Free API API-571 Practice Exam with Questions & Answers | Set: 2

API RP 571 provides detailed insights:

“Brittle fracture generally occurs at low temperatures, specifically below the ductile-to-brittle transition temperature as defined by the Charpy impact test.”

“Most carbon and low-alloy steels are ductile above their Charpy transition temperature but may fracture suddenly without ductility below it.”

(Reference: API RP 571, Section 4.2.1.2 – Brittle Fracture)

Hence, option C is correct and technically accurate based on the material behavior curve.

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.

Per API RP 571, susceptibility to hydrogen embrittlement is strongly influenced by:

Material strength level

Microstructure

Heat treatment condition

Heat treatment determines:

Dislocation density

Trap sites for hydrogen

Residual stress state

The modulus of elasticity does not significantly affect hydrogen embrittlement susceptibility, making Option A incorrect.

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrogen Embrittlement

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API RP 571 defines Hydrogen Embrittlement as:

“Improper PWHT can trap hydrogen within the microstructure of carbon steel or fail to relieve residual stresses, both of which increase the risk of hydrogen embrittlement.”

“Moisture in electrodes contributes to hydrogen-induced cracking, but embrittlement from atomic hydrogen is more directly tied to heat treatment and microstructure.”

Thus, option D is the most accurate.

API RP 751, which governs HF alkylation units, specifies:

“The total residuals of chromium, nickel, and copper in carbon steel used in HF service should be less than 0.15 wt.% to minimize corrosion risk.”

“Higher residuals increase the reactivity and corrosion rate of carbon steel in the presence of HF acid.”

(Reference: API RP 751, Section 5.3 – Materials Specification for HF Units)

Therefore, option A is the correct choice.

According to API RP 571, under Oxidation:

“Austenitic stainless steels (300 Series) are generally used for oxidation resistance up to about 1500°F (815°C). At higher temperatures, scaling and loss of protective chromium oxide layers increase.”

“Above these temperatures, specialized high-temperature alloys may be required.”

(Reference: API RP 571, Section 5.1.1 – Oxidation)

Therefore, option C is the correct answer.

API RP 571 and RP 939-C define sulfidation conditions as:

“Sulfidation of iron-based alloys generally becomes significant at temperatures above 500°F (260°C) in hydrocarbon streams with sulfur but without hydrogen.”

“For hydrogen-containing environments, this threshold may be reduced to 450°F (230°C).”

(Reference: API RP 571, Section 4.2.1.1 – Sulfidation; API RP 939-C, Section 3.1)

Therefore, the correct answer is D.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, ammonium bisulfide (NH₄HS) corrosion typically occurs in high-velocity, wet sour service, such as hydroprocessing reactor effluent systems. The damage is characterized by localized wall thinning, often with highly irregular corrosion profiles that vary with flow regime.

API RP 571 emphasizes that the most effective inspection techniques for NH₄HS corrosion are those capable of:

Detecting localized thinning

Identifying corrosion patterns related to flow velocity

Covering large areas efficiently

Ultrasonic scanning (C-scan) and profile radiography are specifically recommended because they:

Reveal localized and preferential metal loss

Allow comparison between high- and low-velocity zones

Are far superior to spot UT, which may miss localized attack

Referenced Documents (Study Basis):

API RP 571 – Section on Ammonium Bisulfide Corrosion

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Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, surface decarburization is a metallurgical degradation mechanism that occurs when carbon is removed from the surface layers of steel due to exposure to oxidizing environments at elevated temperatures. This results in a carbon-depleted surface layer.

Carbon is a primary strengthening element in carbon and low-alloy steels. When carbon is lost from the surface:

Hardness and tensile strength are reduced

Creep resistance and load-carrying capability decrease

The component becomes more susceptible to plastic deformation and failure under stress

API RP 571 states that decarburization leads to loss of mechanical strength, especially critical in high-temperature service, where components already operate close to material limits.

Why the other options are incorrect:

Option A: Stress cracking is not the primary effect; loss of strength is.

Option C: Decarburization does not directly accelerate oxidation or sulfidation, although both may coexist.

Option D: This is incorrect; decarburization is considered detrimental in high-temperature applications.

Referenced Documents (Study Basis):

API RP 571 – Section on Decarburization and Metal Dusting

API Corrosion and Materials Study Guide

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API RP 571 on Chloride Stress Corrosion Cracking (Cl-SCC) clearly states:

“Austenitic stainless steels such as 304 and 316 are highly susceptible to Cl-SCC when subjected to tensile stress and exposed to chloride-bearing environments.”

“Susceptibility is greatest in non-stress-relieved weldments or cold-worked areas.”

Brass and carbon steels are not typical Cl-SCC candidates; low alloy steels such as 1.25Cr-0.5Mo are also not commonly affected.

Thus, option C is the correct answer.