API API-571 - Corrosion and Materials Professional

API RP 571, Naphthenic Acid Corrosion:

“Molybdenum significantly improves resistance to naphthenic acid attack. Alloys such as 317, 316Mo, and Alloy 20 are used in severe NAC environments due to higher Mo content.”

“Chromium and nickel play secondary roles; molybdenum is the key element.”

Answer is A – Molybdenum.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, ammonium bisulfide (NHâ‚„HS) 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 NHâ‚„HS 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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API RP 945, dedicated to environmental cracking in amine units, states:

“Carbon steel is widely used in amine systems but is particularly susceptible to localized corrosion, under-deposit corrosion, and stress corrosion cracking, especially in high-pressure or rich amine environments.”

“Proper control of amine quality, oxygen ingress, and temperature is critical to avoid accelerated corrosion in carbon steel equipment.”

Thus, carbon steels are the most commonly affected materials, making option C correct.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, refractory materials are non-metallic and therefore do not experience metallic corrosion mechanisms such as carburization, sulfidation, or oxidation.

Instead, refractories degrade through:

Dusting (loss of material due to chemical or thermal attack)

Checking (crack formation caused by thermal cycling and differential expansion)

These are the primary and characteristic damage modes of refractories in high-temperature service.

Referenced Documents (Study Basis):

API RP 571 – Section on Refractory Degradation

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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 COâ‚‚ and Hâ‚‚S.

Thus, the correct answer is Option C.

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.

Comprehensive and Detailed Explanation From Exact Extract:

Polythionic acid stress corrosion cracking (PASCC) occurs when sulfide scales on stainless steel surfaces are exposed to oxygen and moisture, most commonly during shutdowns and outages.

Per API RP 571:

Polythionic acids form when iron sulfides react with air and water

Cracking occurs in austenitic stainless steels

Damage is most likely during shutdown, not normal operation

Therefore, PASCC is the damage mechanism directly associated with shutdown exposure conditions.

Referenced Documents (Study Basis):

API RP 571 – Section on Polythionic Acid Stress Corrosion Cracking

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API RP 571 under Sulfidation (High Temperature Sulfidic Corrosion) recommends:

“Thickness monitoring using ultrasonic testing (UT) or radiographic testing (RT) is the most common method for detection of wall loss due to sulfidation.”

“Sulfidation results in uniform thinning, especially in low Cr steels in hydrogen/H₂S environments.”

(Reference: API RP 571, Section 4.2.1.1 – Sulfidation)

Thus, option A is the most appropriate and effective inspection method.

Under Thermal Fatigue and Shock, API RP 571 explains:

“Thermal shock damage often initiates at locations with sudden temperature changes, such as quench zones, cold/hot injection points, and areas subjected to rapid cycling.”

“These are critical inspection points for detecting cracking from thermal shock.”

Thus, option B best matches the described mechanism.