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Polythionic Acid Stress Corrosion Cracking (PASCC) is an intergranular cracking mechanism affecting sensitized austenitic stainless steels (300 series) and Ni alloys (e.g., Alloy 600, 800). It typically occurs during shutdowns or start-ups, when sulfide scale—formed during high-temperature, sulfur-rich operation—reacts with air and moisture, producing polythionic acids (H₂SₓO₆). These acids cause rapid cracking in the presence of tensile stress, particularly adjacent to welds or high-stress zones. Cracks can propagate through the wall within minutes or hours.