What is the purpose of a passivation layer on a metal surface and how does it form?

• A. It forms a protective oxide/hydroxide film that reduces metal dissolution; formation occurs via oxidation in oxidizing environments or via pre-treatment.
• B. It is a protective coating that seals the metal completely.
• C. It accelerates corrosion by increasing surface area.
• D. It is a temporary film that doesn't affect dissolution.

Answer: (B)

Passivation is about forming a protective barrier on the metal surface that slows down dissolution. The surface develops a thin, adherent oxide or hydroxide film when exposed to oxidizing environments. This dense layer acts like a shield, making it harder for metal atoms to go into solution and for corrosive species to reach the underlying metal, so the corrosion rate drops significantly.

This film can form naturally in air or water for many metals (for example, aluminum or chromium-containing steels) and can be enhanced or created deliberately through pre-treatments or anodizing processes. The protective layer is usually very thin and may contain defects, so it isn’t a perfect seal, but it is stable enough to reduce dissolution and to re-form at damaged spots, which is part of its self-healing nature.

In contrast, a completely sealed coating would be a different approach than passivation, and a film that accelerates corrosion or is merely temporary would not provide the sustained protection that a passivating oxide layer offers.