5.2 Thinning, Stress Corrosion Cracking (SCC), High-Temperature, and Environmental Degradation Modes

Core Degradation Classifications in Process Plants

Under API RP 580 (4th Edition) and API RP 571, damage mechanisms affecting fixed equipment in refinery and petrochemical facilities are grouped into four primary degradation modes:

1. General and Localized Thinning (wall loss mechanisms).
2. Environmental and Stress Corrosion Cracking (SCC) (environmental cracking mechanisms).
3. High-Temperature Degradation (thermal, hydrogen, and creep mechanisms).
4. Mechanical and Metallurgical Physical Damage (fatigue, erosion, brittle fracture).

Each degradation mode exhibits distinct physical damage kinetics, requires specific Non-Destructive Examination (NDE) detection methods, and uses fundamentally different mathematical algorithms to calculate Damage Factors ($DF$) under API RP 581.

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1. Thinning Degradation Modes (General and Localized)

Thinning damage mechanisms cause progressive loss of wall thickness from the internal or external surface of pressure-retaining components. Material loss reduces structural load capacity, increasing circumferential hoop stress as governed by Barlow's equation:

$\sigma_h = \frac{P \cdot D_i}{2 t}$

Where $\sigma_h$ is circumferential hoop stress, $P$ is internal pressure, $D_i$ is inside diameter, and $t$ is actual wall thickness. As $t$ decreases, $\sigma_h$ approaches material yield strength, leading to ductile yielding and burst failure.

Major Refining Thinning Mechanisms

• Hydrochloric Acid (HCl) Corrosion: Mineral acid attack occurring in crude unit overhead systems where hydrogen chloride gas dissolves in condensed free water, forming highly aggressive $\text{HCl}(aq)$. Causes severe localized thinning and pitting of carbon steel at dew point condensation locations.
• High-Temperature Sulfidation Corrosion: Reaction of elemental sulfur, hydrogen sulfide ($\text{H}_2\text{S}$), and mercaptans with carbon steel and low-alloy steels at temperatures above $450^\circ\text{F}$ ($232^\circ\text{C}$). Forms iron sulfide ($\text{FeS}$) scale and causes uniform or localized thinning. Metal loss rates follow modified McConomy curves.
• Naphthenic Acid Corrosion (NAC): Severe localized thinning and deep grooving occurring in crude and vacuum distillation units processing high-TAN crudes (Neutralization Number $>0.5\text{ mg KOH/g}$) at temperatures between $440^\circ\text{F}$ and $750^\circ\text{F}$ ($227^\circ\text{C} - 399^\circ\text{C}$) under high fluid velocity.
• Corrosion Under Insulation (CUI): External thinning of carbon steel and low-alloy steel equipment operating between $25^\circ\text{F}$ and $350^\circ\text{F}$ ($-4^\circ\text{C} - 175^\circ\text{C}$) due to water ingress beneath damaged insulation cladding.

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2. Environmental & Stress Corrosion Cracking (SCC) Modes

Environmental cracking mechanisms differ fundamentally from thinning: they produce localized crack-like planar flaws with zero or negligible overall wall loss. SCC requires three simultaneous condition drivers:

1. A susceptible alloy metallurgy.
2. A specific corrosive chemical environment.
3. Sustained tensile stress (either applied operating stress or residual fabrication welding stress).

                    ┌───────────────────────────────────┐
                    │   STRESS CORROSION CRACKING       │
                    │          (SCC TRIAD)              │
                    └─────────────────┬─────────────────┘
                                      │
            ┌─────────────────────────┼─────────────────────────┐
            ▼                         ▼                         ▼
┌───────────────────────┐ ┌───────────────────────┐ ┌───────────────────────┐
│ SUSCEPTIBLE MATERIAL  │ │ CORROSIVE ENVIRONMENT │ │ TENSILE STRESS        │
│ • CS (un-PWHT)        │ │ • Caustic (NaOH)      │ │ • Residual weld stress│
│ • Sensitized 304/316  │ │ • Chlorides (Cl-)     │ │ • Applied thermal     │
│ • High-strength alloy │ │ • Polythionic Acid    │ │ • Internal pressure   │
└───────────────────────┘ └───────────────────────┘ └───────────────────────┘

Major Cracking Mechanisms

• Caustic Stress Corrosion Cracking (Caustic Embrittlement): Intergranular cracking of carbon steel exposed to sodium hydroxide ($\text{NaOH}$) solutions at elevated temperatures ($>115^\circ\text{F} - 180^\circ\text{F}$) without stress relief. PWHT effectively mitigates Caustic SCC by eliminating residual weld tensile stresses.
• Chloride Stress Corrosion Cracking (Cl-SCC): Transgranular branched cracking of austenitic stainless steels (300 series) exposed to aqueous chlorides, dissolved oxygen, and tensile stress at temperatures above $140^\circ\text{F}$ ($60^\circ\text{C}$).
• Polythionic Acid SCC (PTA-SCC): Rapid intergranular cracking of sensitized austenitic stainless steel components (such as hydrocracker furnace tubes) occurring during turnaround shutdowns when iron sulfide scales are exposed to moist air, forming polythionic acids ($\text{H}_2\text{S}_x\text{O}_6$). Mitigated by soda ash washing or continuous dry nitrogen purging.
• Wet \text{H}_2\text{S} Cracking (HIC / SOHIC / SSC): Hydrogen Blistering, Hydrogen-Induced Cracking (HIC), Stress-Oriented Hydrogen-Induced Cracking (SOHIC), and Sulfide Stress Cracking (SSC). Driven by atomic hydrogen ($\text{H}^+$) entry into carbon steel promoted by bisulfide ($\text{HS}^-$) and cyanide poisons in aqueous sour water service.

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3. High-Temperature Degradation Modes

High-temperature degradation mechanisms operate above critical thermal thresholds and alter material microstructures:

• High-Temperature Hydrogen Attack (HTHA): Occurs in carbon steel and low-alloy steels exposed to high partial pressure hydrogen ($P_{\text{H}_2} > 50\text{ psia}$) at temperatures above $400^\circ\text{F}$ ($204^\circ\text{C}$). Atomic hydrogen diffuses into the metal lattice and reacts with dissolved carbon or iron carbides:

$\text{Fe}_3\text{C} + 4\text{H} \rightarrow 3\text{Fe} + \text{CH}_4$

Because methane ($\text{CH}_4$) molecules cannot diffuse out of the steel matrix, enormous internal pressures accumulate, forming methane bubbles, microfissures, and intergranular cracks. HTHA susceptibility is governed by API RP 941 (Nelson Curves).

• Creep and Stress Rupture: Time-dependent permanent plastic deformation occurring at temperatures above $800^\circ\text{F}$ ($427^\circ\text{C}$) for carbon steel or $1000^\circ\text{F}$ ($538^\circ\text{C}$) for 1.25Cr-0.5Mo steel under sustained operating stress, leading to void formation and tertiary rupture.
• Temper Embrittlement: Metallurgical loss of notch toughness in low-alloy steels (e.g., 2.25Cr-1Mo) exposed to temperatures between $650^\circ\text{F}$ and $1070^\circ\text{F}$ ($343^\circ\text{C} - 577^\circ\text{C}$) due to segregation of tramp elements (phosphorus, tin, antimony, arsenic) to grain boundaries.

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Comparison Matrix of Primary Degradation Categories

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Formulating Damage Factors: Thinning vs. Cracking (API RP 581)

Under API RP 581, the mathematical formulation of Damage Factors ($DF$) reflects the distinct physical behavior of thinning versus cracking:

• Thinning Damage Factor ($DF_{\text{thin}}$): Calculated probabilistically based on measured or estimated corrosion rate (CR), age ($t$), starting thickness ($t_{\text{initial}}$), minimum required thickness ($t_{\text{min}}$), and structural variance factor. Performing higher effectiveness inspections (Category A or B) narrows the probability distribution curve of remaining wall thickness, reducing $DF_{\text{thin}}$.
• Cracking Damage Factor ($DF_{\text{scc}}$): Calculated using an environmental susceptibility index (Low, Medium, High) derived from fluid chemistry and stress state (e.g., PWHT vs non-PWHT). $DF_{\text{scc}}$ increases as a function of operating time. Performing volumetric crack detection (Category A inspection using PAUT or WFMT) resets or significantly lowers the calculated $DF_{\text{scc}}$ by verifying crack-free status.