API 571 Damage Mechanisms

Common refining damage mechanisms per API RP 571 with affected materials, critical factors, and inspection methods.

API 571 Damage Mechanisms

API RP 571 — REFINERY DAMAGE MECHANISMS QUICK REFERENCE · Source: API RP 571, 3rd Edition (March 2020). Not a substitute for the standard.

Mechanism	Temp Range	Materials Affected	Critical Factors	Typical Services	Mitigation	API 571 Section
Sulfidation (H-S Corrosion)	450-1000°F	CS, low-alloy Cr-Mo, 300-series SS	H2S content, temp, Si content of CS (<0.10% worst)	Crude/vac units, FCC feed, hydrotreater effluent	Specify Si ≥ 0.10% CS; Cr upgrade (5Cr, 9Cr, 304/321SS); Couper-Gorman curves	4.4.2
High Temp H2 Attack (HTHA)	>430°F w/ H2 partial pressure	CS and C-½Mo (Nelson curves)	H2 partial pressure, temp, time; 'C-½Mo curve' withdrawn in 2014	Hydroprocessing, hydrogen plants, catalytic reformers	Use API RP 941 Nelson curves; upgrade to Cr-Mo (2¼Cr-1Mo or higher); inspect via AUBT/TOFD	5.1.3
Naphthenic Acid Corrosion	425-750°F	CS, 5Cr, 9Cr, 12Cr, 304/316SS	TAN > 0.5 mg KOH/g; high velocity; vapor-liquid boundary	Crude unit HVGO, atm & vac overheads	Upgrade to 316L (Mo ≥ 2.5%) or 317L (Mo ≥ 3.5%); inject inhibitor; crude blending	5.1.3.5
Wet H2S Damage (HIC/SSC/SOHIC)	Ambient-300°F w/ aqueous H2S	CS welds & HAZ	H2S > 50 ppm wt in water, pH, hardness (>22 HRC)	Sour water, amine, LPG, FCC	HIC-resistant CS (NACE MR0103/MR0175); PWHT to hardness < 200 HBW; blistering inspection	5.1.2.3
Corrosion Under Insulation (CUI)	25-350°F (peak ~200°F)	CS, low-alloy steel, 300-SS (SCC)	Wet insulation + oxygen; chlorides on SS (ClSCC)	Cold service lines 25-125°F; hot-cold cycling	Paint/coating; sealed jacketing; TSA; inspect per API 583; remove absorbent insulation	4.3.3
Chloride Stress Corr. Cracking	>140°F	Austenitic SS (300 series, esp 304/316)	Cl- ions, tensile stress, moisture; ppm-level Cl- will crack	Cooling water HX, insulation on hot SS, deionizer regen	Use duplex/super-duplex, Alloy 625/825; limit chlorides; stress relieve	4.5.1
Caustic SCC (Caustic Embrittlement)	>140°F in NaOH	CS, stainless, Ni-Cr-Fe	NaOH concentration, temp, stress; dilution steam joints	Caustic treaters, KO drums with entrained NaOH	PWHT CS below NACE SP0403 curve; alloy upgrade (Alloy 600/625); dilute before reheat	4.5.3
Amine SCC	Amine regen inlet (>140°F MEA)	CS at welds/HAZ	MEA/DEA concentration, temp, dissolved O2	Amine absorber/regenerator, lean-rich HX, reboiler piping	PWHT to 200 HBW max; NACE SP0472; deoxygenate; periodic WFMT	5.1.2.4
Polythionic Acid SCC	Shutdown exposure to air+H2S+H2O	Sensitized 300-SS (304H, 321H, 347H)	Sensitization (600-1500°F), O2 during outage	Hydroprocessing reactor internals, FCC cyclones	NACE SP0170 shutdown procedures; soda ash soak; use stabilized grades (321/347)	5.1.2.1
Creep / Stress Rupture	>850°F for CS; >1100°F Cr-Mo	All materials at elevated T	Time, temp, stress; material ASME creep curves	Heater tubes, reformer tubes, hot hydrogen lines	Stay on or below ASME Section II Pt D creep limit; eddy current tube inspection	4.2.8
Thermal Fatigue	Cycling ≥ 200°F	All metals	Delta-T, cycles to failure, restraint	Heater outlet, quench points, flare tips, jacket cooling	Flexible connections; reduce cycling; inspect weld toes via WFMT	4.2.9
Hydrogen Blistering	Ambient-300°F sour service	CS	H+ diffusion into laminations/inclusions; low pH	Sour water, hydroprocessing effluent	Clean CS per NACE MR0103; surface inspect via UT C-scan	5.1.2.3
MIC (Microbiologically Induced)	50-180°F in stagnant water	All metals	Biofilm, stagnant O2, sulfate-reducing bacteria	Firewater, hydrotest water, underdeposit	Biocide; continuous flow; complete drying after hydrotest	4.3.8
Erosion-Corrosion	All temps	CS, Cu alloys, SS depending on service	Velocity > critical; presence of particulates; flashing	Slurry lines, de-propanizer reboilers, wet gas compressor impellers	Lower velocity; upgrade to Cr alloy; add flow splitter; impingement plates	4.2.14
Refractory Degradation	Varies by refractory type	Firebrick, castables, ceramic fiber	Thermal shock, abrasion, chemical attack	FCC regenerator, sulfur furnaces, hydrogen reformers	Proper curing schedule; anchor design per API 936; inspect hot-face thickness	4.3.2

References

Source: Corrosion_API571_Damage_Mechanisms_Reference_v1.xlsx

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