Corrosion in heating and cooling systems

Most buildings services engineers will have come across a heating or cooling system that has not received water treatment and still appears to function perfectly and another that has apparently been treated but experienced serious corrosion related failures. Why should one be vulnerable and the other not?

The answer is that most common metals are subject to corrosion but the rate of corrosion and risk of failure depends on a variety of factors including:

The chemical and microbiological environment.
The temperature.
The flow rate.
The thickness of the metal.

In many respects water is the ideal heat transfer medium for building services. It has a reasonably high specific heat, is liquid over a convenient temperature range and is non-flammable, non-toxic and freely available. The downside is that water is an electrolyte that facilitates corrosion in metallic pipework and components. One might think that the obvious solution is to use plastic pipework but this can actually increase the risk of corrosion of the corrodible components that remain.

In a steel pipework system, the dissolved oxygen in the system water will be rapidly used up as it reacts with the large area of corrodible surface, but the loss of metal thickness should be insignificant. In a plastic pipework system there are few corrodible components so oxygen concentration will remain higher for longer and the corrodible materials will continue to corrode at a high rate. This means that almost all water-based heating and cooling systems should have some form of water treatment to control corrosion, and this may be even more important in plastic pipework systems.

The usual construction programme for large building projects involves installation and pressure testing of pipework followed by pre-commission cleaning and commissioning several months later. During the gap between pressure testing and pre-commission cleaning the system may be stagnant and may still be contaminated with manufacturing and construction residues. This is an ideal environment for the development of biofilm and corrosion.

In traditional steel pipe systems (using BS 1387:1985 or BS EN 10255:2004 medium or heavy grade pipe) this is not too much of a problem. The relatively thick pipe (at least 3.2 mm for 1 inch nominal bore and larger) can tolerate the initial corrosion due to the oxygen in the fill water and biofilm development during subsequent stagnation conditions. Provided the pre-commissioning cleaning is carried out effectively, ideally with a biocide wash prior to chemical cleaning, there should be minimal impact on the lifetime of the system.

Thin wall steel pipes and steel panel radiators may not be so fortunate. The thickness of 25 mm nominal bore thin wall carbon steel pipe is only 1.5 mm while a typical steel panel radiator is only 1.3 mm thick. If the initial corrosion was spread uniformly across the metal surface it would not be a problem, but what tends to happen is that small patches of the surface become anodic relative to their surroundings and are preferentially corroded leading to rapid localised pitting. If dissolved oxygen levels persist or are replenished due air ingress, continuing additions of fresh water or permeation through non-metallic materials, then the pitting can progress to perforation. Components that should last 25 years can be perforated in a few months. This is one of the most frequent types of corrosion failure and can result in expensive remedial works even before the building is occupied.

Water treatment chemicals work by inhibiting the corrosion process, either by coating the surface of the metal (anodic inhibitors) or by blocking the corrosion reactions (cathodic inhibitors). However, inhibitors are not the solution to poor closed system design or operational deficiencies and certainly won’t work to best effect in a dirty system (i.e. one with a high level of suspended solids and/or biological contamination). Also, the system operation must allow the inhibitors and other water treatment chemicals to be maintained at an effective concentration and circulated throughout the year.

In summary, the factors necessary to avoid pitting corrosion of steel components in closed systems are:

Minimise the delay between first fill and pre-commission cleaning.
Carry out effective pre-commission cleaning of the pipework system.
Establish, monitor and maintain effective water treatment and water quality as soon as possible in the life of the system.
Circulate water throughout the system on a daily basis to avoid stagnation.
Avoid ingress of oxygen from inadequate pressurisation or excessive fresh water additions.

What happens in the first few weeks of the life of the system will influence its fate over the next 25 years. You can’t easily see what is going on inside a pipe but get it wrong and you could be looking at major remedial works in a tenth of that time.

This article originally appeared in the May 2014 edition of BSRIA’s Delta T magazine. It was written by Reginald Brown, Head of Energy and Environment. It has been posted here by --BSRIA 09:12, 8 December 2014 (UTC)

[edit] Related articles on Designing Buildings

[edit] External references

A detailed discussion of corrosion and the use of inhibitors and other chemicals is contained in BSRIA BG50 Water Treatment for Closed Heating and Cooling Systems.
Pre-commissioning cleaning is described in BSRIA BG29 Pre-commission cleaning of pipework systems.
Guidance on the monitoring of water quality in closed systems is contained in these documents and BS 8552 Sampling and monitoring of water from building services closed systems - Code of practice.

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Comments

Corrosion in heating and cooling systems is a common issue that can lead to various problems and reduce the efficiency and lifespan of the equipment. Corrosion occurs when metal components in the system react with water, oxygen, and other substances, resulting in the deterioration of the metal surfaces. Here are some of the effects and consequences of corrosion in heating and cooling systems:

1. **Reduced Efficiency:** Corroded components, such as pipes, heat exchangers, and coils, can develop a layer of scale or rust, hindering the heat transfer process. This reduces the system's efficiency, leading to increased energy consumption and higher utility bills.

2. **Decreased Performance:** Corrosion can obstruct water flow, reducing the heating or cooling capacity of the system. This can result in uneven temperature distribution, reduced comfort levels, and longer heating or cooling times.

3. **Leakage and Water Damage:** Severe corrosion can lead to the formation of pinholes, cracks, or leaks in pipes or heat exchangers. This can cause water or refrigerant leakage, leading to water damage in the surrounding area and potential safety hazards.

4. **Poor Water Quality:** Corrosion products, such as rust and scale, can contaminate the water circulating in the system. This can negatively impact water quality and potentially harm the health of occupants if ingested.

5. **System Breakdown and Costly Repairs:** Advanced corrosion can lead to system failures and breakdowns, requiring costly repairs or replacements of damaged components.

6. **Reduced Lifespan:** Corrosion accelerates wear and tear on the system's components, shortening their lifespan and requiring more frequent replacements.

7. **Loss of Refrigerant:** In cooling systems, corrosion can lead to refrigerant leaks, which not only affect the system's performance but also contribute to environmental issues due to the release of greenhouse gases.

To prevent and manage corrosion in heating and cooling systems, consider the following preventive measures:

1. **Water Treatment:** Implement proper water treatment to minimize the risk of corrosion. Water treatment methods, such as filtration, softening, and chemical treatment, can help control impurities and prevent scaling and corrosion.

2. **Material Selection:** Use corrosion-resistant materials for critical system components, such as stainless steel or coated metals.

3. **System Flushing and Cleaning:** Regularly flush and clean the system to remove accumulated sediment, rust, and other contaminants that contribute to corrosion.

4. **Proper System Design and Installation:** Ensure that the system is designed and installed correctly to minimize areas where water can stagnate, which can lead to increased corrosion.

5. **Monitoring and Maintenance:** Regularly monitor the system's performance and conduct preventive maintenance to detect and address corrosion issues promptly.

6. **Coatings and Linings:** Apply protective coatings or linings on vulnerable surfaces to create a barrier against corrosion.

By taking these preventive measures and addressing corrosion issues promptly, you can enhance the efficiency, reliability, and longevity of your heating and cooling systems while reducing the risk of costly repairs and downtime.