Corrosion and Corrosion Control Fundamentals
What you must understand
Offshore corrosion is an electrochemical process: an anodic reaction (metal dissolution, e.g. Fe → Fe²⁺ + 2e⁻) and a cathodic reaction (oxygen reduction in aerated seawater, hydrogen evolution in anaerobic/sour service) coupled through the electrolyte. Candidates should be fluent in the galvanic series (nobility ordering in seawater), the distinction between general wastage, pitting, crevice, galvanic, microbiologically influenced corrosion (MIC), and stress corrosion cracking (SCC), and the environmental drivers: dissolved oxygen, temperature, salinity, flow velocity, pH, and sulfate-reducing bacteria activity under deposits.
Know the control hierarchy: material selection, coatings/barriers, cathodic protection, inhibitors, and design detail (drainage, avoidance of crevices, dissimilar-metal separation). Understand how temperature and oxygen interact - warm seawater accelerates kinetics but lower oxygen solubility can offset this - and why splash zone corrosion is severe (alternate wetting, high oxygen, no continuous CP).
Traps in questions
Watch for distractors that confuse the galvanic series with the EMF series, or that claim CP protects the splash zone. Questions often test whether a mechanism is anodic (metal loss at the anode) or cathodic. MIC is frequently mislabeled as purely chemical - it is biologically driven and often localized under deposits.
Worked example / rule of thumb
A steel structure coupled to a large stainless or bronze component: the less noble metal (steel) becomes the anode and corrodes faster because the cathode area is large. Rule of thumb: the unfavorable area ratio is a small anode to a large cathode - always the dangerous configuration.