How to: control corrosion in central heating systems

About the problem

Since corrosion of metals within a central heating system is often the factor which dictates its useful lifespan, and different metals are vulnerable in different ways, it is vitally important to take action to protect all central heating system metals against corrosion.

Corrosion will occur naturally in central heating systems and the consequences when boilers and radiators fail is both disruptive and costly for homeowners and poses major problems for installers. So it is important to understand the causes and impact of corrosion of different system metals.

Copper
Things to know about copper in central heating systems:

• Copper is a noble metal, which means it is not easily corroded but when copper is corroded, tiny amounts of it dissolve in the system water and this subsequently leads to corrosion of other metals around the system. Flux residues or other aggressive contaminants can partially corrode copper and introduce dissolved copper to the system water.
• Dissolved copper can ‘plate out’ onto mild steel or aluminium surfaces within the system, usually radiators or the heat exchange surface in the boiler and causes localised corrosion in the form of pits, which eventually result in pin holes and leads to equipment failure.
• When first brought into contact with aerated water, only slight general uniform corrosion of copper takes place. This usually results in a passivating protective oxide layer, preventing further corrosion from occurring. Problems occur when corrosion continues beyond this.

Aluminium
Aluminium corrosion is often the factor which contributes to the end of the useful life of a central heating system.
Things to know about aluminium in central heating systems:

• It is one of the most commonly used system metals in domestic central heating systems. This is because it is highly thermally conductive, light and relatively inexpensive and so is often used to produce highly efficient boilers and radiators.
• It is naturally resistant to corrosion due to a protective film of surface oxide. Disruptions in this film by aggressive contaminants can lead to localised corrosion.
• Aluminium corrosion occurs in small, localised areas, resulting in pits in the metal surface. These can quickly turn into metal perforation and equipment failure.
• The aforementioned protective film is sensitive not just to aggressive contaminants but also to changes in the pH of the water. It is critical that the pH of the water be maintained between 6.5 and 8.5, which is optimum for aluminium to minimise corrosion.
• Base-exchange softened water behaves in a corrosive manner towards aluminium. This means that unless the system has been treated with an appropriate corrosion inhibitor such as Sentinel X100 Inhibitor, the use of this water ought to be avoided.

Steel
Things to know about steel in central heating systems:

• Mild steel is easily corroded in the presence of water and oxygen and copious amounts of iron oxide debris can form. This spalls off into the system water and disrupts valves and other moving parts and eventually settles as black voluminous sludge in radiators and heat exchangers.
• Iron oxide also creates an under-deposit corrosion reaction which self-perpetuates as a result of an oxygen differential. This leads to predominantly localised corrosion, commonly leading to equipment failure. This occurs because under deposit corrosion often progresses into pitting corrosion, which can quickly create perforations in metal.
• Stainless steel is an alloy that is able to protect itself with an effective thin oxide layer. However, aggressive contaminants can penetrate this layer to cause localised corrosion, pitting and eventual perforation.

The solution

Protecting a central heating system and all of its component metals against these kinds of corrosion is as simple as follows:

1. Ensure careful and correct installation and commissioning (or maintenance) of the system to minimise issues of contamination such as flux residues or existing corrosion deposits.
2. Once you are satisfied that you are working with a clean system, add a high quality corrosion inhibitor which offers multi-metal protection such as Sentinel X100 Inhibitor.
3. Check that the inhibitor has been dosed to the correct level to ensure ongoing protection.

With its TripleTech™ Anti-Corrosion Technology, Sentinel X100 Inhibitor protects all of the most commonly used system metals against the effects of corrosion, surpassing Buildcert standards by up to five times on all metals and up to fifty times on aluminium. It is suitable for use in softened water.