API-571 Corrosion and Materials Professional Questions and Answers

According to API RP 571, under the section "Corrosion in Aqueous Environments – Cooling Water Corrosion", one of the key contributors to corrosion in carbon steel and other materials used in heat exchangers is the presence of dissolved oxygen. API RP 571 states:

"Oxygen is a primary contributor to corrosion in cooling water systems. Systems open to the atmosphere are typically more corrosive than closed systems due to the continual replenishment of oxygen."

"Corrosion rates are highest where oxygen concentration is the greatest, especially in systems using untreated or poorly treated water."

"Carbon steel corrodes in the presence of oxygen and water, forming corrosion products that may or may not adhere to the surface."

(Reference: API RP 571, Section 4.3.1.1 – Cooling Water Corrosion)

Therefore, increasing oxygen content directly increases corrosion activity in exchanger tubes, making option C the correct and documented answer.

API RP 571 under Cooling Water Corrosion and Scaling:

“Cooling water scaling tends to occur more rapidly as water temperature increases. To minimize scaling, inlet temperatures to exchangers should be kept below 140°F (60°C) where possible.”

“Above this temperature, calcium carbonate and other salts tend to precipitate more readily.”

Thus, option A is correct.

Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, chloride stress corrosion cracking (Cl-SCC) of austenitic stainless steels is strongly influenced by low pH environments in the presence of chlorides, tensile stress, and elevated temperature.

API RP 571 identifies that:

Chloride SCC can initiate at very low pH levels

pH values near 2 represent a threshold where aggressive chloride attack and cracking become possible

Increasing acidity dramatically increases SCC susceptibility

Therefore, pH = 2 is the lowest and most critical value listed where chloride SCC can begin.

Referenced Documents (Study Basis):

API RP 571 – Section on Chloride Stress Corrosion Cracking

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API RP 578 (Material Verification Program), referenced in API RP 571, specifically calls out sulfidation failures as a major driver for Positive Material Identification (PMI):

“PMI programs are particularly important in services prone to sulfidation where incorrect alloying elements (e.g., carbon steel used in place of low Cr alloy) have led to catastrophic failures.”

“Many failures have occurred when carbon steel was mistakenly installed in place of specified Cr-alloys in sulfidation-prone environments.”

(Reference: API RP 578 and API RP 571, Section 4.2.1.1 – Sulfidation)

Thus, option C is the most appropriate answer.

API RP 571, under "Caustic Corrosion" (also referred to as Caustic Stress Corrosion Cracking or Caustic Cracking), notes that this form of damage is:

“Most commonly associated with the injection points of caustic (e.g., NaOH) in crude units, particularly upstream of desalter vessels and heat exchanger circuits.”

“This damage occurs due to the combination of caustic environment and tensile stress, especially in carbon steel and low alloy steels.”

“Stress relief heat treatment of welds can mitigate susceptibility.”

(Reference: API RP 571, 3rd Edition, Section 4.2.2.3)

Thus, among the options, caustic injections in crude units are the most typical area of concern, making option C the correct answer.

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

“Austenitic stainless steels such as 304 and 316 are highly susceptible to Cl-SCC when subjected to tensile stress and exposed to chloride-bearing environments.”

“Susceptibility is greatest in non-stress-relieved weldments or cold-worked areas.”

Brass and carbon steels are not typical Cl-SCC candidates; low alloy steels such as 1.25Cr-0.5Mo are also not commonly affected.

Thus, option C is the correct answer.

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:

Microbiologically Induced Corrosion (MIC) is one of the most universally applicable corrosion mechanisms, affecting:

Carbon steel

Low-alloy steel

Stainless steels

Copper alloys

Aluminum alloys

According to API RP 571, MIC can occur in any aqueous system where microorganisms are present, regardless of metallurgy.

By contrast:

Sour water corrosion and amine corrosion mainly affect carbon steel

Polythionic acid corrosion is limited to austenitic stainless steels

Referenced Documents (Study Basis):

API RP 571 – Section on Microbiologically Induced Corrosion

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.

Comprehensive and Detailed Explanation From Exact Extract:

Ethanol stress corrosion cracking (SCC) of carbon steel is strongly dependent on the presence of water.

Per API RP 571, moisture content is the primary controlling factor, because:

Water enables formation of corrosive species

Dry or very low-water ethanol does not cause SCC

Even small amounts of water significantly increase cracking risk

Other factors influence severity, but without moisture, ethanol SCC does not occur.

Referenced Documents (Study Basis):

API RP 571 – Section on Ethanol Stress Corrosion Cracking

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Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, graphitization is a high-temperature degradation mechanism that occurs in carbon and carbon-molybdenum steels exposed for long periods at temperatures typically above 800 °F (425 °C).

In this process:

Iron carbides decompose into free graphite nodules

The steel loses strength, ductility, and pressure-retaining capability

Failures can occur without significant wall loss

Graphitization does not occur in aluminum, stainless steels, or nickel-copper alloys such as Monel.

Referenced Documents (Study Basis):

API RP 571 – Section on Graphitization

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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.

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 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.

API RP 571 notes that for detecting existing cracks or flaws that may lead to brittle fracture, especially those that initiate at welds or surface stress points:

“Wet fluorescent magnetic-particle inspection is highly effective for detecting surface-breaking flaws in ferromagnetic materials that are susceptible to brittle fracture.”

“It is sensitive to tight cracks and surface discontinuities that may serve as initiation sites for brittle fracture.”

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

Therefore, option A is correct as it is the most effective NDE technique for detecting early signs of brittle cracking.

API RP 571 under Polythionic Acid Stress Corrosion Cracking (PTA SCC) specifies:

“Cracking is usually intergranular, and often localized, especially in sensitized austenitic stainless steels exposed to sulfur-containing environments during shutdown or cleaning.”

“It is not always visible on inspection and may only become evident when a leak occurs.”

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

Hence, option C is correct as it best describes the detection difficulty and behavior of PTA SCC.

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.

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.

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.

API RP 571 describes temper embrittlement as a subtle metallurgical change that reduces impact toughness without obvious macrostructural change:

“Temper embrittlement is not readily apparent by visual or standard tensile testing. It is confirmed by Charpy impact testing which measures toughness at low temperatures.”

“It typically affects Cr-Mo steels exposed to intermediate temperatures (650°F to 1100°F) for extended times.”

(Reference: API RP 571, Section 4.2.1.4 – Temper Embrittlement)

Thus, the condition is not visually or physically apparent, but is confirmed through impact testing, making option D correct.

As per API RP 571 on Sigma Phase Embrittlement:

“Sigma phase embrittlement occurs in stainless steels and nickel-based alloys due to the formation of a brittle intermetallic phase in the temperature range of 1050°F to 1650°F (565°C to 900°C).”

“This damage mechanism is not easily detected through surface or hardness testing and typically requires metallographic examination to confirm the presence of sigma phase in the microstructure.”

Metallographic testing allows clear identification of sigma phase particles and is thus the most reliable method, making option D 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).

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, hydrofluoric (HF) acid corrosion rates increase significantly once operating temperatures exceed approximately 150 °F (65 °C).

Key points noted in API RP 571:

Below ~150 °F, corrosion rates are relatively low due to formation of protective iron fluoride films.

Above 150 °F, these films become unstable.

Corrosion rates increase rapidly with temperature, especially under high velocity and water contamination conditions.

Why the other options are incorrect:

Option A (100 °F) is below the threshold for accelerated corrosion.

Option C and D are well above the onset temperature but are not the minimum threshold.

API RP 571 explicitly identifies 150 °F (65 °C) as the critical temperature where corrosion acceleration begins.

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrofluoric Acid Corrosion

API Alkylation Unit Corrosion Study Guide

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 Section 5.3.2.3 (Spheroidization):

“Spheroidization is the transformation of the microstructure of carbon and low alloy steels when exposed to elevated temperatures for long durations. The rate of spheroidization depends on temperature, prior microstructure, and exposure time... The microstructure becomes less effective at carrying loads, and strength is reduced.”

Key influencing factors for the rate of spheroidization are:

Temperature (higher accelerates the process),

Microstructure (initial phase distribution and morphology).

Pressure and hydrogen partial pressure are not relevant for this transformation, nor is stress a primary driver.

Therefore, Option D (temperature and microstructure) 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 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.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, corrosion in cooling water systems often increases at low flow velocities due to:

Stagnation

Deposit formation

Increased risk of underdeposit corrosion and MIC

Higher velocities tend to:

Sweep away deposits

Maintain protective films (within erosion limits)

Therefore, decreasing velocity is strongly associated with increased corrosion risk.

Referenced Documents (Study Basis):

API RP 571 – Section on Cooling Water Corrosion

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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, 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.

In API RP 571, Carbonate SCC and Alkaline SCC are both discussed under mechanisms that occur in basic (high pH) environments, often with similar contributing conditions and prevention methods. The document highlights:

“Carbonate SCC is considered a subcategory of alkaline stress corrosion cracking. It results from the concentration of alkaline carbonates under thermal insulation or deposits.”

“Both mechanisms involve the same key parameters: elevated temperatures, tensile stress, and high pH solutions.”

(Reference: API RP 571, 3rd Edition, Section 4.2.2.4 and 4.2.2.3)

Because these mechanisms share similar material susceptibility and operating conditions, they are indeed closely related—option B is therefore the accurate and supported answer.

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.

Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, mechanical fatigue failures are identified by the presence of beach marks or clam shell patterns on the fracture surface.

These features:

Appear as concentric rings

Represent successive crack growth increments

Radiate outward from the crack initiation site

This “clam shell” or “beach mark” appearance is one of the most recognizable signatures of fatigue failure.

Referenced Documents (Study Basis):

API RP 571 – Section on Mechanical Fatigue

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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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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.

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.

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.

Per API RP 571, susceptibility to hydrogen embrittlement is strongly influenced by:

Material strength level

Microstructure

Heat treatment condition

Heat treatment determines:

Dislocation density

Trap sites for hydrogen

Residual stress state

The modulus of elasticity does not significantly affect hydrogen embrittlement susceptibility, making Option A incorrect.

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrogen Embrittlement

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API RP 571 discusses this under Wet H₂S Damage – Hydrogen Blistering, HIC, SOHIC:

“Hydrogen generation is reduced as pH increases. At pH 9 and above, the corrosion potential is less favorable for hydrogen charging and thus permeation into the steel is minimal.”

“Most wet H₂S cracking damage mechanisms are accelerated under acidic conditions (pH < 7).”

Hence, high pH (around 9) environments offer the most protection against hydrogen damage, making option D the correct choice.

According to API RP 571, high-cycle fatigue (HCF) is characterized by very tight, narrow cracks that can escape detection using typical NDE methods due to their minimal opening displacement and fine geometry.

From API RP 571 Section 5.2.1 (Fatigue - High Cycle):

"The cracks are usually tight and may be difficult to detect using conventional NDE techniques such as PT or MT. UT and RT may also have difficulty identifying these small, tight flaws, especially in early stages of propagation."

The primary issue in NDE is not the geometry (such as 90° corners) or ID location, but rather the tightness and early-stage subtlety of the cracks which results in reduced detectability. Therefore, option B is correct as it aligns most accurately with API RP 571's detailed characterization of HCF detection challenges.