How do inhibitors such as nitrite function in closed cooling systems?

• A. They accelerate corrosion by forming aggressive species.
• B. Nitrite-based inhibitors help form a protective oxide film and scavenge oxygen, reducing corrosion rates and maintaining passivity on the metal surface.
• C. They increase electrical conductivity of the coolant.
• D. They corrode copper preferentially to protect steel.

Answer: (B)

Inhibitors like nitrite work in closed cooling systems by two complementary actions that keep metal surfaces passive and corrosion rates down. First, nitrite ions react with dissolved oxygen, effectively scavenging it from the water. Lower oxygen levels slow the cathodic reactions that drive corrosion, which helps prevent metal dissolution. Second, the presence of nitrite promotes the formation and maintenance of a protective oxide film on iron-based metals, often a stable magnetite-type layer. This oxide film adheres to the surface and acts as a barrier to further corrosion, so the metal remains in a passive state rather than continuing to corrode.

So, nitrite inhibitors reduce corrosion by both consuming oxygen that would otherwise accelerate corrosion and by fostering a protective surface film that passivates the metal. The other statements describe effects that don’t align with how nitrite inhibitors function—accelerating corrosion, increasing coolant conductivity, or targeting copper instead of protecting steel—so they aren’t correct.