From API RP 571, brittle fracture prevention measures include:

“Postweld Heat Treatment (PWHT) reduces residual stresses in the heat affected zone and base metal, thus lowering susceptibility to brittle fracture.”

“PWHT is particularly effective when applied to pressure-containing components fabricated from carbon steel or low-alloy steel that will experience low-temperature service.”

(Reference: API RP 571, Section 4.2.1.2 – Brittle Fracture)

Therefore, while other measures may reduce stress or detect flaws, PWHT directly targets one of the root causes: residual stress. Thus, option D is the best prevention method.

According to API RP 571 and API RP 583:

“The most effective mitigation of CUI is the use of appropriate coatings underneath the insulation, which are resistant to moisture and chloride ingress.”

“Inspection and insulation selection are also important but are secondary controls.”

(Reference: API RP 571, Section 4.3.4 – Corrosion Under Insulation; API RP 583)

Hence, option D is correct.

API RP 571 explains under Condensate Corrosion (CO₂ Corrosion):

“In condensate systems, carbon dioxide dissolves in the water forming carbonic acid, which leads to localized corrosion, including grooving and under-deposit attack, especially at low points like the bottom of horizontal piping.”

“This is most commonly observed in steam condensate return lines, and the damage is localized due to stratification.”

This matches the scenario described, where internal grooving at the bottom of steam condensate piping is characteristic of CO₂-induced corrosion. Hence, option A is correct.

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.

API RP 571 – Sulfidation:

“Sulfidation generally results in uniform thinning of iron-based alloys at temperatures above 500°F (260°C).”

“It is classified as a form of general corrosion, although it can be accelerated in turbulent flow areas.”

So, option A is correct.

As defined in API RP 571, under Hydrogen Blistering and HIC/SOHIC:

“SOHIC typically manifests as subsurface stepwise cracking, generally oriented perpendicular to the direction of stress.”

“This mechanism initiates below the surface, typically in weld heat-affected zones, and requires metallographic or advanced ultrasonic methods to detect.”

By contrast, SSC, caustic cracking, and amine SCC are primarily surface-initiated, stress-related cracking mechanisms.

Thus, SOHIC is the subsurface mechanism in this list, making option D correct.

API RP 571 under Polythionic Acid Stress Corrosion Cracking (PTA SCC) specifies:

“Cracking is usually intergranular, and often localized, especially in sensitized austenitic stainless steels exposed to sulfur-containing environments during shutdown or cleaning.”

“It is not always visible on inspection and may only become evident when a leak occurs.”

(Reference: API RP 571, Section 4.2.2.5 – PTA SCC)

Hence, option C is correct as it best describes the detection difficulty and behavior of PTA SCC.