API-571 Corrosion and Materials Professional Question and Answers

According to API RP 571, in the section on Boiler Water Condensate Corrosion:

“The major contributors to condensate corrosion are dissolved CO₂ and O₂. Carbon dioxide forms carbonic acid in the presence of water, lowering pH and causing generalized corrosion.”

“Oxygen causes pitting and localized corrosion unless chemically treated.”

Thus, option B (Carbon dioxide and oxygen) is the primary root cause and the correct answer.

API RP 571 states under Galvanic Corrosion:

“Because galvanic corrosion usually manifests as thinning of the more anodic metal, ultrasonic thickness measurements taken near dissimilar metal joints are an effective way to detect this type of internal corrosion.”

“Surface NDE techniques like liquid penetrant or magnetic particle testing are not effective for detecting wall loss.”

Thus, option D (ultrasonic thickness testing) is correct.

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.

API RP 571 under Oxidation:

“Carbon steels and low alloy steels, such as 9 Cr, have poor oxidation resistance at temperatures above 1000°F (540°C), rapidly losing thickness.”

“Stainless steels with higher Cr and Ni content (like 310) have superior oxidation resistance.”

Thus, 9 Cr steel will corrode fastest at 1350°F, making option D correct.

Comprehensive and Detailed Explanation From Exact Extract:

Polythionic acid stress corrosion cracking (PASCC) occurs in austenitic stainless steels exposed to sulfur-containing environments during shutdowns, particularly when sulfide scales react with oxygen and moisture.

According to API RP 571:

Unstabilized stainless steels, such as Type 304, are most susceptible

Stabilized grades (e.g., Type 321, titanium-stabilized) are more resistant

Carbon steel and Monel are not susceptible to PASCC

Referenced Documents (Study Basis):

API RP 571 – Section on Polythionic Acid SCC

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According to API RP 941, and also noted in RP 571:

“Nickel significantly improves resistance to HTHA (High Temperature Hydrogen Attack) by stabilizing austenitic phases and reducing carbide decomposition.”

Hence, option C is correct.

According to API RP 571 Section 5.3.2.3 (Spheroidization):

“Spheroidization is a microstructural degradation that occurs in carbon steels and low alloy steels when exposed to elevated temperatures for long periods, typically beginning at temperatures as low as 650°F (345°C). It involves the transformation of lamellar pearlite into spherical cementite particles, reducing strength and hardness.”

Thus, Option A (650°F / 345°C) is the minimum threshold temperature for spheroidization to begin.

Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte such as brackish water or seawater. The rate and severity of galvanic corrosion depend on:

The relative position of the metals on the galvanic series (potential difference)

Area ratio between anode and cathode

Conductivity of the electrolyte

API RP 571 explains:

“The more active (anodic) metal in the galvanic couple corrodes preferentially. The more noble (cathodic) metal is protected.”

“The greater the difference in potential between the two metals, the more aggressive the galvanic reaction.”

“Aluminum is highly anodic compared to steel. In seawater or brackish conditions, aluminum will corrode rapidly when electrically coupled to steel, especially if the aluminum surface area is small relative to the steel.”

(Reference: API RP 571, Section 4.3.5.1 – Galvanic Corrosion)

Referring to the galvanic series presented in your table:

Aluminum is much higher (more anodic) than steel.

This makes aluminum the sacrificial metal in such a pair.

This is not the case for brass-to-nickel or cast iron-to-Ni-Resist, which are much closer together on the galvanic scale.

Therefore, among all the listed pairs, Aluminum coupled to Steel creates the most significant galvanic potential difference and is most likely to result in galvanic corrosion in brackish or seawater service.

Correct Answer: B

API RP 571 describes Temper Embrittlement as:

“A metallurgical degradation mechanism that leads to a reduction in fracture toughness due to long-term exposure in the temperature range of 650°F to 1070°F (345°C to 575°C).”

“It affects Cr-Mo steels and is not easily detected except by impact testing.”

Therefore, option C most accurately defines the condition based on the recognized API description.

Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, deaerators often operate in environments where concentrated caustic solutions can form locally due to oxygen scavengers and water chemistry control additives.

When postweld heat treatment (PWHT) is not performed:

High residual welding stresses remain

Conditions are ideal for caustic stress corrosion cracking (Caustic SCC)

API RP 571 identifies deaerators as classic examples of equipment that has experienced caustic SCC when PWHT was omitted.

Referenced Documents (Study Basis):

API RP 571 – Section on Caustic Stress Corrosion Cracking

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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.

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.

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 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.

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 – Sulfidation:

“Sulfidation generally results in uniform thinning of iron-based alloys at temperatures above 500°F (260°C).”

“It is classified as a form of general corrosion, although it can be accelerated in turbulent flow areas.”

So, option A is correct.

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.

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.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, chromium is the most effective alloying element for improving oxidation resistance in steels.

Chromium:

Forms a stable, adherent chromium oxide (Cr₂O₃) scale

Greatly reduces oxidation rates at elevated temperatures

Is the basis for oxidation-resistant alloys and stainless steels

Other alloying elements may provide secondary benefits, but chromium is the primary contributor to oxidation resistance.

Referenced Documents (Study Basis):

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

As per API RP 571 Section 5.3.2.3 (Spheroidization):

“Spheroidization is the transformation of the microstructure of steels into spherical carbides, leading to reduced strength and hardness, especially after extended high-temperature exposure. It’s commonly seen in carbon and low alloy steels in refining heaters and reactors.”

It reduces strength and sometimes hardness.

It increases ductility, not reduces.

SCC potential is not directly tied to this change.

Therefore, Option D (Loss of strength) is correct.

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

“Nickel contents above approximately 35% provide significant resistance to chloride stress corrosion cracking. Alloys such as Alloy 625 and Alloy 825 exhibit excellent resistance.”

“Austenitic stainless steels with lower nickel contents, such as 304 and 316, are more susceptible to Cl-SCC.”

Hence, option C (35%) is correct.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571 and corrosion control best practices, maximum protection of buried or soil-exposed carbon steel is achieved by combining:

A protective coating (primary barrier)

Cathodic protection (CP) (secondary, active protection)

This dual system:

Minimizes current demand

Protects coating defects

Extends service life significantly

Neither coatings nor CP alone provide equivalent long-term reliability.

Referenced Documents (Study Basis):

API RP 571 – Section on Soil-Side Corrosion

API Corrosion Control Study Guide

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According to API RP 571, under Sulfuric Acid Corrosion:

“The most aggressive corrosion typically occurs below 65% H₂SO₄, particularly between 20% to 50% concentration range.”

“Above 95% concentration, sulfuric acid becomes less corrosive to carbon steel due to its dehydrating nature.”

“Dilute sulfuric acid is highly ionized and conducts electricity well, increasing corrosion rates.”

(Reference: API RP 571, Section 4.3.3.2 – Sulfuric Acid Corrosion)

Hence, the most severe corrosion occurs below 65% concentration, making option B the accurate answer.

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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API RP 571 provides the following guidance regarding temper embrittlement:

“For materials that contain impurity levels which make them susceptible to temper embrittlement, the most effective preventive measure is to specify low FATT or Charpy impact energy requirements.”

“These requirements ensure the material retains adequate toughness even if embrittlement occurs.”

“While temper embrittlement itself cannot always be fully prevented during long-term exposure to temperatures between 650°F and 1070°F (343°C–577°C), its effects can be identified and mitigated through impact testing criteria.”

Therefore, option C is correct. Specification of Charpy V-notch testing ensures material selection accounts for embrittlement susceptibility and maintains service integrity.

API RP 571, under Chloride Stress Corrosion Cracking, notes:

“Cracking often appears as highly branched or ‘crazed’ networks on the surface, especially in austenitic stainless steels exposed to chlorides under tensile stress.”

“These cracks often initiate in the heat-affected zones or cold-worked areas and are intergranular in nature.”

This signature ‘crazed’ appearance with branching is diagnostic of Cl-SCC, making option C the correct and most descriptive answer.

According to API RP 571:

“Oxidation is the reaction of metal and oxygen to form oxides. It generally occurs in high temperature environments with excess oxygen.”

“Sulfidation is a high temperature corrosion mechanism that occurs due to interaction between sulfur-containing compounds and metal surfaces, forming metal sulfides. Like oxidation, sulfidation occurs in environments above 500 °F (260 °C).”

“Both oxidation and sulfidation can occur concurrently, particularly in mixed environments of oxygen and sulfur.”

(Reference: API RP 571, Sections 5.1.1 and 5.1.3 – Oxidation & Sulfidation)

These two mechanisms are thermally activated and occur in elevated temperature conditions found in heaters, furnaces, and piping systems. Hence, option B is correct.

As per API RP 571 Section 5.3.2.2 (Sigma Phase Embrittlement):

“Sigma phase embrittlement occurs in austenitic stainless steels (like 300 series) and duplex stainless steels exposed to 1000°F to 1800°F (540°C to 980°C)... This embrittlement results in a significant loss of toughness and ductility.”

Sigma phase is not a concern in Monel (nickel-copper alloy) or 5Cr steels.

400 series are ferritic and not typically affected by sigma phase.

Hence, Option A (300 series stainless steel) is correct.

From API RP 571 Section 5.3.3.2 (Caustic Corrosion):

“Caustic corrosion, also known as caustic gouging, occurs in boiler systems when caustic concentrates in under-deposit areas or due to improper steam drum and boiler water design. Good design and water treatment practices are key preventive measures.”

Temperature control or acid injection are not practical or effective mitigation techniques. Design considerations—such as proper water flow, material selection, and minimizing deposits—are essential.

Hence, the correct answer is Option C.

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:

Amine stress corrosion cracking is most frequently associated with rich amine services, where:

Acid gas loading (Hâ‚‚S / COâ‚‚) is high

Corrosive contaminants and degradation products are present

Higher operating temperatures and stresses exist

Per API RP 571, cracking is far more common in rich amine systems than in lean amine circuits.

Why the other options are incorrect:

Lean amine has lower acid gas content and reduced SCC risk

SCC is not limited to low temperatures

Concentration alone is not the dominant factor

Referenced Documents (Study Basis):

API RP 571 – Section on Amine Stress Corrosion Cracking

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, creep damage is a time-dependent, high-temperature damage mechanism that occurs when materials are exposed to stress at temperatures typically above about 700 °F (370 °C) for extended periods. Creep damage results in void formation, microcracking, grain boundary separation, and eventual rupture.

Once creep damage has occurred, it is considered irreversible metallurgical degradation. API RP 571 clearly states that heat treatments cannot restore creep life because the material’s microstructure has already been permanently damaged.

Option A (PWHT at 1150 °F) may relieve residual stresses but does not heal creep voids or microcracks.

Option B (Solution annealing) is applicable to certain stainless steels but is not effective for reversing creep damage, particularly in low-alloy and Cr-Mo steels.

Option D (Preheating during welding) helps prevent hydrogen-related cracking but has no effect on existing creep damage.

API RP 571 emphasizes that the only effective mitigation for creep damage is to remove the affected material and replace it with sound material, or to retire the component if damage is widespread. Fitness-for-service assessments (API 579-1/ASME FFS-1) may be used to evaluate remaining life, but mitigation requires material removal.

Referenced Documents (Study Basis):

API RP 571 – Section on Creep and Stress Rupture Damage

API Corrosion and Materials Study Guide

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Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, hydrogen embrittlement (HE) of carbon steels is a low-temperature damage mechanism. It occurs when atomic hydrogen enters the steel lattice, reducing ductility and toughness and potentially causing cracking or delayed failure.

API RP 571 clearly differentiates HE from high-temperature hydrogen damage mechanisms (such as HTHA). Hydrogen embrittlement is most severe at ambient to moderately elevated temperatures, typically below about 200 °F (93 °C), where hydrogen mobility and trapping promote embrittlement.

At higher temperatures, hydrogen tends to diffuse out of the steel and embrittlement effects diminish.

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrogen Embrittlement

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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.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, chloride stress corrosion cracking (Cl-SCC) of austenitic (300 series) stainless steels becomes a credible concern when:

Chlorides are present

Tensile stress exists

Metal temperatures exceed approximately 140 °F (60 °C)

Below this temperature, Cl-SCC is far less likely, while susceptibility increases rapidly above this threshold.

Referenced Documents (Study Basis):

API RP 571 – Section on Chloride Stress Corrosion Cracking

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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 details that:

“Sulfide Stress Cracking (SSC) susceptibility increases significantly with increased hardness of the steel.”

“NACE MR0175/ISO 15156 provides hardness limits for carbon and low-alloy steels to avoid SSC in sour environments. These are typically limited to 22 HRC or 248 Brinell.”

“High hardness promotes crack initiation under tensile stress in sour environments (i.e., containing H₂S).”

(Reference: API RP 571, Section 4.2.2.1 – Sulfide Stress Corrosion Cracking)

While hydrogen-induced cracking (HIC) and blistering are related to hydrogen charging, SSC is the only mechanism directly and critically impacted by hardness, making option B 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.

API RP 571 explains that both Sulfide Stress Cracking (SSC) and SOHIC are:

“Strongly influenced by material hardness. Carbon and low alloy steels with hardness exceeding 22 HRC (248 Brinell) are most susceptible.”

“They occur in hardened steels under tensile stress, in the presence of wet H₂S environments.”

(Reference: API RP 571, Sections 4.2.2.1 and 4.2.2.7 – SSC and SOHIC)

Therefore, hardened steels are the common factor for both mechanisms, making option C the correct 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.

Same as above. Sigma phase formation is a microstructural phenomenon and not typically detected by NDE methods like magnetic particle or surface hardness testing.

Therefore, metallographic testing remains the definitive detection method, confirming option D again as the correct answer.