Metallurgy for the Corrosion Technologist
What you must know
Corrosion is an electrochemical process, so metallurgy questions usually test how composition and microstructure change corrosion behavior. Be comfortable with the galvanic series (noble/cathodic vs. active/anodic metals), the difference between the galvanic series and the EMF series, and how an area ratio drives attack: a small anode coupled to a large cathode concentrates current density and produces rapid, localized penetration.
Know the common alloy families and their corrosion signatures. Carbon steel is uniform and general; stainless steels rely on a passive chromium oxide film, so they are vulnerable to chloride pitting, crevice attack, and sensitization (chromium carbide precipitation at grain boundaries during welding, leaving chromium-depleted zones). Austenitic stainlesses are prone to chloride stress-corrosion cracking; ferritic and duplex grades resist it better. Copper alloys suffer dezincification and impingement attack; aluminum depends on a passive film and suffers from alkaline and acidic attack; cast irons graphitize.
Understand heat treatment and welding effects: sensitization, residual stress, and galvanic couples between weld metal and base metal. Know the forms of corrosion (uniform, galvanic, crevice, pitting, intergranular, SCC, erosion-corrosion, microbiologically influenced corrosion) and be able to match a material and environment to a mechanism.
Traps
Questions often present a "more noble metal" and imply it is always the correct choice; the correct answer depends on the coupled area ratio and environment. Another trap is confusing the galvanic series (in a specific electrolyte) with the EMF series (standard conditions). Watch for "which is the anode" wording when two metals are coupled.
Rule of thumb
Bad area ratio = bad outcome: keep the anode large and the cathode small. A small anode to a large cathode (e.g., a stainless fastener in a carbon-steel plate) accelerates attack on the anode.