Corrosion in water systems can mean a reduction in efficiency, it can incur costly maintenance & repairs, and in some cases cause plant shutdowns but as a collective, throughout all sectors, the consequences of the corrosion process have become major problem with many experts estimating the cost of corrosion amounts to billions of euro every year. [1]

In this article we look at how corrosion in water systems (mainly closed systems) is affecting not only the actual system but how it impacts the environment and what’s being done to help reduce the negative environmental impact and economic loss.

Understanding Corrosion

The word corrode is derived from the Latin “corrodere”, which means “to gnaw to pieces.”

Corrosion is the deterioration of a metal by reactions with its’ environment, where the environment is the whole surrounding area in contact with the material. Corrosion, in an aqueous environment, occurs when oxygen enters the system and mixes with the metal and water; creating a “rust” on the surface of the metal.

In the water industry nearly all metals can suffer from corrosion, it is of particular concern in closed loop systems as there is no blowdown, allowing the corrosion products to build up leading to deposition which can worsen the corrosion.

Most of the time corrosion issues are only detected after a failure has occurred which is too late to take preventative action, and so it is vital that corrosion is monitored and maintained.

The BSRAI – have updated their Guidance BG29/2021 and BG50/2021 in 2021. In closed heating and cooling systems, water treatment comprises a number of techniques, including:

• Pre-treatment of fill water
• Removal of dissolved gases
• Chemical water treatment
• pH management
• Solids removal and filtration
• Galvanic anode cathodic protection / electrochemistry
• Bacteria and biofouling inhibition by biocides

Factors that Affect the Rate of Corrosion

There are several factors which influence the rate of corrosion and whether it will form in your system, including (but not limited to):

• Aeration (dissolved O_² in water)
• Flow
• Temperature
• Composition (deposits / hardness / pH)
• Water treatment
• System design

Out of them all, aeration is the most important as corrosion can be prevented by excluding the combination of air & water from the surface of the metal, however, unfortunately, there are always ways for oxygen to enter the system. Oxygen can enter the system through leakage, pump seals, poor maintenance, make up water or design / installation failure. Luckily nowadays corrosion-protective coatings & technologies are not only more readily available but are a more practical and sustainable way of providing a barrier between the metal and its “environment”.

Flow is also an important factor; in moderate flow conditions the effect of corrosion is usually minimal as protective barriers (inhibitors) have the chance to absorb onto the surface. Under low flow conditions, suspended soils are given the chance to deposit onto the surface, leading to blockages or, if bacteria present, microbial influenced corrosion. However, if there are very high flow rates, erosion corrosion (occurs when oxygen is in the system) or cavitation (occurs due to formation of steam bubbles in deaerated water) can occur. The temperature can also accelerate the rate of corrosion in closed loop systems as the oxygen has nowhere to escape. In mild steel, the rate of corrosion is about 0.05-0.15 mm year^-1 at 288 K. When the temperature gets closer to 343 K, the rate of corrosion is 1.8 times greater and thus the corrosion rate goes up by 0.09-0.27 mm year^-1.

Scale deposits and microbial activity also need to be mentioned as these both can cause operational problems due to them reducing the heat transfer efficiency. The continuous fouling of heat exchange surfaces and system piping with corrosion by-products will reduce the systems efficiency and result in higher operating and equipment costs.

With the ongoing demand on supply, it is vital that the limited water supplies we have are utilized and recycled correctly. In a closed loop water system, water is constantly recirculated with very little water being lost, which is one of the biggest advantages of these systems. However, they need to be maintained to ensure everything is being done to minimise the risk of corrosion.

The Effects of Corrosion

Every industry has some form of heating and chilled system and depending on the size of facility there can be some serious consequences. If corrosion is not identified at an early stage, it can cause plant shutdowns, reduction in efficiency, loss or contamination of product and costly maintenance.

No matter the size of a facility, plant failure has big consequences especially when it comes the cost of getting equipment back online:

• All corroded parts need to be replaced and possibly re-designed to allow for corrosion
• Equipment adjacent to corrosion source may need replacing
• Technical experts are required to find a solution and repair the system
• Loss of production & product
• Possible contamination of goods
• General maintenance required to make good any repairs.

The Environmental Effects

Climate change is high on the agenda for most industries these days and  more engineers are focusing on delivering sustainable products and systems which are helping minimise the environmental and societal impact.

Whilst it may not be widely talked about, corrosion in cooling and heating water systems can have a negative impact on the environment. Inhibitors are used in closed systems to help mitigate the risk of corrosion, commercially available corrosion inhibitors have toxic properties and can have a damaging effect to the environment if released.

In large industrial, power, or chemical plants the environmental impact of corrosion can be severe. As the systems efficiency decreases additional energy is required to keep the system going, this will put additional strain on all systems and could result in plant shut down or worst-case scenario – a fire.

There is also the physical aspect of the corroded material to consider. Once it has been removed from the system, along with any other damaged materials, it must be transported and disposed of correctly and not taken to the landfill / dump.

When you consider the harmful impacts happening every day, worldwide, never mind the day-to-day issues; higher energy usage, higher water usage, additional labour & transport, you can see why corrosion mitigation & control is still one of the hot topics for researchers in industries & academia.

We are now seeing manufacturers & infrastructure owners focusing on end-of-life management and making early considerations with respect to corrosion in the design and planning stages.

The Economic Impact of Corrosion

Apart from the environmental consequences, economically the global cost of corrosion exceeds €2.27 trillion, or 3% of global GDP.[2]

While this is not only corrosion in water systems, it shows just how significant the economic and environmental impact of corrosion is.

Research has shown that approximately 1/3 of these costs can be reduced if there was a broader use of corrosion resistant materials. Studies carried out by The Battelle Panel shows that since 1975, the industry has eliminated 35% of avoidable corrosion by improved practices and reclassified 15% of unavoidable costs as avoidable. [3]

New and improved corrosion technology results from research and development, and with advancements in corrosion technology and techniques evolving at such a high rate, there are always new innovations coming onto the market. The proper application of methods to control corrosion (e.g., coatings, inhibitors, and cathodic protection) reduces the cost of corrosion individually and globally and makes a positive contribution to the overall carbon footprint.

Corrosion Control

Increased consumer awareness has allowed corrosion prevention products to develop and improve significantly. Through the application of existing or emerging technologies, advances are being made in all methods for corrosion control and its application. These prevention methods include temporary and permanent coatings, material selection, inhibitors, cathodic protection, and design.

Design

Each closed system differs depending on several factors including size of facility and application. When designing the system consideration should be given for the mechanical and strength requirement, together with an allowance for corrosion.

Material

The design of a system should take into account the materials to be used for various components within the system.

At design stage it is important that the correct metals that make up a system are given thought. At construction stage and pre-commissioning stage that the most suitable cleaner, inhibitor and biocide are selected. For commissioning and hand over it is vital that the water treatment company is aware of all the metals in the system so they can formulate a site specific water treatment plan for the site.

The implications of adding a high pH corrosion inhibitor to a system with aluminium is detrimental.

Inhibitors

The technique of adding inhibitors to the environment of a metal is a well-established method for controlling corrosion. A corrosion inhibitor may act in a number of ways: it can restrict the rate of the anodic process or the cathodic process by simply blocking active sites on the metal surface.

Alternatively, it may act by increasing the potential of the metal surface so that the metal enters the passivation region where a natural oxide film forms.

Cathodic Protection

The protection is achieved by supplying electrons to the metal structure to be protected. There are two ways to cathodically protect a structure; by an external power supply or by appropriate galvanic coupling.

On the ground there are several modifications to the environment which can help control corrosion:

• Lowering the temperature can slow down the rate of corrosion
• Adjusting the concentration of a corrosion solution will reduce the rate of corrosion
• Removal of oxygen or oxidizers in most environments decreases corrosion rate, but it is not favourable for active—passive metal or alloys since they require oxidizers to form and maintain protective films.
• Avoid high velocities as this will increase the rate of corrosion.

To help reduce the cost and the environmental impact of corrosion, and enhance the sustainability of materials, the use of any or all these methods are recommended and could lead to savings from 10 – 35% of the cost of corrosion.  [4]

Monitoring Corrosion

Most heating and cooling systems are continuously in use so it can be difficult to get access to inspect the system, luckily there are a number of methods available to help monitor the effects of corrosion while the systems are operating. It is essential that there is some form of corrosion monitoring in place to allow the onset of corrosion to be detected as early as possible.

There are a number of ways to monitor the effects of corrosion, but by far the most popular way is through the use of corrosion coupons. This is when the metal “coupons” are installed into the system for 3 – 6 months and when removed the coupon is measured and compared to the initial weight. It is a popular choice due to its effectiveness, it being relatively inexpensive, simple to install and less invasive, however there are limitations to this technique:

• The rate of corrosion can only be measured once the coupon is removed – which could be too late in some instances
• The corrosion rate is only measured as an average during the duration it was installed, not providing further analysis to show peak times it could have occurred
• Short term studies usually yield high corrosion rates which are not representative of the actual system

An alternative to using corrosion coupons is the use of an online corrosion monitor. This uses a probe which communicates to its own, individual software and displays accurate, graphical information regarding the corrosion rate in the system.

Every few hours data can be sent to a device (either stationed onsite or remote) which allows site engineers to see if any form of corrosion is starting to develop, giving advanced warning and ample time to act and resolve the issue.

With continued advancements being made in the field of corrosion, even more technologies are becoming available to help reduce corrosion costs and the risk of system failure.

Conclusion

Corrosion has aways been an issue, especially in the water treatment industry, but in recent years with the increasing focus on sustainability and environmental issues it has become something that needs to be addressed by both individuals and corporations.

The cost of corrosion can be extremely high for some businesses and vary from industry to industry. There are several factors which could increase the cost of corrosion within a business including:

• Equipment being designed to last longer = more usage and maintenance
• New government regulations could prohibit the use of time-honoured methods of protection because of safety or environmental damage
• For the consumer, corrosion costs are incurred for purchases of corrosion prevention and control products, maintenance and repair, and premature replacement.

Fortunately, advances in technology are being made every day and with a central focus on corrosion, new, environmentally friendly, corrosion prevention methods will be available to the wider market sooner rather than later.

References

[1] [2] [4] Bowman, E 2016, International Measures of Preventions Application, and Economics of Corrosion Technologies Study

[3] AMS International, 2000, Corrosion Understanding the Basics