API API-571 Corrosion and Materials Professional Exam Practice Test

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.

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.

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.

Comprehensive and Detailed Explanation From Exact Extract:

Reheat cracking is explicitly defined in API RP 571 as cracking that occurs during postweld heat treatment or elevated temperature exposure due to stress relaxation in the heat affected zone of welded components.

Key characteristics per API RP 571 include:

Occurs during PWHT or high-temperature service

Associated with creep-strength-enhanced steels (e.g., Cr-Mo steels)

Driven by stress relaxation and grain boundary weakness

Most common in heavy wall sections

Why the other options are incorrect:

Option A (PWHT cracking) is a non-specific term; API classifies this mechanism as reheat cracking.

Option B (Thermal fatigue) requires cyclic temperature changes, not stress relaxation.

Option D (Temper embrittlement) does not cause cracking during PWHT.

API RP 571 uses reheat cracking as the correct classification.

Referenced Documents (Study Basis):

API RP 571 – Section on Reheat Cracking

API Welding Metallurgy Study Guide

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Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, sulfidation corrosion is controlled by three primary variables:

Sulfur species concentration (e.g., H₂S, sulfur compounds)

Operating temperature

Metallurgy, especially chromium content

Chromium significantly improves sulfidation resistance by forming protective sulfide scales. All three factors must be considered in RBI assessments.

Referenced Documents (Study Basis):

API RP 571 – Section on Sulfidation Corrosion

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API RP 571, under Organic Acid Corrosion, highlights:

“Corrosive organic acids in overhead systems are those that condense at temperatures above the water dew point. These acids condense independently of water and can form highly concentrated corrosive films.”

“When acids condense first, they aggressively corrode carbon steel prior to any water wash mitigation.”

(Reference: API RP 571, Section 4.3.3.9 – Organic Acid Corrosion)

Hence, option D correctly identifies the most aggressive corrosion scenario.

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.

API RP 571 under Oxidation states:

“Resistance to oxidation is improved significantly by increasing chromium content, as it promotes the formation of a protective chromium oxide (Cr₂O₃) layer.”

“Nickel may aid in high-temperature strength but does not directly improve oxidation resistance like chromium does.”

(Reference: API RP 571, Section 5.1.1 – Oxidation)

Hence, option A is correct.

API RP 571 discusses Hydrogen-Induced Cracking (HIC) and Blistering under the section:

“HIC and blistering are most strongly influenced by non-metallic inclusions, particularly elongated manganese sulfide (MnS) inclusions, which serve as trap sites for hydrogen atoms.”

“These inclusions create local planes of weakness where atomic hydrogen recombines into molecular hydrogen (H₂), causing high pressure and cracking.”

(Reference: API RP 571, Section 4.2.2.7 – Hydrogen Blistering and HIC)

Thus, inclusions are the critical material factor, making option A correct.

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.

Comprehensive and Detailed Explanation From Exact Extract:

Concentration Cell Corrosion is defined in API RP 571 as a form of electrochemical corrosion driven by differences in concentration of corrosive species, most commonly oxygen, under deposits or within crevices.

This type of corrosion typically occurs:

Beneath deposits, scale, or fouling

In stagnant zones or crevices

Where oxygen concentration differs between adjacent areas

The area with lower oxygen concentration becomes anodic and corrodes preferentially, while the oxygen-rich area becomes cathodic.

Why the other options are incorrect:

Option B describes galvanic corrosion, not concentration cell corrosion.

Option C refers to high-temperature sulfidation, unrelated to concentration gradients.

Option D describes mechanical damage mechanisms, not electrochemical corrosion.

API RP 571 clearly categorizes concentration cell corrosion as being associated with deposits or crevice conditions, making Option A the correct description.

Referenced Documents (Study Basis):

API RP 571 – Section on Concentration Cell and Crevice Corrosion

API Corrosion Fundamentals Study Guide

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According to API RP 571 Section 5.2.1 (Fatigue – High Cycle and Low Cycle):

“Unsupported small-bore attachments like vents and drains are prone to high-cycle mechanical fatigue due to vibration. These attachments experience cyclic stress, especially near welds or bends.”

Thermal overload is unlikely without over-temperature data.

SSC (Option C) requires a wet H2S environment, which is not indicated.

Weld defects may contribute but are less likely the root cause without other signs.

Hence, mechanical fatigue (Option A) is most probable in this configuration.

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.

From API RP 571 Section 5.3.2.1 (Creep and Stress Rupture):

“Short-term creep rupture is often associated with localized overheating and results in bulging and thinning, often with a characteristic ‘fish-mouth’ rupture appearance. These failures occur under sustained load in a short time (hours or days), typically in high-temperature service due to a sudden increase in temperature or poor heat distribution.”

Thus, Option B accurately describes the mechanism and visual indications of short-term stress rupture.

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.

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, surface decarburization is a metallurgical degradation mechanism that occurs when carbon is removed from the surface layers of steel due to exposure to oxidizing environments at elevated temperatures. This results in a carbon-depleted surface layer.

Carbon is a primary strengthening element in carbon and low-alloy steels. When carbon is lost from the surface:

Hardness and tensile strength are reduced

Creep resistance and load-carrying capability decrease

The component becomes more susceptible to plastic deformation and failure under stress

API RP 571 states that decarburization leads to loss of mechanical strength, especially critical in high-temperature service, where components already operate close to material limits.

Why the other options are incorrect:

Option A: Stress cracking is not the primary effect; loss of strength is.

Option C: Decarburization does not directly accelerate oxidation or sulfidation, although both may coexist.

Option D: This is incorrect; decarburization is considered detrimental in high-temperature applications.

Referenced Documents (Study Basis):

API RP 571 – Section on Decarburization and Metal Dusting

API Corrosion and Materials Study Guide

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According to API RP 571 Section 5.1.3.4 (Naphthenic Acid Corrosion - NAC):

“Sulfur can act as an inhibitor to NAC. Where sulfur content in crude is high, a reduction in naphthenic acid corrosion is sometimes observed, especially in certain parts of vacuum and atmospheric units. However, this is not universally reliable, and corrosion can still occur based on local operating conditions and metal temperatures.”

Thus, Option C (Inhibition) is the correct choice, as sulfur can reduce NAC under some conditions.

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.

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, amine stress corrosion cracking (Amine SCC) is a form of environmentally assisted cracking that occurs in alkanolamine systems (e.g., MEA, DEA, DGA) used for acid gas treating. The cracking mechanism requires the simultaneous presence of tensile stress, susceptible microstructure, and an amine environment.

API RP 571 clearly states that Amine SCC occurs primarily in the weld heat affected zone (HAZ) of carbon and low-alloy steels. The reasons include:

The HAZ contains high residual stresses from welding.

The microstructure in the HAZ is metallurgically altered, making it more susceptible than base metal.

Cracks often initiate adjacent to welds, not in the weld metal itself.

Why the other options are incorrect:

Option A (Weld fusion line) is not the primary cracking location.

Option C (Weld metal) generally has different chemistry and lower susceptibility.

Option D (Base metal) typically has lower residual stress and is less prone.

API RP 571 specifically identifies the weld HAZ as the dominant cracking location for Amine SCC.

Referenced Documents (Study Basis):

API RP 571 – Section on Amine Stress Corrosion Cracking

API Corrosion and Materials Study Guide

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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.

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).

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.

Comprehensive and Detailed Explanation From Exact Extract:

Hydrochloric acid (HCl) corrosion, particularly in crude unit overhead systems, is aggressive toward carbon steel and stainless steels.

According to API RP 571, nickel-based alloys, such as Monel (Ni–Cu alloys), exhibit excellent resistance to hydrochloric acid corrosion and are commonly used for:

Overhead piping

Injection quills

High-risk corrosion zones

Why the other options are incorrect:

Duplex and 300 series stainless steels are highly susceptible to chloride corrosion and SCC.

Copper-based materials alone lack sufficient resistance and structural strength in refinery HCl environments.

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrochloric Acid Corrosion

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API RP 571 on Naphthenic Acid Corrosion (NAC) details:

“NAC is most aggressive in two-phase flow where the shear stress is high, such as in elbows, reducers, and at orifices.”

“High velocity and turbulence in two-phase flow strip protective layers and accelerate corrosion.”

Therefore, option A is the correct and most critical phase.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, corrosion fatigue results from the combined action of a cyclic stress and a corrosive environment. One of the most important distinguishing features between corrosion fatigue and SCC is crack morphology.

Corrosion fatigue cracks are typically transgranular and show little or no branching

Stress corrosion cracking is characterized by extensive crack branching

Therefore, the absence of significant branching is a key diagnostic feature of corrosion fatigue.

Referenced Documents (Study Basis):

API RP 571 – Section on Corrosion Fatigue

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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.

From API RP 571, brittle fracture prevention measures include:

“Postweld Heat Treatment (PWHT) reduces residual stresses in the heat affected zone and base metal, thus lowering susceptibility to brittle fracture.”

“PWHT is particularly effective when applied to pressure-containing components fabricated from carbon steel or low-alloy steel that will experience low-temperature service.”

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

Therefore, while other measures may reduce stress or detect flaws, PWHT directly targets one of the root causes: residual stress. Thus, option D is the best prevention method.

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.

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.

API RP 571 defines Hydrogen Embrittlement as:

“Improper PWHT can trap hydrogen within the microstructure of carbon steel or fail to relieve residual stresses, both of which increase the risk of hydrogen embrittlement.”

“Moisture in electrodes contributes to hydrogen-induced cracking, but embrittlement from atomic hydrogen is more directly tied to heat treatment and microstructure.”

Thus, option D is the most accurate.

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:

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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From API RP 571 Section 5.1.2.1 and RP 941:

“High strength low alloy steels (HSLA) are more susceptible to hydrogen embrittlement, especially during cold work, forming, or welding. These materials, due to their higher tensile strengths, are more prone to cracking when exposed to hydrogen, particularly during fabrication or post-weld heat treatment operations.”

Therefore, Option A (High Strength Low Alloys) is the correct answer due to their known susceptibility during manufacturing and processing.

API RP 571 on Organic Acid Corrosion in Overheads explains:

“Corrosion caused by organic acids (e.g., formic, acetic) in overhead systems is best mitigated by neutralization, usually via injection of alkaline neutralizing amines that raise pH and reduce acidity.”

“Water wash can be helpful but is often insufficient by itself.”

Therefore, the most effective approach is option C – injection of neutralizer.

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, 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.

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

Comprehensive and Detailed Explanation From Exact Extract:

In HF Alkylation units, API RP 571 identifies certain residual alloying elements in carbon steel that can significantly increase corrosion rates in HF acid service.

The most detrimental residual elements are:

Chromium (Cr)

Copper (Cu)

Nickel (Ni)

These elements:

Disrupt formation of protective iron fluoride films

Increase general and localized corrosion rates

Are tightly controlled in carbon steel specifications for HF service

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrofluoric Acid Corrosion

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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 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.

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, carbon steel exhibits a minimum corrosion rate in sulfuric acid at concentrations around 65–70 wt% due to formation of a protective iron sulfate film.

When sulfuric acid concentration drops below approximately 65%, this protective film becomes unstable and corrosion rates increase dramatically.

Referenced Documents (Study Basis):

API RP 571 – Section on Sulfuric Acid Corrosion

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