General Corrosion
General corrosion fundamentals underpin every other domain. Candidates must understand the electrochemical nature of corrosion: an anodic oxidation reaction releasing electrons, a cathodic reduction reaction consuming them, an electrolyte path, and a metallic return path. Removing any element of this circuit stops corrosion. The driving force is the potential difference between the reactions; kinetics (polarization, exchange current density) determine the actual rate.
Traps appear around the corrosion cell components and the role of oxygen and pH. In neutral aerated water, oxygen reduction is the dominant cathodic reaction, so oxygen concentration gradients create differential aeration cells, driving crevice and under-deposit corrosion. Lower pH generally accelerates corrosion of steel; passivating metals can be more sensitive to specific ions than to pH alone. Another trap is confusing corrosion rate units; mils per year (mpy) and millimeters per year (mm/y) are common, and questions may require conversion. Faraday's law links current density to mass loss.
Worked example
A coupon loses 0.25 mm of thickness over one year. The corrosion rate is 0.25 mm/y, which is about 9.8 mpy (since 1 mm/y ≈ 39.4 mpy). Recognizing the conversion avoids unit errors.
Understand the eight common forms of corrosion: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, and stress corrosion cracking. Each has a characteristic morphology and mitigation strategy. Questions often present a scenario and ask which form is occurring; identifying the morphology and the controlling variable (oxygen, chloride, stress, velocity) usually points to the answer.