Surface Functionalisation through Nanoscale Science

Development of self-healing coatings for corrosion protection on metallic structures

Development of self-healing coatings for corrosion protection on metallic structures

Alicja Stankiewicz and Michael B Barker
School of Engineering & the Built Environment, Edinburgh Napier University, 10 Colinton Road, Edinburgh, EH10 5DT, UK


Received 6 December 2015, revised 14 April 2016
Accepted for publication 27 April 2016
Published 15 July 2016

Inspired by biological systems, artificial self-healing materials are designed for repairing local damage caused by external factors. The rapidly expanding field of self-healing systems contains, among others, materials with well-defined surface properties. Undoubtedly, enhancing surface functionalisation, by applying smart coatings, enjoys an extensive interest. The self-healing ability is particularly essential property for corrosion protection strategies, especially when the
use of one of the most effective corrosion systems, based on chromium(VI) compounds, is now banned by the current registration, evaluation, authorisation and restriction of chemicals
legislation. Self-healing protective coatings are produced using macromolecular compounds, ceramics, metals and composites. Considering the wide range of available materials, the number of potential combinations seems to be unlimited. The self-healing action of such coatings is activated by appropriate stimuli: temperature changes, radiation, pH changes, pressure changes and mechanical action. In this paper, the research and practical implications of the various approaches to achieving self-healing functionality of protective coatings, as well as potential developments in this area, are explored.

Keywords: self-healing coating, protective coating, corrosion
(Some figures may appear in colour only in the online journal)

1. Introduction

In selecting a construction material one has to face a com- promise between the desired properties and price. Very often the somewhat inadequate surface properties of magnesium and aluminium alloys, steel and composites, can be enhanced by covering their surface with a protective coating. This approach, apart from saving the costs of using more expen- sive materials, imparts desired properties such as hardness or abrasion resistance. Preventing and mitigating corrosion issues of structural and functional materials are a vast cost to modern industrialized economies. According to data pub- lished by the Corrosion Committee of the Surface Engineer-ing Division (the Institute of Materials, Minerals and Mining (IOM3, UK), annual costs connected with chemical degra- dation or electrochemical corrosion to the UK economy are estimated between 2% and 3% of gross national product. A similar situation is observed in USA and Japan [1]. More specifically it can be said, that corrosion costs around £600

Often in these figures, the costs related to equipment idle time are highlighted. However, replacing, repairing or restoring corroded equipment also requires sub- stantial additional manpower and investment in new capital equipment. Nowadays, protective coatings have to meet the requirements of the current legislation embodied in the registration, evaluation, authorisation and restriction of che- micals, which prevents the use of harmful substances including hexavalent chromium, the main compound employed in such coatings until recently.

Furthermore, the surface finishing sector is aware that it is not only the man- ufacture of the product, but also its use in service and its disposal at end-of-life that can impact the environment det- rimentally. Consequently, there is a pressing need to render production methods less environmentally damaging without compromising on the product life cycle. The circumstances mentioned above have become the impulse for the develop- ment of alternative solutions [2]. Currently, coatings should not only fulfil requirements mentioned above but should also be easily manufactured, demonstrate reduced material usage and installation costs and exhibit enhanced durability. Therefore a strong emphasis is given by academic institutions as well as industry sector, to investigating and developing self-healing and smart coatings, combining various func- tionalities for improved corrosion protection.

Number of publications on self-healing protective coatings in years 1984–2014, according to data provided by Scopus.
Figure 2. Global contribution to research on self-healing protective coatings, according to data provided by Scopus.

2. Literature data

Scientists are intensively trying to mimic natural processes and incorporate them into engineering materials. According to the data provided by Scopus, one of the largest abstract and citation databases of peer-reviewed literature, there have been 542 articles on self-healing protective coatings published between 1984 and November 2015. In fact, as figure1 demonstrates, the growth in the number of articles was vir- tually exponential until 2014. However there is a slight downward trend this year with 61 articles published by November 2015 some 24 fewer than at the same time in 2014. The first publications describing self-healing protective coatings appeared in the middle of the 1980s. However those articles dealt with coatings containing carcinogenic hex- avalent chromium. The self-healing action of a chromium- containing coating was based on the formation of a protective oxide film in the damage area. The presence of Cr(VI) pro- vides a self-repair mechanism by reduction to Cr(III) during oxidative attack. A breakthrough in self-healing coatings’ technology was the work of White et al The authors presented the results of research on a polymer coating with self-repair functionality resulting from the action of polymerising agents loaded into microcapsules [3]. Besides a microencapsulated healing agent, a catalytic chemical trigger was incorporated within an epoxy matrix. Embedded microcapsules were rup- tured mechanically and released the healing agent into the crack area through capillary action. The polymerisation of the healing agent was then initiated by the action of the catalyst in the epoxy coating which subsequently led to the bonding of the crack faces. Additionally a healing concept of using living polymerization catalysts—without terminated chain-ends, enabled repeated healing activity.

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