According to API RP 571, under Naphthenic Acid Corrosion (NAC):
"The most severe corrosion is typically found in areas of high velocity and turbulence, such as at pump around nozzles, transfer lines, elbows, reducers, and orifices."
"NAC is characterized by uniform thinning and can accelerate significantly where flow-induced erosion is present." (Reference: API RP 571, Section 4.3.4.1 - Naphthenic Acid Corrosion) This clearly establishes that high velocity and turbulence increase NAC severity, making option D 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 emphasizes under Hydrogen Stress Cracking - Hydrofluoric Acid (HF):
"Cracking in carbon steel is influenced by high hardness and tensile stress in the presence of wet HF acid."
"Controlling weld and heat-affected zone hardness to below 200 BHN is a key mitigation measure, as higher hardness significantly increases susceptibility." Steel hardness and strength are thus critical for cracking in HF environments, making option C 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.

Same as above. Sigma phase formation is a microstructural phenomenon and not typically detected by NDE methods like magnetic particle or surface hardness testing.
Therefore, metallographic testing remains the definitive detection method, confirming option D again as the correct answer.