API-571 Exam Dumps : Corrosion and Materials Professional

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

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, 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.