effect of temperature on aerated water

below 60 °C

In a closed container, heat will cause bicarbonates in the water to convert to carbonates leading to scaling and then to carbonate equilibrium. However, although corrosivity in relation to steel is higher, the steel can be protected by the CaCO₃ deposit. Conversely, when water that has been heated and balanced through a CaCO₃ deposit is cooled back to its original temperature, it will become aggressive and, in the absence of precipitation, will allow corrosion to develop.

Consequently, two processes will take place in an industrial and, therefore, open system :

• chemical effect: the processes are identical to those that take place in a closed recipient but the equilibrium is shifted by escaping CO₂, therefore in the direction of increased scaling properties;
• electrochemical effect: the possible coexistence of hot zones (with a lower pH) and cold zones (with a higher pH) create surface heterogeneity combining with hot anodic zones and cold cathodic zones.

Thus, a 20°C temperature differential can create a 55 mV potential differential. At the high points in circuits, the formation of carbon dioxide pockets and of acid condensation water films lead to the creation of extremely active anodic zones.

above 60 °C

Oxygen degassing takes over and results in the formation of differential aeration couples that are added to existing couples, creating a much greater potential. This generates a very much greater level of corrosion.

Consequently, protection for hot water systems in buildings creates far greater problems than those affecting industrial systems (higher gradients).