Under what condition does simple electrochemical corrosion occur?

Simple electrochemical corrosion occurs when a metal is freely corroding. This process takes place when there is an electrochemical reaction between the metal and its surroundings, typically in the presence of moisture and an electrolyte. Under these conditions, the metal atoms lose electrons and dissolve into the environment, leading to deterioration of the metal structure.

The term "freely corroding" indicates that the metal is not receiving any protective treatments or coatings that would inhibit this electrochemical process. It is a natural occurrence when metals are exposed to corrosive elements, where they tend to seek a stable, lower-energy state through oxidation.

Other scenarios mentioned, such as when metal is fully protected or when there are specific positive or negative potential differences, do not promote simple electrochemical corrosion. Full protection would imply that there are measures in place (like coatings or cathodic protection) to prevent corrosion, while positive or negative potential differences relate more to electrochemical potential and may influence corrosion but do not define the corrosion process itself as being 'simple' or 'freely corroding.'