Material Selection
Material selection balances corrosion resistance, mechanical properties, cost, and availability. Candidates must understand the galvanic series, passivity, and how environment (pH, chloride, temperature, oxygen, flow) drives degradation modes: uniform corrosion, pitting, crevice, galvanic, stress corrosion cracking (SCC), and erosion-corrosion.
Know key families. Carbon steel is economical but requires protection. Stainless steels rely on a chromium oxide passive film; chloride and low oxygen can break it, causing pitting and crevice attack. Austenitic grades (e.g., 304, 316) are susceptible to chloride SCC; 316 adds molybdenum for better chloride resistance. Duplex and super-duplex offer higher strength and chloride resistance. Nickel alloys, titanium, and copper alloys serve specialized environments.
Traps: choosing a material by corrosion rate alone without considering localized attack, ignoring temperature effects (higher temperature worsens many modes), and forgetting galvanic coupling (a noble material coupled to an active one accelerates the active one). Also watch for sensitization of austenitic stainless steel from welding, and for velocity limits in copper alloys (erosion-corrosion).
Worked example: A 304 stainless tank handles chloride-bearing water at 60 °C. 304 is prone to chloride pitting and SCC under these conditions; upgrade to a duplex or higher-molybdenum grade, or use a coated carbon steel with CP, depending on economics. Rule of thumb: if chlorides and elevated temperature coexist, do not default to 304.
Understand material selection charts, corrosion allowances, and life-cycle cost comparison.