Corrosion detection: what lies beneath?

Louise Davis

Jonathan Douglas reveals a new sensor and modelling technique being developed to detect corrosion under insulation (CUI)

With an estimated US$2.2 trillion spent repairing, replacing and maintaining equipment and materials affected by corrosion(1) it is a major concern for organisations across a huge range of sectors. From oil and gas companies, through process, marine and automotive businesses, to the nuclear industry – all need to take action to combat corrosion. But although rust can provide a clear signal of external corrosion, identifying corrosion under insulation (CUI) is more difficult. A test on one area of pipework may come back negative for CUI, while the trigger factors that cause it may be occurring just a few metres away.

The potential for extensive structural damage to pipeline makes CUI a major worry for the oil and gas industry. Companies are facing a huge challenge in identifying the electrochemical changes under pipeline lagging, as there are more than 2.5 million kms of pipework in the USA and the UK(2) alone. Logistically, the schedule for checking pipework may have to factor in complex issues including remote locations, access problems, shutdown requirements and the risk of damaging the insulation – making it expensive and time-consuming.

Even the lagging material itself may promote CUI, through its free chloride chemical content. Coatings are often promoted as a solution to protect against corrosion – the application of ceramic, polyurethane or enamel, for example – as they can suppress the corrosion-forming electrochemical reactions, or form a seal against air and water incursion. Cathodic or anodic protection, using an external power supply, is also used to protect pipework, as is metal plating. But there are problems associated with each approach: whether poor abrasion resistance or strength of adhesion of coatings; environmental concerns around the chemicals used; or the complex and costly requirements for their application. Furthermore, even pipework that has had a protective layer applied will still need regular checks, and this may require a shut down of equipment that impacts upon production.

A sensor-based approach

Non-intrusive sensors, which monitor the environment under lagging for changes in chemical composition, and provide an indication that corrosion may be developing, are an ideal solution. Frazer-Nash Consultancy, working with the University of Southampton, has developed a new approach that uses innovative sensor and logging technologies, supported by degradation and probabilistic corrosion models, to identify the condition of a metal asset under insulation. The sensors are installed across the network and deliver continuous monitoring and detection remotely, minimising external access and disturbance.

Using a microelectrode array that detects and monitors the metal ions present in a microenvironment, the boron-doped diamond sensor can identify multiple potential corrosion factors. It can measure the chemical environment, including the pH balance and presence of oxygen; the types of cations that are being removed from the surface and their mechanisms; and when the surface of a metallic structure is wet. This information allows an operator to assess the corrosion type being caused, enabling timely intervention and thus reducing failure frequency and its associated operational costs.

There are many additional benefits to using sensors to monitor corrosion. The continuous monitoring of an asset’s condition delivers confidence in its integrity status, while providing information about any impending degradation. Non-invasive and low cost, they can check known corrosion hotspots, difficult to access locations and essential safety critical areas. Through enabling proactive inspections, the sensors also support through-life asset management processes, as users can carry out maintenance and replacement of ageing equipment based upon its recorded condition rather than a fixed timescale.

The complementary advanced modelling software uses the sensor data to create a probabilistic corrosion model of the area being monitored, based on degradation mechanisms. With inspection required reduced to a minimum, costs are reduced, while rapid download of the recorded electrochemical data empowers quick decision-making, based upon the projected rate of degradation. Potentially, users may be able to save the cost of replacement, through taking remedial action before failure. Safety can also be facilitated, through the combination of sensors and modelling, as the active monitoring of the asset condition delivers early warning of faults or corrosion areas. The data gathered can even inform future design requirements.

What’s next?

Currently, the sensor has been constructed, and has been tested within a laboratory environment. The results have demonstrated that the technology and methodology is capable of sensing in situations analogous to under insulation. The next stage is to undertake ‘proof of concept’ testing, in a real-life environment, and Frazer-Nash and the University of Southampton are seeking partners that would be interested in taking part in this activity via the ITF. The data gathered will be used to demonstrate the value of the modelling software. It is hoped that, following field testing, the sensor and software can be launched to market and will benefit the oil and gas industry, and others struggling with the problem of detecting CUI.

Jonathan Douglas is group leader of the Materials Performance group at Frazer-Nash

References: (1) Hays, George F., Now is the Time, World Corrosion Organization; (2) US Central Intelligence Agency, The World Factbook

Recent Issues