Free Practice Questions for API API-571 Exam

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.

From API RP 571 Section 5.1.1.3 (Amine Corrosion):

“Amine corrosion is a form of general or localized corrosion caused by the presence of amine solutions used to remove acid gases (H₂S and CO₂)... Corrosion is most severe in areas where acid gas loading is high.”

While temperature and concentration are contributing factors, the primary driver is dissolved acid gases such as CO₂ and H₂S.

Thus, the correct answer is Option C.

According to API RP 571 Section 5.1.3.4 (Naphthenic Acid Corrosion - NAC):

“Sulfur can act as an inhibitor to NAC. Where sulfur content in crude is high, a reduction in naphthenic acid corrosion is sometimes observed, especially in certain parts of vacuum and atmospheric units. However, this is not universally reliable, and corrosion can still occur based on local operating conditions and metal temperatures.”

Thus, Option C (Inhibition) is the correct choice, as sulfur can reduce NAC under some conditions.

Hydrogen Stress Cracking (HSC) is a subclass of environmental cracking that occurs in certain materials when hydrogen is present. According to API RP 571, HSC typically affects high-strength low alloy steels and carbon steels, particularly under tensile stress in a hydrogen-containing environment. This includes areas like weld-affected zones and hard spots in steels.

From API RP 571 Section 5.1.2.1 (Hydrogen-Induced Cracking):

"Hydrogen stress cracking (HSC) can occur in carbon steel and low alloy steels in environments containing hydrogen at ambient to moderately elevated temperatures (up to 200°F or 93°C)... Hardness is a critical factor for HSC. High hardness in welds and heat affected zones (HAZ) are most susceptible."

Additionally, API RP 939-C does not directly define HSC but reinforces the relationship between hydrogen exposure and metal integrity, particularly under conditions not exceeding 450 °F (230 °C) for hydrogen services.

Thus, option A is correct, as it directly reflects the metal types most susceptible to HSC in operational environments defined by API RP 571.

According to API RP 571 and API RP 751:

“In HF alkylation units, carbon steel can be used if weld hardness is controlled to ≤ 200 Brinell (BHN). Exceeding this can significantly increase the corrosion rate and susceptibility to cracking.”

“Hard welds act as preferential corrosion sites due to microstructural inhomogeneity and stress concentration.”

(Reference: API RP 571, Section 4.3.3.4 – Hydrofluoric Acid Corrosion; API RP 751, Section 5.3 – Materials of Construction)

Thus, option D is correct.

As per API RP 571 Section 5.1.4.2 (Metal Dusting):

“Metal dusting is a type of carburization attack that typically occurs in high carbon activity environments, with temperatures generally between 900°F and 1200°F (480°C to 650°C), although damage has been reported as low as 600°F (315°C).”

Therefore, the commonly accepted operational range is best described by Option A: 600°F–1200°F (315°C–650°C).

API RP 571 on Oxidation indicates:

“Oxidation is a surface loss phenomenon that generally results in uniform thinning. Ultrasonic thickness measurements are typically used to monitor metal loss in oxidizing environments.”

“Metallography may be used to confirm oxide scale structure but is not necessary for routine evaluation.”

Hence, option B is the most appropriate standard method.

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.

In API RP 571, under the section Polythionic Acid Stress Corrosion Cracking, it is stated:

“Sensitization of austenitic stainless steels occurs when the material is exposed to temperatures in the range of 800°F to 1500°F (427°C to 816°C), which causes chromium carbide precipitation at grain boundaries, depleting the adjacent areas of chromium.”

“Once sensitized, the steel becomes susceptible to attack in the presence of polythionic acids, particularly in environments containing sulfides and oxygen.”

(Reference: API RP 571, Section 4.2.2.5 – PTA SCC)

This makes option B (750°F to 1500°F / 400°C to 815°C) the most accurate and standard-aligned answer.