Materials Science and Corrosion
Core concepts
Material selection and corrosion control determine plant life and safety. Know the families of engineering materials - carbon and low-alloy steels, stainless steels (austenitic, ferritic, duplex), nickel alloys, copper alloys, polymers, ceramics and composites - and their property trade-offs (strength, ductility, temperature limits, weldability, cost). Understand crystal structures, phase diagrams (especially iron–carbon), heat treatment, and mechanical testing (tensile, hardness, impact, creep). Corrosion forms: uniform, galvanic, pitting, crevice, intergranular (sensitisation), stress corrosion cracking, erosion-corrosion, and hydrogen damage. Know the electrochemical basis, passivity, Pourbaix diagrams, and protection methods (coatings, inhibitors, cathodic protection, anodic protection, material selection, design).
Traps
Do not assume stainless steel is immune; chlorides cause pitting and SCC of austenitic grades. Galvanic corrosion attacks the anodic (less noble) metal; area ratio matters - small anode to large cathode is severe. Sensitisation occurs from chromium carbide precipitation at grain boundaries after welding. Erosion-corrosion worsens with velocity; impingement differs from cavitation.
Worked rule of thumb
In a galvanic couple, the more active metal (lower in the galvanic series) corrodes. A large cathode coupled to a small anode gives rapid localised attack; design to avoid this. For high-chloride service, prefer duplex or super-duplex stainless or nickel alloys over 304/316.
Exam strategy
Identify the corrosive environment first, then match the degradation mechanism to a material and a mitigation strategy.