API-571 Exam Dumps - Corrosion and Materials Professional

API RP 571 under Nitriding (Section 4.2.16):

“Nitriding occurs at elevated temperatures and is most severe at temperatures above 900°F (480°C).”

“This mechanism is associated with the reaction of ammonia or other nitrogen-containing compounds with the steel surface, leading to brittle nitride formation.”

Therefore, the correct answer is D.

API RP 571 notes in the context of Hydrochloric Acid Corrosion in overhead condensers:

“Titanium is highly resistant to low pH acidic aqueous phases, including hydrochloric acid formed during condensation in overhead systems.”

“Stainless steels like 316 and 410 are not suitable in the presence of free chlorides at low pH and elevated temperatures.”

(Reference: API RP 571, Section 4.3.3.3 – Hydrochloric Acid Corrosion)

Thus, Titanium is the preferred alloy under the described acidic and high-temperature conditions, making option B correct.

According to API RP 941 and API RP 571, High Temperature Hydrogen Attack (HTHA) is highly influenced by hydrogen partial pressure at elevated temperatures:

“The likelihood and rate of HTHA increase with both temperature and hydrogen partial pressure. Operating beyond established material limits on the Nelson Curves can lead to internal decarburization and methane formation in the steel.”

(Reference: API RP 571, Section 4.2.1.3 – High Temperature Hydrogen Attack)

Therefore, among all listed damage types, HTHA is the most directly affected by high hydrogen partial pressures, making option D 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.

Comprehensive and Detailed Explanation From Exact Extract:

Naphthenic Acid Corrosion (NAC) occurs in high-temperature refinery streams (typically 450–750 °F / 230–400 °C) containing organic acids. According to API RP 571, NAC is particularly aggressive toward carbon steels and low-alloy steels, and resistance improves with increasing chromium and molybdenum content, but is best with titanium.

Titanium exhibits exceptional resistance to NAC due to the formation of a stable, protective oxide film that is not attacked by naphthenic acids. API RP 571 identifies titanium as one of the most resistant materials available for severe NAC environments.

Why the other options are less suitable:

Option B (9 Cr-1 Mo steel) has limited resistance and is still susceptible at higher TAN and velocities.

Option C (317 stainless steel) offers improved resistance but can still experience attack under severe NAC conditions.

Option D (321 stainless steel) has lower molybdenum content than 317 SS, making it less resistant.

API RP 571 explicitly notes that titanium provides superior resistance compared to stainless steels and Cr-Mo alloys in NAC service.

Referenced Documents (Study Basis):

API RP 571 – Section on Naphthenic Acid Corrosion

API Corrosion and Materials Selection Study Guide

According to API RP 571, under Naphthenic Acid Corrosion (NAC):

“The most severe corrosion is typically found in areas of high velocity and turbulence, such as at pump around nozzles, transfer lines, elbows, reducers, and orifices.”

“NAC is characterized by uniform thinning and can accelerate significantly where flow-induced erosion is present.”

(Reference: API RP 571, Section 4.3.4.1 – Naphthenic Acid Corrosion)

This clearly establishes that high velocity and turbulence increase NAC severity, making option D correct.

Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, sulfidation corrosion rates are significantly accelerated by the presence of hydrogen. Hydrogen promotes:

Breakdown of protective sulfide scales

Increased diffusion of sulfur into the metal

Higher metal loss rates

This phenomenon is often referred to as hydrogen-assisted sulfidation and is commonly observed in refinery process units.

Moisture is more closely associated with oxidation mechanisms rather than high-temperature sulfidation.

Referenced Documents (Study Basis):

API RP 571 – Section on Sulfidation and High-Temperature Corrosion

According to API RP 571 and API RP 751 (HF Alkylation Units):

“HF acid corrosion rates increase with elevated temperature and higher water content. This is because water acts as an ionizing agent that facilitates the acid’s attack on steel.”

“Corrosion is minimized when water content is strictly controlled, usually between 0.5–1.5%.”

“Contrary to some assumptions, increasing HF acid concentration actually decreases corrosivity.”

Hence, option B is correct.