Materials Selection and Corrosion
What this domain tests
Materials selection questions test damage-mechanism recognition: uniform corrosion, pitting, crevice attack, chloride stress-corrosion cracking (SCC), caustic SCC, sulfidation and erosion-corrosion. You must match alloy to stream chemistry, temperature and welding condition.
Core rules and concepts
- Carbon steel with corrosion allowance is common for alkaline cooking liquor where pH is high and chloride is low. It is not acceptable for hot chloride or low-pH bleach service without protection.
- 304L/316L are general stainless choices, but 304 is vulnerable to chloride SCC above roughly 50-60 degrees C; 316L improves pitting resistance, not immunity to SCC.
- Duplex 2205 gives higher strength and better chloride resistance; superduplex and 6Mo are used for severe chloride. Titanium resists chlorine dioxide and chlorate but can be attacked by dry chlorine or hydrogen.
- Sensitization of austenitic welds raises intergranular corrosion risk; use low-carbon grades, stabilized grades or proper heat treatment.
- PREN = %Cr + 3.3 x %Mo + 16 x %N is a ranking index, not a service limit.
Worked example
Compare 316L and 2205. 316L: 17 + 3.3(2.5) + 16(0.05) = 26.1. 2205: 22 + 3.3(3.0) + 16(0.15) = 34.3. Higher PREN predicts better pitting resistance, but at 80 degrees C with 1,000 mg/L chloride, SCC and crevice geometry may still rule out both without a corrosion test or liner.
Traps that cost marks
Treating PREN as a pass/fail limit; ignoring temperature and crevices; using 304 in hot chloride wash water; forgetting weld sensitization; assuming titanium is universal in bleaching.