Recognising the different types of corrosion in water systems, understanding how and why they occur, and their effect on system metallurgy is an essential stage in the development of any effective water treatment programme for the control of corrosion in steam, cooling and closed systems.
If corrosion starts to develop in a water system it can quickly start to become a serious problem that can end up costing business owners dear, in terms of down-time, additional resources and money. Worldwide, the costs related to corrosion in water systems run into billions of pounds every year.
Corrosion in water systems
Metal loss caused by corrosion can reduce the reliability and overall lifespan of a water system and its components including distribution pipework and any associated plant and equipment. Corrosion in water systems can increase maintenance expenses and cause equipment to fail before it should. It can also reduce heat transfer efficiencies, lead to a build-up of deposits, restrict water flow and simply stop the system working efficiently. Understanding the different types of corrosion that can occur in a water system, and what causes them can help address the issues before they spiral out of control.
What are the different types of corrosion in water systems?
Corrosion experts typically class corrosion into two broad categories, generalised and localised corrosion.
What is galvanic corrosion?
Galvanic corrosion occurs because of a chemical reaction between two metals in a solution which acts as a conductor. The chemical differences between the two metals in effect create a simple battery, where one metal deteriorates and the other is protected from corrosion.
The speed of the chemical reaction will depend on the types of metals involved. You can use a galvanic potential chart to help predict whether corrosion is likely to be an issue, and how quickly the corrosion is likely to happen. Galvanic corrosion can be controlled by using a device such as a dielectric union to keep the two metals apart. They are typically made from galvanised steel and lead-free brass and act to break the circuit and stop electrolysis occurring.
What is pitting corrosion?
Pitting corrosion is one of the hardest forms of metallic corrosion to predict. It’s also one of the most destructive.
Pitting corrosion occurs when specific areas of metal experience intense, localised corrosion but the majority of the surface is unaffected.
Some of the more common risk factors associated with pitting corrosion are stagnant water conditions and low velocity water flow.
Additionally, the presence of corrosive ions such as chlorides, sulphates and oxygen can increase the risks significantly. Once a pit has formed as a depression in the surface of the metal, any liquid trapped within it becomes even more corrosive and makes the problem much worse.
Under-deposit corrosion is also sometimes known as crevice corrosion. It is a type of localised corrosion which occurs in a shielded area such as a crevice. Under the deposit, any trapped solution starts to concentrate and over time becomes more and more corrosive.
Certain metals and alloys such as aluminium, copper and steel which use an oxide film to protect them are more likely to suffer under-deposit corrosion. It is more commonly found at the end of the lines, on lower levels of process runs, or at points in the system where the flow of water is slower. The most effective way of preventing under-deposit corrosion in water systems is to keep all metal based systems clean and free of contamination.
Microbiologically influenced corrosion (MIC)
MIC or microbiologically influenced corrosion in water systems is a term used to label any type of corrosion which is primarily caused by bacteria which can include pseudomonas, sulphate reducing bacteria and nitrite reducing bacteria.
MIC is often a factor in corrosion tubercules forming (see below), and can be a factor in other types of corrosion too. Bacteria however do not directly attack the metal. The corrosion is caused by the by-products of the process which result from bacterial growth reacting with the metal.
Erosion-corrosion in water systems
Erosion-corrosion is a combination of abrasion and corrosion from the flow of water around the system. Nearly every metal can suffer from some degree of erosion-corrosion. However, soft metals such as those which depend on a passive layer to protect against corrosion are particularly susceptible.
Erosion-corrosion in water systems can also be caused or made worse by fluids containing suspended solids. The easiest way to prevent erosion-corrosion is to consider it at the design stage. Create a system which will maintain the speed of flow, taking into account the metals being used and how often the flow changes direction.
An alternative approach is to remove or limit the suspended solids using a filtration system such as a side stream filter.
What is tuberculation?
A tubercule is the name given to an accumulation of corrosion formed from by-products and other deposits. They tend to affect iron components and form over areas of lost metal. The composition of the tubercule will depend on the chemical composition of the water, the dissolved oxygen concentration, flow, water temperature and rate of corrosion. As the metal is removed, the turbercules develop and corrosion starts to build up. Usually, the corroded areas underneath tubercules are irregular and broad depressions. Don’t automatically assume that all iron oxide deposits which are red or brown in colour are tubercules. If you’re not seeing any metal loss underneath then the iron might have accumulated from elsewhere in the water system.
Expert industrial water treatment and corrosion control
Water Treatment Services offer a comprehensive range of industrial water treatment solutions for the management and treatment of corrosion in water systems. Our experts can help you optimise costs and operational efficiencies, reduce downtime, achieve water and energy savings, and increase plant reliability and safety.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians, in-field specialists and consultants we can offer professional, cost effective water treatment and engineering services throughout the UK and Internationally.
Contact us today to learn how our industrial water management solutions can help you.
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