A guide to using cathodic protection systems in hazardous environments
18 November 2020
For mission-critical steel structures, like industrial pipelines, tanks and bridges, it is commonplace for cathodic protection (CP) systems to be used to protect key assets from the ongoing threat of corrosion. However, this can be easier said than done if those assets are situated in hazardous environments where sources of ignition could spark a major explosion.
Image 1 - BLB1
Here, David Celine, Managing Director at remote monitoring specialist Omniflex, discusses the best practice for using CP systems in hazardous environments.
Corrosion is a natural, electrochemical process where metals are gradually degraded and destroyed as part of two simultaneous chemical reactions. A well-known example of this is rust, where iron is electrochemically converted by oxygen and water into hydrated iron oxides. This involves two reactions, an anodic oxidation that corrodes the metal structure and a cathodic reduction, occurring at the same time, with electron transfer between the anodic and cathodic cell through an electrolyte. CP systems are used to control the corrosion by ensuring that the structure you want to protect remains the cathode in any electrochemical reaction.
It is commonplace today to see structures that were designed over 50 years ago with a 50-year life expectancy being rehabilitated and having cathodic protection installed to extend their life for a further fifty years. Structures that are key targets for life extension are steel and concrete wharves. Major ports such as NSW Ports and the Port of Melbourne, two of the largest port operations in Australia, have both embarked on major life extension projects that involve significant cathodic protection programmes.
Traditionally, protecting critical infrastructures using CP systems has been done following one of two approaches. The first is a passive technique, called galvanic CP, where a buried or submerged steel structure is connected to a metal alloy with a more negative electrode potential than it, such as magnesium, aluminium or zinc. This guarantees that the metal structure is always the cathode of the electrochemical cell, and the metal alloy becomes a sacrificial anode that is consumed by corrosion, rather than damaging the structure.
Image 2 - Remote Monitoring Unit
Most often, galvanic CP installations are not regularly monitored, if at all, on the assumption that the simplicity of the systems will ensure their ongoing performance and protection. But in a world of increasing requirements for compliance, performance guarantees and reporting, strategic assets do require more regular monitoring. This normally involves regular physical visits to sites by experts. These sites can often be remotely located and involve safety risks to access, all of which add to the ongoing cost of ensuring protection from corrosion.
The second approach to using CP systems to protect against corrosion is called impressed current cathodic protection (ICCP). In this method, current is applied from an external source to ensure that the metal structure remains cathodic with respect to its environment. Traditionally, galvanic CP systems haven’t been able to offer significant protection for larger structures, like industrial pipelines, bridges or ports, so ICCP is normally used in these situations instead.
Both types of cathodic protection rely on the principle of ensuring that the steel remains more negative than its surroundings. If the steel is more negative than its environment by 800 mV, it can be safely assumed that corrosion of the steel will have been halted. However, this is particularly challenging when dealing with structures constructed using pre-stressed concrete.
For pre-stressed concrete, high tensile steel reinforcing rods are put under tension prior to pouring the concrete. This presents a problem when using CP systems on these structures because the high tensile steel is prone to hydrogen embrittlement where the crystalline structure of the steel can be compromised, causing it to lose tensile strength and snap. For this reason, it is vital that the voltage applied to the steel is carefully controlled and doesn’t cause it to be over 1 V more negative than its environment, so that hydrogen evolution does not occur.
Bringing the classic together
Image 3 - Hybrid Anodes
UK-based Concrete Preservation Technologies Limited recently developed a hybrid anode system that combines the properties of galvanic and ICCP protection. It works by drilling holes in the structure in a matrix and inserting the specially designed anodes directly into the structure. Then the anodes are connected using titanium wire and a voltage is applied to force salt migration from the steel to the anode and passivate the zone. Once the zone is sufficiently “charged”, the power source is disconnected, and the sacrificial anodes are left to operate galvanically and provide passive CP protection to the structure.
Using this hybrid CP system allows larger structures to take advantage of galvanic protection that wasn’t possible before. However, because the technology is relatively new, there is still some uncertainly about whether it can be used to protect assets from corrosion over a period of 50 years because it hasn’t been field tested over that length of time yet. To provide asset managers with the reassurance they need, remote monitoring technologies can be implemented to monitor system performance and corrosion levels 24/7 on a permanent basis.
Benefits of remote monitoring
Remote monitoring of CP systems offers several key benefits for enterprises and asset owners. Firstly, as regulations continue to evolve, data accessibility and transparency are becoming increasingly important, and cloud-based remote monitoring platforms provide managers with a single, easy-to-access repository for all live and historical CP data.
Secondly, by automatically monitoring and recording data relating to asset performance and system status, all abnormal events, like power outages or system failures, can be reported directly to all relevant personnel without delay. This enables site managers and engineers to take appropriate action immediately, minimising the chance of a negative outcome like high maintenance costs or having to face the prospect of unplanned downtime.
Image 4 - Engineer working on system
Thirdly, ongoing maintenance costs are lower when remote monitoring technologies are used to monitor CP systems. This is because there is a reduced need to physically inspect systems in difficult-to-access areas or those in hazardous environments. Furthermore, the duration of any on-site inspections is lessened because preliminary testing can be done remotely before the site visit, reducing the overall cost and minimising disruption.
Bringing it all together – A customer case study
Throughout the last decade, investigations have been conducted to determine the level of corrosion of the structures at NSW Ports, the Port Authority of New South Wales in Australia (which includes Sydney Harbour and Port Botany, Australia’s largest container port). These reports established that chlorine-induced corrosion was setting in on some of the structures. In 2019, NSW Ports embarked on a two-year program of works to rehabilitate the structures and combat corrosion levels at the ports Bluk Liquid Berth 1 (BLB1) which had been operational since 1979.
NSW Ports commissioned Ian Godson, from the Melbourne-based consultancy Infracorr, to design a CP system for use at BLB1, which is situated at Port Botany and houses hazardous gas tanks and pipelines. The project also included the repair of defective concrete structures which were suffering from the effects of corrosion and concrete spalling within the many pre-stressed beams and headstocks of the various bridges and catwalks at the port.
The system needed to be designed to allow for tight control of currents and voltages used throughout the site for two key reasons. Firstly, Port Botany is NSW’s main bulk liquid and gas port and BLB1 is a key part of this facility, playing an active role in loading and unloading hazardous liquids and gases. These hazardous materials can be present in the environment on an ongoing basis, meaning that any stray sparks caused by excess voltages and currents on site could become an ignition source and cause a major explosion.
Image 5 - BLB1 layout
Secondly, because many of the structures present are constructed from pre-stressed concrete, it was extremely important that all electrical currents applied were carefully controlled. As we discussed earlier, to control corrosion in steel it is necessary to make it more negative than its environment by 800 mV. However, if the charge applied results in the steel being more than 1 V negative than its environment, hydrogen embrittlement can occur which leads to failure of the steel and catastrophic failure of the structure with long-term structural damage that isn’t easily repaired.
The system designed by Ian Godson, for use at BLB1, was a hybrid CP system which would use remote monitoring technology to provide asset managers with ongoing reassurance that systems were operating as intended and corrosion levels were under control. To deliver this, Godson requested the assistance of Omniflex to advise on the hazardous area and remote monitoring aspects of the design.
Because of the low currents required to meet the prestressed steel and hazardous area limitations, the hybrid CP system required an initial power up phase of three to four months before the external power source was disconnected and the system left to operate galvanically. The system comprises of nearly 35,000 embedded hybrid anodes that were installed within the structures at the berth and is designed to control corrosion for 50 years. Because of the ongoing operational nature of the berth, extra controls were put in place to manage activities across the site while works took place.
Because this was the first large scale implementation of hybrid CP used within a working hazardous area anywhere in the world, some of the components required certification for the design to meet the requirements of AS60079 as an intrinsically safe certified system. Most countries have their own certification process so it can be costly and time consuming to get installations like this one certified as being intrinsically safe.
In this case, the hybrid anodes were from a UK supplier, Omniflex’s technology is manufactured in South Africa and the project is in Australia, so navigating certifying bodies was challenging. However, the design was approved and certified as being intrinsically safe for use in areas classified as Zone 1 hazardous for gas group IIB in the BLB1 project, where there is the potential for exposure to volatile, flammable substances.
David Celine - Managing Director of Omniflex
System performance and corrosion levels are continuously monitored 24/7 using 24 remote monitoring units (RMUs) that are situated around the site, each with the capacity to monitor 16 structures.
Crucially, these works ensure that the integrity of BLB1 is maintained and the berth remains reliable and available to handle NSW’s growing bulk liquid trade volumes for the next 50 years.
About the author:
David Celine is Managing Director of Omniflex Pty Ltd, a position he has held for over 20 years. David graduated as an electronics engineer and has spent his entire working career in the development of industrial measurement and control instrumentation products, with specialisation in recent years in remote monitoring and control. Omniflex has been developing web based remote monitored and controlled CP systems in Australia and abroad for the corrosion markets since 2008 and are considered a leading innovator world-wide in the development and adoption of robust solutions to the problems of remote control of cathodic protection systems.
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