corrosion and materials degradation Review Review of Recent Developments in the Formulation of Graphene-Based Coatings for the Corrosion Protection of Metals and Alloys Bronach Healy, Tian Yu, Daniele da Silva Alves and Carmel B. Breslin * Department of Chemistry, Maynooth University, Maynooth, Co. Kildare, Ireland; [email protected] (B.H.); [email protected] (T.Y.); [email protected] (D.d.S.A.) * Correspondence: [email protected] Received: 26 August 2020; Accepted: 23 September 2020; Published: 25 September 2020 Abstract: Corrosion is a naturally occurring phenomenon and there is continuous interest in the development of new and more protective coatings or films that can be employed to prevent or minimise corrosion. In this review the corrosion protection afforded by two-dimensional graphene is described and discussed. Following a short introduction to corrosion, the application of graphene in the formulation of coatings and films is introduced. Initially, reduced graphene oxide (rGO) and metallic like graphene layers are reviewed, highlighting the issues with galvanic corrosion. Then the more successful graphene oxide (GO), functionalised GO and polymer grafted GO-modified coatings are introduced, where the functionalisation and grafting are tailored to optimise dispersion of graphene fillers. This is followed by rGO coupled with zinc rich coatings or conducting polymers, GO combined with sol-gels, layered double hydroxides or metal organic frameworks as protective coatings, where again the dispersion of the graphene sheets becomes important in the design of protective coatings. The role of graphene in the photocathodic protection of metals and alloys is briefly introduced, while graphene-like emerging materials, such as hexagonal boron nitride, h-BN, and graphitic carbon nitride, g-C3N4, are then highlighted. Keywords: corrosion protection; graphene; epoxy coatings; zinc-rich coatings; corrosion; coatings; conducting polymers 1. Introduction Corrosion is a naturally occurring phenomenon that results in the oxidation of metals or alloys and this is normally accompanied by dissolution of the metal or alloy. It can affect the performance of materials used in many technological fields, ranging from light-weight aluminium alloys employed in the aerospace industry [1], metals and alloys used in the electronics industry [2], transportation [3], pipeline [4], wastewater treatment [5], coatings and linings for food storage [6], pulp and paper [7], fuel cells [8] and refining industries [9]. Many of the traditional corrosion protective strategies, for example, those employing chromates, especially hexavalent chromium [10], are no longer viable due to environmental considerations. Consequently, there is an ever-increasing interest in developing new and environmentally acceptable corrosion-control technologies. The corrosion reaction can be represented as two half-cell reactions, with the oxidation reaction corresponding to the oxidation of the metal, M, Equation (1), while the reduction reaction, which consumes the electrons, is typically the reduction of oxygen at neutral pH values, Equation (2), or under acidic conditions, the reduction of H+ becomes the predominant reaction, Equation (3). n+ M M + ne− (1) ! Corros. Mater. Degrad. 2020, 1, 296–327; doi:10.3390/cmd1030015 www.mdpi.com/journal/cmd Corros. Mater. Degrad. 2020, 1 297 Corros. Mater. Degrad. 2020, 3 FOR PEER REVIEW 2 + O2 +24H ++ 4e−− 2H22O (2) O + 4H + 4e ! 2H O (2) + 2H2H++ +2e 2e− H H22 (3)(3) ! TheThe sites sites for for the the oxidation oxidation reaction reaction are are termed termed anodic anodic sites, sites, while while the the reduction reduction reactions reactions occur occur at cathodicat cathodic sites sites on aon surface. a surface. These These anodic anodic and and cathodic cathodic sites sites can can be be spatially spatially separated separated and and fixed fixed at at locations,locations, such such as as heterogeneities heterogeneities or or defects defects on on the the surface. surface. This This normally normally leads leads to to localised localised corrosion, corrosion, suchsuch as as pitting, pitting, crevice, crevice, intergranular intergranular or or galvanic galvanic corrosion corrosion [11 [11]]. A. A schematic schematic representation representation of of pitting pitting corrosioncorrosion is is shown shown in in Figure Figure1a. 1a. This This localised localised attack attack gives gives rise rise to the to formationthe formation of deep of deep pits, pits, Figure Figure1c, while1c, while the pitthe contentspit contents are are acidified acidified as aas result a result of hydrolysisof hydrolysis reactions. reactions. Alternatively, Alternatively, if if the the cathodic cathodic andand anodic anodic sites sites fluctuatefluctuate randomlyrandomly acrossacross thethe surface, then uniform uniform or or nearly nearly uniform uniform corrosion corrosion is isgenerally generally observed observed as as illustrated illustrated in in Figu Figurere 11b.b. TheseThese corrosioncorrosionreactions reactions will will propagate propagate with with the the oxidationoxidation reaction, reaction, Equation Equation (1),(1),and and reduction reduction reaction, reaction, Equations Equations(2) (2) or or (3), (3), occurring occurring until until the the conductingconducting path path is is interrupted interrupted and and this this is is one one way way in in which which the the corrosion corrosion reaction reaction can can be be halted. halted. FigureFigure 1. 1.( a()a Schematic) Schematic representation representation of of pitting pitting attack attack and and micrographs micrographs illustrating illustrating ( b()b general-like) general-like dissolutiondissolution and and (c ()c pitting) pitting attack. attack. VariousVarious coatings coatings and and surface surface modifications modifications have have been been applied applied to to limit limit this this conducting conducting pathway pathway andand protect protect the the underlying underlying substrate. substrate. These These approaches approaches include include the the deposition deposition of metallicof metallic coatings coatings or compositesor composites [12], [12] polymeric, polymeric coatings coatings [13] and [13] conversion and conversion coatings coatings [14]. Electroplating [14]. Electroplating is a well-known is a well- andknown versatile and versatile method formethod forming for metallicforming coatingsmetallic coatings and this and approach this approach is applicable is applicable to any system to any thatsystem can that be electrochemicallycan be electrochemically reduced reduced from thefrom ionic the ionic to the to metallicthe metallic state state when when present present in in the the electrolyteelectrolyte solution solution [ 15[15]].. However,However, certaincertain metalsmetals suchsuchas as aluminium, aluminium, titanium titanium and and magnesium magnesium cannotcannot be be electrodeposited electrodeposited from from aqueous aqueous solutions solutions as as the the competing competing hydrogen hydrogen ion ion reduction reduction reaction reaction isis thermodynamically thermodynamically favouredfavoured andand willwill occuroccur inin preferencepreference toto thethe reductionreduction ofof thethe metalmetal ions.ions. Nevertheless,Nevertheless, this this can can be be achieved achieved in in ionic ionic liquids liquids or or in in conducting conducting organic organic electrolytes electrolytes where where the the hydrogenhydrogen ion ion reduction reduction reaction reaction is is negligible negligible [16 [16].]. Polymeric Polymeric coatings, coatings, which which include include paints, paints, lacquers lacquers andand varnishes, varnishes, normally normally protect protect the the substrate substrate from from corrosion corrosion by by acting acting as as a a barrier, barrier, preventing preventing the the reactants,reactants, water, water, oxygen oxygen and and ions, ions, from from reaching reaching the the protected protected substrate. substrate. The The protective protective properties properties of theseof these coatings coatings typically typically improve improve as the as thickness the thickness of the coating of the iscoating increased, is increased, while fillers while and fillers pigments and canpigments be added can to be increase added to the increase diffusion the path diffusion for water path and for oxygen.water and Conversion oxygen. Conversion coatings is coatings the term is usedthe term to describe used to coatings describe where coatings the where substrate the takessubstrate part takes in the part coating in the formation coating formation reactions, reactions, with the bestwith known the best being known the commercialbeing the commercial phosphate phosphate conversion conversion coatings. coatings. InIn more more recent recent years, years, there there has has been been considerable considerable interest interest in in using using graphene graphene to to provide provide corrosion corrosion protectionprotection and and it it has has been been employed employed to to provide provide protective protective metallic metallic coatings coatings or or composites composites and and also also addedadded to to various various polymeric polymeric coatings. coatings. The The level level of of interest interest in in graphene graphene in in the the field field of of corrosion corrosion science science isis readily readily evident evident from from Figure Figure2, 2 with, with the the number number of of publications publications