Thermal Fatigue & Thermal Shock 13.3 Corrosion Fatigue

14 Chapter 14: Metallurgical Phase Embrittlement & Cast Iron Degradation

14.1 885 °F (475 °C) Embrittlement 14.2 Sigma Phase Embrittlement 14.3 Graphitic Corrosion of Cast Irons

15 Chapter 15: Thermal Degradation of Microstructure & Grain Structure

15.1 Graphitization 15.2 Spheroidization (Softening) 15.3 Temper Embrittlement & Strain Aging

16 Chapter 16: Toughness Loss, Welding & Dissimilar Metal Cracking

16.1 Brittle Fracture 16.2 Dissimilar Metal Weld (DMW) Cracking 16.3 Stress Relaxation Cracking (Reheat Cracking)

17 Chapter 17: Environmental Embrittlement & Specialized Hydriding

17.1 Hydrogen Embrittlement (HE) 17.2 Liquid Metal Embrittlement (LME) 17.3 Titanium Hydriding

10.3 Sulfide Stress Cracking (SSC)

Key Takeaways Explain This Topic AI

Sulfide Stress Cracking (SSC, API RP 571 Section 3.67) is a severe form of hydrogen embrittlement cracking occurring in high-strength steels or hard weld heat-affected zones (HAZs) under the combined action of tensile stress and wet H2S electrochemical corrosion.

Hardness is the primary metallurgical governing factor: SSC susceptibility escalates dramatically when carbon steel hardness exceeds 22 HRC (Rockwell C 22, equivalent to 237 HBW / 248 HV), the foundational threshold codified in NACE MR0103 / ISO 17945 and NACE MR0175 / ISO 15156.

API RP 571 notes that SSC usually occurs at temperatures below about 180 °F (82 °C); susceptibility decreases as temperature rises because hydrogen diffuses out of the steel faster.

Susceptibility is highest in untempered martensite and bainite; resistance comes from tempered, fine-grained microstructures, controlled carbon equivalent (CE <= 0.43), hardness limits (NACE SP0472 commonly limits refinery weld deposits to 200 HBW), and PWHT where needed.

Wet Fluorescent Magnetic Particle Testing (WFMT) utilizing an AC electromagnetic yoke is the industry standard and most sensitive non-destructive examination technique for detecting surface-breaking SSC cracks on internal process-wetted surfaces.

Last updated: September 2026

• Mechanistic Definition and Physics of Sulfide Stress Cracking (SSC)
• 1. Phenomenological Description (API RP 571 Section 3.67)

Sulfide Stress Cracking (SSC) is a catastrophic, brittle failure mechanism resulting from the synergistic combination of tensile stress and hydrogen embrittlement in an aqueous hydrogen sulfide ( H 2 S H_2S H 2 ​ S ) environment.

Unlike Hydrogen Blistering, HIC, and SOHIC—which involve internal void pressurization by molecular hydrogen gas ( H 2 H_2 H 2 ​ )— SSC does not involve the formation of molecular gas cavities .

Instead, SSC is a true hydrogen embrittlement phenomenon operating directly at the sub-microscopic crystal lattice level. Dissolved atomic hydrogen enters the stressed metallic lattice, migrates to regions of intense triaxial tensile stress, and lowers the interatomic cohesive strength of the metal, leading to sudden, brittle crack initiation and rapid propagation at stresses far below the material's nominal yield strength.

2. The Hydrogen Embrittlement Mechanism: HEDE and HELP Models

At the metallurgical and crystal lattice scale, SSC operates via two widely accepted scientific mechanisms:

Hydrogen-Enhanced Decohesion (HEDE) :

Interstitial atomic hydrogen ( H ∙ H^\bullet H ∙ ) dissolved in the body-centered cubic (BCC) ferrite or martensite lattice is drawn by stress gradients toward regions of maximum triaxial tensile stress (such as the hydrostatic stress field immediately ahead of a sharp crack tip or notch).

The accumulation of high local hydrogen concentrations at grain boundaries and atomic planes weakens the electron bonds between adjacent iron atoms.

When local tensile stresses exceed this reduced interatomic bond strength, the lattice ruptures via brittle cleavage along crystallographic planes or intergranular separation along prior austenite grain boundaries.

Hydrogen-Enhanced Localized Plasticity (HELP) :

Dissolved hydrogen shields the elastic stress fields of crystal dislocations, lowering the barrier for dislocation motion and localized plastic shear.

Highly localized micro-plastic deformation occurs along narrow slip bands at crack tips, leading to premature micro-void coalescence and brittle-like macro-fracture with negligible overall macroscopic elongation. THE SSC INITIATION & PROPAGATION CYCLE

• Wet Sour Environment (H2S + H2O)
• │
• ▼ Cathodic Corrosion: 2H+ + 2e- → 2H• (Poisoned by S2-, HS-, CN-)
• High Surface Concentration of Nascent Atomic Hydrogen (H•)
• │
• ▼ Interstitial Lattice Diffusion into High-Strength / Hard Steel
• Permeation Toward Regions of Maximum Triaxial Tensile Stress (Crack Tips, Hard HAZ)
• │
• ▼ Lattice Trapping at Hard Microstructures (Untempered Martensite / >22 HRC)
• Severe Embrittlement (Interatomic Bond Weakening via HEDE / HELP Models)
• │
• ▼ Applied Mechanical Load + Un-Relieved Welding Residual Stress
• Sudden, Catastrophic Brittle Crack Initiation & Rapid Transgranular / Intergranular Fracture
• Critical Factors Governing SSC Susceptibility

Susceptibility to Sulfide Stress Cracking is governed by a precise interplay of material hardness, metallurgical microstructure, tensile stress magnitude, environmental pH, and operating temperature.

1. Material Hardness and Strength Thresholds (The 22 HRC Rule)

Material hardness is the single most critical metallurgical variable dictating SSC vulnerability.

The 22 HRC Threshold : Decades of laboratory testing and refinery operating experience establish that carbon and low-alloy steels exhibit high resistance to SSC when bulk hardness and localized weld heat-affected zone (HAZ) hardness remain ≤ 22 HRC \le 22\text{ HRC} ≤ 22 HRC (Rockwell C 22) .

Hardness Equivalents : 22 HRC corresponds to approximately 237 HBW 237\text{ HBW} 237 HBW (Brinell Hardness) and 248 HV 248\text{ HV} 248 HV (Vickers Hardness) .

When steel hardness exceeds 22 HRC, the critical stress intensity factor required for crack initiation ( K I S S C K_{ISSC} K I SSC ​ ) drops catastrophically, permitting cracks to propagate at stresses representing a tiny fraction of the material's yield strength.

High-Strength Alloys : High-strength carbon steels (yield strength > 80 ksi / 550 MPa >80\text{ ksi} / 550\text{ MPa} > 80 ksi /550 MPa ) are inherently prone to SSC in sour environments unless specifically quenched, tempered, and qualification-tested.

2. Metallurgical Microstructure

The crystalline microstructure of the steel dramatically influences hydrogen trap density and embrittlement resistance:

Untempered Martensite & Bainite (Extremely Susceptible) : Produced when high-carbon-equivalent weldments or rapid cooling cycles occur. The body-centered tetragonal (BCT) lattice of untempered martensite contains high internal strain, high dislocation density, and low fracture toughness, making it exceptionally vulnerable to SSC.

Tempered Martensite / Tempered Bainite (Moderately Resistant) : Subjecting martensite to high tempering temperatures relieves lattice strain and precipitates fine, spherical carbides.

Normalized / Annealed Ferrite-Pearlite (Highly Resistant) : Fine-grained, equiaxed ferritic-pearlitic microstructures exhibit superior SSC resistance, provided localized hard bands are absent.

3. Operating Metal Temperature Envelope

Unlike most corrosion mechanisms that accelerate exponentially as temperature rises, SSC exhibits an inverted temperature dependency :

Maximum Susceptibility : SSC is most likely near ambient temperature; RP 571 notes that SSC usually occurs below about 180 °F (82 °C) .

High-Temperature Drop-Off : As temperature rises, susceptibility decreases because hydrogen diffuses out of the steel faster.

Physical Reason : At elevated temperatures, the thermal kinetic energy of hydrogen atoms increases, accelerating hydrogen diffusion rates out of the steel lattice and preventing hydrogen from remaining trapped in high-stress crack tip fields. Furthermore, the cohesive bonding strength of the iron lattice improves, and plastic slip blunts crack tips.

Sub-Freezing Temperatures : Below 32 ∘ F ( 0 ∘ C ) 32\text{ }^\circ\text{F} (0\text{ }^\circ\text{C}) 32 ∘ F ( 0 ∘ C ) , water freezes, eliminating the aqueous electrolyte necessary for electrochemical corrosion and hydrogen charging.

4. Environmental pH, H 2 S H_2S H 2 ​ S Partial Pressure, and Cyanides

Low pH Regime (pH 2 to 4) : Severe hydrogen charging occurs due to massive hydrogen ion ( H + H^+ H + ) availability. SSC can initiate at lower stress levels and lower hardness thresholds.

Near-Neutral Regime (pH 5 to 7) : Moderate charging; standard 22 HRC hardness controls provide robust protection.

Alkaline Sour Water Regime (pH > 8.5) with Cyanides : In refinery Fluid Catalytic Cracking (FCC) and coker gas recovery plants, alkaline sour water contains ammonium bisulfide ( N H 4 H S NH_4HS N H 4 ​ H S ) and cyanides ( C N − CN^- C N − ). Cyanides dissolve protective iron sulfide films, driving severe hydrogen flux and inducing SSC in steels even at near-neutral or slightly alkaline bulk pH.

Industry Standards: NACE MR0103 / ISO 17945 vs NACE MR0175 / ISO 15156

Refinery inspection and maintenance personnel must distinguish between the two primary international standards governing sour service materials:

Feature / Scope NACE MR0103 / ISO 17945 NACE MR0175 / ISO 15156

Application Scope Petroleum Refining & Downstream Plants (process units, treaters, sour water systems). Upstream Oil & Gas Production (wellhead, flowlines, gathering, offshore platforms).

Environmental Definition Defines sour service based on process unit experience, dissolved H 2 S H_2S H 2 ​ S ( > 50 ppmw >50\text{ ppmw} > 50 ppmw ), or p H 2 S > 0.05 psia p_{H_2S} > 0.05\text{ psia} p H 2 ​ S ​ > 0.05 psia . Uses environmental severity regions (0, 1, 2, 3) plotted against in-situ pH and H 2 S H_2S H 2 ​ S partial pressure.

Carbon Steel Hardness Limit ≤ 22 HRC \le 22\text{ HRC} ≤ 22 HRC (237 HBW / 248 HV) for base metal, weld metal, and HAZ. ≤ 22 HRC \le 22\text{ HRC} ≤ 22 HRC (237 HBW / 248 HV) for base metal, weld metal, and HAZ.

Weld Hardness / PWHT Controls weld hardness through procedure qualification and production testing (refinery carbon steel weld deposits are commonly limited to 200 HBW per NACE SP0472); PWHT is one way to meet the limits. Permits as-welded construction when qualified procedures meet the hardness limits (for example, 250 HV10 for many carbon steel applications).

Carbon Equivalent (CE) Control Strongly recommends limiting C E ≤ 0.43 CE \le 0.43 CE ≤ 0.43 to prevent hardenability issues during field welding. Sets limits on chemical composition, C E CE CE , and welding heat input.

Bolting Requirements Requires exposed bolting to meet its hardness limits, typically by using ASTM A193 Grade B7M ( ≤ 22 HRC \le 22\text{ HRC} ≤ 22 HRC ) for exposed sour bolting. Mandates Grade B7M or specific corrosion-resistant alloys (CRAs) for sour exposure.

• Morphology and Affected Refinery Equipment
• 1. Crack Morphology & Initiation

Initiation Site : Cracking invariably initiates on the internal, process-wetted surface at locations of high stress and localized hardness.

Crack Profile : Cracks are sharp, tight, and show zero macroscopic plastic deformation or necking. The failure appears brittle to the naked eye.

Microscopic Propagation Path : Cracks can propagate transgranularly (cleaving through grain bodies) or intergranularly (following prior austenite grain boundaries), often displaying multiple fine branches.

Vulnerable Weld Locations : Hard root passes, un-tempered cap passes, and localized hard spots in the weld HAZ.

2. Affected Plant Units and Equipment

Hydroprocessing Units (Hydrotreaters & Hydrocrackers) : High-pressure sour gas separators, recycle gas compressors, amine wash columns, and sour water flash vessels operating below 150 °F.

Fluid Catalytic Cracking (FCC) Gas Plants : Fractionator overhead condensers, accumulator drums, deethanizer overhead systems, and wet gas compressor interstage vessels containing wet H 2 S H_2S H 2 ​ S and cyanides.

Amine Treating Units : Rich amine piping, amine flash drums, regenerator overhead condensers, and sour gas contactor bottoms.

Sour Water Strippers (SWS) : Overhead condensers, reflux drums, and pump suction lines exposed to concentrated H 2 S H_2S H 2 ​ S and N H 4 H S NH_4HS N H 4 ​ H S .

Fasteners and Bolting : Pressure-retaining flange bolting exposed to sour process leaks, internal bolting in valves, and pump casing studs. Standard ASTM A193 Grade B7 bolts (hardness up to 35 HRC) undergo rapid, catastrophic SSC failure when wetted by sour fluid.

Engineering Prevention, Mitigation & Materials Control

Strict Hardness Enforcement ( ≤ 22 HRC \le 22\text{ HRC} ≤ 22 HRC ) :

Specify that all carbon steel base metals, welding filler metals, and production weld heat-affected zones maintain a maximum hardness of 22 HRC (237 HBW / 248 HV) .

For refinery carbon steel welds, RP 571 points to NACE SP0472 , which commonly limits weld deposit hardness to 200 HBW .

Post-Weld Heat Treatment (PWHT) :

PWHT tempers hard martensitic microstructures in weld metal and HAZ, converting hard, brittle phases into tough, tempered ferrite and spheroidized carbides. Simultaneously, PWHT relieves residual welding stresses from > 50 ksi >50\text{ ksi} > 50 ksi down to < 15 to 20 ksi <15\text{ to }20\text{ ksi} < 15 to 20 ksi .

Chemistry and Hardenability Control :

Restrict carbon content and control Carbon Equivalent ( C E CE CE ) per the International Institute of Welding (IIW) formula: C E = C + M n 6 + C r + M o + V 5 + N i + C u 15 ≤ 0.43 CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15} \le 0.43 CE = C + 6 M n ​ + 5 C r + M o + V ​ + 15 N i + C u ​ ≤ 0.43

Controlling C E ≤ 0.43 CE \le 0.43 CE ≤ 0.43 prevents the formation of hard, untempered martensite during typical welding cooling cycles.

Bolting Upgrades (B7M Specification) :

In sour service or areas subject to sour leaks, replace standard ASTM A193 Grade B7 bolts with ASTM A193 Grade B7M fasteners (or ASTM A320 Grade L7M for low temperature).

Grade B7M fasteners receive 100% individual production hardness testing to guarantee that hardness does not exceed 22 HRC (235 HBW maximum) .

Corrosion-Resistant Alloys (CRAs) :

In severe sour environments exceeding carbon steel capabilities, utilize solid or clad CRAs:

Nickel-Base Alloys : Alloy 625 (UNS N06625) and Alloy C-276 (UNS N10276) provide near-complete immunity to SSC.

Duplex Stainless Steels : 2205 Duplex (UNS S31803 / S32205) per NACE MR0103 limits.

Process Inhibition & Temperature Management :

Water washing and ammonium polysulfide ( A P S APS A PS ) injection to scrub cyanides and maintain protective iron sulfide films.

Controlling process chemistry so hydrogen charging stays low.

Non-Destructive Examination (NDE) Methodologies

NDE Technique Target & Sensitivity Operational Capabilities & Limitations

Wet Fluorescent Magnetic Particle Testing (WFMT) Surface-Breaking SSC Cracks Gold Standard / Preferred Method ; uses an AC electromagnetic yoke and fluorescent magnetic particles viewed under ultraviolet (UV-A) light (black light, ≥ 1000 μ W/cm 2 \ge 1000\ \mu\text{W/cm}^2 ≥ 1000 μ W/cm 2 ) in a darkened area. Exceptional sensitivity for fine, tight surface-breaking SSC cracks in weld root and HAZ.

Internal Angle-Beam Shear Wave UT Subsurface & Surface Flaw Detection Angle-beam shear wave (45°, 60°, 70°) transducers locate cracking in weld roots and HAZ from external or internal surfaces when internal access for WFMT is restricted.

Phased Array UT (PAUT) Volumetric Crack Sizing & Profiling High-resolution sectorial scanning and Total Focusing Method (TFM) map crack depth, branching, and tip locations in complex weld geometries.

Field Hardness Testing Hardness Verification ( ≤ 22 HRC \le 22\text{ HRC} ≤ 22 HRC ) Portable hardness testing methods (TeleBrineller, Ultrasonic Contact Impedance [UCI], Equotip Leeb) verify that base metal, weld caps, and HAZ meet the 22 HRC / 237 HBW threshold.

Liquid Penetrant Testing (PT) Surface-Breaking Cracks Low sensitivity for tight, oxide-filled SSC cracks compared to WFMT; generally not recommended for carbon steel in sour service unless magnetic particle testing is impossible.