In modern households hot water is self-evident. As steel is the standard raw material for boilers there is a question of corrosion resistance. The corrosion of steel because of water is an electro-chemical process with two reactions.
1. Anodic oxidation of metal to metal oxides dissolving the metal
2. Cathodic reduction of oxygen dissolved in water
If it is possible to stop one of these reactions you can avoid corrosion. This is not possible during the production of hot water for the household since there is always oxygen in the water and it is not a closed circuit. The most save and economic solution is to coat the steel boiler with a passive layer.
Porcelain Enamel proved its worth
If porcelain enamel is melted onto the steel at about 830-850°C it is more than a coating. During the enamelling process a chemical and physical compound between steel and the glass-like enamel. The boiler gets its strength from the steel and the necessary corrosion resistance from the glass coating. The melted-on layer avoids any rusting. Porcelain enamel acts like an insulation because there is no absorption of water or electric conduction. The glass lined steel is temperature resistant from -60°C to +450°C and extreme temperature changes are also no problem. Special enamels were developed. These provide an optimal adhesion to the steel and fulfill every requirement for boilers. They show a very high resistance against water, steam and acids and they are physiologically harmless. The layer thickness should be more than 0,15mm at an average of 0,5 mm. The melting of the enamel onto the steel is a dynamic process. During the burning process first gases and steam exhaust; after this the enamel melts, flows and gets hard again during the cooling process. The output is the glass lined steel. The enamel layer itself would provide a corrosion protection of 99% but to reach 100% anodes are installed.
Enamel provides highest protection for over- and underground hydrants, pipe systems, valves and connection parts because it is resistant to acid soils, frost, heat, wind and snow.