Which of the following is NOT a characteristic of passivating inhibitors?

Passivating inhibitors are substances that help protect metal surfaces from corrosion by creating a passive layer. The characteristics of these inhibitors typically include promoting passivation, being anodic in nature, and often proving to be cost-effective for industrial applications.

The assertion that they form soluble films is not accurate regarding the primary functioning of passivating inhibitors. Instead, passivating inhibitors create thin, protective films that are usually insoluble in the corrosive environment, thus adhering to the surface of the metal. This film formation helps to reduce the metal's reactivity and provides a barrier against corrosion.

In contrast, promoting passivation means they encourage the metal surface to transition into a non-reactive state, which is a key characteristic of these inhibitors. Being anodic in nature indicates that they can impede anodic reactions, thereby decreasing the rate of oxidation. Additionally, being economical is a significant advantage of using these inhibitors, as it makes them practical for widespread industrial use.

Thus, the characteristic of forming soluble films does not align with the typical properties of passivating inhibitors, making it the correct answer to the question.