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)
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.
Question # 36
The most effective mitigation for amine stress corrosion cracking is:
“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.
