“We need stainless steel flanges” – this is the most common initial requirement when facing corrosive media. However, “stainless steel” is a vast family, and the wrong choice can lead to catastrophic failure. For instance, 304 flanges are susceptible to stress corrosion cracking in environments containing chlorides, while 316 flanges may fail quickly in hot concentrated sulfuric acid. A systematic material selection should follow this path:
- Define the Corrosion Environment Spectrum: Precisely document the chemical composition, concentration, temperature, pressure, pH of the medium, and the presence of solid particles (erosion-corrosion). Pay special attention to trace impurities (e.g., chloride, fluoride ions), which are often the culprit behind failures.
- Prioritize Failure Modes: Is it general corrosion, pitting, crevice corrosion, or stress corrosion cracking? Different materials have vastly different resistances to these failure modes. For example, Duplex Stainless Steel 2205 offers superior resistance to chloride-induced stress corrosion cracking and pitting compared to 316L.
- Consider the Non-Contact Environment: Parts of the flange exposed to the atmosphere (external surface, bolts) are equally important. Salt-laden atmospheres in coastal plants or acidic atmospheres in chemical plants require different protective considerations than the process side, potentially leading to a “clad or coated” design.
- Total Life Cycle Cost Assessment: The initial purchase cost is just the tip of the iceberg. Calculate the total cost, including installation, maintenance, unplanned downtime, and eventual replacement. Sometimes, higher initial investment materials (e.g., Hastelloy) or lined flanges (e.g., PTFE lined) prove to be the most economical choice over the project’s entire lifecycle.
For critical applications, strongly consider conducting corrosion coupon testing to evaluate candidate materials under actual or simulated service conditions. This is the final, reliable defense against theoretical selection errors.