Heating Corrosion Protection
Most installers are aware that heating systems need to be protected from corrosion; this ensures system longevity and the components within the heating system, boiler, pumps valves etc. but up to now we have always been told the only way to do this is by adding chemicals, well I am of the opinion that this is not the only way to protect a heating system and there are more eco-friendly ways to achieve the same goal, we are told to add a bottle of inhibitor to the system water, by different means and that this will protect the system and that is all we are told, a very simplistic approach in my opinion, there is a bit more to water in a heating system than that, it is a science, the use of de-mineralised water to fill the heating system is enough amongst other accustomed practices such correct usage of materials and good workmanship to protect the system, this is based on guidance of the German VDI2035 below is a bit more of a scientific approach and explanation of the heating water and the use of demineralised water, from our German Colleague Daniel Schneider.
Demineralised -water removes all ions that are dissolved in water. Thus the conductivity is low and the hardness has been removed. As there is no hardness, there is no risk of scale. This avoids the usage of scale inhibitors.
Pure demineralised water has a pH of 7. Pure demineralised water has no pH buffer. If there is an acidic reaction in the system nothing is there to buffer the pH. In normal water calcium and magnesium or sodium acts as the pH buffer -> the alkali metals. As demineralised water has no alkali dissolved the pH will drop in case of acidic reactions. Carbon dioxide for example would lower the pH. As normally carbon dioxide gasses out of the system when the water is heated >60°C the pH even of demineralised water rises to 8.2. You can simulate this by heating demineralised water in a microwave and measure the pH afterwards. 8.2 is normally enough but still there is no buffer! This is why the TUV Germany suggests mixing demi-water with tap water. You will lower the conductivity of the water, you will lower the hardness of the water but you will still have alkali in the water that act as pH-buffer.
If you heat up the system with such a water the pH will stabilise naturally at >8.5. You need no chemicals for this reaction.
Natural corrosion protection of metals
When metals come in contact with water they react with the initial corrosion that builds up a thin layer of oxides. If this layer is kept stable the metals get passivated and are protected against corrosion naturally. No oxygen can attack the metal because it is protected by the thin oxide layer on its surface.
The pH has an influence on the natural passivation. If the pH is ideal, the oxide layer can be formed and kept stable. The researcher Pourbaix has done lots of work on this and has created the Pourbaix diagrams that show the electrochemical equilibrium of metals – that means the state when they are corroded, passive or immune against corrosion. You will find that the pH has an important function.
The conductivity has an influence on the natural passivation. When the conductivity is too high the natural layer is not formed on the metal itself but in the water. The flow of electrons is too quick so the layer cannot be built up on the metal. If the conductivity is low it is guaranteed that the oxide layer is built up on the metal surface.
The amount of dissolved solids in the water influences the formation of these layers as well – there are a lot of studies about it.
Too hot water, turbulences, high volume flow or erosion can destroy the natural layer as well.
Under normal conditions you will fill the system with clean water, so that there is no erosion. Only if there is corrosion then metallic particles un-dissolved in water can cause erosion. In modern systems there should be no problem with too much heat, apart from in the boiler itself, and no problem with high volume flow. Turbulences are avoided as there is no scaling as long as the systems are built correctly.
Oxygen and other gases
If there is oxygen, corrosion can take place if the metals are not passivated! As soon as the natural layer is stable, no oxygen attack is possible. Only that much oxygen will ingress into the system as much oxygen is either consumed by corrosion or degassed with a de-aerator for example. If the metals are stable you can reach equilibrium with a certain level of oxygen in the system but with no corrosion! As with oxygen, carbon dioxide can ingress as well. But it degasses in the same way!
The function of conductivity on oxygen corrosion
If the conductivity of the water is high the ohmic resistance is very low. Metals can lose electrons rather quickly and consume oxygen – as long as they are not passive! If the ohmic resistance of the water is high, the conductivity of the water is low. This makes it harder for metals to loose electrons. Though corrosion is relative unlikely. It is known that the lower the conductivity of the water, the more oxygen can be tolerated without causing corrosion!
The effect of conductivity on galvanic corrosion
As the galvanic corrosion needs a short circuit the water has to act as the electrolyte that transports the electrons. If the conductivity is low enough – the ohmic resistance high enough – the water cannot transport these electrons. Risk of Galvanic corrosion is unlikely or rather low. As the modern installer is aware of different metals, he should avoid galvanic elements by the correct usage of materials.
Under normal conditions demineralised water is a part of corrosion protection. If the pH is kept stable there will be no corrosion.
Under normal conditions it is enough to mix demineralised – and tap water. This is exactly what the VDI2035 says to do and what the common experience in Germany proves