To exclude oxygen and moisture from the equipment internal atmosphere.
When austenitic stainless steels (like 304 or 316) are exposed to temperatures between 425°C and 815°C (800°F to 1500°F) during fabrication, welding, or high-temperature service, chromium carbides precipitate along the grain boundaries. This leaves the adjacent zones depleted of chromium (sensitization), making them vulnerable to corrosion. nace sp0170 pdf
What is Polythionic Acid Stress Corrosion Cracking (PTA-SCC)? To exclude oxygen and moisture from the equipment
Safer for short-term excursions; still requires caution in long-term high-temp service. Implementation Checklist for Refinery Turnarounds or high-temperature service
Engineers and inspectors search for the because it provides the specific "how-to" for preventing this specific type of failure. It isn't just theoretical; it is a practical guide.