API 571 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Root cause versus contributing cause

The root cause is the primary condition or action that initiated a failure; contributing causes are secondary conditions that accelerated or enabled it but would not alone have caused the failure.

Corrosion rate mpy formula

mpy = 534 × weight loss (mg) / [density (g/cm³) × area (in²) × time (hours)]; this converts measured mass loss to an average penetration rate for uniform corrosion.

Remaining life calculation

Remaining life equals (current thickness minus minimum required thickness) divided by the corrosion rate, using consistent units and a corrosion rate based on a representative operating period.

Fitness-for-service assessment

API 579/ASME FFS-1 assesses whether equipment with flaws or damage is fit for continued service, defining remaining strength and acceptable future operating limits.

Damage mechanism review

A damage mechanism review systematically connects process conditions, materials of construction, and corrosion loops to identify credible damage mechanisms and to define inspection and monitoring requirements.

Ductile versus brittle fracture

Ductile fracture shows dimpled rupture and plastic deformation; brittle fracture shows cleavage facets, little deformation, and rapid propagation, often below the ductile-to-brittle transition temperature.

Fatigue beach marks

Fatigue cracks initiate at stress concentrations under cyclic loading and propagate, leaving macroscopic beach marks or microscopic striations before final overload fracture.

Creep temperature threshold

Creep becomes significant above about 0.4 of the absolute melting temperature; time-dependent deformation can cause bulging, stress-rupture cracking, and dimensional change.

Thermal fatigue cracking

Cyclic thermal gradients restrain expansion and contraction, producing fatigue cracking at stress concentrations such as nozzles, welds, and mixing points.

Erosion versus erosion-corrosion

Erosion is mechanical metal loss from particle or droplet impact; erosion-corrosion combines mechanical removal with electrochemical corrosion, often producing grooves, horseshoe patterns, or thinning downstream of restrictions.

Galvanic corrosion area ratio

When dissimilar metals are coupled in an electrolyte, the less noble anodic metal corrodes faster; a small anode coupled to a large cathode produces severe localized attack.

Pitting corrosion mechanism

Breakdown of a passive film in chloride-containing or stagnant environments creates localized deep penetrations; the pitting factor is the ratio of deepest pit depth to average penetration.

Crevice corrosion conditions

Crevice corrosion occurs in tight gaps where oxygen and ion movement are restricted, creating differential aeration and acidification; gaskets, lap joints, and deposits are common sites.

Corrosion under insulation

Corrosion under insulation occurs where water enters insulation or jacketing, typically in the 50–175°C (122–350°F) range; carbon steel thins and austenitic stainless steel may suffer chloride SCC.

Under-deposit corrosion

Deposits create differential aeration and concentration cells, leading to localized pitting or grooving beneath sludge, scale, or corrosion products; inspection requires deposit removal and localized NDE.

Stress corrosion cracking triad

Stress corrosion cracking requires a susceptible material, a tensile stress (residual or applied), and a specific corrosive environment; removing any one of these factors prevents cracking.

Chloride SCC of stainless

Austenitic stainless steels crack in chloride environments above about 60°C (140°F), especially where residual stresses and crevices exist; cracking is transgranular and branching.

Sulfide stress cracking

Sulfide stress cracking is hydrogen embrittlement cracking of high-strength or hard steels in wet H2S service; NACE MR0175/ISO 15156 limits hardness, stress, and environment to control it.

Polythionic acid SCC

Stainless steels with sensitized grain boundaries crack in polythionic acids formed during shutdown when sulfides, oxygen, and moisture react; prevention includes neutralization and dry purge.

Caustic SCC

Carbon steel and some stainless steels crack in concentrated sodium hydroxide at elevated temperature; residual stress and caustic concentration govern susceptibility, with a temperature-concentration threshold.

Amine stress corrosion cracking

Alkanolamine units can crack carbon steel in lean or rich amine service at high temperature; cracking is often intergranular and associated with residual weld stresses and H2S.

Sulfidation high-temperature corrosion

Sulfidation occurs when sulfur compounds react with iron or alloys at elevated temperature, forming iron sulfide scale; rates increase with H2S concentration and temperature, typically from 260–540°C.

Carburization damage mechanism

Carbon ingress at high temperature forms carbides, increasing hardness and reducing ductility; it can lead to cracking and metal dusting in carbon monoxide or hydrocarbon environments.

Metal dusting

Metal dusting is catastrophic carburization and oxidation in carbon-monoxide-rich gases at 400–800°C (750–1470°F), producing pitting and disintegration into fine metal particles.

Hydrogen attack Nelson curves

At high temperature and hydrogen partial pressure, atomic hydrogen reacts with carbides to form methane, causing internal fissures and loss of strength; Nelson curves define safe operating limits.

Sensitization of austenitic stainless

Chromium carbides precipitate at grain boundaries during exposure to 425–815°C (800–1500°F), depleting chromium and making austenitic stainless steel susceptible to intergranular corrosion.

Spheroidization and graphitization

Spheroidization coarsens carbides and lowers strength at elevated temperature; graphitization converts carbides to graphite in carbon steels above about 425°C (800°F), reducing creep strength.

Crude unit overhead corrosion

Hydrogen chloride from salt hydrolysis and H2S form corrosive aqueous condensate in crude unit overheads; ammonium chloride deposition and water dew point determine attack, controlled by desalting and neutralizers.

Naphthenic acid corrosion

Naphthenic acids corrode carbon steel and low-alloy steels at high velocity and 220–400°C; attack is often localized at elbows and weld heat-affected zones, and higher-molybdenum alloys resist it.

Ammonium bisulfide corrosion

Ammonium bisulfide in hydroprocessing and sour water service causes severe localized corrosion and erosion-corrosion; velocity, concentration, and hydrogen partial pressure govern severity, requiring alloy selection and water washing.

Risk-based inspection planning

Risk-based inspection per API 580/581 prioritizes inspection by risk (likelihood times consequence), defines inspection intervals, and selects NDE methods with adequate effectiveness for credible damage mechanisms.

Inspection effectiveness categories

API 581 ranks inspection effectiveness as highly effective, usually effective, fairly effective, and poorly effective, based on the NDE method, coverage, and ability to detect the specific damage mechanism.