coatings Review Hybrid Sol Sol-Gel‐Gel Coatings: Smart and Green Materials for Corrosion Mitigation Rita B. Figueira 1,*, Isabel R. Fontinha 1, Carlos J. R. Silva 2 and Elsa V. Pereira 1 Rita B. Figueira 1,*, Isabel R. Fontinha 1, Carlos J. R. Silva 2 and Elsa V. Pereira 1 1 LNEC, Laboratório Nacional de Engenharia Civil, Av. Brasil 101, Lisboa 1700‐066, Portugal; 1 [email protected], Laboratório (I.R.F.); Nacional [email protected] de Engenharia (E.V.P.) Civil, Av. Brasil 101, Lisboa 1700-066, Portugal; 2 [email protected] de Química, (I.R.F.);Universidade [email protected] do Minho, (E.V.P.) Campus de Gualtar, Braga 4710‐057, Portugal; 2 [email protected] de Química, Universidade do Minho, Campus de Gualtar, Braga 4710-057, Portugal; * Correspondence:[email protected] [email protected] or [email protected]; Tel.: +351‐218‐443‐634 or +351‐966‐430‐142 * Correspondence: rmfi[email protected] or rita@figueira.pt; Tel.: +351-218-443-634 or +351-966-430-142 Academic Editor: Naba K. Dutta Received:Academic 17 Editor: December Naba 2015; K. Dutta Accepted: 11 March 2016; Published: March 2016 Received: 17 December 2015; Accepted: 11 March 2016; Published: 16 March 2016 Abstract: CorrosionCorrosion degradation degradation of of materials materials and and metallic metallic structures structures is one is one of the of major the major issues issues that givethat giverise riseto depreciation to depreciation of assets, of assets, causing causing great great financial financial outlays outlays in in their their recovery recovery and and or prevention. Therefore,Therefore, the the development development of active of active corrosion corrosion protection protection systems systems for metallic for substrates metallic substratesis an issue is of an prime issue importance. of prime importance. The promising The promising properties properties and wide and application wide application range of range hybrid of hybridsol-gel-derived sol‐gel‐derived polymers polymers have attracted have significant attracted attentionsignificant over attention recent decades. over recent The combinationdecades. The of combinationorganic polymers of organic and inorganic polymers materials and inorganic in a single materials phase provides in a single exceptional phase provides possibilities exceptional to tailor possibilitieselectrical, optical, to tailor anticorrosive, electrical, and optical, mechanical anticorrosive, properties and for diversemechanical applications. properties This for unlimited diverse applications.design concept This has unlimited led to the design development concept ofhas hybrid led to coatings the development for several of applications, hybrid coatings such for as severaltransparent applications, plastics, glasses,such as andtransparent metals to plastics, prevent glasses, these substrates and metals from to prevent permeation, these mechanical substrates fromabrasion, permeation, and corrosion, mechanical or even abrasion, for decorative and functions.corrosion, Nevertheless, or even for the decorative development functions. of new Nevertheless,hybrid products the requires development a basic of understanding new hybrid products of the fundamental requires a basic chemistry, understanding as well as of of the fundamentalparameters that chemistry, influence as the well processing as of the parameters techniques, that which influence will briefly the processing be discussed. techniques, Additionally, which willthis reviewbriefly be will discussed. also summarize Additionally, and discuss this review the most will promising also summarize sol-gel coatingsand discuss for corrosionthe most promisingprotection ofsol steel,‐gel coatings aluminium, for corrosion and their protection alloys conducted of steel, at aluminium, an academic and level. their alloys conducted at an academic level. Keywords: hybrid; coatings; sol-gel; corrosion; steel; green materials; aluminium; metallic Keywords: hybrid; coatings; sol‐gel; corrosion; steel; green materials; aluminium; metallic 1. Introduction Introduction The major degradation mechanism of metallic materials is corrosion (Figure 11).). CorrosionCorrosion cancan be generally defined defined as the reaction of a metallic material with its environment and the products of this reaction may be solid, liquid, or gaseous [1–4]. [1–4]. Both the physical and chemical properties of the products are important since they may influence influence the subsequent rate of reaction. In In the the case case of of steel, steel, −´ Cl , ,OO2,2 ,and and H H2O2O species, species, as as well well as as electron electron transport, transport, all all play play extremely extremely important important roles roles in the corrosion process [[5–9].5–9]. Figure 1. Corrosion of steel in a bridge: ( a)) general view; ( (b)) detail of the thickness of the corrosion layer formed. Coatings 2016, 6, 12; doi:10.3390/coatings6010012 www.mdpi.com/journal/coatings Coatings 2016, 6, 12; doi:10.3390/coatings6010012 www.mdpi.com/journal/coatings Coatings 2016, 6, 12 2 of 19 CorrosionCoatings 2016, prevention6, 12 in a specific environment is usually performed using a corrosion2 resistantof 19 material, or at least one with exceptional lifetime. There are several methods for protection against corrosionCorrosion that are based prevention on electrochemical in a specific environment principles is [usually9]. Alternatively, performed using some a ofcorrosion the methods resistant adopt Coatings 2016, 6, 12 2 of 19 the obviousmaterial, task or at of least separating one with the exceptional metal from lifetime. the environment.There are several The methods success for of protection this depends against upon the chemicalcorrosionCorrosion or that electrochemical are prevention based on electrochemicalin a resistance specific environment of principles the protective is usually[9]. Alternatively, layer.performed In particular usingsome a of corrosion the situations methods resistant adopt it may be possiblethematerial, obvious to adjust or task theat leastof environment separating one with theexceptional in metal order from lifetime. to the turn environment. There it less are corrosive. several The successmethods Generally, of for this protection corrosiondepends against upon prevention the chemical or electrochemical resistance of the protective layer. In particular situations it may be technologycorrosion uses that one are or based more on of electrochemical the following principles methods [9]. [2 ,Alternatively,5,7,8,10,11]: some of the methods adopt possiblethe obvious to adjust task theof separating environment the metal in order from to the turn environment. it less corrosive. The success Generally, of this corrosion depends uponprevention the 1. Adoptingtechnologychemical metalsusesor electrochemical one with or more elements of resistance the following that of enrich the methods protective the surface [2,5,7,8,10,11]: layer. with In particular a corrosion-resistant situations it may component be possible to adjust the environment in order to turn it less corrosive. Generally, corrosion prevention during1. Adopting the corrosion metals with process; elements that enrich the surface with a corrosion‐resistant component technology uses one or more of the following methods [2,5,7,8,10,11]: 2. Additionduring of the aqueous corrosion inhibitors, process; which adsorb strongly on the metal surface and prevent the reaction2. 1. AdditionAdopting with of the metalsaqueous oxidizing with inhibitors, elements agent; andwhichthat enrich adsorb the stronglysurface with on the a corrosion metal surface‐resistant and component prevent the during the corrosion process; 3. Depositionreaction of with protective the oxidizing coatings. agent; and 3. 2. DepositionAddition of protectiveaqueous inhibitors, coatings. which adsorb strongly on the metal surface and prevent the The choicereaction of with the mostthe oxidizing effective agent; method and is not always simple and easy. This choice is partly 3. TheDeposition choice of of the protective most effective coatings. method is not always simple and easy. This choice is partly governedgoverned by the by actualthe actual environmental environmental conditions conditions andand partly by by economic economic considerations. considerations. The Thelatter latter The choice of the most effective method is not always simple and easy. This choice is partly includeinclude not onlynot only the the initial initial cost cost of application application but but also also the replacement the replacement of corroded of corroded parts and, parts in some and, in governed by the actual environmental conditions and partly by economic considerations. The latter somecases, cases, renewal renewal of ofthe the protecting protecting medium. medium. These These are frequently are frequently the determining the determining factors and factors the and include not only the initial cost of application but also the replacement of corroded parts and, in some the practicing corrosion engineer will use economic parameters as familiarly as scientific data in practicingcases, renewal corrosion of the engineer protecting will medium. use economic These are parameters frequently asthe familiarly determining as factorsscientific and data the in determiningdeterminingpracticing their theircorrosion choice choice (Figures engineer (Figures2 willand 2 and 3use). 3). economic parameters as familiarly as scientific data in determining their choice (Figures 2 and 3). Figure 2. (a) In situ studies of coatings for steel protection