formation of protective layers and passivation

There are three states that determine a metal/water-based solution combination’s tendency to corrosion.

Immunity: corrosion is a thermodynamic impossibility.

Corrosion: corrosion is a thermodynamic possibility and will take place without any interference.

Passivation: corrosion is a thermodynamic possibility but the rate of corrosion will be slowed down and even blocked by the processes that are responsible for slowing it down.

In the case of iron, potential and pH zones that define the three states quoted above are themselves defined by the Pourbaix diagram (chapter fundamental physical-chemical engineering processes applicable to water treatment, figure 90).

Many metals used in construction will react sufficiently with water to ensure that the creation of a state of immunity is difficult or even impossible. That is why anti-corrosion protection often requires the creation of a state of passivation. This state is obtained either through the material’s intrinsic qualities (e.g. stainless steel), or by modifying the composition of the corroding fluid (e.g. remineralisation in order to protect drinking water networks – see natural water used in carbon steel passivation and neutralisation – remineralisation).