API 571 Sample Flashcards
Use the 6 interactive API 571 flashcards below to review key damage mechanisms fast. Each card flips to show the answer plus the matching API 571 section reference, so you can double-check the source and lock in the concept. Flip back and keep cycling through the cards until it becomes automatic.
API 571 – Interactive Flashcards (Practice Mode)
Click a card to reveal the answer and reference numbers. Click again to flip back.
Flashcard 1 • Salts vs Acid Under-Deposit
Salt deposit is visible… but what is the corrosion driver?
In a reactor effluent circuit you find hygroscopic salt deposits and localized pitting under the deposits.
Explain the most likely “pair” of mechanisms working together and why the corrosion can accelerate after wash water.
Answer: Ammonium chloride / amine hydrochloride deposit corrosion can cause localized under-deposit attack, and aqueous hydrochloric acid can form beneath these deposits when water is available (including injected wash water). HCl gas is typically not corrosive when dry, but becomes very corrosive when water is present.
Ref: API 571 3.6.1–3.6.3 & 3.37.3
Flashcard 2 • NH4HS vs NH4Cl (Look-alikes)
High turbulence + threshold clue
Corrosion is worst at elbows/inlets/outlets and other turbulence points. The stream is alkaline sour water, and someone mentions “a concentration threshold” where carbon steel becomes corrosive. Identify the mechanism and list the critical factors that API 571 emphasizes.
Answer: Ammonium bisulfide corrosion (alkaline sour water). Critical factors include NH4HS concentration, H2S partial pressure, velocity/wall shear stress and localized turbulence, pH, temperature, alloy composition, and flow distribution. For carbon steel, solutions below about 2 wt% NH4HS are not generally corrosive; above that, corrosion increases.
Ref: API 571 3.5.1 & 3.5.3
Flashcard 3 • “Spider-web” Cracks Under Insulation
Cracks with no visible corrosion
An insulated 300-series stainless line shows spider-web/craze cracking with little or no visible corrosion.
The insulation gets wet. What is the damage mechanism, what temperature range is most concerning, and which alloys are NOT susceptible?
Answer: External chloride stress corrosion cracking. The operating temperature range of most concern is 140 °F (60 °C) to 400 °F (205 °C). Carbon steels, low-alloy steels, and 400 series stainless steels are not susceptible to chloride SCC.
Ref: API 571 3.17.5–3.17.6
Flashcard 4 • Amine SCC vs “Looks like Wet H2S”
How do you confirm, and what PWHT target matters?
Non-PWHT’d carbon steel in amine service develops cracks near welds that look similar to wet H2S cracking on the surface.
What metallography description confirms the mechanism, and what minimum stress-relief temperature is recommended?
Answer: Amine SCC: metallography typically shows intergranular, oxide-filled cracking with some branching. Recommended minimum stress-relief temperature is 1175 ± 25 °F (635 ± 15 °C) for carbon steel welds (including repair and attachment welds).
Ref: API 571 3.3.5 & 3.3.6
Flashcard 5 • NAC vs “TAN-only” Thinking
Why TAN can mislead + where NAC hits hardest
A crude/vac circuit shows worst metal loss in two-phase, high-velocity/turbulent areas. The crude is low sulfur.
Explain why whole-crude TAN can be misleading, give the main temperature band, and name one material strategy API 571 highlights.
Answer: NAC: whole-crude TAN correlation with corrosion rate is poor because severity depends on the naphthenic acids present in the actual stream/cut (TAN measures total acids, and different naphthenic acids have different corrosivity). NAC primarily occurs above 425 °F (220 °C), has been reported as low as 350 °F (175 °C), and severity increases up to about 750 °F (400 °C). Material strategy: use alloys with increasing molybdenum content (e.g., higher-Mo stainless/nickel alloys) for improved resistance.
Ref: API 571 3.46.3 (d–l) & 3.46.2
Flashcard 6 • “Two Mechanisms at Once” on Stainless
Pitting + cracking in the same circuit
In an overhead/effluent circuit where aqueous HCl is present, stainless steel shows pitting, and cracking is suspected at elevated temperature.
What combined damage picture does API 571 warn about for stainless in this environment?
Answer: Aqueous HCl can drive pitting/attack in stainless steels, and 300 series stainless may also experience chloride SCC if temperature is sufficiently high. API 571 also states 300 and 400 series stainless steels are not usefully resistant to HCl across concentrations/temperatures, so relying on stainless alone is not a protection strategy here.
Ref: API 571 3.37.3 & 3.37.5(b), plus linkage to 3.17
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