Free Practice Questions for API API-571 Exam

Comprehensive and Detailed Explanation From Exact Extract:

Caustic stress corrosion cracking (Caustic SCC) occurs when carbon and low-alloy steels are exposed to caustic environments in the presence of tensile stress.

Per API RP 571, the most effective and proven prevention method is postweld stress-relief heat treatment, which:

Reduces residual welding stresses

Eliminates one leg of the SCC triangle (stress)

Why other options are incorrect:

300 series stainless steels are also susceptible under some caustic conditions.

SCC can occur even at low caustic concentrations.

SCC has been observed below 150 °F in high-stress areas.

Referenced Documents (Study Basis):

API RP 571 – Section on Caustic Stress Corrosion Cracking

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API RP 571 – Hydrogen Stress Cracking – HF Acid Service:

“HF acid environments can cause hydrogen stress cracking (HSC) in carbon steel, particularly in areas with high hardness or residual stress.”

“This is distinct from general corrosion and requires specific inspection focus.”

So, option A is correct.

API RP 751 (referenced in RP 571) lists materials appropriate for HF acid alkylation service. It specifies:

“Chrome-moly steels such as ASTM A-193 B5 (5% Cr) bolts are preferred for their resistance to HF acid-induced cracking.”

“Standard low alloy steels (e.g., B7) are not considered adequately resistant.”

(Reference: API RP 571 summary & API RP 751, Section 5.6.3 – Bolting Materials)

Therefore, the correct and industry-approved answer is option A.

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).

API RP 571 states under the section Phosphoric Acid Corrosion:

“Corrosion usually occurs when the acid dries out and forms concentrated phosphoric acid deposits, which are very aggressive to carbon steel.”

“Drying or concentration occurs due to poor flow distribution or operational upsets. Localized areas may experience acid concentration due to vaporization.”

(Reference: API RP 571, Section 4.3.3.8 – Phosphoric Acid Corrosion)

Therefore, the most aggressive corrosion occurs when the acid dries out, making option D correct.

API RP 571, in its discussion on Wet H₂S Damage, including HIC, SOHIC, and SSC, consistently identifies a shared mechanism:

“All wet H₂S-related damage mechanisms involve the absorption of atomic hydrogen, formed at the steel surface by the corrosion reaction in the presence of H₂S.”

“The absorbed hydrogen may diffuse into the steel, accumulate at trap sites, and cause cracking or blistering.”

(Reference: API RP 571, Section 4.2.2.7 – Wet H₂S Damage Mechanisms)

Thus, hydrogen absorption and permeation is a unifying mechanism across all these damage modes. Therefore, option D is correct.

API RP 571 on Thermal Fatigue emphasizes:

“Thermal fatigue results from cyclic thermal stresses during startup, shutdown, or transient operations.”

“The best mitigation strategy is to control heating and cooling rates to reduce temperature differentials and resulting stress.”

(Reference: API RP 571, Section 4.2.1.5 – Thermal Fatigue)

Hence, option D correctly identifies the preventive action.

Comprehensive and Detailed Explanation From Exact Extract:

Amine stress corrosion cracking is most frequently associated with rich amine services, where:

Acid gas loading (H₂S / CO₂) is high

Corrosive contaminants and degradation products are present

Higher operating temperatures and stresses exist

Per API RP 571, cracking is far more common in rich amine systems than in lean amine circuits.

Why the other options are incorrect:

Lean amine has lower acid gas content and reduced SCC risk

SCC is not limited to low temperatures

Concentration alone is not the dominant factor

Referenced Documents (Study Basis):

API RP 571 – Section on Amine Stress Corrosion Cracking

According to API RP 571 Section 5.1.2.6 (Carbonate Stress Corrosion Cracking - CSCC):

“The cracking is usually intergranular and is most commonly detected using wet fluorescent magnetic-particle testing (WFMT) after surface cleaning. PT may not be sensitive enough to detect fine cracks formed by this mechanism.”

Because CSCC cracks are typically surface-connected and very fine, WFMT offers the best visibility, especially after proper surface prep.

Therefore, Option D is correct.

Comprehensive and Detailed Explanation From Exact Extract:

Microbiologically Induced Corrosion (MIC) is one of the most universally applicable corrosion mechanisms, affecting:

Carbon steel

Low-alloy steel

Stainless steels

Copper alloys

Aluminum alloys

According to API RP 571, MIC can occur in any aqueous system where microorganisms are present, regardless of metallurgy.

By contrast:

Sour water corrosion and amine corrosion mainly affect carbon steel

Polythionic acid corrosion is limited to austenitic stainless steels

Referenced Documents (Study Basis):

API RP 571 – Section on Microbiologically Induced Corrosion