Metallic corrosion is the result of the metal interacting with the environment, the metal either dissolves or forms an insoluble product such as an oxide.

The main type of reaction is electrochemical, metal passing into solution at the anode of a cell and a reaction taking place at the cathode to remove the electrons produced in the anodic reaction. Ions in the water increase its conductivity so assisting the flow of corrosion currents, additionally chlorides and sulphites will penetrate any protective films formed at the anode.

If a soluble compound is formed at the anode the metal remains exposed and is continuously dissolved as long as the cathode reaction continues to maintain the corrossion current.

To control corrosion the anodic and/or cathodic reactions should be inhibited as far as possible, the anodic reaction is often inhibited by the formation of a layer of magnetite, this is formed by alkaline conditions and a chemically reducing environment in the boiler water.

Local concentrations of sodium hydroxide should also be avoided by controlling the pH value of the boiler water with sodium phosphate so that in the event of total evaporation no free sodium hydroxide would be formed.