Sol–gel coatings on metals for corrosion protection

Progress in Organic Coatings 64 (2009) 327–338 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat Review Sol–gel coatings on metals for corrosion protection Duhua Wang, Gordon. P. Bierwagen ∗ Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58105, USA a r t i c l e i n f o Article history: Received 31 October 2007 Received in revised form 12 August 2008 Accepted 12 August 2008 Keywords: Sol–gel Corrosion resistance Protective coatings a b s t r a c t Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates. Further, the sol–gel method is an environmentally friendly technique of surface protection and had showed the potential for the replacement of toxic pretreatments and coatings which have traditionally been used for increasing corrosion resistance of metals. This review covers the recent developments and applications of sol–gel protective coatings on different metal substrates, such as steel, aluminum, copper, magnesium and their alloys. The challenges for industrial productions and future research on sol–gel corrosion protective coatings are also briefly discussed. © 2008 Published by Elsevier B.V. Contents 1. 2. 3. 4. 5. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General background of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Brief history of sol–gel chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Preparation of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corrosion protective sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Steel substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Metal oxide coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Organic–inorganic hybrid sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Inhibitor doped sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4. Inorganic zinc-rich coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Aluminum substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Metal oxide coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Organic–inorganic hybrid sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Hybrid sol–gel magnesium-rich coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Copper and magnesium substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges and future studies of sol–gel corrosion protective coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Basic theory studies of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Optimization and new synthesis routes of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. New raw materials and multiple component systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction Metals, such as iron, aluminum, copper and magnesium and their alloys are used in a myriad of structural, marine, aircraft appli- ∗ Corresponding author. E-mail address: Gordon.Bierwagen@ndsu.edu (Gordon.P. Bierwagen). 0300-9440/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2008.08.010 327 328 328 329 329 329 329 331 332 332 333 333 334 335 336 337 337 337 337 337 337 337 cations and cultural heritage, etc. While these metals are useful because of their physical characteristics, such as stiffness and high strength to weight ratios, they are highly susceptible to corrosion in aggressive environments. Corrosion is always the major reason of energy and material loss. It was reported that 1/5 of energy globally and average 4.2% of gross national product (GNP) is lost each year due to corrosion [1] and the economic impact of corrosion is estimated to be greater than $100,000,000,000 per year in the United 328 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 States alone [2]. This cost includes the application of protective coatings (paint, surface treatment, etc.), inspection and repair of corroded surfaces and structures, and disposal of hazardous waste materials. A generic way to protect metals from corrosion is to apply protective films or coatings, which also permit the desired properties of the substrate to be coated through the chemical modification of the coatings [3,4], such as mechanical strength, optical appearance, bioactivity, etc. There are several techniques for the deposition of coatings on metals, including physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition, plasma spraying and sol–gel process. There are many advantages using sol–gel coatings, several most important features are listed as follows [5,6]: (A) Sol–gel processing temperature generally is low, frequently close to room temperature. Thus thermal volatilization and degradation of entrapped species, such as organic inhibitors, is minimized. (B) Since liquid precursors are used it is possible to cast coatings in complex shapes and to produce thin films without the need for machining or melting. (C) The sol–gel films are formed by “green” coating technologies: It uses compounds that do not introduce impurities into the end product as initial substances, this method is waste-free and excludes the stage of washing. Ten year ago, Guglielmi [7] has already discussed the potential of sol–gel coatings as a corrosion inhibiting system for metal substrates. Since then, a great deal of work has been done to make various sol–gel based protective coatings. This review will introduce the basic chemistry involved in sol–gel processes, then the progress and development of sol–gel protective coatings on metal substrate, such as steel, aluminum, etc. Finally some problems and future work on sol–gel coatings will be summarized briefly. 2. General background of sol–gel coatings 2.1. Brief history of sol–gel chemistry The sol–gel process is a chemical synthesis method initially used for the preparation of inorganic materials such as glasses and ceramics [8]. And this process can be traced back to 1842, when French chemist, J.J. Ebelmen reported the synthesis of uranium oxide by heating the hydroxide, but the aging and heating process last almost a year to avoid cracking which made it difficult for wider application and did not catch many eyes that time [9]. It was not until 1950s, when R. Roy and his colleague changed the traditional sol–gel process into the synthesis of new ceramic oxides, making the sol–gel silicate powders quite popular in the market [10–12]. In 1971, the production process of so-called low-bulk density silica involving the hydrolysis of tetraethoxysilane (TEOS) in the presence of cationic surfactants was patented [13]. In the middle Fig. 1. Hydrolysis and condensation involved in making sol–gel derived silica materials. 329 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 1980s, many material scientists and chemists, represented by H. Schmidt and G.L. Wilkes started to synthesis organic–inorganic hybrid materials (OIHMs) by sol–gel process and published a series of pioneering research articles [14–17]. Since then, sol–gel technology has attracted a great deal of attention, especially in the fields of ceramics, polymer chemistry, organic and inorganic chemistry, physics and played an indispensable role in preparing novel OIHMs [5,18,19]. 2.2. Preparation of sol–gel coatings The sol–gel process can be described as the creation of an oxide network by progressive condensation reactions of molecular precursors in a liquid medium [18]. Basically, there are two ways to prepare sol–gel coatings: the inorganic method and the organic method. The inorganic method involves the evolution of networks through the formation of a colloidal suspension (usually oxides) and gelation of the sol (colloidal suspension of very small particles, 1–100 nm) to form a network in continuous liquid phase. But the most widely used method is the organic approach, which generally starts with a solution of monomeric metal or metalloid alkoxide precursors M(OR)n in an alcohol or other low-molecular weight organic solvent. Here, M represents a network-forming element, such as Si, Ti, Zr, Al, Fe, B, etc.; and R is typically an alkyl group (Cx H2x+1 ). Generally, the sol–gel formation occurs in four stages: (a) hydrolysis, (b) condensation and polymerization of monomers to form chains and particles, (c) growth of the particles, (d) agglomeration of the polymer structures followed by the formation of networks that extend throughout the liquid medium resulting in thickening, which forms a gel. In fact, both the hydrolysis and condensation reactions occur simultaneously once the hydrolysis reaction has been initiated. As seen in Fig. 1, both the hydrolysis and condensation steps generate low-molecular weight by-products such as alcohol and water. Upon drying, these small molecules are driven off and the network shrinks as further condensation may occur. These processes are basically affected by the initial reaction conditions, such as pH, temperature, molar ratios of reactants, solvent composition, etc. Readers may refer to other studies and reviews for a more complete understanding of the entire sol–gel process [6–8,18,19]. A sol–gel coating can be applied to a metal substrate through various techniques, such as dip-coating and spin-coating, which are the two most commonly used coating methods. Spraying [20,21] and electrodeposition [22–24] also emerged recently and could be the major sol–gel coating application methods in the future. But whatever technique is used, after the coating deposition, there is a substantial volume contraction and internal stress accumulation due to the large amount of evaporation of solvents and water. Cracks are easy to form due to this internal stress if the film formation conditions are not carefully controlled. Usually the curing and heat treatment of sol–gel coatings vary substantially depending on different microstructures, quality requirement and practical application. The formation of silica sol–gels also holds true for non-silicate inorganic alkoxides. In fact, metal alkoxides of titanium, zirconium, tin or aluminum are much more reactive towards water than alkoxysilanes due to the lower electronegativity and higher Lewis acidity [8,25]. But it is that the reaction is quite gentle and mild makes the alkoxysilanes studied most extensively in the formation of sol–gel materials, especially OIHMs. Alkoxysilanes, including tetraoxy silicate (Si(OR)4 ) and organically modified silicates (Ormosils, R’n Si(OR)4−n or (RO)3 Si R’Si(OR)3 ) have been the most widely used metal-organic precursors for preparation of hybrid materials by sol–gel processing. Table 1 and Fig. 2 lists some Table 1 Abbreviation, chemical name and functional group of some commonly used alkoxysilane precursors for sol–gel protective coating Abbreviation Chemical name TEOS TMOS MTES MTMS VTMS PTMS PHS Tetraethyl orthosilicate Tetramethyl orthosilicate Methyl triethoxysilane Methyl trimethoxysilane Vinyl trimethoxysilane Phenyl trimethoxysilane Diethylphosphonatoethyl triethoxysilane 3-Aminopropyl trimethoxysilane 3-(2-Aminoethyl)aminopropyl trimethoxysilane 3-Glycidoxypropyl trimethoxysilane ␥-Methacryloxypropyl trimethoxysilane ␥-Mercaptopropyl trimethoxysilane Bis-[3-(triethoxysilyl)propyl]tetrasulfide APS AEAPS GPTMS MAPTS MPTMS BTSTS Functional group MethylMethylVinylPhenylPhosphonatoAminoAminoGlycidoMethacryloxyMercaptoSulfide- of the most commonly used alkoxysilanes in sol–gel protective coatings area. 3. Corrosion protective sol–gel coatings 3.1. Steel substrates Steel and stainless steel are widely used in different industrial fields because of their mechanical and corrosion properties. However, they still tend to corrode in the presence of halide ions. The corrosion resistance behavior of sol–gel coatings or thin films deposited onto steel substrate has been extensively studied [26–45], as summarized in Table 2 following the time of publication. 3.1.1. Metal oxide coatings SiO2 , ZrO2 , Al2 O3 , TiO2 and CeO2 , etc. all have very good chemical stability and can provide effective protection to metal substrate. SiO2 can improve the oxidation and acidic corrosion resistance of metals under different temperatures due to its high heat resistance and chemical resistance [29,34]. Vasconcelos et al. made SiO2 coating on AISI 304 stainless steel using tetraethyl orthosilicate (TEOS) as chemical precursor [34]. It was found that the coating contained Si, O and Fe elements and formed a transition layer between steel substrate and SiO2 layer. The obtained sol–gel silica coatings were homogeneous, free of cracks. Samples were tested in 1 mol/L H2 SO4 solution and 3.5% NaCl solution, both corrosion potential increased and corrosion current density decreased, indicating this 100 nm thin SiO2 layer improved the anti-corrosion performance of stainless steel substrate. ZrO2 has a high expansion coefficient very close to many bulk metals, which can reduce the formation of cracks during high temperature curing process [26,36]. ZrO2 also shows good chemical stability and high hardness [35] which makes it a good protective materials. Perdomo et al. [31] made ZrO2 coatings on 304 stainless steel by sol–gel method using zirconium propoxide as precursor and densified in air and in oxygen-free (argon or nitrogen) atmospheres. The corrosion behavior of the stainless steel substrate was studied by potentiodynamic polarization curves. It was found that the ZrO2 coatings extended the lifetime of the material by a factor of almost eight in a very aggressive environment, independently of the preparation procedure. In order to improve the adhesion between protective organic coating and metal substrate, Fedrizzi 330 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 Fig. 2. Chemical structure of some commonly used alkoxysilane precursors for sol–gel protective coating. et al. [35] prepared ZrO2 sol–gel coating on low carbon steel sheets, then applied polyester organic coating onto the ZrO2 layer. According to adhesion testing, the samples pretreated with ZrO2 layer showed promising performance, in comparison with commercial chemical treatments, such as tricationic phosphate and iron phosphate pretreatment. Li et al. [36] also reported on thin ZrO2 sol–gel film on mild steel sheets, and found that ZrO2 layers heat-treated at 400 ◦ C and 800 ◦ C were homogeneous, crack-free and increased the corrosion resistance of the mild steel by a factor of 6.3 and 2.3, respectively. Al2 O3 is a well-known insulator and has very low conductivity for transmitting electrons, which is ideal for protective coatings. Masalski et al. [33] prepared two-, four- and six-layer Al2 O3 coatings on AISI 316 stainless steel in order to improve its local anti-corrosion ability. It was found that the cathode current density varied with sintering temperature: higher sintering temperature (within the range 500–850 ◦ C), the lower cathode current density values, but also the lower breakdown potentials. The author believed that at higher temperatures conversion of ␥-Al2 O3 (less resistant to aggressive agents) into the ␣-Al2 O3 modification (corundum, more resistant to aggressive agents) proceeds more readily. However, on the other hand, an increase in the sintering temperature resulted in a marked increased on the anodic branch of the polarization curve and thus increased the number of defects in the coating. TiO2 has excellent chemical stability, heat resistance and low electron conductivity, making it an excellent anti-corrosion material. But pure TiO2 film is mostly used in catalyst chemistry. Very few TiO2 films have been reported as protective coatings on steel substrate [28]. CeO2 is in the similar situation, although widely used in optics, catalyst chemistry, pigments, superconductors and sensors, cerium is more popular in hybrid sol–gel coatings as corrosion inhibitors [41,44], which will be discussed later. Two and multiple-component oxide coatings can overcome the limitation of single-component oxide layers, broaden their application areas and improve the comprehensive protective ability of steel substrates. Early works, such as Atik et al. [26] reported 70SiO2 -30TiO2 and 75SiO2 -25A12 O3 acting very efficiently as corrosion protectors of 316L stainless steel substrates in aqueous NaCl and acid media at room temperature. The films could increase the lifetime of the substrate by a factor of up to 10 in 3% NaCl and 5 in 15% H2 SO4 solutions. In order to improve 331 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 Table 2 Corrosion protective sol–gel coatings on steel substrates Composition and precursors Steel substrate Coating method Thickness (␮m) Reference and year ZrO2 TiO2 -SiO2 Al2 O3 -SiO2 316L SS Dip-coating 0.4–0.6 [26] 1995 ZrO2 -PMMA 316L SS Dip-coating 0.2 [27] 1997 CeO2 TiO2 304 SS Dip-coating 0.5 [28] 1997 SiO2 SiO2 -CaO-P2 O5 316L SS Dip-coating 0.4–1.4 [29] 1998 CH3 -SiO2 B2 O3 -SiO2 MgO-SiO2 304 SS, 430 SS Dip-coating 0.2–2 [30] 1998 ZrO2 ZrO2 -PMMA Al2 O3 SiO2 ZrO2 ZrO2 TEOS-MAPTS 304 SS 316L SS 316L SS 304 SS Carbon steel Mild steel 304 SS Dip-coating Dip-coating Dip-coating Dip-coating Dip-coating Dip-coating Dip-coating 0.7 0.2–1.0 2.0–3.0 0.15 0.3–0.6 0.2 [31] 1998 [32] 1999 [33] 1999 [34] 2000 [35] 2001 [36] 2001 [37] 2001 TEOSMAPTS 304 SS 316L SS Dip-coating 0.2 [38] 2003 SiO2 -Na2 O Zinc-plated steel Electrodepositing 1.0 [22] 2003 APS AEAPS GPTMS MAPTS Iron plate Dip-coating 10–12 [39] 2003 SiO2 -PMMA SiO2 -PVB 304 SS, Zinc-plated steel Dip-coating 1.0 [40] 2004 Cerium-APS TEOS-MAPTS TEOS-MTES Cerium-TEOS-MTES CaO-P2 O5 Carbon steel Carbon steel Galvanized steel 304 SS 316L SS Dip-coating Brushing Dip-coating Spin-coating Spin-coating 2.1–2.5 N/A 4.0 1.9–2.0 1.0 [41] 2005 [42] 2006 [43] 2006 [44] 2006 [45] 2007 the bioactivity and corrosion resistance of an implant material, Vijayalakshmi and Rajeswari [45] recently reported the preparation of CaO-P2 O5 coating on 316L stainless steel. The sol–gel film had combined effects of good adherence with higher corrosion resistance acting as a diffusion barrier and could be used as a potential material for implantation purposes. Similar SiO2 -CaO-P2 O5 coating was also studied to improve the corrosion resistance and bioactivity of stainless steel implant material [29]. 3.1.2. Organic–inorganic hybrid sol–gel coatings From the studies above, the inorganic oxide coatings can provide good protection on metal substrates. But there are still some major drawbacks of these coatings, from the standpoint of corrosion resistant layers: (1) oxide films are brittle and thicker coatings (>1 ␮m) are difficult to achieve without cracking; (2) relatively high temperatures (400–800 ◦ C) are often required to achieve good properties [8]. To overcome the limitation of pure inorganic sol–gel coatings, such as brittleness and high temperature treatment, much work has been done to introduce organic component into the inorganic sol–gel to form the organic–inorganic hybrid sol–gel coatings. These materials turned out to be among the most interesting areas of coatings science in last decade [27,32,39–44]. Though many organic (polymeric/oligomeric) species have been successfully incorporated within inorganic networks by different synthetic methods, they are classified into three major approaches according to the chemical bond between inorganic and organic phases: (1) mix organic component directly into the inorganic sol–gel system, the product is a simple mixture, and there is no chemical bonding between organic and inorganic components; (2) utilize already existing functional groups within the polymeric/oligomeric species to react with the hydrolized of inorganic precursors, thus introducing chemical bonding between them; (3) use alkoxysilanes R’n Si(OR)4−n as the sole or one of the precursors of the sol–gel process with R’ being a second-stage polymerizable organic group often carried out by either a photochemical or thermal curing following the sol–gel reaction, e.g. methacryloxy group in MAPTS (see Table 1 and Fig. 2). Atik et al. [27] made hybrid coatings of polymethylmethacrylate (PMMA) and ZrO2 onto 316L stainless steel. Coatings’ anticorrosion behavior was analyzed in 0.5 M H2 SO4 solution through potentiodynamic polarization curves at room temperature. The coatings act as geometric blocking layers against the corrosive media and increase the lifetime of the substrate up to a factor 30. Messaddeq et al. [32] analyzed the microstructure of ZrO2 -PMMA coating by scanning electron (SEM) and atomic force microscopy (AFM) and found that zirconium concentrated domains were surrounded by continuous PMMA secondary phase domains. Maximum corrosion resistance of the substrate was observed for the coating containing 17 vol.% PMMA. Higher PMMA volume made thicker coatings but tended to form a single-phase structure at the micrometer scale and their adhesion to the substrate was worse resulting in the breakdown and the peeling of the coating during the electrochemical testing. Similarly, a SiO2 -PVB (polyvinyl butyral) hybrid 332 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 vided good protection for the zinc coating underneath and steel substrate at the same time. Fig. 3. AFM images of 90% TEOS–10% MAPTS sol–gel coating on a stainless steel substrate [37]. coating was also deposited on Zn-plated steel substrate [40]. The results of salt spray testing of substrates coated with hybrid sol–gel films indicated that SiO2 -1 wt% PVB coating was the relatively best one against corrosion and crack-free even its thickness was close to 1 ␮m. Chou et al. [37,38], prepared hybrid sols by copolymerizing TEOS and MAPTS with a two-step acid-catalyst process. Then hybrid coatings were dip-coated on 304 stainless steel substrates and annealed at 300 ◦ C for 30 min. The resultant coatings were relatively dense, uniform and defect free (Fig. 3) and provided excellent corrosion protection probably because the dense physical barrier coatings separated the anode from the cathode. Ref. [42] also reported that the TEOS-MAPTS hybrid coatings were uniform and crack-free while the pure inorganic coatings from TEOS had apparent cracks on the surface (Fig. 4). Duran and coworkers [43] used TEOS and MTES to form a thick hybrid sol–gel coating (thickness around 4 ␮m) under basic catalyst and found that the corrosion mechanisms for sol–gel galvanized steels did not change with respect to the uncoated steel. The sol–gel coating pro- 3.1.3. Inhibitor doped sol–gel coatings Besides the organic component, other additives, such as inhibitors, can also be incorporated into the sol–gel system to increase the corrosion resistance of the metal substrates. Sugama [41] made hybrid coatings to protect steel and aluminum against corrosion by adding about 20 wt% of Ce acetate as corrosion inhibitor into 3-aminopropyl trimethoxysilane (APS) sol. The cerium compound could minimize the content of non-reacted water-soluble APS and form a passive Ce3+ oxide film insensitive to Cl− dissolution over the metal surface. The coating thickness was about 2.5 ␮m and extended the useful lifetime of steel exposed in a salt-fog chamber from only ∼10 h to ∼768 h, and aluminum panels from ∼40 h to ∼1440 h. Duran and coworkers [44] also studied cerium ions loaded hybrid silica sol–gel coatings deposited on AISI 304 stainless steel substrates. It was found that coatings prepared with the Ce (III) salt enhance the corrosion resistance through a barrier effect, but developed defects later with immersion time, showing no apparent inhibiting effects. At the same time, coatings obtained from Ce (IV) chemicals enhanced the coating performance, probably due to the formation of Ce(OH)3 on the surface through a chemical/electrochemical mechanism involving a redox reaction between Cr and Ce ions. Through these studies of sol–gel coatings on steel substrates (stainless steel, carbon steel and zinc steel), it was clear that sol–gel coating can provide effective protection against corrosive media in practical service conditions. The corrosion resistance of steel substrates was substantially improved. 3.1.4. Inorganic zinc-rich coatings In addition to the corrosion inhibitors, sacrificial metal pigments such as zinc, aluminum, magnesium and their alloy particles can also be included into the sol–gel coating formula according to the different metal substrate to be protected. Generally, steel substrate is often a suitable target under this cathodic protection. Fig. 4. Surface morphologies sol–gel coatings from (a) TEOS; (b) TEOS (enlarged); (c) TEOS-MAPTS; (d) TEOS-MAPTS (enlarged) [42]. 333 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 Inorganic zinc-rich coatings are a widely used and unique class of coatings that provide cathodic protection to ferrous and steel substrates. The coatings have been categorized by Steel Structures Painting Council (SSPC) into three major groups: (1) post-cured, water-borne alkali metal silicates; (2) self-cured, water-borne alkali metal silicates; (3) self-cured, solvent-borne alkyl silicates. Of the three types, the self-cured, solvent-borne alkyl silicates have the greatest commercial usage by far [46]. The most common binder precursors are TEOS and its oligomers derived from it by controlled partial hydrolysis with a small amount of water. Ethyl or isopropyl alcohol is used as the principal solvent, since an alcohol helps maintain package stability. After application, the alcohol evaporates, and water from the air completes the hydrolysis of the oligomer to yield a film of polysilicic acid partially converted to zinc salts. TEOS can undergo hydration and condensation processes and form a complex polysiloxane network in atmosphere, and its final hydration products are SiO2 and water [47–49]. Unlike the general sol–gel protective coatings, the protection ability of the inorganic zinc-rich coatings is more relied on the cathodic protection of zinc pigments, rather than the barrier properties of the sol–gel binder. And the current research of these coatings is mainly focused on the effects of pigment volume concentration [46,50–52], size and shape of zinc pigments [51], zinc alloy pigments [53,54] and extenders [55]. The studies on using different alkoxysilane precursors or hybrid binder materials have not been reported in the last decade. 3.2. Aluminum substrates Aluminum and its alloys are obvious target substrates for corrosion studies due to their widespread applications [56,57]. The low cost, lightweight, high thermal and electrical conductivity grant aluminum a remarkable industrial and economical importance. Many of its applications are practicable due to its natural tendency to form a passivating Al2 O3 oxide layer, which can also be artificially generated by anodizing the substrate. However, this passivating layer deteriorates in aggressive media, such as chloride, which results in pitting corrosion [58–62]. Although current chromate conversion coatings function very well in corrosion protection, the US Environmental Protection Agency is planning to totally ban the use of chromates in coating materials in the near future because of their extremely toxic effect. A broad range of research is being actively pursued to develop less toxic and environmentally benign organic coatings for corrosion protection. One relatively new but very promising approach is sol–gel coating and has been extensively studied in the last ten years [20,23,39–41,63–85], as summarized in Table 3 following the time of publication. 3.2.1. Metal oxide coatings Similar to steel substrates, there have been some studies focusing on using pure inorganic oxide coatings, such as SiO2 , ZrO2 , on aluminum substrates. Thim et al. [64] dipped aluminum (98% Al) surfaces into silicic acid aqueous solution containing urea as Table 3 Corrosion protective sol–gel coatings on aluminum substrates Composition and precursors Al substrate Coating method Thickness (␮m) Reference and year Al2 O3 -TEOS-GPTMS ZrO2 -TEOS-GPTMS Al plate Spin-coating 7 [63] 1998 SiO2 SiO2 -ZrO2 ZrO2 -TiO2 -soybean oil ZrO2 -TEOS-GPTMS SiO2 -vinylpolymer Cerium-SiO2 -epoxy Cerium-ZrO2 -GPTMS Al plate Al 2024-T3 Al plate Al 2024-T3 Al 2024-T3 Al 2024-T3 Al 2024-T3 Dip-coating Dip-coating Blade-casting Dip-coating Dip-coating Dip-coating Dip-coating N/A 0.1 45–95 3–4 3–4 2-3 2–3 [64] 2000 [65] 2001 [66] 2001 [67] 2001 [68] 2001 [69] 2001 [70] 2001 Aminosilane-epoxy Epoxysilane-epoxy Al 2024-T3 Al 7075-T6 Spraying 30–50 [71] 2001 TEOS-GPTMS Al 2024-T3 Spraying 2.2 [20] 2001 TMOS-GPTMS-amine cross-linkers Al 2024-T3 Dip-coating 1 [72] 2003 [73] 2003 APS AEAPS GPTMS MAPTS Al plate Dip-coating 10–12 [39] 2003 TEOS MTES PTMS Al electrode Electrodeposition 0.16–0.18 [23] 2003 SiO2 -BTSTS Al 2024-T3 Al 7075-T6 Al 6061-T6 Al 5005 Dip-coating 0.4–0.6 [74] 2003 SiO2 -PMMA SiO2 -PVB Al alloy (ADC12) Dip-coating 0.1–0.3 [40] 2004 TMOS-GPTMS-organic inhibitor Al 2024-T3 Dip-coating 1 [78] 2004 [79] 2005 Bis-silane inhibitor Cerium-APS ZrO2 -TEOS-MAPTS ZrO2 -GPTMS inhibitor Al 2024-T3 Al 3003 Al disk Al 2024-T3 Dip-coating Dip-coating Spin-coating Dip-coating 0.3 2.1–2.5 1.9–7.5 1.8–2.0 [80] 2005 [41] 2005 [81] 2005 [84] 2007 TEOS-GPTMS-PDMS Al 2024-T3 Al 6061-T6 Spin-coating N/A [85] 2007 334 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 dry chemical control agent (DCCA) to obtain SiO2 films. But its adhesion was quite limited and coating developed defects during the heat treatment process. Then anodic polarization pretreatment was used on aluminum surfaces before depositing SiO2 coatings, and corrosion resistance was improved. Yang et al. [65] investigated the corrosion behavior of 3.4SiO2 -1ZrO2 sol–gel coating on Al 2024-T3 substrate under immersion in dilute Harrison’s solution [3.5 g/L (NH4 )2 SO4 , 0.5 g/L NaCl]. SEM, AFM, EIS (electrochemical impedance spectroscopy) and XPS (X-ray photoelectron spectroscopy) were used during the evolution of the coating system under immersion. It was found that pitting corrosion and degradation products on the sol–gel coating surface developed after 2 days of immersion, the impedance increased, and it was conjectured that aluminum oxide and silicon oxide may form a stable mixed oxide barrier layer at the interface after initial corrosion, which prohibits further pitting corrosion. 3.2.2. Organic–inorganic hybrid sol–gel coatings Hybrid sol–gel coatings are much more popular than pure inorganic oxide layers in terms of the corrosion protection of metal substrates for two main reasons. First, hybrid coatings can easily form a thicker coat in micrometer scale without cracks and much lower curing temperature is needed (usually < 100 ◦ C) than 400–800 ◦ C for sintering of oxide layers. Second, the hybrid sol–gel system has much more flexibility in adaptation of anti-corrosion additives, such as inhibitors, pigments, etc., so the overall corrosion protection ability of the sol–gel system can be substantially improved. Early studies, such as Chen et al. [63] made hybrid nanocomposite protective coatings on aluminum substrates by the incorporation of nanosized particles (Al2 O3 , ZrO2 , SiO2 ) into hybrid sol–gel matrices. The obtained coatings were thick (7 ␮m), dense, smooth and inhibited corrosion. Sol–gel/epoxy resin hybrid coatings were formulated and studied at Boeing Company [71]. The hybrid coating showed enhanced mechanical strength such as hardness and abrasion resistance and passed wet adhesion testing when cured at elevated temperatures (80 ◦ C). However, watersensitivity remains for most of the room temperature cured hybrid coatings. Sayilkan et al. [39] developed hybrid sol–gel coatings from alkoxysilane precursors, such as APS, AEAPS, GPTMS and MAPTS. Coatings were sprayed onto aluminum substrates, then cured by UV or thermal source. Standard ASTM tests showed the coatings had very good adhesion on surface as well as high mechanical, chemical, and thermal stability. Soucek and co-workers [66] developed the ZrO2 -TiO2 -soybean oil hybrid coatings. The beneficial synergistic effects of using mixed metal oxides as the inorganic phase of hybrid coatings were observed in tensile modulus and fracture toughness, but the adhesion decreased when double sol–gel precursors were used. Also, a UV-curable hybrid coating based on epoxynorbornene linseed oils (ENLO) was prepared using the in situ method of TEOS oligomers [82]. It was found that the formulations including ∼10 wt% TEOS oligomers provided the optimum balance of the properties such as Young’s modulus, tensile strength, thermo-mechanical properties, fracture toughness, and protective properties. Soucek et al. [83] indicated later that both the inorganic and organic phases must have fast curing kinetics in order for the hybrid coatings having the inorganic particles to be uniformly dispersed in the organic phase at nanoscale level. Van Ooij and co-workers have been using bis-silanes to make sol–gel protective coatings on metal substrates [74–77]. The bissulfur silane (bis-[3-(triethoxysilyl)-propyl]-tetrasulfide, BTSTS) as a sol–gel precursor with later addition of silica nanoparticles to make hybrid protective coatings on aluminum alloy substrates was reported [74,75]. It was found that the bis-sulfur silane film was thickened and strengthened by loading silica nanoparticles into the film up to 15 ppm in the corresponding silane solution. But extra amount of silica only further hardens the surface but not the interfacial layer and may form a porous film, which promotes film delamination. According to his studies, a general accepted bonding mechanism of silanes to metal surfaces is illustrated in Fig. 5. Silanols (SiOH) spontaneous adsorption onto the metal surface through hydrogen bonds before condensation and upon drying, forming covalent metallo-siloxane bonds (MeOSi) at the coating metal interface. Later, Van Ooij and co-workers [80] added corrosion inhibitors (tolytriazole, benzotriazole molecules and inorganic cerium salts) to the sol–gel films and studied their corrosion properties in 0.5 M Fig. 5. Simplified schematic of bonding mechanism between silane molecules and metal surface hydroxide layer: (a) before condensation: hydrogen-bonded interface; (b) after condensation: covalent-bonded interface [74]. D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 NaCl solution. It was found that the organic inhibitor tolytriazole improved the overall corrosion resistance of the Al 2024-T3 alloy but did not impart a self-healing effect to the films, while the inorganic cerium inhibitor showed the property of protecting fresh exposed metal surface. ZrO2 -TEOS-MAPTS hybrid coating was deposited onto aluminum disks by spin-coating technique [81]. Coating thickness as a function of the spin-coating rotational speed and the chemical solcomposition was investigated. It was found that the ZrO2 /MAPTS molar ratio had significant influence on the sol viscosity and thus on the coating thickness. Higher ZrO2 contents seemed to have a positive influence on the machinability but at the same time the brittleness of the coatings increased. Wu et al. [85] also reported the GPTMS/TEOS molar ratio R had a crucial influence to the corrosion resistance of hybrid TEOS-GPTMS-PDMS sol–gel coatings, coatings had the relatively best anti-corrosion performance when R = 4. The Air Force Research Laboratory of the United States have been investigating sol–gel derived anti-corrosion coatings on aerospace aluminum substrates for long time, and has developed high quality, environmental benign corrosion protective coatings by a sol–gel method [20,67–73,78,79]. Voevodin et al. formulated and tested hybrid sol–gel coatings from ZrO2 -TEOS-GPTMS [67] and SiO2 -vinylpolymer [68] systems. A two-stage mechanism in pit development was observed for a sol–gel coated Al 2024-T3 samples. An initial low current stage was associated with electrolyte penetration and pit initiation through the coating to the aluminum substrate surface, and the second high current stage was associated with active pit growth in the metal surface regions. The growth of pits caused coating cracking and even delamination by the pressure of corrosion products and gases. These studies can help to identify the ways for the improvement of corrosion protection through sol–gel coatings: elimination of cracks or defects in the coating or use of corrosion inhibitors to extend or terminate the initial stage of pitting development. Rare earth elements, such as cerium, yttrium, hafnium, can improve coating properties by unifying crystal size, reducing amorphous structures and brittleness upon heating treatment [28]. Kasten et al. [69,70] added small amount of cerium (1–3 wt%) into hybrid sol–gel systems. Cerium showed effective cathodic inhibition in aluminum when good dispersion was achieved. Voevodin et al. [72,73] developed sol–gel surface treatment coatings based on the Self-assembled Nanophase Particle (SNAP) approach in order to replace the traditional chromate-based surface treatments on aircraft aluminum alloys. Through the in situ formation in an aqueous sol–gel process, the functionalized silica nanoparticles could cross-link with each other to form a dense, elastic thin film well covered and adhered onto aluminum alloy substrate surface. From the EIS bode plots of Fig. 6, the low frequency impedance modulus, |Z|, initially as high as on the order of 107  cm2 and maintained that value for almost 2 months, and still had 1 × 106  cm2 after 80 days of immersion. Considering the thickness of the SNAP films was only around 1 ␮m, their barrier properties was quite amazing and showed good potential as longterm corrosion protection coatings. Khramov et al. [78] proposed to add corrosion inhibitors into hybrid sol–gel systems to improve coatings’ anti-corrosion ability. TMOS and GPTMS were first hydrolyzed under acidic conditions, the sol was diluted and aged for 3 days, then the organic inhibitor, ␤-cyclodextrin and cross-linker (diethylenetriamine DETA) were added sequentially. Finally, coating was deposited onto Al 2024T3 substrate by dip-coating technique. It was found that not only the hybrid coating had very good barrier properties, but also the organic inhibitors could gently release in the region of damaged area, providing self-healing ability of the localized corrosion attack 335 Fig. 6. EIS bode plots for SNAP films as a function of immersion time in dilute Harrison’s solution [72]. in corrosive media. Though the way of adding inhibitors is simple and easy to achieve, it is very difficult to control the release of internal inhibitors to the surface. The author use ␤-cyclodextrin to form complexes with inhibitors and insured the slow release of the inhibitor and its continuing delivery to corrosion sites. Later, Khramov et al. [79] also tried to deliver inhibitors by the ionexchange strategy, but resulted in poorer performance compared with the ␤-cyclodextrin complexation strategy. 3.2.3. Hybrid sol–gel magnesium-rich coatings By analogy to the formulation of zinc-rich coatings for the protection of steel, magnesium-rich, magnesium pigmented coatings can also be formulated for the corrosion protection of aluminum alloy. Bierwagen and the corrosion group at NDSU are leading this Mg-rich coatings research and a series of articles have been published [86–88] on their properties. Recently, hybrid sol–gel Mg-rich primers were developed from MAPTS-TEOS-PVB-Mg particles system. Firstly, MAPTS and TEOS were hydrolyzed under the acidic condition, then small amount of PVB powder was added into the sol, finally certain amount of magnesium pigments were mixed and sprayed onto the aluminum alloy substrates. The resultant coatings are 45–55 ␮m thick and crack-free. Fig. 7 shows the SEM image of this hybrid Mg-rich coating. The pigment volume concentration Fig. 7. SEM image of hybrid sol–gel Mg-rich primer (35% PVC). 336 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 Table 4 Corrosion protective sol–gel coatings on Cu and Mg substrates Composition and metal-organic precursors Cu and Mg substrate Coating method GPTMS, MAPTS SiO2 -ZrO2 GPTMS-MTMS SiO2 -MAPTS-MPTMS TEOS-PHS Bronze Copper Bronze copper Mg AZ91D Mg AZ31B Spraying Dip-coating Brushing Spraying Dip-coating (PVC) was 35%, which was a little lower than the critical volume pigment concentration (42% in this formulation), making a rough but still non-porous surface. Coatings were evaluated by ASTM standard tests and found that this hybrid coating had excellent chemical resistance, abrasion resistance and high corrosion resistant (Fig. 8) which was due to the combination of barrier properties from sol–gel binder and cathodic protection by magnesium particles. Currently, sol–gel coatings have been used on aerospace aluminum alloy substrates for corrosion protection [89–91]. The corrosion events from etching electrolytes, temperature gradients or mechanical stresses can be controlled and limited substantially. The traditional chromate pretreatment on aluminum alloys can be replaced by this environmentally benign sol–gel protective coatings. 3.3. Copper and magnesium substrates Sol–gel coating were also investigated for the protection of copper and magnesium surfaces, and several studies are summarized in Table 4. Copper and bronze are popular metal materials for sculptures and kitchen utensils owing to their beautiful appearance and low chemical reactivity. But in wet environment, their corrosion process will be accelerated by forming hydroxides and harmful complexes. SiO2 sol–gel coatings have been reported as barrier layer on copper [7,92,93], but the coatings tended to peel off when raising temperature up to 400 ◦ C since the thermal expansion coefficients of SiO2 and Cu are quite different. Boysen et al. [94] formulated the inorganic SiO2 -ZrO2 coating on copper surface. The interface adhesion and stability problems were expected to be solved by using high Thickness (␮m) 10–12 1–3 5–10 21–23 0.6–0.7 Reference and year [93] 1997 [94] 1999 [95] 2003 [21] 2005 [98] 2006 thermal expansion coefficient ZrO2 component. SEM showed the coating surface was uniform, defects-free and well adhered to the substrate surface. Bescher et al. [95] reported using GPTMS + MTMS as precursors to form hybrid sol–gel coatings on copper and bronze surfaces. The coatings had a strong adhesion on the substrates and can be applied as thicker layers (5–10 ␮m). SEM showed almost no corrosion products appeared after two years exposure to high sulfur/humidity conditions. The anti-corrosion ability was further improved after top-coated with a fluoropolymer layer. The author believed this is because of the high degradation resistance of the fluoropolymer. Magnesium and its alloys have many useful properties, such as high strength-to-weight ratio, good thermal conductivity, high damping characteristics, good machinability and attracted a revival of interest in industrial applications recently [96]. However, magnesium and its alloys are highly susceptible to corrosion, which limits their practical application [97]. Tan et al. [21] reported using 68 wt% MAPTS-2% MPTMS-30% SiO2 as hybrid coating materials, photoinitiator was also added for later UV curing process. The resulted coating was defects-free and could be as thick as 23 ␮m, providing excellent physical barrier against corrosive attack. Khramov et al. [98] used phosphonate functional silane (diethylphosphonatoethyl triethoxysilane, PHS) and found its phosphonate groups have more affinity on the magnesium surface than silane headgroups (Fig. 9), thus forming a sol–gel coating with phosphonate binding to magnesium surface. The improved corrosion protection of phosphonate-containing coatings as compared to pure silica sol–gel coatings has been observed and explained by the favorable combination of barrier properties of the organo-silicate matrix with strong chemical bonding of phosphonate groups to the magnesium substrate. Fig. 8. Surface images of hybrid sol–gel Mg-rich primer (35% PVC) after 700 h of Prohesion® exposure (left) and salt spray test (right), the scribe length was 5 cm. D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 337 post treatment, especially in hybrid coating synthesis. So the optimization of current synthesis routes and design of new gelation and heat treatment methods are the keys to promote the development and applications of sol–gel coatings. Electrochemical deposition is a promising method of making sol–gel coatings. One can form a sol–gel layer by controlling the solution pH, colloid concentration, deposition voltage and time. Compared with the traditional dip-coating, spin-coating and spraying methods, the structure and properties of final coat are more controllable from electrodeposited coat. Furthermore, the coat can be deposited on complex shapes. Veeraraghavan et al. [22] reported the electrodeposition of SiO2 layers on galvanized steel surface from aqueous solutions. The single SiO2 layer was 1 ␮m thick, crack-free and provided high corrosion resistant to the zinc steel underneath. The author indicated that aqueous solutions were much more stable than the conventional sol system, which was thermodynamically unstable (viscosity changes with time), and the resulted electrodeposition film was more reliable and controllable in properties. Sheffer et al. [23] also electrodeposited hybrid sol–gel films on aluminum electrodes and compared the difference between the conventional dip-coating method and the electrodeposition approach for depositing sol–gel films, indicating a clear advantage of the latter. 4.3. New raw materials and multiple component systems Fig. 9. Schematic of sol–gel processing of hybrid coatings with phosphonate functional silane [98]. 4. Challenges and future studies of sol–gel corrosion protective coatings With the extensive studies in the last decade or so, sol–gel coatings, especially hybrid systems for corrosion protection are being investigated and developed rapidly and already have some commercial applications. But on the whole, this sol–gel technique is still in the initial stage, facing many difficulties and challenges for large-scale industrial production. 4.1. Basic theory studies of sol–gel coatings The interface properties of sol–gel coatings, such as adhesion and delamination, are the crucial factors to the quality of the coatings, but the systematic theory of how to evaluate the feasibility and stability of sol–gel coatings has not been established. So much more studies should be focused on the effects of adding various precursors and functional additives to the film formation process and anti-corrosion performance of the coating. In addition, the parameters to sol formation (temperature, pH, H2 O/-OR molar ratios, solvents, sol aging, etc.), kinetics of hydrolysis and condensation, gelation and curing process, cracks formation and crystal transition during the heat post treatment, are all very important for the overall understanding of the film formation theory of sol–gel coatings and further design and control of coating compositions and properties. The traditional sol–gel precursors, such as TEOS, TMOS, are often expensive and toxic to some extent. So more environmental benign precursors or industrial colloid particles, such as silicate, titanate, can be used to replace the traditional precursors in the formation of sol–gel protective coatings without changing the overall properties a lot. Metal particles and pigments can also be incorporated in the coating system by uniformly dispersing in the sol to increase coating thickness and toughness. Metal-rich sol–gel coatings, containing zinc or magnesium particles, which can combine the barrier properties of sol–gel layer and cathodic protection of sacrificial particles, are also very promising in corrosion protective coating areas. 5. Conclusions Sol–gel protective coatings on metal and alloy surfaces can improve their corrosion resistance in various corrosive mediums and practical applications. And the replacement of high corrosion resistant, environmental friendly sol–gel coatings to traditional chromates coatings and pretreatments on metal surfaces can be expected in the near future. Beside the resistance to corrosion, sol–gel coatings can also provide high oxidation, abrasion, water resistant, and many other useful properties. With the further studies of sol–gel technique and related characterization technologies, sol–gel protective coatings will have wider and more practical applications. Acknowledgments The authors are grateful to the Air Force Office of Scientific Research (Grant # 49620-02-1-0398) for the funding provided and to Dr. Scott Payne (USDA/NDSU) for the assistance in the SEM study. 4.2. Optimization and new synthesis routes of sol–gel coatings References Current synthesis route of sol–gel coatings requires long time processing flow, e.g. the sol aging alone often needs several hours to a few days, which is not favorable to plant production. Phase separation is another common phenomenon during the curing and heat [1] T.N. Nguyen, J.B. Hubbard, G.B. McFadden, J. Coatings Technol. 63 (1991) 43. [2] T.L. Metroke, R.L. Parkhill, E.T. Knobbe, Prog. Org. Coat. 41 (2001) 233. [3] R.G. Buchheit, R.P. Grant, P.F. Hlava, B. Mcjebzue, G.L. Zender, J. Electrochem. Soc. 144 (1997) 2621. 338 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327–338 [4] G.P. Bierwagen, L. He, J. Li, L. Ellingson, D.E. Tallman, Prog. Org. Coat. 39 (2000) 67. [5] C.J. Brinker, A.J. Hurd, P.R. Shunrk, J. Non-Cryst. Solids 147 (1992) 424. [6] J.D. Wright, N.A.J. Sommerdijk, Sol–Gel Materials Chemistry and Applications, CRC Press, OPA Overseas Publishers Association, 2001. [7] M. Guglielmi, J. Sol–Gel Sci. Technol. 8 (1997) 443. [8] J. Wen, G.L. Wilkes, Chem. Mater. 8 (1996) 1667. [9] J.J. Ebelmen, Ann. Chem. Phys. 5 (1842) 199. [10] R. Roy, J. Am. Ceram. Soc. 39 (4) (1956) 145. [11] R. Roy, E.F. Osborn, Am. Miner. 39 (1954) 853. [12] R. Roy, Science 238 (1987) 1664. [13] V. Chiola, J.E. Ritsko, C.D. Vanderpool, US Patent 3, 556,725 (1971). [14] G.L. Wilkes, B. Orler, H. Huang, Polym. Preparation 26 (1985) 300. [15] H. Schmidt, J. Non-Cryst. Solids 73 (1985) 681. [16] H. Schmidt, G. Philipp, J. Non-Cryst. Solids 63 (1984) 283. [17] H. Schmidt, H. Scholze, H. Kaiser, J. Non-Cryst. Solids 63 (1984) 1. [18] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Harcourt Brace Jovanovich (Academic Press, Inc.), Boston, 1990. [19] L.L. Hench, Sol–Gel Silica, Properties, Processing, Technology Transfer, Noyes Publications, 1998. [20] R.L. Parkhill, E.T. Knobbe, M.S. Donley, Prog. Org. Coat. 41 (2001) 261. [21] A.L.K. Tan, A.M. Soutar, I.F. Annergren, Y.N. Liu, Surf. Coat. Technol. 198 (2005) 478. [22] B. Veeraraghavan, B. Haran, D. Slavkov, S. Prabhu, B. Popov, B. Heimann, Electrochem. Solid-State Lett. 6 (2003) B4. [23] M. Sheffer, A. Groysman, D. Mandler, Corros. Sci. 45 (2003) 2839. [24] Y. Castro, B. Ferrari, R. Moreno, A. Duran, J. Sol–Gel Sci. Technol. 26 (2003) 735. [25] L.L. Beecroft, C.K. Ober, Adv. Mater. 7 (1995) 1007. [26] M. Atik, P. Neto, L.A. Avaca, M.A. Aegerter, Ceram. Int. 21 (1995) 403. [27] M. Atik, F.P. Luna, S.H. Messaddeq, M.A. Aegerter, J. Sol–Gel Sci. Technol. 8 (1997) 517. [28] A. Nazeri, P.P. Trzaskoma, D. Bauer, J. Sol–Gel Sci. Technol. 10 (1997) 317. [29] P. Galliano, J.J.D. Damborenea, M.J. Pascual, A. Duran, J. Sol–Gel Sci. Technol. 13 (1998) 723. [30] M. Mennig, C. Schelle, A. Duran, J.J. Damborenea, M. Guglielmi, G. Brustain, J. Sol–Gel Sci. Technol. 13 (1998) 717. [31] F. Perdomo, P.D. Lima, M.A. Aegerter, L.A. Avaca, J. Sol–Gel Sci. Technol. 15 (1999) 87. [32] S.H. Messaddeq, S.H. Pulcinelli, C.V. Santilli, A.C. Guastaldi, Y. Messaddeq, J. Non-Cryst. Solids 247 (1999) 164. [33] J. Masalski, J. Gluszek, J. Zabrzeski, K. Nitsch, P. Gluszek, Thin Solid Films 349 (1999) 186. [34] D.C.L. Vasconcelos, J.A.N. Carvalho, M. Mantel, W.L. Vasconcelos, J. Non-Cryst. Solids 273 (2000) 135. [35] L. Fedrizzi, F.J. Rodriguez, S. Rossi, F. Deflorian, R.D. Maggio, Electrochim. Acta 46 (2001) 3715. [36] H. Li, K. Liang, L. Mei, S. Gu, S. Wang, J. Mater. Sci. Lett. 20 (2001) 1081. [37] T.P. Chou, C. Chandrasekaran, S.J. Limmer, G.Z. Cao, J. Non-Cryst. Solids 290 (2001) 153. [38] T.P. Chou, C. Chandrasekaran, G.Z. Cao, J. Sol–Gel Sci. Technol. 26 (2003) 321. [39] H. Sayilkan, S. Sener, E. Sener, M. Sulu, Mater. Sci. 39 (2003) 733. [40] S. Ono, H. Tsuge, Y. Nishi, S. Hirano, J. Sol–Gel Sci. Technol. 29 (2004) 147. [41] T. Sugama, J. Coat. Technol. Res. 2 (October) (2005) 649. [42] L. Jianguo, G. Gaoping, Y. Chuanwei, Surf. Coat. Technol. 200 (2006) 4967. [43] A. Conde, J.D. Damborenea, A. Duran, M. Menning, J. Sol–Gel Sci. Technol. 37 (2006) 79. [44] A. Pepe, M. Aparicio, A. Duran, S. Cere, J. Sol–Gel Sci. Technol. 39 (2006) 131. [45] U. Vijayalakshmi, S. Rajeswari, J. Sol–Gel Sci. Technol. 43 (2007) 251. [46] Z.W. Wicks, F.N. Jones, S.P. Pappas, Org. Coat. Sci. Technol. (1998) 129–140 (second edition). [47] Geeta Parashar, Prog. Org. Coat. 42 (2001) 1. [48] T. Ginberg, J. Coat. Technol. 53 (June) (1981) 677. [49] R. Aelion, J. Am. Chem. Soc. 72 (1950) 5705. [50] C. Hare, J. Protect. Coat. Linings (September) (2001) 54. [51] C. Hare, J. Coat. Technol. 72 (November) (2000) 21. [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] M. Mitchell, Protect. Coat. Europe (PCE) (July) (2001) 12. C. Hare, Modern Paint Coat. 4 (1982) 48. S. Feliu, Corrosion 57 (July (7)) (2001) 591. Tsai. Wen-Ta, Appl. Surf. Sci. 187 (2002) 154. G.P. Bierwagen, D.E. Tallman, Prog. Org. Coat. 41 (2001) 201. J.H. Osborne, K.Y. Blohowiak, S.R. Taylor, C. Hunter, G. Bierwagen, B. Carlson, D. Bernard, M.S. Donley, Prog. Org. Coat. 41 (2001) 217. A.J. Aldykiewicz Jr., H.S. Isaacs, A.J. Davenport, J. Electrochem. Soc. 142 (1995) 3343. R.G. Buchheit, R.P. Grant, P.F. Hlava, B. McKenzie, G.L. Zender, J. Electrochem. Soc. 144 (1997) 2621. B. Mazurkiewicz, A. Piotrowski, Corros. Sci. 23 (1983) 697. G.S. Chen, M. Gao, R.P. Wei, Corrosion 52 (1996) 8. V. Guillaumin, G. Mankowski, Corros. Sci. 41 (1999) 421. Y. Chen, L. Jin, Y. Xie, J. Sol–Gel Sci. Technol. 13 (1998) 735. G.P. Thim, M.A.S. Oliveria, E.D.A. Oliveria, F.C.L. Melo, J. Non-Cryst. Solids 273 (2000) 124. X.F. Yang, D.E. Tallman, V.J. Gelling, G.P. Bierwagen, L.S. Kasten, J. Berg, Surf. Coat. Technol. 140 (2001) 44. R.L. Ballard, J.P. Williams, J.M. Njus, B.R. Kiland, M.D. Soucek, Eur. Polym. J. 37 (2001) 381. N.N. Voevodin, N.T. Grebasch, W.S. Soto, L.S. Kasten, J.T. Grant, F.E. Arnold, M.S. Donley, Prog. Org. Coat. 41 (2001) 287. N. Voevodin, C. Jeffcoate, L. Simon, M. Khobaib, M. Donley, Surf. Coat. Technol. 140 (2001) 29. L.S. Kasten, J.T. Grant, N. Grebasch, N. Voevodin, F.E. Arnold, M.S. Donley, Surf. Coat. Technol. 140 (2001) 11. N.N. Voevodin, N.T. Grebasch, F.E. Arnold, M.S. Donley, Surf. Coat. Technol. 140 (2001) 24. Y. Joshua Du, M. Damron, G. Tang, H. Zheng, C.J. Chu, J.H. Osborne, Prog. Org. Coat. 41 (2001) 226. N.N. Voevodin, V.N. Balbyshev, M. Khobaib, M.S. Donley, Prog. Org. Coat. 47 (2003) 416. A.N. Khramov, V.N. Balbyshev, N.N. Voevodin, M.S. Donley, Prog. Org. Coat. 47 (2003) 207. V. Palanivel, D. Zhu, W.J. Van Ooij, Prog. Org. Coat. 47 (2003) 384. D. Zhu, W.J. Van Ooij, Corros. Sci. 45 (2003) 2177. D. Zhu, W.J. Van Ooij, Corros. Sci. 45 (2003) 2163. D. Zhu, W.J. Van Ooij, Prog. Org. Coat. 49 (2004) 42. A.N. Khramov, N.N. Voevodin, V.N. Balbyshev, M.S. Donley, Thin Solid Films 447 (2004) 549. A.N. Khramov, N.N. Voevodin, V.N. Balbyshev, R.A. Mantz, Thin Solid Films 483 (2005) 191. V. Palanivel, Y. Huang, W.J. Van Ooij, Prog. Org. Coat. 53 (2005) 153. W. Datchary, A. Mehner, H.W. Zoch, D.A. Lucca, M.J. Klopfstein, R. Ghisleni, D. Grimme, E. Brinksmeier, J. Sol–Gel Sci. Technol. 35 (2005) 245. Z. Zong, J. He, M.D. Soucek, Org. Coat. 53 (2005) 83. M.D. Soucek, Z. Zong, A.J. Johnson, JCT Res. 3 (2006) 133. M.L. Zheludkevich, D.G. Shchukin, K.A. Yasakau, H. Mohwald, M. Ferreira, Chem. Mater. 19 (2007) 402. K.H. Wu, C.M. Chao, T.F. Yeh, Surf. Coat. Technol. 201 (2007) 5782. M. Nanna, G.P. Bierwagen, J. Coat. Technol. 1 (2) (2004) 52. D. Battocchi, G.P. Bierwagen, Corros. Sci. 48 (2006) 1292. D. Battocchi, G.P. Bierwagen, Corros. Sci. 48 (2006) 2226. US Patent No. 6077885, 2000. US Patent No. 6579472, 2003. US Patent No. 7141306, 2006. O. de Sanctis, L. Gomez, A. Marajofsky, C. Parodi, N. Pellegri, A. Duran, J. Non Cryst. Solids 121 (1990) 315. M. Pilz, H. Romich, J. Sol–Gel Sci. Technol. 8 (1997) 1071. W. Boysen, A. Frattini, O. Sanctis, Surf. Coat. Technol. 122 (1999) 14. E. Bescher, J.D. Mackenzie, J. Sol–Gel Sci. Technol. 26 (2003) 1223. J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. F. Hehmann, J. Mater. Sci. 24 (1989) 2369. A.N. Khramov, V.N. Balbyshev, L.S. Kasten, R.A. Mantz, Thin Solid Films 514 (2006) 174.