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 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.

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 on Thermal Fatigue:

“Mixing of hot and cold streams, particularly where hydrogen is present, can induce thermal cycling stresses, leading to thermal fatigue cracking, especially at fittings like tees.”

“Hydrogen embrittlement typically affects stressed zones, but this pattern strongly indicates thermal fatigue.”

So the correct answer is C – Thermal fatigue.

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 explains the typical location of dissimilar metal weld (DMW) cracks as follows:

“DMW cracking usually occurs in the heat-affected zone (HAZ) on the ferritic side of the weld.”

“The difference in thermal expansion coefficients, grain structures, and creep rates between the ferritic and austenitic materials contributes to the development of high stress and strain concentrations in the HAZ of the ferritic material.”

(Reference: API RP 571, Section 4.2.1.6 – Dissimilar Metal Weld Cracking)

Thus, these cracks do not occur in the center of the weld or on the austenitic side, but rather at the ferritic HAZ, making option A 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.

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 and API RP 751 (HF Alkylation Units):

“HF acid corrosion rates increase with elevated temperature and higher water content. This is because water acts as an ionizing agent that facilitates the acid’s attack on steel.”

“Corrosion is minimized when water content is strictly controlled, usually between 0.5–1.5%.”

“Contrary to some assumptions, increasing HF acid concentration actually decreases corrosivity.”

Hence, option B is correct.

API RP 571 identifies these parameters as critical to the corrosion rate of carbon and low alloy steels in sour water environments:

“The extent of corrosion in sour water is influenced by pH, H₂S content, temperature, liquid velocity, and oxygen contamination.”

“These factors control the formation and stability of protective iron sulfide layers.”

(Reference: API RP 571, Section 4.3.2.1 – Sour Water Corrosion)

Therefore, the correct answer is A.

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.

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.

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.

According to API RP 571, Section 4.2.1.1 – Sulfidation, and also echoed in API RP 939-C, the presence of certain compounds significantly increases sulfidic corrosion:

“Chlorides are known to accelerate sulfidation and other high-temperature corrosion mechanisms by forming low-melting eutectics and breaking down protective scales on alloy surfaces.”

“Chlorine-containing species also interfere with the formation of stable sulfide or oxide scales, leading to more aggressive metal loss.”

Therefore, chlorides are the most critical accelerant among the listed substances in high-temperature sulfur compound environments, making option C correct.

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.

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 5.3.2.3 (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.

According to API RP 571 and API RP 583 (CUI-specific guidance):

“Neutron backscatter techniques are effective in detecting moisture within insulation systems, which can indicate areas of concern for corrosion under insulation (CUI).”

“This method provides non-invasive, quick identification of wet insulation without removing cladding.”

(Reference: API RP 571, Section 4.5.1 – Corrosion Under Insulation and API RP 583 – CUI Detection Techniques)

Thus, neutron backscatter is the preferred NDE method for moisture detection, making option B correct.

According to API RP 571 Section 5.3.2.1 (Creep):

“Creep damage is exponentially dependent on temperature. A small increase in temperature (e.g., 25°F or 15°C) can reduce remaining life by more than half. This is because the creep rate increases rapidly with temperature, especially above design limits.”

Stress is a contributing factor, but the dominant and most sensitive variable is temperature.

Thus, Option C (increase in temperature of 25°F/15°C) is the correct answer.

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.

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.

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.

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.

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

“Postweld heat treatment (PWHT) does not significantly reduce susceptibility to HIC, as the mechanism is primarily driven by hydrogen charging in wet H₂S environments and steel cleanliness, not residual stresses.”

“PWHT is more effective for SSC, SOHIC, and other stress-driven mechanisms.”

Thus, HIC will not benefit much from PWHT, making option C correct.

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.

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.

API RP 571 – Polythionic Acid Stress Corrosion Cracking:

“Polythionic acid SCC may be mistaken for wet H₂S damage because both involve cracking in austenitic stainless steels under certain conditions.”

Thus, option D is correct.

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.