API 571 cheat sheet

API 571 groups damage mechanisms into general, mechanical and metallurgical, uniform and localized corrosion, environment-assisted cracking, and high-temperature corrosion/degradation.

Failure analysis sequence: compile operating and service history, document damage visually, perform NDE and metallography, identify the mechanism, determine root cause, then define corrective action.

Uniform corrosion rate: mpy = 534 × weight loss (mg) / [density (g/cm³) × area (in²) × exposure time (h)].

Remaining life = (current thickness − minimum required thickness) / corrosion rate; minimum thickness comes from design, code, or a fitness-for-service assessment.

Risk = likelihood × consequence; API 580/581 RBI ranks equipment and sets inspection intervals from this product.

Fitness-for-service per API 579/ASME FFS-1 evaluates flaws and damage against remaining strength and future operating limits.

A damage mechanism review links process conditions, materials of construction, and corrosion loops to credible mechanisms and required inspection/monitoring.

Inspection effectiveness levels are highly effective, usually effective, fairly effective, and poorly effective; the selected method must be capable of detecting the credible mechanism.

Ductile fracture exhibits dimpled rupture and plastic deformation; brittle fracture exhibits cleavage facets, low energy absorption, and little deformation, often below the ductile-to-brittle transition temperature.

Fatigue initiates at stress concentrations under cyclic stress, propagates with striations or beach marks, and ends in an overload zone when the remaining section is too small.

Creep is significant above about 0.4 Tm (absolute melting temperature); tertiary creep leads to bulging, dimensional change, and stress-rupture cracking.

Thermal fatigue results from cyclic restrained thermal expansion; ratcheting is progressive plastic deformation under cyclic loading.

Erosion severity increases with velocity, particle concentration, particle size, and impact angle; erosion-corrosion adds electrochemical metal loss to mechanical removal.

Cavitation damages surfaces by vapor-bubble collapse, producing pitting, deep gouges, and rough metal loss.

Hydrogen blistering and HIC occur when atomic hydrogen diffuses into steel and recombines at inclusions or laminations; SOHIC is aligned cracking near welds.

Mechanical overload occurs when applied stress exceeds yield or ultimate strength, producing gross deformation or fracture; buckling is compressive instability.

Tempered martensite embrittlement and 885°F embrittlement reduce toughness; material and post-weld heat treatment limits control susceptibility.

Uniform corrosion causes general wall thinning; the rate is calculated from thickness measurements over a known time interval.

Galvanic corrosion requires dissimilar metals and an electrolyte; the less noble metal is anodic, and a small anode coupled to a large cathode gives severe localized attack.

Pitting is localized breakdown of a passive film in chloride or stagnant conditions; pitting factor = deepest pit depth / average penetration.

Crevice corrosion occurs in restricted geometries with differential aeration, such as gaskets, lap joints, and deposit-covered surfaces.

Under-deposit corrosion forms concentration cells and differential aeration beneath sludge or scale, producing localized pitting or grooving; deposits must be removed for reliable inspection.

CUI occurs where water enters insulation or jacketing, typically at 50–175°C (122–350°F); carbon steel thins and austenitic stainless steel can suffer chloride SCC.

Microbiologically influenced corrosion is associated with sulfate-reducing bacteria and stagnant nutrient-rich water, producing localized pitting and sulfide-containing deposits.

Atmospheric corrosion is governed by time of wetness, chloride deposition, and pollutants; ISO 9223 assigns corrosivity categories.

Soil corrosion depends on soil resistivity, moisture, oxygen content, and stray currents.

SCC requires three simultaneous conditions: susceptible material, tensile stress (residual or applied), and a specific corrosive environment.

Chloride SCC of austenitic stainless steels occurs above about 60°C (140°F); residual stresses and crevices increase susceptibility, and cracking is transgranular and branching.

Caustic SCC of carbon steel is governed by sodium hydroxide concentration and temperature; post-weld heat treatment reduces residual stress.

Amine SCC affects carbon steel in alkanolamine units, particularly lean or rich amine at high temperature; hardness and residual stress control susceptibility.

Wet H2S damage includes sulfide stress cracking, hydrogen-induced cracking, and SOHIC; NACE MR0175/ISO 15156 sets hardness, stress, and environmental limits.

Hydrogen embrittlement affects high-strength steels; atomic hydrogen enters the lattice and reduces ductility, so hardness control and bake-out are used.

Polythionic acid SCC occurs in sensitized stainless steels during shutdown when sulfides, moisture, and oxygen form polythionic acids; neutralization and dry purge prevent it.

Carbonate SCC affects carbon steel in CO2-containing, high-pH environments; stress relief and coatings reduce risk.

Liquid metal embrittlement occurs when a liquid metal contacts a susceptible solid metal, such as zinc on austenitic stainless steel or copper on steel.

Sulfidation forms iron sulfide scale when sulfur compounds react with steel at elevated temperature; rate increases with H2S concentration and temperature, typically 260–540°C.

High-temperature oxidation forms oxide scale with parabolic growth kinetics; scale spalling exposes fresh metal and accelerates loss.

Carburization introduces carbon at high temperature, forming carbides that increase hardness and reduce ductility; CO and hydrocarbon gases are common sources.

Metal dusting is catastrophic carburization/oxidation in CO-rich gas at 400–800°C (750–1470°F), producing pitting and metal disintegration.

Hydrogen attack occurs when high-temperature, high-pressure hydrogen reacts with carbides to form methane, causing internal fissures; Nelson curves define allowable temperature and hydrogen partial pressure.

Sensitization precipitates chromium carbides at grain boundaries during 425–815°C (800–1500°F) exposure, depleting chromium and promoting intergranular corrosion.

Sigma phase forms in stainless steels at roughly 540–900°C, causing embrittlement and reduced corrosion resistance.

Graphitization converts carbides to graphite in carbon steel above about 425°C (800°F), reducing creep strength; spheroidization coarsens carbides and lowers strength.

885°F embrittlement affects ferritic stainless steels at 400–540°C, reducing toughness.

Crude unit overhead corrosion is driven by HCl from salt hydrolysis, H2S, and ammonium chloride deposition; water dew point and desalting/neutralizer control determine severity.

Naphthenic acid corrosion occurs at 220–400°C and high velocity; attack concentrates at elbows and weld heat-affected zones, and molybdenum-bearing alloys improve resistance.

Hydroprocessing reactors and exchangers are susceptible to ammonium bisulfide corrosion and erosion-corrosion; velocity, NH4HS concentration, and hydrogen partial pressure govern severity.

Catalytic reforming involves chloride and hydrogen; austenitic stainless steels are used, and chloride SCC and chloride corrosion are key concerns.

Amine units are susceptible to amine SCC and corrosion in lean/rich amine service; stress relief and hardness control are required.

Sour water stripper units handle NH4HS and H2S; carbon steel corrosion is controlled by velocity limits.

Delayed cokers experience erosion, thermal fatigue from coke drum cycling, and sulfidation.

Alkylation units use HF or sulfuric acid; carbon steel and Monel are common for HF service, with corrosion and cracking concerns.

RBI per API 580/581 sets inspection intervals from risk (likelihood × consequence) and requires inspection methods with adequate effectiveness; TMLs and IOWs per API 584 monitor damage.