Corrosion in district cooling systems
District cooling is becoming an increasingly popular way of cooling properties and special facilities. In such cooling systems, corrosion represents a serious risk to the integrity of the steel pipes used. The conditions in district cooling systems differ from those in district heating systems and are in many ways unique. In order to mitigate the threat posed by corrosion many factors have to be considered and controlled.
Corrosion takes many forms (see the info box for an overview of the corrosion process). General or uniform corrosion proceeds uniformly over the entire surface area, as the oxidation and reduction occur on the same surface while the anode and cathode change their positions continuously. In water, though, corrosion tends to occur more often in its local forms. Pitting corrosion often originates from a scratch or a crack in a protective film and results in holes in the metal surface. A scratch or a pit functions as a local anode while the surface around it serves as a cathode. In district cooling systems local corrosion appears often as crevice corrosion where a crevice, joint or a hidden pit under a deposit can create a microenvironment that enhances the corrosion attack.
Galvanic corrosion can take place where different metals are present in electrical contact with each other - the less noble metal will become an anode and corrode. There are many components made of different materials in a district cooling system, which means that galvanic corrosion is also a potential threat in cooling networks.
Moreover, pipelines in a cooling system can also be damaged by erosion corrosion. This is due to chemical corrosion as well as mechanical wearing caused by high-velocity fluids in the system. Loose particles, such as solid corrosive products, that are drifting in the flow can accelerate the wearing effect.
If a pipeline is exposed to residual or applied stresses in a corrosive environment stress corrosion cracking (SCC) may be induced. High pH levels and nitrite ion concentrations increase the risk of SCC for carbon steel pipes.
The role of microbes in corrosion
Corrosion that is induced or affected by microbes, i.e. microbiologically influenced corrosion (MIC), has often been given too little attention. There is much evidence indicating that microbes can play a highly significant role in corrosion attacks. Many microbes are tolerant of harsh conditions and can live in unfriendly environments such as those found in district cooling systems. However, relating the observed corrosion attack to microbes can be difficult; finding microbes at the damaged site does not necessarily prove that microbes are to blame.
There are several ways microbes affect corrosion and not all of these are fully understood. Microbes essentially influence corrosion by changing the electrochemical conditions at the metal-solution interface. Micro-organisms, of which the most abundant are bacteria, live on a pipe surface in the form of a biofilm. Microbes can produce organic or inorganic acids and extracellular corrosive metabolites or concentrate chloride, oxygen, hydrogen or metal ions under the biofilm. The biofilm can act as a barrier that blocks the free movement of molecules and ions, thus affecting the concentrations of chemical substances. Consequently, the conditions under a biofilm can differ essentially from the conditions elsewhere in the system, potentially creating a corrosive microenvironment.
The role of microbes is made even more complicated by the fact that they can also inhibit corrosion. They can, for example, consume oxygen that is needed for cathodic reaction, produce inhibitory metabolites, or stabilize the protective film on the metal surface. Some microbes can also produce antibiotics that inhibit the growth of corrosion-inducing microbes.
A district cooling system is mostly an oxygen depleted environment. The most important microbes causing MIC under anaerobic conditions are sulfate reducing bacteria (SRB). They can promote corrosion in different ways and cause severe corrosive damage. It has to be remembered, though, that microbial communities consist of many different microbes and there are numerous complicated biochemical reactions and interactions involved.
Factors affecting the corrosion rate
The maintenance of proper water chemistry in piping systems is crucial in minimizing the corrosion risk. There are, however, many elements in water chemistry that interact, thereby making the system highly complex. Optimal control over the corrosion rate in a cooling network may still require some fine-tuning of the conditions and different elements in the system.
The two most critical factors in respect of the corrosion rate are the oxygen content and pH level. Oxygen typically accelerates corrosion markedly, but the relation is not that simple: a small amount of oxygen promotes the formation of an oxide layer that can protect the metal surface from corrosion. This kind of protective or passivating film on the surface of carbon steel usually consists mostly of magnetite Fe3O4 which - when conditions are favourable - forms a dense layer that adheres tightly to the steel surface. However, excess oxygen or destabilization or destruction of the magnetite layer can increase the corrosion rate. In district heating systems hydrazine is used as a scavenger for residual oxygen, but its reaction rate is very slow in cooling network temperatures, which means that it does not really work in such conditions. There is still a lack of a simple and practical oxygen scavenger for district cooling systems.
Typically fluid acidity speeds up corrosion. A high hydrogen ion concentration accelerates the cathodic reaction, but a low pH also destabilizes the magnetite layer which makes the metal surface more susceptible to corrosion attacks. This is why water is kept alkaline in cooling systems. A very high pH can, however, also damage or destabilize the oxide layer. Thus, the optimal pH in respect of corrosion control is usually 9-10.
Water treatment is also used to gain low hardness and low conductivity. Chloride and sulfate ions are examples of strongly corrosive elements, but some salts that affect conductivity can also be corrosion inhibitors. A temperature rise is known to accelerate the corrosion rate as it usually does for chemical reaction rates. However, the final effect is a sum of several factors. As the temperature rises the diffusion rate of oxygen increases, oxygen solubility decreases, pH falls, conductivity is enhanced and the properties of corrosion products can change.
The temperature also affects the growth rate of microbes. In district cooling systems temperatures are relatively cool and the differences are not that large: the temperature is usually about 8 °C in supply pipes and about 16 °C in return pipes.
Other factors in the system that affect the corrosion rate are e.g. the flow rate, materials and compounds that act as nutrients for microbes, and different metals that can cause galvanic corrosion. Loose particles such as magnetite deposits can induce corrosion through erosion and also by sedimentation – as particles deposit on the pipe surface they can form pockets that function as a suitable microenvironment for corrosion to develop or microbes to grow. In addition, differences in altitudes in the network can affect the distribution of gases, including oxygen.
Can plastic pipes be a corrosion risk?
In the future, high-density polyethylene (HDPE) plastic pipes may be used in district cooling systems. HDPE pipes have many advantages compared to steel pipes: They are light and flexible and therefore easy to install. They are also not vulnerable to corrosion nor do they erode easily. Moreover, the heat conductivity of HDPE is much lower than that of steel, which reduces the need for insulation.
The problem with HDPE pipes in water systems is that plastic allows some oxygen diffusion, which may lead to corrosion of steel parts. The level of oxygen diffusion is dependent on the material density, the wall thickness, the size of the exposed area, temperature, and whether the pipes are buried in soil or placed in tunnels or cellars.
Studies indicate, however, that there is only minor oxygen diffusion through HDPE pipes in district cooling systems. Oxygen leaks through pipe accessories during the network operation, in fact, pose a much larger corrosion risk. Oxygen diffusion should not be a problem with HDPE pipes as long as there is sufficient steel in the network.
It is clear that corrosion control in district cooling systems requires careful water quality management, prevention of oxygen leaks, particle and sediment removal, and appropriate installation work to avoid contamination. Biocides are not usually a long-term solution in MIC mitigation. A much more sustainable way is to control the habitat conditions of microbes.
Author: Anita Vuorenmaa