What type of surface contaminant is difficult to remove by cleaning parts in a vapor degreaser?

Choosing rust as the difficult surface contaminant to remove by cleaning parts in a vapor degreaser is insightful because rust typically represents a layer of oxidation that adheres strongly to metal surfaces. Vapor degreasing works primarily by utilizing a solvent in a vapor phase to dissolve and remove organic compounds like oils and greases effectively. However, rust is a result of a chemical reaction between iron and oxygen, forming iron oxide, which does not dissolve in solvents the same way grease and oil do.

The presence of rust can create a textured surface that can trap debris, making it resistant to straightforward solvent cleaning. While other contaminants such as dust, grease, and oil are more soluble in the solvent used in vapor degreasers, rust generally requires a more aggressive method for removal, such as mechanical abrasion or chemical rust removers, rather than just vapor degreasing. This underscores the limitations of vapor degreasing with respect to certain types of inorganic contaminants.