In general, corrosion of a material can be considered to be an interaction of the material (e.g. steel, brass, aluminium) with its environment (e.g. water). This causes damage to the material, resulting, among other things, in a deterioration of the mechanical strength of the material.

The most common types of corrosion are the impairment of metal surfaces by oxygen and water in the air (dry corrosion), such as the rusting of iron or the formation of a green layer of copper carbonate on copper. Corrosion can, however, also occur in an aqueous environment and at high temperatures, whereby ceramic materials and even polymers can be affected.

In an aqueous environment, the interaction between the material and water is of an electrochemical nature, which means that the phenomena that occur are governed by both chemical and electrical laws. The most common corrosion phenomenon in heating engineering is still the deterioration of steel through oxygen. In systems where there is little or no steel, the oxygen will react with the next least noble metal.

The main component of steel is iron (Fe), which, when brought into contact with pure water, tends to allow the iron particles on its surface dissolve in the water as divalent, positively charged particles (iron ions Fe²⁺). This can happen because their energy level in water is lower than in the parent material. The steel is then left with the remaining negative particles (electrons e), and, as a result, is now negatively charged. The overall system, with the parent material and iron ions dissolved in water, thereby remains electrically neutral.

As more and more iron ions become dissolved in the water, and as the parent material, therefore, becomes more and more negatively charged, an increasing potential difference will be created between the steel and the positive iron particles that have been transferred to the solution. Over time, this difference becomes so significant that it prevents any further iron particles dissolving in the water: the chemical energy that is released through the dissolution is insufficient to overcome the increasing difference in electrical potential. At first sight, an electrochemical equilibrium between the negatively charged steel and the positive dissolved particles is thereby created. In this situation, the impairment of the steel element would be limited to a small loss of material.

In practice, however, we find that the water used in central heating installations is very different from the ideal, pure water described above. It contains many dissolved substances, including oxygen. The latter will cause several chemical reactions in the vicinity of the negative parent material, resulting in the consumption of electrons on the one hand, and the oxidisation of the iron ions dissolved in the water on the other, causing them to disappear from the aqueous solution as insoluble substances.

Due to this combination of reactions, the original electrical equilibrium is upset, with the result that iron particles will again leave the parent material. In this way, the deterioration of the steel element will continue as long as oxygen remains present in the system. Corrosion problems with steel in heating installations can therefore almost always be traced back to the influx of oxygen into the installation.

By applying the mass balance to the reaction of iron with oxygen, it is possible to determine the quantity of iron that will be affected by a given quantity of oxygen: each gram of oxygen that is present in the water reacts with 2.6 grams of iron, and forms 3.6 grams of fine, granular black iron oxide, which is called magnetite because it can be magnetised.

As already mentioned corrosion is a complex process. The chemistry of the water also plays a part in the corrosion potential of the installation. Factors such as pH, conductivity, dissolved minerals and salts all have an influence. Analysing the water and 'optimising' its values can be beneficial but can differ a lot depending on the materials of the components such as brass, copper and Aluminium. As a rule, the likelihood of corrosion decreases greatly with a fall in conductivity of the water. PH values in the alkaline range are advantageous with regards to the durability of copper and steel but not for aluminium. There is a common misconception that by replacing steel in systems corrosion problems will not occur. However, this is not correct as the oxygen will also react with the other metals in the system. Often it is not possible to avoid steel altogether and these few parts will be under severe attack. It, therefore, makes sense to have as much steel in an installation as possible which will consume the dissolved oxygen quickly and without damaging components. Buffer tanks which are usually made from steel and carbon steel pipework should be incorporated as much as possible. Note that the amount of magnetite formed is always the same as it is directly related to the amount of oxygen that reacts. More detailed Information and recommendations can be found in the VDI 2035 .

Correct pressure control is vital in closed heating and cooling systems. If the pressure drops it is possible that a negative pressure or vacuum will exist at the very top of the system. If the pressure in the system at any point is less than the surrounding atmosphere air may be drawn into the installation. The amount of air drawn in can be very substantial if automatic air vents have been fitted at the top of the system. But also smaller amounts of air can be sucked in through screwed joints or valve seal glands etc. The oxygen contained in the air will greatly accelerate the corrosion process. It is therefore paramount that a positive pressure must be maintained everywhere in the system at all times by the expansion vessel. All expansion vessels will lose gas charge over time. The time it takes depends on the type of vessel and it´s quality. Gas charge is lost by diffusion of gas through the membrane or through the crimped seals and air valves.

When the vessel is fitted for the first time it is vital that the pressure is checked and set to P0 for the specific installation. This is best done before the system is filled with water. If the pre-charge pressure is set to high or too low, it cannot maintain reliable system pressure.

When the pre-charge pressure is too low too much water will enter the vessel while the system is still cold [STEP 1]. When the water heats up and expands, the expanded volume will be pushed into the vessel. However, as there is now insufficient volume left for the expanded water (the acceptance of the vessel is now much too high) the pressure increases beyond the calculated end pressure Pe. (Pressure Factor Pf too high). In the worst case, the pressure will be high enough that the safety valves open and spill water to limit the pressure [STEP 2]. At this stage, the heating system will function normally and a casual inspection of the pressure gauges will show that everything is normal. However when the system now cools there is insufficient water volume and pressure in the expansion vessel to maintain a positive pressure at the top of the installation. Air may now be sucked in leading to accelerated corrosion [STEP 3]. At this stage, systems are often topped up with water to re-pressurise them. However, this will start the cycle from new with the additional corrosion potential due to the added fresh water.

This is the opposite scenario but with the same effect. If the gas pressure in the vessel is too high not sufficient or even no water enters the vessel during the cold fill [STEP 1]. When the water heats up the pressure rises too high as it needs to overcome the excessive gas pressure in the vessel. Once again the pressure may exceed the safety valve pressure Psv [STEP 2]. When the system cools again there is now no or insufficient water in the vessel to compensate for the reduced volume and therefore pressure cannot be maintained. It is worth remembering that a vessel that has no water in it cannot pass its pressure onto the system. As soon as the vessel is empty the system pressure at the top of the installation will be zero and can quickly become negative resulting in air being sucked in [STEP 3].

All vessels will lose pre-charge pressure over time. Therefore the pre-charge pressure needs to be checked at regular intervals at least once per year. It is not possible to determine the state of the gas pressure by looking at a system pressure gauge. These read system pressure, not gas charge pressure. Therefore the vessel needs to be isolated from the system by closing the lockshield valves which are commonly fitted. Then the vessel needs to be drained. Once empty the gas pressure can be measured with a tyre gauge.

If the vessel is sized too small it cannot accommodate the expanded water volume. When the system heats up and water is forced into the vessel the pressure will increase rapidly as the pressure factor will quickly be exceeded. This will result in safety valves lifting and spilling water to control the maximum pressure. Now when the system cools again there is not enough water in the vessel to maintain the pressure at the top of the installation and air may be drawn in leading to accelerated corrosion rates.

On the other hand, if the vessel is sized too large full pressure control is still guaranteed. It, therefore, follows that a vessel cannot be too large.

Drinking water contains approx 10mg/l of dissolved oxygen which starts to react with the steel in an installation as soon as it comes into contact with it. When the installation is being filled for the first time the corrosion process starts immediately. Provided the system is properly vented after filling and positive pressure is maintained the oxygen dissolved in the fill water will soon be used up by the corrosion process without causing any harm to the system. (Note: in a system which contains only plastic or non-ferrous materials and very little steel, all the corrosion will be concentrated on this part.) Oxygen dissolved in the fill water will only become a problem if the system is continuously topped up to compensate for water losses. Water may be lost in heating systems for a number of reasons. The obvious ones are leaks or frequent maintenance where the system is being drained and re-filled. The designer of the installation should ensure that components such as pumps can be replaced without having to drain the complete system. Automatic water make-up can make leaks less obvious unless water consumption is closely monitored. The less obvious cause of losing water is down to poor pressure control where water is lost through safety. The German VDI2035 allows a maximum of 2x the system volume during the lifetime of an installation. If water loss exceeds 10% per annum this should be urgently investigated.

Oxygen diffusion is a well know problem with some plastics and rubbers. Many modern plastic pipes are therefore now coated with oxygen diffusion barrier to reduce oxygen diffusion. Multi-layer pipes using an aluminium foil virtually eliminate this phenomenon altogether. However other materials such as rubbers also allow oxygen to diffuse from the atmosphere into the system water. Components made of rubber such as hoses and bellows are often used as flexible connections but constitute a relatively small overall surface. Another rubber component found in virtually every system is the membrane in the expansion vessel. The membranes need to be elastic so that they can stretch when the vessel takes in water. The ´stretchy´ rubbers such as EPDM are much more open to diffusion than non-elastic rubbers such as butyl. That is why some vessels are charged with nitrogen instead of air. Others use bags and not membranes which do not need to stretch and can, therefore, use more diffusion tight rubbers e.g. Butyl. It is often not understood that gases can diffuse through materials even if the pressure is higher on the water side than it is on the atmospheric side. The fact that some materials are water-tight but not gas-tight can be seen with high tech fabrics such as Goretex but usually, with fabric, the pressure is equal on both sides. However, as the dissolved oxygen in water is consumed by corrosion the partial pressure of oxygen in the water is lower than the partial pressure of oxygen in the atmosphere. Now if the molecular structure allows oxygen molecules to penetrate, oxygen will diffuse from the outside into the water even if this is at higher pressure. If not prevented, this is a continuous cycle as the diffused oxygen is consumed by the corrosion process and replenished from the atmosphere.

The job of a pump or circulator is to move the water around the system. In doing so the pump has to overcome only the friction losses and not the static hight pressure of the system. Therefore there is a higher pressure at the discharge side of the pump compared to the suction or return side. The difference in pressure is referred to as the pump head. Due to the friction losses in the system, the pressure reduces around the system. It follows that there is a point in the system where the pressure is the same no matter if the pump runs or not, this is called the neutral point. This point is determined by the location of the expansion vessel. (provided there is a single expansion vessel connection) It is therefore important that the expansion vessel is connected on the suction side of the pump. That way the pump has to push the water around the system and generates a positive pump head. If the vessel is on the discharge side, the pump it has to ´suck´ the water around the system. This can cause negative pressure at high points and air entering the system as a consequence. When this happens the remedy is frequently to install a bigger pump which only aggravates the problem.

All system corrode to some extent. If the corrosion rate is low this is not detrimental to the installation. However, the corrosion levels can rise quickly when there are problems such as a defective expansion vessels or water is lost due to a leak. It is often not recognised that oxygen reacts very quickly when it comes into contact with steel. A good example where fast corrosion is visible are the brake discs on a car which go rusty shortly after a rain shower. It, therefore, makes sense to monitor the corrosion rate of a heating system continuously and during the entire lifetime of the system. Nowadays there are low cost highly accurate sensors on the market which record the corrosion rate and sound an alarm if a set threshold is exceeded. Corrosion sensors are an early warning system for problems such as:

• a defective membrane in the expansion vessel
• loss of gas pressure in the expansion vessel
• loss of pressure due to leaks
• excessive topping up due to leaks or maintenance
• excessive ingress of oxygen due to permeable materials such as plastics and rubbers.
• high oxygen levels
• Incorrect pump position

There are a number of ways that corrosion can be monitored either directly or indirectly. Corrosion coupons is a direct method whereas water sampling and analysis is an indirect method.

Both methods share the same problem as they are only snapshots of the condition or corrosion activity when the water sample is taken or when the coupons are measured.

Modern sensors have made it possible to monitor the system and water condition continuously and record this data to give a complete picture over the lifetime of an installation. They can pick up small changes in corrosion activity or changes in water condition which allows operators to investigate the causes and act on them much more quickly thus preventing costly corrosion damages.

Risycor is a patented direct corrosion sensor that is based on a corrosion coupon but can measure the mass of the coupon online and continuously. It is a low-cost sensor that reacts sensitively to increasing oxygen levels and therefore increased corrosion in the system water. Corrosion speed is logged every 7 hours and stored in the memory. The data can be downloaded locally or remotely via a cloud portal. The generated graphics can easily be analysed and show if corrosion levels are safe and if not help with diagnosing the problems.

There are a number of systems on the market that use sensors to measure water quality such as pH, conductivity, dissolved oxygen, redox potential etc. Data from these sensors is the recorded and stored and made available locally or via the cloud.

The disadvantage of these systems is that they are costly and therefore more suited to large installations and they require a high level of expertise and experience to analyse the large amounts of data they generate. Examples of these systems are:

• Hevasure
• Fe-Quan