Using PLCs in cathodic protection systems

Corrosion of steel and reinforced concrete in structures such as bridges, jetties, shipping vessels and pipelines can lead to catastrophic failures if not maintained correctly. Employing the use of an appropriate corrosion mitigation method will reduce damage to the structures, increasing their operating life and lowering maintenance costs. Impressed current cathodic protection systems are one of the most effective methods of mitigating corrosion in metal and concrete reinforced structures, however these systems can be complex so must be designed and implemented correctly.

Corrosion of bridges, jetties, ships and pipelines

Steel and steel reinforced concrete is used extensively in structures and machines such as bridges, jetties, pipelines, boats and shipping vessels. Many of these structures and machines are located outdoors and exposed to all manner of weather and environmental conditions, especially when they are located near, or over, large bodies of water such as rivers, lakes and oceans.

Without some form of corrosion protection, the structures or machines will deteriorate to the point where they are no longer able to withstand loads within their original design limits and pose a risk of partial or total failure between the fixed elements.

One form of corrosion protection involves coating the metal surfaces with some type of sealer in the form of corrosion resistant paint, however where the structures are in contact with moisture (from sea, land or air) this method does not provide long term effectiveness on it’s own due a constant electrical potential that is naturally generated between materials within the structures and machines, so corrosion will still occur.

Where steel reinforced concrete is involved, it is impossible to obtain full corrosion protection with paint alone as concrete is naturally hydroscopic, meaning it absorbs moisture, also concrete can develop cracks that allow moisture to come in contact with the internal reinforcing steel and cause unseen internal corrosion. The logistics of applying corrosion resistant coating or some form of sealing medium to the steel reinforcing prior to the construction process would likely make it logistically difficult and cost prohibitive for a majority of projects.

Another form of corrosion protection is cathodic protection, this is by far the best way, and in some cases the only way, to reduce the corrosion effects due to it’s ability to ‘balance’ the electrical potential between materials. Cathodic protections comes in two forms, passive and active.

What is cathodic protection?

Cathodic protection (CP) is a corrosion management technique where metal structures have some form of sacrificial material (passive) or carefully controlled impressed current (active) systems.

Passive cathodic protection takes the form of galvanic anodes attached a specific point or multiple points around the structure which are ‘sacrificed’ to protect the structural steel from corrosion. These sacrificial anodes are alloys typically composed of zinc, aluminum and/or magnesium, in this form they are commonly used where they can be easily replaced as required.

Active cathodic protection uses some form of control system to introduce ‘impressed’ voltages and currents to the structure in order to balance and offset the naturally generated electrical potential. Transformer-rectifier units are used in Impressed Current Cathodic Protection (ICCP) systems and are often custom manufactured and include a number of features, such as remote monitoring and control, integral current interrupters and different types of electrical enclosures.

On ICCP systems, the output DC negative terminal is connected to the structure, the output DC positive cable is connected to the anodes and the AC cable is connected to the input terminals. The System output needs to be optimised to supply enough current to provide protection to the structure. Some units are designed with taps on the windings and jumper terminals so the voltage output can be selected, while some applications are made with solid state circuits to automatically adjust the operating voltage to ensure optimum current output is maintained.

Some systems can be fitted with analogue and/or digital meters to locally show operating voltage and current output, for larger systems information and status of the system can be sent to a centralised SCADA system, where operators can monitor and take necessary action to ensure the ICCP system is functioning correctly.

For large complex structures such as bridges or submerged structures such as jetties, ICCP systems are often designed with multiple independent zones of anodes, with each zone connected to its own transformer-rectifier (T/R). These systems are typically technically challenging and usually require the use of an automatically controlled DC power source with remote monitoring and operation.

Using PLCs to control ICCP

In the past, power supply control systems for ICCP applications were programmed in BASIC on a PC, however with technology advances the stability of these systems are no longer guaranteed due to the complexity and sometimes instability of Windows operating environment.

Programmable logic controllers (PLCs) have become widely adopted as the control preference due to their ability to be programmed specifically to meet the project requirements while performing to a high level of reliability due to the quality of the PLC hardware and software.

There are two main approaches for the use of PLC’s for controlling and monitoring ICCP systems. The first approach is to adapt a system from an existing control set-up, such as a building control system. These systems are typically provided by large electronics companies to undertake tasks such as maintaining heat and light at predetermined levels. Modules can then be added on which are controlled by a PLC. This has been a popular approach as it has a low capital cost.

The second approach is to custom design the system for the specific task of monitoring and providing current to an ICCP application. While this approach is more expensive initially, it allows the system and associated modules to be optimised for the task. The greatest benefit of the second approach is the systems can be designed to provide optimal current to the application. If there is too little current, it will not provide enough protection however too much current can cause material degradation and premature failure. The main power supplies also require constant monitoring to ensure there are no power failures, as this will result in the ICCP system being ineffective.

To ensure effective operation of an ICCP system, the PLC will ideally be programed to interrupt the current for all output zones simultaneously, then take instant ‘off’ values for each of the reference electrodes. This is difficult to achieve in practice, as it requires coordination of the timing system, synchronising of the switching ‘off’ output zones along with the ability to provide essential information such as system health and status.

Experience matters

Selecting an experienced control system engineering company, such as Automation IT, is important to ensure the ICCP system is designed, engineered and implemented correctly for the long term integrity of the protected structures from the effects of corrosion.

Automation IT’s experienced and RPEQ certified engineers are able to design, implement and commission a functional and reliable PLC controlled ICCP system based on a client’s specific requirements and what hardware/software is the most appropriate for a given application.

A holistic approach is taken when designing an ICCP system for a client, encompassing the entire design process, from the initial project scope to understanding the process under control, the hardware elements, installation procedures, programming requirements, documentation along with back up support.

Automation IT’s solutions comply with all laws, standards and warranties, giving clients peace of mind that their ICCP system will be designed correctly the first time.