Material selection in chemical plants starts with four dimensions:

1) Medium: whether it is an oxidizing acid, reducing acid, chlorine-containing, or fluorine-containing. 2) Concentration: dilute sulfuric acid and concentrated sulfuric acid have completely different material requirements. 3) Temperature: as temperature rises, the risk of pitting corrosion and stress corrosion cracking increases. 4) Flow rate: high flow rates cause erosion-corrosion, while low flow rates tend to cause deposition and crevice corrosion.

Common progression: 304 → 316L → 2205 → 904L/6Mo → nickel-based alloys. Each step up significantly increases cost, so corrosion testing should be performed or operating data from similar plants should be referenced.