Which statement correctly differentiates active corrosion from passive corrosion?

• A. Active corrosion involves ongoing metal dissolution.
• B. The formation of a protective oxide film that slows attack.
• C. Passive corrosion occurs when metal dissolves rapidly.
• D. Corrosion that stops once a protective coating is applied.

Answer: (A)

Active corrosion means the metal keeps dissolving in the corrosive environment; the attack continues as long as conditions remain harsh. Passive corrosion, on the other hand, involves the formation of a protective film that slows or stops further attack. The statement that describes ongoing metal dissolution directly captures the difference between active and passive, so it’s the best choice. The other statements describe features of passive behavior or are incorrect: a protective oxide film that slows attack is a hallmark of the passive case, not the differentiating factor; passive corrosion does not occur when metal dissolves rapidly; and saying corrosion stops once a protective coating is applied ignores real-world realities where coatings can fail or have defects and corrosion can continue at a reduced rate.