Polythionic acid stress corrosion cracking is:

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.