API-571 Exam Dumps : Corrosion and Materials Professional

API RP 571 on Thermal Fatigue emphasizes:

“Thermal fatigue results from cyclic thermal stresses during startup, shutdown, or transient operations.”

“The best mitigation strategy is to control heating and cooling rates to reduce temperature differentials and resulting stress.”

(Reference: API RP 571, Section 4.2.1.5 – Thermal Fatigue)

Hence, option D correctly identifies the preventive action.

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).

Comprehensive and Detailed Explanation From Exact Extract:

Naphthenic Acid Corrosion (NAC) occurs in high-temperature refinery streams (typically 450–750 °F / 230–400 °C) containing organic acids. According to API RP 571, NAC is particularly aggressive toward carbon steels and low-alloy steels, and resistance improves with increasing chromium and molybdenum content, but is best with titanium.

Titanium exhibits exceptional resistance to NAC due to the formation of a stable, protective oxide film that is not attacked by naphthenic acids. API RP 571 identifies titanium as one of the most resistant materials available for severe NAC environments.

Why the other options are less suitable:

Option B (9 Cr-1 Mo steel) has limited resistance and is still susceptible at higher TAN and velocities.

Option C (317 stainless steel) offers improved resistance but can still experience attack under severe NAC conditions.

Option D (321 stainless steel) has lower molybdenum content than 317 SS, making it less resistant.

API RP 571 explicitly notes that titanium provides superior resistance compared to stainless steels and Cr-Mo alloys in NAC service.

Referenced Documents (Study Basis):

API RP 571 – Section on Naphthenic Acid Corrosion

API Corrosion and Materials Selection Study Guide