Choosing Alloys for Chemical Processing

## The Selection Process 1. Define the environment (acid type, concentration, temperature, contaminants) 2. Consult isocorrosion diagrams (0.1 or 0.5 mm/year curves) 3. Consider upset conditions (worst-case temperature, concentration spikes) 4. Evaluate economics (cheap alloy + corrosion allowance vs expensive alloy + long life) ## Alloy Selection by Acid Type ### Sulfuric Acid | Concentration | Alloy | Notes | |-------------|-------|-------| | 0-5% (dilute) | 316L, Alloy 20 | Aeration increases attack | | 5-85% | Alloy 20 (UNS N08020) | The classic sulfuric acid alloy (35Ni-20Cr-3.5Cu-2.5Mo) | | 85-98% | Carbon steel | Passive film in concentrated H2SO4 | ### Hydrochloric Acid The most aggressive common acid. Up to 40 degrees C: Hastelloy B-3 or C-276. Tantalum is immune at all concentrations below 150 degrees C. Non-metallic linings (PTFE, glass) are widely used. ### Nitric Acid An oxidizing acid. 304L stainless is the standard up to 65% at boiling. Nickel alloys (Alloy 600) are NOT suitable. ## Caustic Service (NaOH, KOH) Below 50 degrees C: carbon steel. 50-120 degrees C: stress-relieved carbon steel. Above 120 degrees C: Nickel 200 or Monel 400. ## Clad Construction For large vessels, explosion-clad or weld-overlay construction provides carbon steel strength with CRA liner corrosion resistance at a fraction of solid CRA cost. Common combinations: carbon steel + 316L, carbon steel + Alloy 625, carbon steel + Alloy 20.