What is the difference between protective inhibitors and corrosion inhibitors in open versus closed systems?

• A. Filming inhibitors form protective films on metal surfaces; volatile inhibitors provide vapor-phase protection. In open systems, dosing is continuous; in closed systems, inhibitors are dosed to maintain protective concentrations.
• B. Filming inhibitors are only for open systems; volatile inhibitors are only for closed systems.
• C. Volatile inhibitors form films; filming inhibitors vaporize to protect.
• D. There is no difference between inhibitor types.

Answer: (A)

The idea being tested is how inhibitors protect metal surfaces and how system openness affects how you dose them. Filming inhibitors protect by forming and maintaining a protective film on the metal surface, which reduces the rate of electrochemical reactions at the surface. Volatile inhibitors protect by vapor-phase transport; they evaporate into the gas phase and diffuse to protected surfaces, which can be especially useful in areas not easily reached by liquid contact.

In open systems, water and chemical losses occur continuously (through evaporation, bleed-off, or drainage), so inhibitors must be replenished on a continuous basis to keep the protective concentration in the system. In closed systems, losses are much lower, so inhibitors can be dosed to maintain a protective concentration without constant addition, often with less frequent or periodic dosing.

Thus, the best explanation contrasts film-forming and vapor-phase protection and ties dosing strategy to whether the system continually loses water and additives (open) or retains them more, allowing maintenance with intermittent dosing.