Definition - What does Passive-Active Cell mean?
A passive-active cell is the concentration cell series where metals rely on an ultra-bonded passive film (typically an oxide) for protection against corrosion. Mainly, salts that accumulate on the metal substrate will have the film broken with water. In turn, the active metal that is underneath the existing film will be open to corrosion.
This will result in an electrical potential and this will exist in the cathode area, or the passive film, and the anode, or smaller area, referred to as active metal. Thus, rapid pitting of the active metal will result.
Corrosionpedia explains Passive-Active Cell
In a passive-active cell, corrosion is accelerated due to the differences that exist in the environment between separated areas on a particular metal. These environmental differences result in elevated electrochemical activity. Passive-active concentration cells obtain electricity from free energy that can be generated from chemical concentration difference. This energy is attained from the thermal energy that cells absorb in the form of heat during the flow of energy. Any condition that builds a variation in the environment can lead to corrosion or concentration cell damage.
This type of corrosion is similar to galvanic corrosion. However, in this case the key force is the potential difference of a metal exposed to dissimilar environments, whereas galvanic corrosion is caused by the potential difference that exists between two unlike metals subjected to one environment.
Corrosion due to passive-active cells occurs most often in industrial settings involving pipelines.