Anticorrosion thin film smart coatings for aluminum alloys

CHAPTER 16

Anticorrosion thin film smart coatings for aluminum alloys Tiago L.P. Galvão1, Anissa C. Bouali2, Maria Serdechnova2, Kiryl A. Yasakau1, Mikhail L. Zheludkevich2 and João Tedim1 1

Department of Materials and Ceramic Engineering, CICECO-Aveiro Institute of Materials, University of Aveiro, Aveiro, Portugal Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung, Geesthacht, Germany

2

Contents 16.1 Introduction 429 16.2 State-of-the-art of surface thin films or pretreatments in transportation industries 430 16.2.1 Sol gel technologies with controlled release additives 432 16.2.2 Anodic films and LDH pretreatments with controlled release of corrosion inhibitors 440 16.2.3 Chitosan and other environmentally friendly approaches as polymer reservoirs for anticorrosion applications 445 16.3 Conclusion 449 References 449

16.1 Introduction The use of coatings for corrosion protection is among one of the most widely used approaches to limit the degradation of active metal alloys when exposed to different environmental conditions. The level of protection required varies depending on the application under consideration, with the protection of structural parts and components used in transportation industries being among the most complex and challenging. The design and development of Cr(VI)-free coatings are currently posing several challenges to original equipment manufacturers (OEMs) since legal restrictions are imposed on the use of chromate-based conversion coatings. While in the aeronautical industry it is still possible to use these pretreatments, REACH regulations have banned their use in the automotive sector. It is expected that the aeronautical industry will follow suit Advances in Smart Coatings and Thin Films for Future Industrial and Biomedical Engineering Applications DOI: https://doi.org/10.1016/B978-0-12-849870-5.00007-0

© 2020 Elsevier Inc. All rights reserved.

429

430

Advances in Smart Coatings and Thin Films

despite a recent extension. Therefore surface pretreatment and coating producers are racing against time to find appropriate solutions capable of performing as well as Cr(VI)-based systems. In this chapter, we will address the state-of-the-art as well as new developments concerning new films directly grown or applied onto the metal surface as a first protective layer. Of particular relevance are the socalled smart coatings, that are responsive coatings which actively protect the metallic substrate upon external stimuli. The chapter is structured into three main sections. The first one succinctly describes the current technologies available in the transportation industry regarding thin films and pretreatments. The second addresses new concepts and materials that can impart responsive feedback and, therefore, a high protection level under demanding conditions with a particular focus on hybrid sol gel, anodic and chitosan-based coatings. The third and final section summarizes some future trends as well as some challenges associated with the industrial implementation of smart thin coating films.

16.2 State-of-the-art of surface thin films or pretreatments in transportation industries Pretreatments are commonly applied to metallic substrates with the main aim to enhance adhesion and stability of the interface between the metal and the subsequent polymer layers. Aluminum, in particular, is pretreated for most applications using chemical conversion or thin-layer application approaches. Moreover, different anodization approaches are widely used in industry and can also be classified as electrochemical conversion processes. Depending on the electrolyte composition and electrical parameters the obtained anodic layers have different pore structures and can offer various properties such as an additional barrier or good adhesion to the adhesive or polymer coatings. The main chemistries of the electrolytes currently used for the anodization of Al-based substrates include chromic acid, sulfuric acid, phosphoric acid, and some organic acids such as tartaric acid. Traditionally used chromic acid anodizing (CAA) is currently going through a progressive ban in various countries due to the serious environmental and health issues associated with Cr(VI) compounds. Sulfuric acidbased electrolytes can be used to produce pretreatments before painting or as standalone coating systems, depending on the process parameters. Tartaric sulfuric acid anodizing (TSA) has become commonly accepted by the aeronautical industry as a pretreatment before the application of a

Anticorrosion thin film smart coatings for aluminum alloys

431

polymer coating system, while phosphoric acid-based processes (phosphoric acid anodizing [PAA], phosphoric/sulfuric acid anodizing [PSA]) are more commonly used to form highly developed porosity, which is essential to ensure high adhesion for adhesive bonding. The anodic coatings are addressed in more detail in section 16.2.2. Aluminum is considered to be a part of complex multimaterial structures mainly utilized in aeronautical and automotive designs. In the case of aircraft structures, different materials are pretreated separately before assembly, while multimaterial car bodies are subjected to the commonly used automotive pretreatment processes such as phosphotation [1]. The suggested tricationic phosphotation ensures acceptable coverage of multimaterial assemblies, although adequate surface cleaning and activation is needed [2]. Deposition of a manganese phosphate coating on aluminum under electrochemical polarization was also demonstrated as a promising approach [3]. However, currently the automotive industry is considering the movement toward phosphate-free surface treatment technologies. Zirconate or titanate based layers have recently been considered as potential alternatives to the phosphate treatment in the automotive industry. The formed thin-oxide layers can provide good adhesion to the applied e-coat. The used baths usually contain fluorozirconates or fluorotitanates or a mixture of both. Additionally, organic additives are often added in order to improve corrosion resistance as well as a small amount of hydrofluoric acid, which activates the surface [4]. The obtained coatings usually consist of two layers, namely the inner aluminum oxide layer (60 90 nm) and the porous outer layer (30 nm) composed of hydrated ZrO2 or TiO2 [5]. The respective conversion process is considered to be electrochemically driven. Therefore the presence of active cathodic intermetallics at the aluminum alloy surface plays an essential role for the formation of a uniform continuous conversion layer. At the initial stage the surface is activated by the fluorides, which attack the native oxide film. During the subsequent stages the conversion layer grows laterally, spreading from the cathodic intermetallics where increased pH values are observed [6]. The Ti or Zr based conversion layers can also be applied to other metallic substrates in multimaterial assemblies. Such pretreatments are also relevant for high-strength aluminum alloys used in aeronautical structures. The coating can be applied by dipping or spraying. Lia et al. [5] compared the commercial Zr/Ti-based conversion coating (Bonderite M-NT 5200, Henkel Corp.) on AA2024-T3 with the one based on Cr (III) chemistry (TCP; trivalent chromium process). It was demonstrated

432

Advances in Smart Coatings and Thin Films

that the 5200 provides lower corrosion inhibition potential than that of TCP coating (Bonderite M-CR T5900, Henkel). TCP pretreatments are currently among those that are close to acceptance by aircraft manufacturers as an alternative for chromate-based pretreatments. TCP conversion layers normally are part of the fluorozirconate family [7]. However, the thickness of the formed layer is considerably higher than that of the fluorozirconate pretreatments. The layer is normally composed of particles with B100 nm diameter, similar to chromate conversion films [8]. The Cr (III) species additionally provide an active protection effect by increasing the breakdown potential and inhibiting the cathodic oxygen reduction reaction. However, the inhibiting action of Cr(III) is currently discussed to be related with the presence of a certain amount of Cr(VI) species generated as a result of redox processes with components surrounding the environment. Qi et al. have clearly shown a presence of up to 1% Cr(VI) species in a TCP layer and provided evidence of the role of oxygen in Cr (VI) formation using Raman spectroscopy measurements [9]. Therefore the environmental and health concerns known for Cr(VI) can also be partially relevant for TCP-based conversion layers as well. Another alternative pretreatment that can confer an additional inhibition effect, is based on rare earth (RE) compounds, specifically Ce chemistry. RE-cations are known to confer an efficient inhibition of corrosion processes on aluminum alloys, especially those with active cathodic intermetallics. The revolutionary works of Hinton suggested cerium-based conversion layers as pretreatments [10], although the kinetics of the conversion process driven by the electrochemical processes at the alloy surface was found to be relatively slow, thereby limiting its application for transportation industries. Later versions include the addition of H2O2 in order to accelerate the conversion layer formation [11]. Another type of alternative pretreatment is based on sol gel siloxane chemistry and can ensure reasonable barrier properties together with acceptable adhesion to the polymer coatings. Some sol gel products are widely applied in the aircraft industry, such as those originating from the Boeing patent [12] licensed by a number of companies worldwide. The deeper focus on the sol gel-based layers is given in section 16.2.1.

16.2.1 Sol gel technologies with controlled release additives Sol gel coatings are one of several pretreatments particularly developed for aluminum alloys in substitution of the chromate-based ones [13]. Such

Anticorrosion thin film smart coatings for aluminum alloys

433

pretreatments play a significant role in the multilayer corrosion protection scheme in which adhesion and stability of the interface between the metal and top organic coatings is of crucial importance. The nature of sol gelbased materials is such that it permits the development of chemical bonds with the metallic surface by forming stable oxygen bonds, for instance M O Si [14]. The formed chemical bonds provide a strong adhesion toward the substrate, thereby efficiently binding the sol gel coating to the metal surface. According to the second role of pretreatment, which is to ensure adhesion toward organic layers, strong adhesion is also created with organic functional groups, which are typically present in organic primer coatings [15]. There are different organic groups capable of chemical reactivity such as epoxy, acrylate, amine, and isocyanate are typically contained in organic polymeric coatings. A type of sol gel which contains both inorganic and organic groups is called a hybrid sol gel and is mostly acknowledged for its adhesion, functional compatibility, mechanical properties, and corrosion resistance [16]. The presence of organic groups in a sol gel matrix greatly improves the ability of the sol gel coating to withstand excessive stress upon drying of the sol gel coating. Sol gel coatings possess good barrier properties at low thickness, thereby making them good candidates for corrosion protective coatings. However, the efficiency can be lost upon the breakdown of the coating, which will undoubtedly lead to the development of corrosion. In some specific cases where a sol gel coating can be modified by healing polymeric materials [17] the damage done to a sol gel coating can be healed, thus allowing for the recovery of coating barrier properties. If the damage has not been healed and corrosive electrolyte borders the metal surface causing corrosion attack on the metal, a standalone sol gel coating cannot provide efficient self-healing of the corrosion process. Thus in order to impart active corrosion protective functionality to a sol gel, corrosion inhibitors must be included in the sol gel matrix. Several criteria necessary for selection of corrosion inhibitors can be stated, including: (1) a corrosion inhibitor should be highly effective in corrosion inhibition of the metal being protected; (2) inhibitors must have little or no chemical interactions with the sol gel matrix; (3) the content of the inhibitor in the coating matrix should be lower than the critical concentration of inhibitor; and (4) an inhibitor should have enough solubility in a corrosive environment to cause an inhibiting effect and have controlled kinetics of release from the coating.

434

Advances in Smart Coatings and Thin Films

In the course of the synthesis process the inhibitor’s molecules can chemically interact with the sol gel precursors resulting in the improper build-up of the matrix and poor protective nature of the coatings [18]. A poor stability of the coating matrix could be caused because of chemical interactions with inhibitors, particularly with organic inhibitors, and excessive concentration of the inhibitor beyond the critical inhibitor concentration point [19]. Usually, the high solubility of corrosion inhibitors in a sol gel matrix limits the maximal concentration of the inhibitor (often much less than 1 wt.% in the dry coating) in the matrix and, because of that, the overall corrosion protection rendered by the coating remains low [19,20]. At the same time the release should be as minimal as possible under the conditions when there is no corrosion process at the metal surface. Clearly, inhibitive pigments comprised of inorganic compounds having a low-solubility product may be used as inhibitive additives, although the main problem is the large size of such pigments, which are often higher than the thickness of sol gel coatings (typically 1 10 μm). The last criterion may be satisfied by preparing sparingly soluble corrosion inhibitors with a sufficiently small size and avoiding agglomeration at the stage of synthesis and coating application. A few examples where poorly soluble inorganic compounds have been used as corrosion protective pigments in sol gel coatings can be found in Refs. [21,22]. In the latter work the cerium molybdate nanowires (CMN) pigment was synthesized and mixed with the sol gel matrix made of zirconium(IV) propoxide and 3-glycidoxypropyltrimethoxysilane precursors. Sol gel coatings were produced by a dip-coating method on AA2024 aluminum alloy coupons. Electrochemical methods of analysis coupled with microscopy observations revealed that the coatings containing CMN pigment possessed high-corrosion protection efficiency. Scanning vibrating electrode technique (SVET) analysis (Fig. 16.1) displays lower local ionic currents measured across the two circular defects made in the (A) bare sol gel and (B) CMN containing sol gel coatings. A SVET study coupled with other chemical analysis methods demonstrated an efficient protection of AA2024 by the sol gel coatings. The release rate of inhibitive species from CMN powder was low in a diluted NaCl solution due to the low solubility of the nanowires. On the other hand, Ce31 release from CMN depends on the concentration of the aggressive ions, that is, Na1, and increases with an increase of the sodium cations [22]. Although sparingly soluble compounds may demonstrate efficient corrosion performance when incorporated into protective coatings, the

Anticorrosion thin film smart coatings for aluminum alloys

435

Figure 16.1 Optical photographs, SVET maps, and selected profiles of ionic currents representing the maximal anodic and cathodic activities at artificial defects during 7 days of immersion in a 0.5 M NaCl corrosive medium. (A) Blank sol gel coating, and (B) sol gel coating doped with cerium molybdate nanowires (CMN) (Sg_CMNx2). Reproduced with permission from K.A. Yasakau, S. Kallip, M.L. Zheludkevich, M.G.S. Ferreira, Active corrosion protection of AA2024 by sol gel coatings with cerium molybdate nanowires, Electrochim. Acta 112 (2013) 236 246.

inhibitor release from such systems may be not sufficient for efficient long-term protection. On the other hand, a hefty proportion of soluble inorganic and organic corrosion inhibitors cannot be simply mixed with the sol gel matrix due to the high tendency for associated damage to the matrix. Therefore a more sophisticated approach employs specifically designed nanocontainers and nanocomposites that may freely carry corrosion inhibitive molecules. Carriers that hold inhibitive molecules by adsorbing them on the surface include oxide nanoparticles, mesoporous silica [23 25], and hollow nanocontainers such as halloysites, hollow

436

Advances in Smart Coatings and Thin Films

silica, and polymeric shell nanocontainers [26,27]. Cationic and anionic exchange inorganic nanocontainers are quite unique classes of materials and are described by minerals such as zeolites, bentonites, and hydrotalcites. In such types of materials, cations or anions are held by electrostatic force of the oppositely charged walls of the inorganic materials and, thus, variously charged ions may be incorporated into such containers [28]. Polymeric shell nanocontainers can be created by using positively [poly(ethylene imine)] and negatively [poly(styrene sulfonate)] charged polyelectrolyte multilayers utilizing the so-called layer-by-layer (LbL) process [29]. One of the advantages of this process is that various inorganic and organic inhibitors can be incorporated between the polyelectrolyte layers. The important point is that different factors such as local changes of pH, local changes of ionic strength, temperature, magnetic field, and light can be relevant triggers for opening the polyelectrolyte layer and releasing the inhibitive molecules to the damaged site or to the site subjected to the “trigger” event. The assembly of nanocontainers and the sol gel matrix shown in Fig. 16.2 demonstrate a self-healing mechanism triggered by pH increase in the place of cathodic activity on the alloy surface. The release of an inhibitor in a cathodic zone reduces the overall

Figure 16.2 Scheme of the controllable release of the inhibitor from the LbL nanocontainers and the “smart self-healing” process. Reproduced with permission from M.L. Zheludkevich, D.G. Shchukin, K.A. Yasakau, H. Möhwald, M.G.S. Ferreira, Anticorrosion coatings with self-healing effect based on nanocontainers impregnated with corrosion inhibitor, Chem. Mater. 19 (3) (2007) 402 411.

Anticorrosion thin film smart coatings for aluminum alloys

437

corrosion rate, as was demonstrated by measuring the ionic current density maps on the surface of AA2024 using the SVET technique [30]. Mesoporous silica nanoparticles have a number of advantages compared to bulk oxide nanoparticles. Mesoporous containers have an increased surface area, easy surface functionalization, and relatively easy manufacturing process. Borisova et al. [24] investigated the influence of concentration of mesoporous silica nanocontainers loaded with 2-mercaptobenzothiazole (MBT) corrosion inhibitor in a hybrid epoxy-based sol gel coating. It has been demonstrated that at too low (0.04 wt.%) or too high (0.8 1.6 wt.%) concentrations of the nanocontainers inhibiting the performance of the coatings was poor. In the former case the inhibitor concentration was apparently too low as it was not sufficient to provide any effective inhibition. In the latter case, too many nanocontainers significantly deteriorated the sol gel matrix causing significant corrosion attack. Thus there is an optimum concentration of nanocontainers (about 0.7 wt.% in the coating matrix) at which the corrosion performance of the coating system is at the highest level, as determined with electrochemical impedance spectroscopy (EIS) and SVET analysis. A manufacturing process of hollow silica nanocontainers (HMSNs) having controllable pH dependent release of inhibitors has been presented by Chen and Fu [31]. HMSNs were prepared following several steps: first synthesizing hematite spheres as a template material and then coating them with silica layer, afterwards dissolving the hematite template and then modifying the outer shell. After the modification process the resulting nanocontainers were loaded with benzotriazole (BTA). At the end of the process the containers containing the inhibitor were capped with α-cyclodextrin. Release studies displayed a well-defined release of BTA, which increased either at more acidic or more alkali pH. A zirconia silica sol gel formulation was mixed with either type of HMSNs and applied onto AA2024. Electrochemical tests employing electrochemical impedance spectroscopy were performed on the coated substrates and provided evidence of active corrosion protection of the coatings containing HMSNs. Another type of nanocontainers comprises polymeric rigid core shell type capsules which can be prepared by polymerization methods at the oil water interface of emulsion droplets. The polymeric shell is more rigid and lacks somewhat intelligent release properties compared with the LbL coated or silica modified capsules. Nevertheless such an approach has found many applications in the design of self-healing coatings [32]. In

438

Advances in Smart Coatings and Thin Films

application to corrosion protection of metals the use of polymeric capsules is viable within certain conditions. Sol gel coatings require a small size of capsules, otherwise it can disrupt the coating matrix. An approach using polymeric capsules for the storage and release of MBT corrosion inhibitors was recently reported by Maia et al. [33]. Polyurea microcapsules (PUMC) were synthesized following an oil-in-water microemulsion. MBT was dissolved in the oil phase during the synthesis step and was incorporated in the capsules upon the polymerization of the shell. These capsules were subsequently added to a protective sol gel coating applied onto AA2024 (Fig. 16.3). The corrosion protection properties at localized

Figure 16.3 A scheme for the incorporation of polyurea capsules to a sol gel coating and SVET maps after 72 h of immersion in 0.05 M NaCl and ionic current profiles of a blank coating (left) and a coating containing capsules (right). Reproduced with permission from F. Maia, K.A. Yasakau, J. Carneiro, S. Kallip, J. Tedim, T. Henriques, et al., Corrosion protection of AA2024 by sol gel coatings modified with MBT-loaded polyurea microcapsules, Chem. Eng. J. 283 (2016) 1108 1117.

Anticorrosion thin film smart coatings for aluminum alloys

439

defect sites were examined by SVET. The sample with MBT PU microcapsules clearly performed better than the others as the maximum anodic current density slightly decreased to 3.4 μA/cm2, while the current density for the reference sol gel coating increased up to 27.4 μA/cm2 (Fig. 16.3). The systems presented show, in spite of being efficient additives in the active corrosion protection of aluminum, certain challenges when considering industrial applications, namely, lack of mechanical or chemical stability or a complex preparation process, among others. Certainly, for the large-scale production of nanocontainers a more efficient and robust process has to be developed. Halloysite nanotubes may be considered as viable and inexpensive inorganic nanocontainers which exist in nature. Halloysites are two-layered aluminosilicates with a hollow tubular structure [34]. Owing to their tubular structure, a suspension of halloysites in a solution of a corrosion inhibitor allows the latter to easily enter inside the hollow space upon placing the suspension under vacuum. However, in order to ensure that the corrosion inhibitor stays inside, it is beneficial to cover halloysites with a LbL multilayer coating or using a “stopper” at both ends of the halloysite tube, as was suggested by Abdullayev et al. [35]. Cation-exchange and anion-exchange compounds have been studied as effective containers for corrosion inhibitive species in different works and are promising systems for industrial applications [28,36 38]. In the context of sol gel coating technology, ion exchange containers are able to enhance corrosion protective properties of the coatings by providing controllable release of inhibitive species. The concentration of the released substance is determined according to the ion exchange equilibrium between the species in solution and the adsorbed ones. In general there are more anionic inhibitors (including many organic compounds and inorganic anions) than cationic ones (mainly precipitating inhibitors from the group of lanthanides). Therefore anion exchanging hydrotalcite-type materials such as layered double hydroxides (LDH) are a more promising and versatile type of inhibitor containers. The main challenges in the preparation of sol gel coatings containing inorganic powders are agglomerates formation and compatibility of LDH pigment with the coating system [36,38]. Sol gel coatings containing LDH/inhibitor additives are able to restrain the aggressive anions existing in a corrosion solution and suppress corrosion activity at the metal surface by the released corrosion inhibitors. Having inhibitor loading and a relatively easy and inexpensive

440

Advances in Smart Coatings and Thin Films

manufacturing route, LDH’s have high potential as promising intelligent nanocontainers for building smart protective coatings [39].

16.2.2 Anodic films and LDH pretreatments with controlled release of corrosion inhibitors As mentioned earlier, anodizing processes are currently well-established in industry for diverse applications, especially in transportation. In general, anodizing involves the growth of a metal-oxide film by passing an electrical current through an acid or alkaline bath in which the metal part is immersed [40]. Several metals, namely aluminum, magnesium, titanium, and their alloys can be used for anodizing. However, only aluminum has found a large-scale usage, since the process parameters for its anodization can be tuned into producing a wide variety of coating properties [41]. Although lightweight metals, such as aluminum alloys, are largely used due to their intrinsic properties, anodizing treatments provide additional superior performance mainly in terms of corrosion and wear resistance [42 44]. Besides conventional anodizing, the plasma electrolytic oxidation (PEO) process has also been intensively used for light metals over the past decade [45]. The main difference between PEO and conventional anodization depends on the applied voltage. Indeed, the PEO process calls for significantly higher voltages leading to the generation of micro-arc discharges that break down the natural oxide layer of the metal. Due to the extreme conditions governing the PEO process (i.e., temperature and pressure), thick and hard ceramic-like coatings with attractive abrasion and corrosion resistance are produced. Contrary to conventional anodizing, the PEO coatings are characterized by a less homogeneous structure with relatively higher porosity [40]. The durability and longevity of both conventional anodic and PEO coatings are in a direct relation with the access and absorption of aggressive chemicals from the surrounding environment through the pores, discharge channels, or other generated defects during or after processing. To reduce this effect, two main approaches have been reported in the literature. The first, which concerns mostly the PEO coatings, directly affects the morphology of the final coating. It involves optimization of the processing parameters in terms of electrolyte concentration and composition (including addition of particles) on the one hand, and temperature, voltage, and current on the other hand [44 46]. The second approach utilizes

Anticorrosion thin film smart coatings for aluminum alloys

441

the sealing posttreatment to further improve the corrosion resistance of the obtained anodic/PEO coatings. Immersion in boiling hot water or steam is among the most commonly used sealing posttreatments, whereby oxide (such as aluminum oxide) is converted into its hydrated form, boehmite, and the resulting swelling product fills up the pores improving the barrier properties [47] (Fig. 16.4). However, this method cannot offer additional active corrosion protection. Low-temperature sealing methods, on the other hand, occur by an impregnation mechanism. They involve the use of chemicals such as dichromate, nickel acetate, and cold nickel fluoride that present higher corrosion performance and can be stacked in the pores of the anodic layer [48]. Yet, these posttreatments need to be replaced due to environment and health concerns. As mentioned earlier, chromate-based compounds are being banned not only for postsealing, but for anodizing electrolytes as well, and they are being replaced in some industries by REACHconforming alternatives based on tartaric (TSA) or phosphoric (PSA) sulfuric acid anodizing [49]. Currently, a number of works reporting on the progress in the development of novel green solutions for sealing anodized/PEO coatings can be found. The first direction is addressed to the utilization of rare earth metal salts (REMS). The utilization of REMS for the corrosion protection of aluminum alloys (AA2024, AA6061, and AA7075) has been successfully

Figure 16.4 Schemes representing (A) an anodized AA2024 without sealing, (B) with hot water/steam sealing, (C) with REMS (cerium) sealing, and (D) with organic coating and inhibitors sealing.

442

Advances in Smart Coatings and Thin Films

demonstrated in many papers and have been suggested as potential candidates for the substitution of chromate salts. Cerium salts, for example, belong to the class of REMS and are widely used in the surface treatment for corrosion protection of AA2024 alloy [50,51]. It was suggested that the increase of local pH around the intermetallic (S-phase) due to cathodic processes lead to the formation of cerium hydroxide deposits, which, in turn, hinder the corrosion process [51]. Similar corrosion resistance improvement was achieved by adding RE compounds into a sealing solution for anodized aluminum alloys [48,52] and anodized magnesium alloys [53,54]. The pores and defects of the anodic film would be considered as microunits for inhibitor storage in this case (Fig. 16.4). In a different manner, the plugging of pores in the anodic layer can be achieved by means of organic or hybrid coatings. The present design has the advantage of combining the adhesion properties offered by the anodic film with the barrier and sealing character of the organic coating [55 57]. For this purpose, some authors have used sol gel films, resulting on a smooth glasstype layer, effective against corrosion protection [57], or as an entrapment layer to inhibitors that were absorbed into the pores and defects of the anodic coating, hence providing additional active corrosion protection in the case of scratches or external impacts [58]. More recently, an entirely new sealing method has been introduced. It consists of the use of hydrotalcite-like structures known as LDH. These clay-based lamellea can exist either as powders or as conversion films directly grown on the surface of metallic substrates. LDH-based additives or pigments are well-known systems with a proven capacity to work in several polymer matrices and release, on demand, corrosion inhibitors, as described in previous works [59,60]. In spite of some early attempts to grow LDH layers directly from the substrate with the ability to release corrosion inhibitors on demand as reported in the early 2000s [61], only one decade later the controlled release ability of LDHs directly grown on the surface of metal substrates has been shown in the works of Tedim and Zheludkevich (Fig. 16.5). According to these studies, it is possible to grow and intercalate corrosion inhibitors that confer active corrosion protection to the substrate, even in conditions where defects have been inflicted [62 64]. Although LDH conversion films present remarkable corrosion protective features, which equal or even surpass some existing commercial solutions, it must be optimized to take into account the control of surface coverage and adhesion

Anticorrosion thin film smart coatings for aluminum alloys

443

Figure 16.5 (A) SEM images of bare substrate, and (B) Zn Al LDH intercalated with V2O74 . Schematic presentation of selectively deposited LDH films. Reproduced with permission from J. Tedim, M.L. Zheludkevich, A.N. Salak, et al., Nanostructured LDHcontainer layer with active protection functionality, J. Mater. Chem. 21 (39) (2011) 15464 15470.

properties to become a true contender against existing commercial products. For the sealing of anodic layers it is more interesting to obtain an in situ LDH conversion film directly grown on the pores and defects of the anodic film since its fabrication engages a direct reaction with the metal matrix (that is the precursor), resulting in an improved adhesion between the LDH and anodic layer due to the strongly formed chemical bonds. For example, Zhang et al. [65] managed to grow ZnAl LDH intercalated with laureate anions directly by simple anion exchange of laureate with ZnAl LDH NO3. The films were fabricated on a porous anodic alumina/aluminum (PAO/Al) substrate. The aim was to make the LDH film superhydrophobic, which consequently acquired a much better corrosion resistance than anodic PAO film alone or only sealed with ZnAl LDH NO3. In order to achieve the same superhydrophobicity, Li et al. [66] went further and investigated the role of various cations (Mg21, Co21, Ni21, and Zn21) besides the intercalation of 1H, 2H, 2Hperfluorodecyltrimethoxysilane (PFDTMS). All four mixtures showed distinct LDH morphologies and displayed good superhydrophobic properties.

444

Advances in Smart Coatings and Thin Films

Figure 16.6 Photographs of samples (A) HWS, (B) LDH NO3, and (C) LDH VOx (60 min treatment) after 168 h exposure to salt-spray tests according to ISO 9227. Reproduced with permission from B. Kuznetsov, M. Serdechnova, J. Tedim, M. Starykevich, S. Kallip, M.P. Oliveira, et al., Sealing of tartaric sulfuric (TSA) anodized AA2024 with nanostructured LDH layers, RSC Adv. 6 (17) (2016) 13942 13952.

More recently, Kuznetsov et al. [67] relied on the concept of the “smart” release of corrosion inhibitors to achieve active corrosion protection of the TSA anodized AA2024. In brief, ZnAl LDH was obtained by hydrothermal treatment of TSA anodized AA2024 samples in a Zn21containing bath. The resulting ZnAl LDH films were loaded with vanadate as a well-known corrosion inhibitor for AA2024. The corrosion evaluation revealed a noticeable improvement in corrosion resistance compared to traditional hot water sealing (HWS) (Fig. 16.6). Similarly, Mata et al. [68] suggested an even more competitive system to HWS. The TSA sealing approach was based on a low-temperature growth of hierarchically organized LiAl LDHs that were further loaded with inhibiting vanadate species. The structure, morphology, and corrosion assessment showed a direct relation between the well-densely packed LDH flakes at the porous oxide layer plus a drastic barrier property enhancement of TSA. Alongside conventional anodizing, a promising outcome from the sealing of PEO layers using in situ grown LDH flakes was attained. The combination of the passive and active properties coming from PEO and LDH, respectively, results in an overall enhanced corrosion resistance. One of

Anticorrosion thin film smart coatings for aluminum alloys

445

the earliest works on in situ ZnAl LDH growth on silicate-phosphatebased PEO coated AA2024 was proposed by Serdechnova et al. [69]. This work was an extension of the one described on TSA anodized AA2024 [67] and presented the active anticorrosion potential of LDH for PEO coated aluminum alloys. Furthermore, Chen et al. [70] reported similar results obtained on a silicate-based PEO treated AA2024. Nonetheless, after a thorough investigation of ZnAl LDH growth on four voltages controlled PEO coatings (350 V, 400 V, 450 V, and 500 V), Mohedano et al. [71] discovered that under higher voltages (thicker and more stable PEO is formed) LDH seem to form the least. The explanation of this effect was associated with the mechanism of LDH formation and is strongly dependent on the liberation of Al(OH)21 cations from the PEO coating. Overall, LDHs possess complex, but extremely attractive features that can be resumed by the microscopic control of their layered chemical composition, the nature of the interlayer anions, as well as the possibility to vary their crystallite size and composition [72], placing them in the list of the most technologically promising inorganic layered materials.

16.2.3 Chitosan and other environmentally friendly approaches as polymer reservoirs for anticorrosion applications Chitosan (CTS) is a green material that has the economic advantage of reducing the future costs of environmental issues. Its application is the result of material scientists having to find sustainable materials that contribute to protect the fragile ecological equilibrium of our planet, which is being exhausted by the growing number of inhabitants. In this section we show the work done to make CTS a high-performance coating to extend the lifecycle of everyday structures. CTS is a polymer-based coating that consists of naturally occurring compounds. It is part of an effort to use safer corrosion protective technologies, such as incorporating less-toxic corrosion inhibitors and relying on water-based coatings to reduce VOC emissions. Chitin is a long-chain polymer of N-acetylglucosamine, which can be deacetylated to obtain CTS. Depending on the degree of deacetylation of chitin, the polymeric structure of CTS is composed of randomly intercalated deacetylated (D-glucosamine) and acetylated (N-acetyl-D-glucosamine) units. Chitin is the second-most abundant naturally occurring polysaccharide, after cellulose, despite being produced annually as much as

446

Advances in Smart Coatings and Thin Films

the last, which points to the cost effectiveness of using CTS. Moreover, CTS is also toxicologically safe, biodegradable, and biocompatible [73]. These properties make CTS a suitable candidate to be used as a coating material in fields where biocompatibility is of uttermost importance, such as for food packaging and biomedical applications. Indeed, its antifungal and antimicrobial properties, together with the possibility to regulate its permeability to carbon dioxide and oxygen, make it ideal to substitute synthetic materials of unnatural provenience in food packaging [74]. Besides promoting cell growth, it has mechanical properties similar to those of bones, such as high compression strength and elastic modulus [75], which led material scientists to use it for biomedical applications. Using CTS as a coating for alloys used in dental, orthopedic, and other implantable devices allows to make them more biocompatible and corrosion resistant [76]. To the best of our knowledge, Sugama and Cook were the first to apply CTS as a protective coating for metallic alloys [77]. However, after depositing and drying CTS on the metallic substrate, they verified that the coating adsorbed water from the atmosphere forming a biodegradable film, which otherwise would be inadequate for corrosion protection. To address this issue, Sugama and Cook modified the structure of CTS. Since CTS is a positively-charged polyelectrolyte soluble at pH bellow 6.5, Sugama and Cook used a negatively-charged polyelectrolyte to modify its molecular configuration and improve the thermal behavior, water adsorption, and interfacial properties, thereby making it suitable for the corrosion protection of aluminum. Another important factor for the use of CTS in biomedical applications was its ability to trap active organic compounds, such as antibiotic and antiinflammatory drugs [76], enabling their controlled delivery when needed. This ability to immobilize other agents led Kumar and Buchheit [78] to implement a strategy that constitutes the basis of self-healing coatings, which entails the immobilization of corrosion inhibitors (vanadate, in the early study of Kumar and Buchheit) onto the CTS host structure to be released upon the onset of corrosion. Once corrosion of AA2024T3 started and the pH increased, vanadate was released to inhibit corrosion. Interestingly, they also verified that when the pH was readjusted to a lower value, CTS was able to reabsorb vanadate onto its structure, probing the reversibility of the protection mechanism. Kumar and Buchheit were also pioneers in functionalizing the surface of CTS, with (3-glycidoxypropyl)-trimethoxysilane (GPTMS), to promote its adhesion to the substrate.

Anticorrosion thin film smart coatings for aluminum alloys

447

Taking advantage of the promising self-healing ability of CTS, but also considering its high permeability to water and other small molecules [79] affecting its barrier properties [80], Zheludkevich et al. [81] designed a coating solution that did not need the chemical modification of CTS. They employed CTS as a thin film directly onto the substrate, which was capable of immobilizing a corrosion inhibitor (cerium (III)) and ensure its prolonged delivery once corrosion started. On top of this thin film, a hybrid sol gel layer was employed to provide enhanced barrier properties to slow the diffusion of aggressive species. It was demonstrated that the immobilization mechanism of the cerium inhibiting cations was by complexation onto the chitosan structure, while the electrochemical study revealed the active inhibition mechanism for AA2024-T3. The active corrosion protection conferred by Ce31 on the CTS coating matrix was confirmed in a new study by a research group at the University of Aveiro [82]. Therein CTS was modified with 2,2,3,3-tetrafluoropropyl ether (GTFE) to provide a standalone coating solution. However, its barrier properties were not consistent with a high-performance coating, being more adequate for a temporary coating or an intermediate layer. Afterward, combining the protonation of chitosan at low pH with the high solubility at high pH of a different corrosion inhibitor, MBT, a new coating system was obtained, which was able to release the inhibitor under local acid and basic conditions once corrosion ensued [83]. In order to improve the barrier properties of CTS GTFE coatings, its surface was functionalized with different agents, poly(ethylene-alt-maleic anhydride) (PEMA) and poly(maleic anhydride-alt-1-octadecene) (POMA), while maintaining the successful impregnation of MBT into CTS [84]. The functionalization of CTS with GTSE improved the interaction of the coating with MBT, allowing it to enhance its loading content in the matrix, thus, providing a more efficient active protection of the metallic substrate. As for the interfacial modification of CTS GTSE with PEMA and POMA, it allowed an increase in the contact angle of the coating (higher hydrophobicity) resulting in stronger barrier properties. Therefore the experimental iterations developed at the University of Aveiro allowed to obtain a standalone “green” coating for AA2024, which was based on CTS GTSE MBT PEMA and CTS GTSE MBT POMA with enhanced active corrosion protection and an hydrophobic surface, thereby providing a pathway to improve the performance of CTS coatings [85].

448

Advances in Smart Coatings and Thin Films

The concept of modifying the structure of CTS coatings to improve corrosion resistance was recently applied for magnesium alloy sheets, AZ31, used in biomedical applications [86]. Pozzo et al. [86] identified the crosslinking of the CTS structure using genipin as a key factor to enhance the performance of the resulting coatings. The higher the degree of crosslinking, the lower the degree of swelling of the protective films, and the better the barrier properties. Moreover, beneficial interfacial properties, such as higher hydrophobicity, were also obtained after the modification of CTS with genipin. Another approach to address the shortcomings of CTS coatings for corrosion protection has been the use of other components to obtain higher-performance composite coatings. Some promising solutions include the use of cellulose acetate and hydroxyapatite, which allow to maintain the biocompatibility of CTS, while improving its corrosion resistance [87]. In fact, some authors have probed the role of hydroxyapatite [88], calcium phosphate [89], or functionalized multiwall carbon nanotubes (MWCNTs) [90] in reducing the porosity of CTS coatings with a direct impact in lowering the diffusion of the solvent and improving its barrier properties. Nevertheless, the most disruptive advances in CTS composite coatings involve its inclusion into polyurethane networks to obtain self-repairing coatings [91]. In conclusion, to take advantage of the biocompatibility of CTS-based coatings, it is essential to improve their barrier properties. To this effect several research groups have shown different paths, such as functionalizing the polymeric structure, improving the crosslinking, and enhancing the hydrophobicity of the surface, including CTS as an intermediate layer and/or using CTS in composite coatings (Fig. 16.7).

Figure 16.7 State-of-the-art in CTS coating developments.

Anticorrosion thin film smart coatings for aluminum alloys

449

16.3 Conclusion In this chapter, different thin smart coating films were presented. Some of these systems are already at a more mature stage of development at industrial level (e.g., sol gel technologies) whereas others can be implemented with fairly straightforward modifications in already existing processes (e.g., sealing of anodizing layers with LDHs), and some are yet to be validated at the pilot scale (chitosan). The replacement of existing industrial pretreatments is not straightforward. If it is true that the different systems presented show-controlled release ability and superior active corrosion protection, other parameters must come into play when taking them into product validation. From a simple perspective, any attempt of introducing new materials in the supply chain, requires primarily technical feasibility of the upscaling and cost-effectiveness of the proposed solutions, not to mention its incorporation and compatibility with other coating layers used in paint schemes. On the other hand, there are other performance parameters such as adhesion to the substrate and to the subsequent polymeric layers, which can impair the industrial implementation of some of these technologies. At the same time, there is still a lot of fundamental research to do when considering vehicles produced with a multitude of metals and composites. The understanding of coupled or interrelated degradation mechanisms within such structures will demand further developments in the selection of materials used as coating components on these structures. Although there is still a lot to do to validate new solutions within industrial environment, it is expected that the urgent need to find more effective and environmentally sustainable Cr(VI)-free solutions for aluminum alloys will continue to push for further developments in this area. The challenge is the combination of performance with environmental friendliness in a synergistic manner, while contextualizing the selection of new materials within important societal challenges such as critical raw materials and circular economy.

References [1] T. Sankara Narayanan, Surface pretreatment by phosphate conversion coatings—a review, Adv. Mater. Sci. 9 (2005) 130 177. [2] I. Van Roy, H. Terryn, G. Goeminne, Study of the phosphating treatment of aluminium surfaces: role of the activating process, Colloids Surf. A: Physicochem. Eng. Aspects 136 (1 2) (1998) 89 96.

450

Advances in Smart Coatings and Thin Films

[3] S. Shanmugam, K. Ravichandran, T.S.N. Sankara Narayanan, M.H. Lee, Deposition of manganese phosphate coating on aluminium: a novel electrochemical approach, in: Proceedings of the Materials Science & Technology 2014, October 12 16, 2014, Pittsburgh, USA. [4] J.H. Nordlien, J.C. Walmsley, H. Østerberg, K. Nisancioglu, Formation of a zirconium titanium based conversion layer on AA 6060 aluminium, Surf. Coat. Technol. 153 (1) (2002) 72 78. [5] L. Li, B.W. Whitman, G.M. Swain, Characterization and performance of a Zr/Ti pretreatment conversion coating on AA2024-T3, J. Electrochem. Soc. 162 (6) (2015) C279 C284. [6] F. Andreatta, I. de Graeve, H. Terryn, J.H.W. de Wit, L. Fedrizzi, SKPFM and SEM study of the deposition mechanism of Zr/Ti based pre-treatment on AA6016 aluminum alloy, Surf. Coat. Technol. 201 (18) (2007) 7668 7685. [7] C.A Matzdorf, J.L. Green, M.J. Kane, Pretreatment for aluminum and aluminum alloys, U.S. Patent No. 6,375726 (2002). [8] Y. Guo, G. Frankel, Characterization of trivalent chromium process coating on AA2024-T3, Surf. Coat. Technol. 206 (19 20) (2012) 3895 3902. [9] J.-T. Qi, T. Hashimoto, J.R. Walton, X. Zhou, P. Skeldon, G.E. Thompson, Trivalent chromium conversion coating formation on aluminium, Surf. Coat. Technol. 280 (2015) 317 329. [10] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, The inhibition of aluminum alloy corrosion by cereus cations, Met. Forum 7 (1984) 211. [11] B. Valdez, S. Kiyota, M. Stoytcheva, R. Zlatev, J.M. Bastidas, Cerium-based conversion coatings to improve the corrosion resistance of aluminum alloy 6061-T6, Corros. Sci. 87 (2014) 141 149. [12] K.Y. Blohowiak, J.H. Osborne, K.A. Krienke, Sol gel coated metal, The Boeing Company, US 5939197 (1999). [13] R. Twite, G. Bierwagen, Review of alternatives to chromate for corrosion protection of aluminum aerospace alloys, Prog. Org. Coat. 33 (2) (1998) 91 100. [14] C.J. Brinkerand, G.W. Scherer, Sol Gel Science: The Physics and Chemistry of Sol Gel Processing, Academic Press, San Diego, CA, 1990. [15] A. Sabata, W. Van Ooij, R. Koch, The interphase in painted metals pretreated by functional silanes, J. Adhes. Sci. Technol. 7 (11) (1993) 1153 1170. [16] G. Critchlow, D. Brewis, Review of surface pretreatments for aluminium alloys, Int. J. Adhes. Adhes. 16 (4) (1996) 255 275. [17] M. Abdolah Zadeh, A.C.C. Esteves, S. van der Zwaag, S.J. Garcia, Healable dual organic inorganic cross-linked sol gel based polymers: crosslinking density and tetrasulfide content effect, J. Polym. Sci. A: Polym. Chem. 52 (14) (2014) 1953 1961. [18] V. Moutarlier, B. Neveu, M.P. Gigandet, Evolution of corrosion protection for sol gel coatings doped with inorganic inhibitors, Surf. Coat. Technol. 202 (10) (2008) 2052 2058. [19] K.A. Yasakau, M.L. Zheludkevich, O.V. Karavai, M.G.S. Ferreira, et al., Influence of inhibitor addition on the corrosion protection performance of sol gel coatings on AA2024, Prog. Org. Coat. 63 (3) (2008) 352 361. [20] D. Raps, T. Hack, J. Wehr, M.L. Zheludkevich, A.C. Bastos, M.G.S. Ferreira, et al., Preparation and electrochemical study of cerium silica sol gel thin films, J. Alloys Compd. 380 (1 2) (2004) 219 224. [21] D. Raps, et al., Electrochemical study of inhibitor-containing organic inorganic hybrid coatings on AA2024, Corros. Sci. 51 (5) (2009) 1012 1021. [22] K.A. Yasakau, S. Kallip, M.L. Zheludkevich, M.G.S. Ferreira, Active corrosion protection of AA2024 by sol gel coatings with cerium molybdate nanowires, Electrochim. Acta 112 (2013) 236 246.

Anticorrosion thin film smart coatings for aluminum alloys

451

[23] M.L. Zheludkevich, R. Serra, M.F. Montemor, K.A. Yasakau, I.M.M. Salvado, M. G.S. Ferreira, Nanostructured sol gel coatings doped with cerium nitrate as pretreatments for AA2024-T3: corrosion protection performance, Electrochim. Acta 51 (2) (2005) 208 217. [24] D. Borisova, H. Möhwald, D.G. Shchukin, Influence of embedded nanocontainers on the efficiency of active anticorrosive coatings for aluminum alloys part I: influence of nanocontainer concentration, ACS Appl. Mater. Interfaces 4 (6) (2012) 2931- 2939. [25] P. Balan, R.K. Singh Raman, E.S. Chan, M.K. Harun, V. Swamy, Effectiveness of lanthanum triflate activated silica nanoparticles as fillers in silane films for corrosion protection of low carbon steel, Prog. Org. Coat. 90 (2016) 222 234. [26] C.D. Ding, Y. Liu, M.D. Wang, T. Wang, J.J. Fu, Self-healing, super hydrophobic coating based on mechanized silica nanoparticles for reliable protection of magnesium alloys, J. Mater. Chem. A 4 (21) (2016) 8041 8052. [27] D.G. Shchukin, S.V. Lamaka, K.A. Yasakau, M.L. Zheludkevich, H. Möhwald, M. G.S. Ferreira, Active anticorrosion coatings with halloysite nanocontainers, J. Phys. Chem. C 112 (4) (2018) 958 964. [28] E.L. Ferrer, A.P. Rollon, H.D. Mendoza, U. Lafont, S.J. Garcia, Double-doped zeolites for corrosion protection of aluminium alloys, Microporous Mesoporous Mater. 188 (2014) 8 15. [29] D.G. Shchukin, M.L. Zheludkevich, K.A. Yasakau, S.V. Lamaka, M.G.S. Ferreira, H. Möhwald, Layer-by-layer assembled nanocontainers for self-healing corrosion protection, Adv. Mater. 18 (13) (2006) 1672 1678. [30] M.L. Zheludkevich, D.G. Shchukin, K.A. Yasakau, H. Möhwald, M.G.S. Ferreira, Anticorrosion coatings with self-healing effect based on nanocontainers impregnated with corrosion inhibitor, Chem. Mater. 19 (3) (2007) 402 411. [31] T. Chen, J. Fu, An intelligent anticorrosion coating based on pH-responsive supramolecular nanocontainers, Nanotechnology 23 (50) (2012) 505705. [32] Y.-K. Song, C.-M. Chung, Repeatable self-healing of a microcapsule-type protective coating, Polym. Chem. 4 (18) (2013) 4940 4947. [33] F. Maia, K.A. Yasakau, J. Carneiro, S. Kallip, J. Tedim, T. Henriques, et al., Corrosion protection of AA2024 by sol gel coatings modified with MBT-loaded polyurea microcapsules, Chem. Eng. J. 283 (2016) 1108 1117. [34] R.R. Price, B.P. Gaber, Y. Lvov, In-vitro release characteristics of tetracycline HCl, khellin and nicotinamide adenine dineculeotide from halloysite: a cylindrical mineral, J. Microencapsulation 18 (6) (2001) 713 722. [35] E. Abdullayev, R. Price, D. Shchukin, Y. Lvov, Halloysite tubes as nanocontainers for anticorrosion coating with benzotriazole, ACS Appl. Mater. Interfaces 1 (7) (2009) 1437 1443. [36] M.L. Zheludkevich, S.K. Poznyak, L.M. Rodrigues, D. Raps, T. Hack, L.F. Dick, et al., Active protection coatings with layered double hydroxide nanocontainers of corrosion inhibitor, Corros. Sci. 52 (2) (2010) 602 611. [37] S.A.S. Dias, S.V. Lamaka, C.A. Nogueira, T.C. Diamantino, M.G.S. Ferreira, et al., Sol gel coatings modified with zeolite fillers for active corrosion protection of AA2024, Corros. Sci. 62 (2012) 153 162. [38] K.A. Yasakau, J. Tedim, M.L. Zheludkevich, M.G.S. Ferreira, Active corrosion protection by nanoparticles and conversion films of layered double hydroxides, Corrosion 70 (5) (2013) 436 445. [39] J. Sinko, M.W. Kendig, Pigment grade corrosion inhibitor host guest compositions and procedure. US7 (2010) 662-312 B2. [40] A. Lugovskoy, M. Zinigrad, Plasma electrolytic oxidation of valve metals, in: Y. Mastai (Ed.), Materials Science—Advanced Topics, vol. 201, InTech, 2013, pp. 85 102.

452

Advances in Smart Coatings and Thin Films

[41] S.A. Salman, M. Okido, Anodization of magnesium (Mg) alloys to improve corrosion resistance, Corrosion Prevention of Magnesium Alloys, Woodhead Publishing Series in Metals and Surface Engineering, 2013, pp. 197 231. [42] M. Mohedano, X. Lu, E. Matykina, C. Blawert, R. Arrabal, M.L. Zheludkevich, Plasma electrolytic oxidation of metals and alloys. Encyclopedia of Interfacial Chemistry, Surface Science and Electrochemistry. 2018, pp. 423 438. Available from: https://Doi.10.1016/B978-0-12-409547-2.13398-0. [43] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (2 3) (1999) 73 93. [44] X. Lu, M. Mohedano, C. Blawert, E. Matykina, R. Arrabal, K.U. Kainer, et al., Plasma electrolytic oxidation coatings with particle additions a review, Surf. Coat. Technol. 307 (2016) 1165 1182. [45] E. Matykina, R. Arrabal, P. Skeldon, G.E. Thompson, Investigation of the growth processes of coatings formed by AC plasma electrolytic oxidation of aluminium, Electrochim. Acta 54 (27) (2009) 6767 6778. [46] B. Mingo, R. Arrabal, M. Mohedano, Y.L. Lamazares, E. Matykina, A. Yerokhina, et al., Influence of sealing post-treatments on the corrosion resistance of PEO coated AZ91 magnesium alloy, Appl. Surf. Sci. 433 (2018) 653 667. [47] L. Hao, B.R. Cheng, Sealing processes of anodic coatings—past, present, and future, Met. Finish. 98 (12) (2000) 8 18. [48] F. Mansfeld, C. Chen, C.B. Breslin, D. Dull, Sealing of anodized aluminum alloys with rare earth metal salt solutions, J. Electrochem. Soc. 145 (8) (1998) 2792 2798. [49] G. Studer, J.-F. Meryet, D. Simon, Flight airworthiness support technology (FAST), Airbus Tech. Mag. 54 (2014). 23 p. [50] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, L. Wilson, XPS and SEM characterization of hydrated cerium oxide conversion coatings, Surf. Interface Anal. 23 (78) (1995) 540 550. [51] K.A. Yasakau, M.L. Zheludkevich, S.V. Lamaka, M.G.S. Ferreira, Mechanism of corrosion inhibition of AA2024 by rare-earth compounds, J. Phys. Chem. B 110 (11) (2006) 5515 5528. [52] A. Carangelo, M. Curioni, A. Acquesta, T. Monetta, F. Bellucci, Cerium-based sealing of anodic films on AA2024T3: effect of pore morphology on anticorrosion performance, J. Electrochem. Soc. 163 (14) (2016) C907 C916. [53] N. Van Phuong, B.R. Fazal, S. Moon, Cerium- and phosphate-based sealing treatments of PEO coated AZ31 Mg alloy, Surf. Coat. Technol. 309 (2017) 86 95. [54] L. Pezzato, K. Brunelli, R. Babbolin, P. Dolcet, M. Dabal, Sealing of PEO coated AZ91 magnesium alloy using La-based solutions, Int. J. Corros. 2017 (2017). [55] G.P. Shulman, A.J. Bauman, Organic acid sealants for anodized aluminum—a new method for corrosion protection, Met. Finish. 7 (93) (1995) 18 19. [56] M. Zemanova, M. Chovancova, Sol gel method for sealing anodized aluminum, Met. Finish. 101 (12) (2003) 14 16. [57] V.R. Capelossi, M. Poelman, I. Recloux, R.P.B. Hernandez, H.G. de Melo, M.G. Olivier, Corrosion protection of clad 2024 aluminum alloy anodized in tartaric sulfuric acid bath and protected with hybrid sol gel coating, Electrochim. Acta 124 (2014) 69 79. [58] J. Yang, C. Blawert, S.V. Lamaka, D. Snihirova, X. Lu, S. Di, et al., Corrosion protection properties of inhibitor containing hybrid PEO-epoxy coating on magnesium, Corros. Sci. 140 (2018) 99 110. [59] D.E. Evans, R.C.T. Slade, Structural aspects of layered double hydroxides, Struct. Bond. 119 (2005) 1. [60] P. Nalawade, B. Aware, V.J. Kadam, et al., Layered double hydroxides: a review, J. Sci. Ind. Res. 68 (4) (2009) 267 272.

Anticorrosion thin film smart coatings for aluminum alloys

453

[61] W. Zhang, R. Buchheit, Hydrotalcite coating formation on Al Cu Mg alloys from oxidizing bath chemistries, Corrosion 58 (7) (2002) 591 600. [62] J. Tedim, M.L. Zheludkevich, A.N. Salak, et al., Nanostructured LDH-container layer with active protection functionality, J. Mater. Chem. 21 (39) (2011) 15464 15470. [63] J. Tedim, A.C. Bastos, S. Kallip, M.L. Zheludkevich, M.G.S. Ferreira, Corrosion protection of AA2024-T3 by LDH conversion films: analysis of SVET results, Electrochim. Acta 210 (2016) 215 224. [64] J. Tedim, M.L. Zheludkevich, A.C. Bastos, A.N. Salak, A.D. Lisenkov, M.G.S. Ferreira, Influence of preparation conditions of layered double hydroxide conversion films on corrosion protection, Electrochim. Acta 117 (2014) 164 171. [65] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum, Angew. Chem. 120 (13) (2008) 2500 2503. [66] Y. Li, S. Li, Y. Zhang, M. Yu, J. Liu, Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy, Mater. Lett. 142 (2015) 137 140. [67] B. Kuznetsov, M. Serdechnova, J. Tedim, M. Starykevich, S. Kallip, M.P. Oliveira, et al., Sealing of tartaric sulfuric (TSA) anodized AA2024 with nanostructured LDH layers, RSC Adv. 6 (17) (2016) 13942 13952. [68] D. Mata, M. Serdechnova, M. Mohedano, C.L. Mendis, S.V. Lamaka, J. Tedim, et al., Hierarchically organized Li Al LDH nano-flakes: a low-temperature approach to seal porous anodic oxide on aluminum alloys, RSC Adv. 7 (56) (2017) 35357 35367. [69] M. Serdechnova, M. Mohedano, B. Kuznetsov, C.L. Mendis, M. Starykevichc, S. Karpushenkov, et al., PEO coatings with active protection based on in-situ formed LDH-nanocontainers, J. Electrochem. Soc. 164 (2) (2017) C36 C45. [70] F. Chen, P. Yu, Y. Zhang, Healing effects of LDHs nanoplatelets on MAO ceramic layer of aluminum alloy, J. Alloys Compd. 711 (2017) 342 348. [71] M. Mohedano, M. Serdechnova, M. Starykevich, S. Karpushenkov, A.C. Bouali, M. G.S. Ferreira, et al., Active protective PEO coatings on AA2024: role of voltage on in-situ LDH growth, Mater. Des. 120 (2017) 36 46. [72] C. Liu, D. He, Q. Yan, Z. Huang, P. Liu, D. Li, et al., An investigation of the coating/substrate interface of plasma electrolytic oxidation coated aluminum, Surf. Coat. Technol. 280 (2015) 86 91. [73] P.K. Dutta, M. Ravikumar, J. Dutta, Chitin and chitosan for versatile applications, J. Macromol. Sci. C: Polym. Rev. 42 (3) (2002) 307 354. [74] T. de Moraes Crizel, A. de Oliveira Rios, V.D. Alves, N. Bandarra, M. MoldãoMartins, S. Hickmann Flôres, Active food packaging prepared with chitosan and olive pomace, Food Hydrocolloids 74 (2018) 139 150. [75] J. Yang, T. Long, N.-F. He, Y.-P. Guo, Z.-A. Zhu, Q.-F. Ke, Fabrication of a chitosan/bioglass three-dimensional porous scaffold for bone tissue engineering applications, J. Mater. Chem. B 2 (38) (2014) 6611 6618. [76] M. Finˇsgar, A.P. Uzunali´c, J. Stergar, L. Gradiˇsnik, U. Maver, Novel chitosan/diclofenac coatings on medical grade stainless steel for hip replacement applications, Sci. Rep. 6 (2016) 26653. [77] T. Sugama, M. Cook, Poly(itaconic acid)-modified chitosan coatings for mitigating corrosion of aluminum substrates, Prog. Org. Coat. 38 (2) (2000) 79 87. [78] G. Kumar, R.G. Buchheit, Development and characterization of corrosion resistant coatings using the natural biopolymer chitosan, ECS Trans. 1 (9) (2006) 101 117. [79] M.Z. Elsabee, E.S. Abdou, Chitosan based edible films and coatings: a review, Mater. Sci. Eng. C 33 (4) (2013) 1819 1841.

454

Advances in Smart Coatings and Thin Films

[80] O. Lundvall, M. Gulppi, M.A. Paez, E. Gonzalez, J.H. Zagal, J. Pavez, et al., Copper modified chitosan for protection of AA-2024, Surf. Coat. Technol. 201 (12) (2007) 5973 5978. [81] M.L. Zheludkevich, J. Tedim, C.S.R. Freire, S.C.M. Fernandes, S. Kallip, A. Lisenkov, et al., Self-healing protective coatings with “green” chitosan based prelayer reservoir of corrosion inhibitor, J. Mater. Chem. 21 (13) (2011) 4805 4812. [82] J. Carneiro, J. Tedim, S.C.M. Fernandes, C.S.R. Freire, A.J.D. Silvestre, A. Gandini, et al., Chitosan-based self-healing protective coatings doped with cerium nitrate for corrosion protection of aluminum alloy 2024, Prog. Org. Coat. 75 (1 2) (2012) 8 13. [83] J. Carneiro, J. Tedim, S.C.M. Fernandes, C.S.R. Freire, A. Gandini, M.G.S. Ferreira, et al., Chitosan as a smart coating for controlled release of corrosion inhibitor 2-mercaptobenzothiazole, ECS Electrochem. Lett. 2 (6) (2013) C19 C22. [84] J. Carneiro, J. Tedim, S.C.M. Fernandes, C.S.R. Freire, A. Gandini, M.G.S. Ferreira, et al., Functionalized chitosan-based coatings for active corrosion protection, Surf. Coat. Technol. 226 (2013) 51 59. [85] J. Carneiro, J. Tedim, M. Ferreira, Chitosan as a smart coating for corrosion protection of aluminum alloy 2024: a review, Prog. Org. Coat. 89 (2015) 348 356. [86] L. de Y. Pozzo, T.F. da Conceição, A. Spinelli, N. Scharnagl, A.T.N. Pires, Chitosan coatings crosslinked with genipin for corrosion protection of AZ31 magnesium alloy sheets, Carbohydr. Polym. 181 (2018) 71 77. Available from: https:// Doi.10.1016/J.CARBPOL.2017.10.055. [87] Z. Zhong, J. Qin, J. Ma, Cellulose acetate/hydroxyapatite/chitosan coatings for improved corrosion resistance and bioactivity, Mater. Sci. Eng. C 49 (2015) 251 255. [88] M. Vardaki, D. Ionita, A.B. Stoian, I. Demetrescu, Increasing corrosion resistance of a ZrTi alloy with a bioinspired coating with low porosity, Mater. Corros. 68 (9) (2017) 988 994. [89] J. Zhang, C.-S. Dai, J. Wei, Z.-H. Wen, Study on the bonding strength between calcium phosphate/chitosan composite coatings and a Mg alloy substrate, Appl. Surf. Sci. 261 (2012) 276 286. [90] H. Ashassi-Sorkhabi, R. Bagheri, B. Rezaei-moghadam, Sonoelectrochemical synthesis of PPy MWCNTs chitosan nanocomposite coatings: characterization and corrosion behavior, J. Mater. Eng. Perform. 24 (1) (2015) 385 392. [91] B. Ghosh, M.W. Urban, Self-repairing oxetane-substituted chitosan polyurethane networks, Science 323 (5920) (2009) 1458 1460.