Question:
Why are coatings often supplemented with cathodic protection to protect against corrosion?
Answer:
Corrosion is one of the most critical failure mechanisms in structures, installations, components, and mechanical systems, in which materials go through decay or deterioration, which in turn compromises the integrity of these structures or systems. Deterioration is the cause of the formation of oxides, hydroxides, or sulphides, which are naturally more stable forms of any refined material. There are several reasons for corrosion to initiate and propagate, these include environmental and operational conditions, material properties, and electrochemical activation. Although several factors are involved in initiating and propagating corrosion, a key factor is the availability of an active metal surface, which incubates electrochemical changes leading to corrosion.
Therefore, the primary focus, as a cost effective, efficient, and reliable technique, is to convert the active metal surface areas into passive surface areas. This helps stop or decelerate corrosion, and can be done by providing a protective current, which in turn reduces the potential of a metal surface. This results in cathodic protection, hence stopping or significantly reducing electrochemical changes, in other words, any corrosive attack is halted. There are two conventional methods by which passivation of metal surfaces is achieved. These are commonly known as (i) sacrificial anode cathodic protection, and (ii) impressed current cathodic protection. The above methods have been widely used in several industrial applications, such as petrochemical, marine, and infrastructure. Another effective technique for mitigating corrosion is to use a coating. The coating acts as an anti-corrosive protective layer, a barrier, or a sacrificial layer over the metal. Coatings offer several benefits by protecting materials, enhancing surface characteristics, and avoiding or reducing the risks of failure. Corrosion, however, becomes more complex in terms of its failure mechanisms when structures and components are dynamically loaded as in marine structures. This will cause failures such as corrosion fatigue and stress corrosion cracking.
Or when static structures are exposed to more aggressive environments, corrosion and corrosion failures will significantly accelerate. The combination of both coatings and cathodic protection will enhance metal resistance against corrosion. Therefore, a combination of both is widely used. These preventative methods also assist at the design stage in reducing the required weight of material for required operating lifetime and can thus significantly reduce fabrication and later transport costs with net-zero type benefits.
A more complex corrosion mechanism will occur when components are dynamically loaded and are subject to relative motion. This adds more complexity to attempts at stopping or controlling corrosion, because other mechanical and physical factors are now combined and are contributing to a more complex form of deterioration. In such instances, the use or application of cathodic protection becomes complicated, challenging, and in some cases, it becomes almost impossible to meet specified CP criteria. Therefore, more robust methods of enhancing corrosion resistance through material development, advanced coatings and coating techniques, corrosion monitoring, and prognostic measures, have been developed over the past several years. It is more pragmatic to provide bespoke solutions for specific applications.
If the components are interacting, then surface wear will occur. In a scenario where corrosion is absent, the wear purely results from mechanical loading and surface deterioration. However, in the presence of corrosive species, corrosion will also occur. This leads to a wear-corrosion mechanism.
Recent Coating Developments
Researchers have been developing advanced coatings to withstand both corrosion and wear in challenging and harsh environmental and operational conditions subject to design life requirements. Recent developments in nanocoatings and nanocomposite coatings [5], have shown that more attractive coating solutions are available for applications in more complex mechanical and chemical conditions. These nanocomposite coatings are, for example, Ni/Al2O3, Ni/SiC, Ni/ZrO2, Ni/Graphene (GPL), and several others. Such coatings have been developed in order to be subjected to corrosion while incorporating the effects of key mechanical properties. These newly developed nanocomposite coatings have been tested according to ASTM B117,
salt spray testing [1].
Further study of the above nanocomposite coatings has been conducted within the wear context [2].
A comprehensive study of the above nanocoatings at atomic surface layers, incorporating corrosive fluids, and using a numerical approach, was conducted [3].
It is well known that the durability and reliability of complex interacting systems are very important from a cost viewpoint and within a wider sustainability context. These interacting systems are subject to corrosion failures and are therefore a major concern for industry professionals. It is important to fully diversify design parameters.
Remote Monitoring
Further work has been performed to predict corrosion in dynamically corrosive environments by considering physical and mechanical characteristics. Researchers have recently developed and patented a new corrosion sensor that could improve the safety and reliability of large structures such as bridges, aircraft, military vehicles, and gas pipelines. The device can detect defects and risks in major infrastructure at a much earlier stage than the methods that are currently used. As well as improving safety, it could reduce the need for time consuming repairs, which can come at a significant cost and inconvenience to industries and the public.
In summary, it evident that the primary objective of an industrial coating is to prevent corrosion, and to withstand a variety of hazardous chemicals. Choosing the right coating is just as important as choosing the coating itself, a wrongly specified coating can lead to a wide range of problems, from maintenance to premature failure. No coating is completely free of defects, even when freshly applied. Faults can occur during the production of the coating as well as during handling and improper application of the coating. A defect may also arise during the course of service. The most common causes of coating failures include inadequate surface preparation, a non-friendly environment, poor formulation, and an inefficient application technique. Coatings with high efficiency are more expensive. Thicker coatings, the use of sophisticated inspection methods, and fixing specific defects, all result in higher cost and critically weight. Sometimes it is advantageous to use another protection method to supplement coatings, which is why we use cathodic protection and especially in marine situations. There is an overall benefit when a good coating application is combined with cathodic protection [4].
Prof. Zulfiqar Khan, Bournemouth University NanoCorr, Energy & Modelling (NCEM) Research Group.
References
1. Nazir, M.H., Khan, Z.A., Saeed, A., Bakolas, V., Braun, W., Bajwa, R. and Rafique, S., 2017. Analyzing and modelling the corrosion behavior of Ni/Al2O3, Ni/SiC, Ni/ZrO2 and Ni/graphene nanocomposite coatings. Materials, 10 (11).
2. Nazir, M.H., Khan, Z.A., Saeed, A., Bakolas, V., Braun, W. and Bajwa, R., 2018. Experimental analysis and modelling for reciprocating wear behaviour of nanocomposite coatings. Wear, 416-417, 89-102.
3. Nazir, M.H., Khan, Z.A., Saeed, A., Siddaiah, A. and Menezes, P.L., 2018. Synergistic wear-corrosion analysis and modelling of nanocomposite coatings. Tribology International, 121, 30-44.
4. L.L. Sheir, R.A. Jarman, and G.T. Burstein; “Corrosion”, Volume 2: “Corrosion Control”, 3rd edition, Butterworth Heinemann, ISBN 0-7506-1077-8.
5. https://www.digitaljournal.com/pr/news/theexpresswire/nano-coating-market-by-2031
Continuity Straps across Flanges for providing Cathodic Protection.
