Free API API-571 Practice Exam with Questions & Answers | Set: 4

According to API RP 571 and API RP 583 (CUI-specific guidance):

“Neutron backscatter techniques are effective in detecting moisture within insulation systems, which can indicate areas of concern for corrosion under insulation (CUI).”

“This method provides non-invasive, quick identification of wet insulation without removing cladding.”

(Reference: API RP 571, Section 4.5.1 – Corrosion Under Insulation and API RP 583 – CUI Detection Techniques)

Thus, neutron backscatter is the preferred NDE method for moisture detection, making option B correct.

According to API RP 571 Section 5.1.2.1 (Hydrogen-Induced Cracking) and Publ 939-A, blistering in carbon and low alloy steels exposed to wet H2S environments can be exacerbated by the presence of cyanides. These accelerate hydrogen entry into steel, increasing the chance of hydrogen blistering.

API RP 571 states:

“HIC and blistering are forms of hydrogen damage that occur in wet H2S environments. The presence of cyanides can significantly increase hydrogen entry and the risk of blistering.”

Publ 939-A (Research on Cracking in Wet H2S Service) also notes:

“Cyanides act as catalytic agents in hydrogen charging, which promotes internal pressure build-up and increases blistering.”

Thus, cyanides (Option B) are the most critical in enhancing blistering in H2S environments.

In steam systems, the absence of a steam trap allows condensate to accumulate, which can lead to corrosion from oxygen and carbon dioxide dissolved in the water—a classic case of condensate corrosion.

From API RP 571 Section 5.3.3.1 (Condensate Corrosion):

“If steam lines or equipment like soot blowers do not have proper drainage or steam traps, condensate may accumulate... Corrosion results from the presence of dissolved oxygen and/or carbon dioxide in the condensate.”

Thus, the correct answer is Option D (condensate corrosion).

As per API RP 945 (4th Edition, 2022):

“Post-weld heat treatment (PWHT) is the most effective method for reducing residual stresses that contribute to amine SCC. PWHT minimizes the stress intensity required for crack initiation and growth.”

Lowering temperature helps but is not always feasible.

Water content and amine concentration affect SCC but are not as impactful as PWHT.

Thus, Option A (Post-weld heat treatment) is the most effective mitigation.

According to API RP 571, in the section on Boiler Water Condensate Corrosion:

“The major contributors to condensate corrosion are dissolved CO₂ and O₂. Carbon dioxide forms carbonic acid in the presence of water, lowering pH and causing generalized corrosion.”

“Oxygen causes pitting and localized corrosion unless chemically treated.”

Thus, option B (Carbon dioxide and oxygen) is the primary root cause and the correct answer.

API RP 571 on Thermal Fatigue:

“Mixing of hot and cold streams, particularly where hydrogen is present, can induce thermal cycling stresses, leading to thermal fatigue cracking, especially at fittings like tees.”

“Hydrogen embrittlement typically affects stressed zones, but this pattern strongly indicates thermal fatigue.”

So the correct answer is C – Thermal fatigue.

API RP 571, under Organic Acid Corrosion, highlights:

“Corrosive organic acids in overhead systems are those that condense at temperatures above the water dew point. These acids condense independently of water and can form highly concentrated corrosive films.”

“When acids condense first, they aggressively corrode carbon steel prior to any water wash mitigation.”

(Reference: API RP 571, Section 4.3.3.9 – Organic Acid Corrosion)

Hence, option D correctly identifies the most aggressive corrosion scenario.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, corrosion fatigue results from the combined action of a cyclic stress and a corrosive environment. One of the most important distinguishing features between corrosion fatigue and SCC is crack morphology.

Corrosion fatigue cracks are typically transgranular and show little or no branching

Stress corrosion cracking is characterized by extensive crack branching

Therefore, the absence of significant branching is a key diagnostic feature of corrosion fatigue.

Referenced Documents (Study Basis):

API RP 571 – Section on Corrosion Fatigue

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API RP 571 discusses Hydrogen-Induced Cracking (HIC) and Blistering under the section:

“HIC and blistering are most strongly influenced by non-metallic inclusions, particularly elongated manganese sulfide (MnS) inclusions, which serve as trap sites for hydrogen atoms.”

“These inclusions create local planes of weakness where atomic hydrogen recombines into molecular hydrogen (H₂), causing high pressure and cracking.”

(Reference: API RP 571, Section 4.2.2.7 – Hydrogen Blistering and HIC)

Thus, inclusions are the critical material factor, making option A correct.

According to API RP 571, under the section "Corrosion in Aqueous Environments – Cooling Water Corrosion", one of the key contributors to corrosion in carbon steel and other materials used in heat exchangers is the presence of dissolved oxygen. API RP 571 states:

"Oxygen is a primary contributor to corrosion in cooling water systems. Systems open to the atmosphere are typically more corrosive than closed systems due to the continual replenishment of oxygen."

"Corrosion rates are highest where oxygen concentration is the greatest, especially in systems using untreated or poorly treated water."

"Carbon steel corrodes in the presence of oxygen and water, forming corrosion products that may or may not adhere to the surface."

(Reference: API RP 571, Section 4.3.1.1 – Cooling Water Corrosion)

Therefore, increasing oxygen content directly increases corrosion activity in exchanger tubes, making option C the correct and documented answer.