Chemical equipment selection depends on medium, concentration, temperature and impurities:

• 316L: general organic acids, mild media, ambient-temperature tanks and piping; moderate cost. • 2205 duplex: chloride-containing media, pressure equipment and seawater cooling; high strength and good stress-corrosion resistance. • 904L (N08904): high nickel and molybdenum for aggressive acids such as sulphuric and phosphoric; higher cost, used at critical locations. • 6% Mo super-austenitic (254SMO etc.): high chloride, seawater, pulp and other severe environments.

Selection process:

1) Define medium composition, concentration, temperature, flow rate and impurities (chloride, oxygen). 2) Check corrosion data or run coupon tests rather than guessing. 3) Assess the effect of welding and heat treatment on corrosion (sensitisation, sigma phase). 4) For pressure equipment, select and calculate per code (GB/T 150, ASME VIII). 5) Design to avoid crevices, trapped liquid and stress concentration.

Purchasing reminder: require material certificates and corrosion data; use third-party inspection and welding procedure qualification for critical equipment.