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According to API RP 571, under the section "4.2.1.2 Brittle Fracture", the following conditions are explicitly identified as most critical for brittle fracture risk:

"Most processes run at elevated temperature, so the main concern is for brittle fracture during startup, shutdown, hydrotest, or other situations where equipment may be exposed to low temperatures."

"Equipment fabricated from materials susceptible to brittle fracture (e.g., carbon steel) is most vulnerable when exposed to low temperatures combined with high stress or pressure."

"A brittle fracture is characterized by a sudden and rapid crack propagation with little or no plastic deformation."

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

Thus, while other options represent stress events, the main concern specifically noted by the standard is during start-up and shutdown—especially due to cooling and repressurization at low temperatures—making option A the most accurate choice.

As per API RP 571 Section 5.3.2.3 (Spheroidization):

“Spheroidization is the transformation of the microstructure of steels into spherical carbides, leading to reduced strength and hardness, especially after extended high-temperature exposure. It’s commonly seen in carbon and low alloy steels in refining heaters and reactors.”

It reduces strength and sometimes hardness.

It increases ductility, not reduces.

SCC potential is not directly tied to this change.

Therefore, Option D (Loss of strength) is correct.

API RP 571 – Polythionic Acid Stress Corrosion Cracking:

“Polythionic acid SCC may be mistaken for wet H₂S damage because both involve cracking in austenitic stainless steels under certain conditions.”

Thus, option D is correct.

From API RP 571 Section 4.2.3 (Galvanic Corrosion):

“Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The more anodic metal corrodes preferentially.”

Contact without insulation and in presence of an electrolyte (like water or brine) is key to galvanic action.

Insulating or coating prevents electrical connection or electrolyte contact, reducing corrosion risk.

Hence, Option D correctly describes the condition that increases galvanic corrosion potential.