API Corrosion and Materials Professional API-571 Exam Dumps: Updated Questions & Answers (May 2025)

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.

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.

API RP 571 under Ammonium Chloride Corrosion details:

“The corrosion results from the deposition of ammonium chloride salts from high-temperature process streams as they cool.”

“This typically occurs in crude unit overheads or other systems where salts condense out as the stream temperature drops below their dew point.”

“Corrosion becomes especially aggressive when the salts are wetted by condensation.”

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

Therefore, option A is technically accurate and supported.

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.

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, 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:

“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.

In steam systems, the absence of a steam trap allows condensate to accumulate, which can lead to corrosion from oxygen and carbon dioxide dissolved in the water—a classic case of condensate corrosion.

From API RP 571 Section 5.3.3.1 (Condensate Corrosion):

“If steam lines or equipment like soot blowers do not have proper drainage or steam traps, condensate may accumulate... Corrosion results from the presence of dissolved oxygen and/or carbon dioxide in the condensate.”

Thus, the correct answer is Option D (condensate corrosion).

API RP 571 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.

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.