API-571 Corrosion and Materials Professional Questions and Answers

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.

API RP 571 under Thermal Fatigue highlights:

“Differential thermal expansion between dissimilar materials (e.g., carbon steel and stainless steel) in welds causes high cyclic stresses that can result in thermal fatigue cracking.”

“These stresses typically occur during transient thermal cycling such as startup/shutdown operations.”

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

Thus, the correct mechanism resulting from differential expansion in bimetallic welds is thermal fatigue, making option B correct.

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, 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 Brittle Fracture notes:

“The primary factor influencing brittle fracture is material toughness, especially at low temperatures.”

“Materials with low fracture toughness are susceptible to catastrophic failure when stressed below their ductile-to-brittle transition temperature.”

Hence, 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.

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.

Dissimilar Metal Cracking (DMC) is a form of high-temperature cracking that frequently occurs in weld joints between carbon steel and austenitic stainless steel (such as Type 316 SS) due to differences in thermal expansion coefficients and mechanical properties.

From API RP 571 Section 5.2.3.1 (Dissimilar Metal Weld Cracking):

“Cracking frequently occurs in weld joints between ferritic and austenitic materials such as carbon steel to 300 series stainless steels due to thermal expansion mismatch during high temperature operation.”

Therefore, Option D (Carbon steel to 316 stainless steel) is the most susceptible combination and the correct answer.

From API RP 571 and RP 932-B:

“In hydroprocessing reactor effluent systems, the most common corrosion mechanism is ammonium bisulfide (NH₄HS) corrosion, which forms in the presence of hydrogen sulfide and ammonia.”

“NH₄HS attacks carbon steel aggressively under turbulent flow and at low pH.”

Correct answer is B – ammonium bisulfide corrosion.

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.

API RP 571 emphasizes under Hydrogen Stress Cracking – Hydrofluoric Acid (HF):

“Cracking in carbon steel is influenced by high hardness and tensile stress in the presence of wet HF acid.”

“Controlling weld and heat-affected zone hardness to below 200 BHN is a key mitigation measure, as higher hardness significantly increases susceptibility.”

Steel hardness and strength are thus critical for cracking in HF environments, making option C correct.

According to API RP 571:

“Ammonium chloride corrosion is most aggressive when dry salts are exposed to a small amount of free water. This condition allows the formation of a concentrated acidic aqueous solution, which is highly corrosive to carbon steel.”

“The corrosion mechanism is activated especially during startup and shutdown when condensation may occur on salt deposits.”

(Reference: API RP 571, Section 4.3.3.1 – Ammonium Chloride Corrosion)

Hence, while salt deposition begins as temperatures drop, the most aggressive corrosion happens when water is introduced to dry salt, making option D correct.

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.

API RP 571 on HTHA states:

“Acoustic emission testing is not currently a proven or reliable method for identifying HTHA in refinery components.”

“HTHA typically occurs internally, not as surface blisters, and is not limited to welds. Austenitic (300 series) stainless steels are generally resistant at normal refinery conditions.”

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

Thus, option A is the most accurate statement.

According to API RP 571, under the section "4.2.1.2 Brittle Fracture", the following conditions are explicitly identified as most critical for brittle fracture risk:

"Most processes run at elevated temperature, so the main concern is for brittle fracture during startup, shutdown, hydrotest, or other situations where equipment may be exposed to low temperatures."

"Equipment fabricated from materials susceptible to brittle fracture (e.g., carbon steel) is most vulnerable when exposed to low temperatures combined with high stress or pressure."

"A brittle fracture is characterized by a sudden and rapid crack propagation with little or no plastic deformation."

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

Thus, while other options represent stress events, the main concern specifically noted by the standard is during start-up and shutdown—especially due to cooling and repressurization at low temperatures—making option A the most accurate choice.

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.

According to API RP 571, under the section on High Temperature Hydrogen Attack (HTHA) and Decarburization:

“Decarburization is typically confirmed by metallographic examination. This method reveals carbon loss and microstructural changes such as spheroidization or softening of pearlite.”

“Tensile or impact testing may not reveal early-stage decarburization, especially in partial layers.”

Therefore, metallographic testing is the standard and most direct method to confirm decarburization damage, making option D correct.

According to API RP 571, in the section on "Chloride Stress Corrosion Cracking (Cl-SCC)", several critical factors are outlined that contribute to the initiation and propagation of this mechanism. The document states:

“Critical factors include the presence of chlorides (even at ppm levels), oxygen, elevated temperatures, tensile stress, and susceptible materials such as austenitic stainless steels and some nickel alloys.”

“Oxygen is an accelerant and promotes pitting that can lead to SCC initiation.”

(Reference: API RP 571, 3rd Edition, Section 4.2.2.2)

Therefore, the presence of oxygen is a well-documented critical factor in Cl-SCC. It acts in conjunction with chlorides to initiate localized pitting, which then progresses into cracking. Hence, option B is the correct choice.

From API RP 571 Section 5.1.2.5 (Polythionic Acid Stress Corrosion Cracking):

“PTASCC typically occurs on the surface of stainless steels that have been sensitized. Since cracks are open to the surface, liquid penetrant testing (PT) is the most effective detection method.”

Magnetic particle testing is ineffective for non-magnetic materials like austenitic stainless steels.

UT is less effective for fine surface-connected cracks.

Hardness testing is unrelated to detecting cracking.

Thus, Option C (Liquid penetrant testing) is the correct and preferred method for detecting PTASCC.

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 571 states in the section on Creep Damage:

“Creep damage in carbon steels generally becomes a concern above 700°F (371°C), especially for steels with higher tensile strength (greater than 60 ksi).”

“For prolonged exposure above this temperature, time-dependent deformation can lead to material degradation and cracking, especially at weldments and stress concentrations.”

Thus, the lowest threshold for creep in high-strength carbon steels is 700°F (371°C), making option B correct.

Chloride Stress Corrosion Cracking (Cl-SCC) is a serious form of corrosion primarily affecting austenitic stainless steels and some duplex stainless steels, particularly when exposed to chloride-containing environments at elevated temperatures.

According to API RP 571 Section 5.1.2.3 (Chloride Stress Corrosion Cracking – Cl-SCC):

“Austenitic stainless steels (e.g., 300 series such as Types 304 and 316) are the most susceptible to Cl-SCC... Duplex stainless steels have greater resistance but are not immune, especially at temperatures >150°F (65°C)... Ferritic stainless steels (400 series) are generally not susceptible.”

Thus, Option A (400 Series Stainless Steel) is not susceptible to chloride SCC, making it the correct answer.

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 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).

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.

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.

API RP 571 describes under Caustic Corrosion:

“One of the significant contributing factors is caustic accumulation in low-flow or stagnant areas, especially in heat-traced lines or around steam injection points.”

“Localized heating can concentrate caustic solutions, promoting corrosion even in carbon or stainless steels.”

Thus, option C (heat-traced equipment) is the correct and technically accurate choice.

From API RP 571 Section 5.1.2.7 (Amine Stress Corrosion Cracking) and API RP 945 (Avoiding Environmental Cracking in Amine Units):

MEA (Monoethanolamine) systems are most aggressive due to:

“Their high reactivity and degradation potential producing corrosive by-products that promote cracking.”

API RP 945:

“SCC in carbon steel is most frequently reported in MEA units, especially where localized concentrations of heat-stable salts exist.”

Thus, Option C (MEA) is the correct answer due to its high SCC risk.

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.

As per API RP 571 and further clarified in API RP 939-C:

“An arbitrary threshold of 50 ppmw H₂S in the aqueous phase is often used to define when carbon and low alloy steels become susceptible to cracking damage in wet H₂S environments.”

“Below this level, the risk of SSC, HIC, and SOHIC is considered lower, although damage has still occurred at lower concentrations depending on stress and metallurgical conditions.”

Therefore, the correct answer is option D (50 ppmw).