Materials and Metallurgy
Materials selection determines whether a component corrodes, passivates, or fails mechanically. Candidates should understand galvanic series relationships, passivity, sensitization, and common alloy families. The galvanic series ranks metals by potential in a given environment; connecting dissimilar metals creates a galvanic couple where the more active (anodic) metal corrodes faster and the more noble (cathodic) metal is protected. The severity depends on the potential difference, the area ratio, and electrolyte conductivity. A classic trap is the area effect: a small anode coupled to a large cathode produces intense, localized attack because the large cathode demands current the small anode cannot supply without high current density.
Stainless steels rely on a chromium-rich passive film. Chlorides can break down passivity and cause pitting; sensitization from improper welding or heat treatment depletes chromium at grain boundaries, promoting intergranular attack. Carbon steel is widely used but depends on coatings, CP, or inhibitors for durability. Questions often test whether a material is appropriate for a given environment rather than asking for a definition.
Worked example
A stainless steel fastener joins a carbon steel plate in seawater. The stainless fastener is cathodic and the large carbon steel plate is anodic, so the plate corrodes near the joint; the unfavorable small-cathode/large-anode geometry is avoided, but the large anodic area still corrodes broadly. The fix is insulation or material matching, not simply more coating on the fastener.
Understand hydrogen embrittlement risk in high-strength steels, dezincification in brasses, and graphitic corrosion in cast iron. Corrosion morphology (uniform, pitting, crevice, galvanic, intergranular) often signals the mechanism and the corrective action.