API-571 Exam Dumps : Corrosion and Materials Professional

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, surface decarburization is a metallurgical degradation mechanism that occurs when carbon is removed from the surface layers of steel due to exposure to oxidizing environments at elevated temperatures. This results in a carbon-depleted surface layer.

Carbon is a primary strengthening element in carbon and low-alloy steels. When carbon is lost from the surface:

Hardness and tensile strength are reduced

Creep resistance and load-carrying capability decrease

The component becomes more susceptible to plastic deformation and failure under stress

API RP 571 states that decarburization leads to loss of mechanical strength, especially critical in high-temperature service, where components already operate close to material limits.

Why the other options are incorrect:

Option A: Stress cracking is not the primary effect; loss of strength is.

Option C: Decarburization does not directly accelerate oxidation or sulfidation, although both may coexist.

Option D: This is incorrect; decarburization is considered detrimental in high-temperature applications.

Referenced Documents (Study Basis):

API RP 571 – Section on Decarburization and Metal Dusting

API Corrosion and Materials Study Guide

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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.

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).