By Mike Yongjun Tan, Professor of Applied Electrochemistry and Corrosion Technologies, Deakin University
Significant improvement in engineering materials, industrial standards and software has facilitated the avoidance of ‘short-term’ failure of engineering structures. However, ‘long-term’ structural failures, in particular those induced by localised forms of corrosion, still remain tenacious threats to the integrity and safety of the huge network of civil and industrial infrastructure assets.
This is evident by many publicly reported catastrophic engineering structure failures, and an enormous number of unreported incidents that occurred on various utility infrastructures, especially those exposed to aggressive and varying environmental conditions. Buried steel pipelines are typical examples of utility infrastructures that are exposed to complex and ‘invisible’ engineering environments.
There are millions of kilometres of pipelines around the world for transporting gases, water, sewage, slurry, beer, hot water or steam, biofuels and hydrogen over various distances. Each square kilometer of a major city could host more than 30km of buried pipes, creating tangled networks of gas lines, water mains and electrical and telecommunications cables.
Effective management and control of complex forms of localised corrosion on pipelines are critical challenges to the maintenance of utility infrastructures that are vital for the provision of the world’s essential services and the maintenance of its economic activities.
In practical industrial environments, corrosion damage could be due to multiple and changing forms of localised corrosion, whose processes and mechanisms can vary significantly over time and location. Buried steel pipelines are affected not only by seasonal changes in soil moisture and oxygen levels, but also by fluctuating stray currents and oscillating mechanical stresses.
Offshore structures such as wind turbines and shore-crossing pipelines are affected by multi-zone and dynamically changing marine environmental conditions. Such complex environmental conditions can not only lead to changes in corrosion rates, but also in corrosion characteristics, mechanisms and patterns.
In some cases, other factors such as microbiological activities in soil and ocean could add further complexity to corrosion processes and mechanisms.
How to detect invisible corrosion
A prerequisite to effectively managing these complex forms of corrosion on buried and submerged pipelines is sufficient corrosion data that provides information about where and how fast it is occurring. Such corrosion data is essential for engineers to alert risks, predict asset maintenance needs, select efficient corrosion control technologies, and monitor the effectiveness of corrosion control methods and materials such as protective coatings, cathodic protection and corrosion inhibitors.
Knowledge of the status of pipeline corrosion is also important for the owners of an engineering asset. Such knowledge would enable owners to prioritize site survey and inspection operations, and to develop maintenance strategies to manage pipeline assets, to predict long-term remnant pipeline life.
For instance, accurate determination of the location of critical localised corrosion would assist pipeline owners to plan the dig-up inspections of unpiggable pipelines, which is an expensive operation, and it is important from asset management perspective to increase the accuracy of identifying dig-up locations.
Unfortunately, such corrosion data is often unavailable. It has been recognized that there is a serious lack of corrosion and materials degradation information on these ‘invisible’ structures, impeding their timely maintenance and protection. In current industrial practice, routine inspection is the most widely used method for identifying localised corrosion on engineering structures.
Risk-based and time-based inspection and fitness-for-service assessments are common methods for the integrity management of engineering structures. These approaches may not be sufficient for detecting, predicting and managing complex localised corrosion under dynamic changes in corrosion environments, mechanisms and kinetics. There is a need for more convenient, reliable and economic asset inspection and management technologies.
The future of corrosion technology
An ‘ideal’ corrosion sensing and control system should be one that not only provides in-situ and site-specific corrosion data required to visualise localised corrosion, but also to use such data to inform corrosion prediction and control. For instance, to guide local coating repair and to regulate local cathodic protection potential and corrosion inhibitors injection.
In this manner, the threat of localised corrosion to the integrity and safety of engineering structures would be minimized and the safe operational life of infrastructure would be maximised. Considering the variable nature of corrosion environments and mechanisms in practical engineering structures, corrosion management and prevention actions may need to be adjusted, based on the prevailing corrosion condition and mechanism.
A prerequisite for effectively doing so is timely knowledge about the initiation, propagation and seriousness of localised corrosion occurring over an engineering structure. In order to meet this need, a novel corrosion probe/sensor has been developed by the Deakin University corrosion research team, supported by the Energy Pipelines Cooperative Research Centre and Future Fuels Cooperative Research Centre.
The corrosion probe was developed for in-situ and site-specific corrosion data acquisition that has enabled the in-situ measurement and monitoring of difficult-to-measure forms of localised corrosion. The function of the localised corrosion probe could be compared with that of sensors for monitoring human health through detecting, diagnosing and preventing cancer and other diseases. The human body itself has many ‘sensors’; the eyes, ears, nose, skin and tongue.
These provide disease information through vision, hearing, smell, touch and taste. Diseases can be further diagnosed and treated through medical testing, doctor’s analysis and the use of various medical treatments. For engineering structures, unfortunately such ‘sensors’ are not naturally available.
Therefore, it is essential to have artificial devices installed for sensing structural health issues, such as complex forms of localised corrosion – which is the prime threat to the integrity of metallic structures.
Corrosion monitoring would make more effective application of corrosion control technologies, such as cathodic protection, coatings and inhibitors which have been widely used to mitigate corrosion. Cathodic protection is probably the most common and effective technology for protecting buried and submerged steel structures from corrosion.
A buried steel pipeline is considered to be fully protected from corrosion if it is cathodically polarised to a standard’ potential of -850 mV vs copper/copper sulphate reference electrode. Unfortunately, in engineering practice, the protectiveness of cathodic protection can be impaired under complex and dynamically changing environmental conditions.
Examples include steel pipelines buried in inhomogeneous soil under the effects of seasonal changes in soil moisture and oxygen levels; cathodic protection disruption; nonuniform coating defects and cathodic shielding under disbonded coatings; fluctuating stray currents and oscillating mechanical stresses.
Overcoming complex corrosion challenges
These complex environmental variables create major challenges. For this reason, cathodic protection design and application are often considered in industry to be an ‘art’ rather than ‘science’. The application of the corrosion probe over the past decade has also enabled an unambiguous understanding of the effects of complex environmental variables on cathodic protection efficiency in order to address major issues in existing cathodic protection criteria and industry standards.
Supported by the Energy Pipelines Cooperative Research Centre and Future Fuels Cooperative Research Centre, Deakin University research team has made major efforts to understand the effects of intermittent cathodic protection and stray currents on cathodic protection efficiency, a persisting issue for decades, by means of the development and use of innovative testing and monitoring methods.
This research has also facilitated major findings on the effects of coating disbondment and cathodic protectionshielding on steel pipeline corrosion, a prime corrosion issue on aged gas pipelines. The ultimate goal of corrosion science and engineering would be to prevent the premature failure of engineering materials and to extend the safe operational life of engineering structures. This includes various utility infrastructures through detecting, mitigating and minimizing corrosion damage.
Industrial application of effective corrosion monitoring sensors and probes will lead to wider availability of real time corrosion data which, in conjunction with data analytics and artificial intelligence technologies, will enable much more reliable corrosion prediction, control and management systems.