Which of the listed conditions is NOT one of the requirements for corrosion to occur?

• A. The presence of a passive oxide film
• B. An electrolyte
• C. A susceptible metal
• D. An electric potential difference

Answer: (A)

Corrosion in metals under electrochemical conditions needs three things: an electrolyte to let ions move, a metal that can be oxidized, and a potential difference that drives the electrochemical reaction between anodic and cathodic sites. A passive oxide film is not required for corrosion; in fact it often protects the metal by acting as a barrier. When the film is intact, corrosion is slowed or prevented, and corrosion only proceeds if the film breaks down or is damaged. Without an electrolyte, ion flow can’t occur; without a driving potential, there’s no net current; and without a susceptible metal, there isn’t a material to corrode. So the presence of a passive oxide film is not a requirement for corrosion to occur.