《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_brittleness4

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS Progress in Organic Coatings xxx(2008)xXx-XXX Progress in Organic Coatings ELSEVIER journalhomepagewww.elsevier.com/locate/porgcoat Review Sol-gel coatings on metals for corrosion protection Gordon p bier Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58105, USA ARTICLE INFO A BSTRACT Sol-gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corro- Received 31 October 2007 sion resistance for metal substrates. Further, the sol-gel method is an environmentally friendly technique eceived in revised form 12 August 2008 of surface protection and had showed the potential for the replacement of toxic pretreatments and coat- Accepted 12 August 2008 ngs which have traditionally been used for increasing corrosion resistance of metals. This review covers he 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 orrosion resistance and future research on sol-gel corrosion protective coatings are also briefly discussed. Protective coatings o2008 Published by Elsevier B V. Contents 1. Introduction 2. 1. Brief history of sol-gel chemistry 3. Corrosion protective sol-gel coatings 3. 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 000000 3.2. 1. Metal oxide coatings 22. Organic-inorganic hybrid sol-gel coatings Hybrid sol-gel magnesium-rich coatings 3.3. Copper and magnesium substrates 4. Challenges and future studies of sol-gel corrosion protective coatings 4. 1. Basic theory studies of sol-gel coating 4. 2. Optimization and new synthesis routes of sol-gel coatings 4.3. New raw materials and multiple component systems 5. Conclusions Acknowledgments 1. Introduction cations and cultural heritage, etc. while these metals are useful because of their physical characteristics, such as stiffness and high Metals, such as iron, aluminum, copper and magnesium and strength to weight ratios, they are highly susceptible to corrosion in their alloys are used in a myriad of structural, marine, aircraft appli- aggressive environments. Corrosion is always the major reason of energy and material loss. It was reported that 1 /5 of energy globally nd average 4. 2% of gross national product(GNP)is lost each year due to corrosion [ 1] and the economic impact of corrosion is esti- E-mail address: Gordon. Bierwagenendsuedu(Gordon.P Bierwag mated to be greater than $100,000,000,000 per year in the United ont matter 2008 Published by elsevier B V Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi: 10.1016/j-porgcoat 2008. 08.010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 Progress in Organic Coatings xxx (2008) xxx–xxx 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 article info Article history: Received 31 October 2007 Received in revised form 12 August 2008 Accepted 12 August 2008 Keywords: Sol–gel Corrosion resistance Protective coatings abstract Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corro￾sion 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 coat￾ings 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. Introduction ............................................................................................................................... . . . . . . . . . . . 00 2. General background of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1. Brief history of sol–gel chemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2. Preparation of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3. Corrosion protective sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Steel substrates............................................................................................................................... . 00 3.1.1. Metal oxide coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1.2. Organic–inorganic hybrid sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1.3. Inhibitor doped sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1.4. Inorganic zinc-rich coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2. Aluminum substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2.1. Metal oxide coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2.2. Organic–inorganic hybrid sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2.3. Hybrid sol–gel magnesium-rich coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3. Copper and magnesium substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4. Challenges and future studies of sol–gel corrosion protective coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.1. Basic theory studies of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.2. Optimization and new synthesis routes of sol–gel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.3. New raw materials and multiple component systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5. Conclusions ............................................................................................................................... . . . . . . . . . . . . 00 Acknowledgments ............................................................................................................................... . . . . 00 References ............................................................................................................................... . . . . . . . . . . . . . 00 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). 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 esti￾mated to be greater than $100,000,000,000 per year in the United 0300-9440/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2008.08.010

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS States alone [2]. This cost includes the application of protective Ten year ago, Guglielmi [7] has already discussed the potential coatings(paint, surface treatment, etc. ) inspection and repair of of sol-gel coatings as a corrosion inhibiting system for metal sub corroded surfaces and structures, and disposal of hazardous waste strates. Since then, a great deal of work has been done to make ls. a generic way to protect metals from corrosion is to various sol-gel based protective coatings. This review will intro- tective films or coa which also permit the desired duce the basic chemistry involved in sol-gel processes, then the s of the substrate to be coated through the chemical mod progress and development of sol-gel protective coatings on metal ication of the coatings 3, 4], such as mechanical strength, optical substrate, such as steel, aluminum, etc. Finally some problems and appearance, bioactivity, etc. future work on sol-gel coatings will be summarized briefly. There are several techniques for the deposition of coatings on metals, including physical vapor deposition(PVD), chemical vapor deposition (CVD), electrochemical deposition, plasma spray- 2. General background of sol-gel coatings ing and sol-gel process. There are many advantages using sol-g 2.1. Brief history of sol-gel chemistry coatings, several most important features are listed as follow: The sol-gel process is a chemical synthesis method initially used for the preparation of inorganic materials such as glasses and (A) Sol-gel processing temperature generally is low, frequently ceramics [ 8. And this process can be traced back to 1842, when close to room temperature. Thus thermal volatilization and French chemist, J.]. Ebelmen reported the synthesis of uranium degradation of entrapped species, such as organic inhibitors, oxide by heating the hydroxide but the aging and heating process last almost a year to avoid cracking which made it difficult for wider (B)Since liquid precursors are used it is possible to cast coatings in application and did not catch many eyes that time [9]. It was not complex shapes and to produce thin films without the need for until 1950s, when Roy and his colleague changed the traditional ng or melting. sol-gel process into the synthesis of new ceramic oxides, making (C)The sol-gel films are formed by "green"coating technologies: the sol-gel silicate powders quite popular in the market [10-12). It uses compounds that do not introduce impurities into the In 1971, the production process of so-called low-bulk density end product as initial substances, this method is waste-free and silica involving the hydrolysis of tetraethoxysilane(teos)in the excludes the stage of washing. presence of cationic surfactants was patented [13. In the middle HYDROLYSIS H2O ROH +五2O HO t ROH H2o HO HO CONDENSATION H+Ro—Si t ROH HOH Fig. 1. Hydrolysis and condensation involved in making sol-gel derived silica materials. Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi:10.1016/ porgcoat.200808010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 2 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 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 mod￾ification 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 spray￾ing 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 sub￾strates. Since then, a great deal of work has been done to make various sol–gel based protective coatings. This review will intro￾duce 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

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS D Wang. Gordon. P. Bierwagen/ Progress in Organic Coatings xxx(2008)xxx-xXx 1980s, many material scientists and chemists, represented by H. Table 1 hmidt and G L. Wilkes started to synthesis organic-inorgani bbreviation, chemical name and functional group of some commonly used hybrid materials(OIHMs) by sol-gel process and published a recursors for sol-gel protectiv ries of pioneering research articles [14-17. Since then, sol-gel Abbreviation Chemical Functional group echnology has attracted a great deal of attention, especially in TEoS Tetraethyl orthosilicate the fields of ceramics, polymer chemistry, organic and inorganic TMOS chemistry, physics and played an indispensable role in preparing MIES novel OIHMs 5, 18, 19 PTMS 2. 2. Preparation of sol-gel coatings phonate- The sol-gel process can be described as the creation of an oxide APS Amino- network by progressive condensation reactions of molecular pre- AEAPS Amino- cursors in a liquid medium[18. Basically, there are two ways to prepare sol-gel coatings: the inorganic method and the organic GPTMS nethod. The inorganic method involves the evolution of networks hrough the formation of a colloidal suspension(usually oxides) MAPTS y-Methacryloxypropyl Methacryloxy and gelation of the sol(colloidal suspension of very small particles, MPTMS 1-100nm)to form a network in continuous liquid phase. But the most widely used method is the organic approach, which generally BIsTs Bis-3-(triethoxysilyl)- Sulfide. 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, of the most commonly used alkoxysilanes in sol-gel protective coatings area. Generally, the sol-gel formation occurs in four stages: (a) hydrolysis, (b)condensation and polymerization of monomers to 3. Corrosion protective sol-gel coatings orm chains and particles. (c) growth of the particles, (d)agglom eration of the polymer structures followed by the formation of 3.1. Steel substrates networks that extend throughout the liquid medium resulting in Steel and stainless steel are widely used in different indu condensation reactions occur simultaneously once the hydrolysis trial fields because of their mechanical and corrosion properties reaction has been initiated. As seen in Fig. 1. both the hydrolysis and However, they still tend to corrode in the presence of halide condensation steps generate low-molecular weight by-products ions. The corrosion resistance behavior of sol-gel coatings or thin such as alcohol and water. Upon drying. these small molecules are films deposited onto steel substrate has been extensively studied driven off and the network shrinks as further condensation may [26-45], as summarized in Table 2 following the time of publication. occur. These processes are basically affected by the initial rea ion conditions, such as pH, temperature molar ratios of reactants, 3. 1. Metal oxide coatings solvent composition, etc. Readers may refer to other studies and Sio2, Zro2 Al2O3, TiO2 and CeO2, etc. all have very good chemical eviews for a more complete understanding of the entire sol-gel stability and can provide effective protection to metal substrate. process6-8.18,19] Sioz can improve the oxidation and acidic corrosion resistance A sol-gel coating can be applied to a metal substrate through of metals under different temperatures due to its high heat resis- various techniques, such as dip-coating and spin-coating, which are tance and chemical resistance[ 29, 34 Vasconcelos et al. made sio the two most commonly used coating methods. Spraying 20, 21 coating on AlSl 304 stainless steel using tetraethyl orthosilicate and electrodeposition [22-24 also emerged recently and could (TEOS)as chemical precursor [34. It was found that the coating be the major sol-gel coating application methods in the future. contained Si,O and Fe elements and formed a transition layer But whatever technique is used, after the coating deposition, there between steel substrate and Sio 2 layer. The obtained sol-gel silica is a substantial volume contraction and internal stress accumula- coatings were homogeneous, free of cracks. Samples were teste tion due to the large amount of evaporation of solvents and water. in 1 mol/ LH2S04 solution and 3. 5% Nacl solution, both corrosion Cracks are easy to form due to this internal stress if the film for- potential increased and corrosion current density decreased, indi- mation conditions are not carefully controlled. Usually the curing cating this 100 nm thin Sio2 layer improved the anti-corrosion and heat treatment of sol-gel coatings vary substantially depend performance of stainless steel substrate. ing on different microstructures, quality requirement and practical ZrOz has a high expansion coefficient very close to many bulk metals, which can reduce the formation of cracks during high tem- The formation of silica sol-gels also holds true for non-silicate perature curing process [ 26, 36. ZrO2 also shows good chemical inorganic alkoxides. In fact, metal alkoxides of titanium, zirco- stability and high hardness 35 which makes it a good protective ium, tin or aluminum are much more reactive towards water materials. Perdomo et al. 31] made Zro2 coatings on 304 stainless than alkoxysilanes due to the lower electronegativity and high steel by sol-gel method using zirconium propoxide as precursor ewis acidity[8, 25]. But it is that the reaction is quite gentle and densified in air and in oxygen-free(argon or nitrogen)atmo- and mild makes the alkoxysilanes studied most extensively in the spheres. The corrosion behavior of the stainless steel substrate was formation of sol-gel materials, especially OIHMs. Alkoxysila studied by potentiodynamic polarization curves. It was found that including tetraoxy silicate(Si(oR)4)and organically modified sil- the Zro coatings extended the lifetime of the material by a factor icates (Ormosils, R'n Si(oR)4-n or(RO)3Si R'Si(OR)3) have been of almost eight in a very aggressive environment, independently the most widely used metal-organic precursors for preparation of of the preparation procedure. In order to improve the adhesion hybrid materials by sol-gel processing. Table 1 and Fig. 2 lists some between protective organic coating and metal substrate, Fedrizzi Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi: 10.1016/j-porgcoat 2008. 08.010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 3 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 pre￾cursors 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 (CxH2x+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) agglom￾eration 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 reac￾tion 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 accumula￾tion due to the large amount of evaporation of solvents and water. Cracks are easy to form due to this internal stress if the film for￾mation conditions are not carefully controlled. Usually the curing and heat treatment of sol–gel coatings vary substantially depend￾ing 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, zirco￾nium, 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 sil￾icates (Ormosils, R’n Si(OR)4−n or (RO)3Si 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 Functional group TEOS Tetraethyl orthosilicate TMOS Tetramethyl orthosilicate MTES Methyl triethoxysilane Methyl￾MTMS Methyl trimethoxysilane Methyl￾VTMS Vinyl trimethoxysilane Vinyl￾PTMS Phenyl trimethoxysilane Phenyl￾PHS Diethylphosphonatoethyl triethoxysilane Phosphonato￾APS 3-Aminopropyl trimethoxysilane Amino￾AEAPS 3-(2-Aminoethyl)aminopropyl trimethoxysilane Amino￾GPTMS 3-Glycidoxypropyl trimethoxysilane Glycido￾MAPTS -Methacryloxypropyl trimethoxysilane Methacryloxy￾MPTMS -Mercaptopropyl trimethoxysilane Mercapto￾BTSTS Bis-[3-(triethoxysilyl)- propyl]tetrasulfide Sulfide￾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 indus￾trial 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 inTable 2 following the time of publication. 3.1.1. Metal oxide coatings SiO2, ZrO2, Al2O3, 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 resis￾tance 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 H2SO4 solution and 3.5% NaCl solution, both corrosion potential increased and corrosion current density decreased, indi￾cating 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 tem￾perature 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) atmo￾spheres. 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

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS TEOS TMOS MTES MTMS VTMS PTMS PHS HaNs HNH AEAPS GPTMS MAPTS MPIMS BTSTS Fig. 2. Chemical structure of some commonly used alkoxysilane precursors for sol-gel protective coating. et al. [35]prepared Zroz sol-gel coating on low carbon steel sheets, readily. However, on the other hand, an increase in the sintering then applied polyester organic coating onto the Zroz layer Accord- temperature resulted in a marked increased on the anodic branch g to adhesion testing, the samples pretreated with Zro2 layer of the polarization curve and thus increased the number of defects owed promising performance, in comparison with commercial in the coating. hemical treatments, such as tricationic phosphate and iron phos- TiO2 has excellent chemical stability, heat resistance and low phate pretreatment. Li et al. 36 also reported on thin ZrO2 sol-g electron conductivity, making it an excellent anti-corrosion mate- im on mild steel sheets, and found that Zroz layers heat-treated rial. But pure TiO2 film is mostly used in catalyst chemistry. Very at 400.C and 800C were homogeneous, crack-free and increased few TiO films have been reported as protective coatings on steel he corrosion resistance of the mild steel by a factor of 6.3 and 2.3, substrate[28. CeOz is in the similar situation, although widely used in optics, catalyst chemistry, pigments, superconductors and sen- Al2O3 is a well-known insulator and has very low conduc- sors, cerium is more popular in hybrid sol-gel coatings as corrosion ivity for transmitting electrons, which is ideal for protective inhibitors (41, 44, which will be discussed later. coatings. Masalski et al. 33 prepared two, four- and six-layer Two and multiple-component oxide coatings can overcome the Al203 coatings on AlSI 316 stainless steel in order to improve its limitation of single-component oxide layers, broaden their appli ocal anti-corrosion ability. It was found that the cathode cur- cation areas and improve the comprehensive protective abili rent density varied with sintering temperature: higher sintering of steel substrates. Early works, such as Atik et al. [ 26] reported temperature(within the range 500-850 C). the lower cathode cur- 70SiO2-30TiO2 and 75SiO2-25A1203 acting very efficiently as rent density values, but also the lower breakdown potentials. The corrosion protectors of 316L stainless steel substrates in aque author believed that at higher temperatures conversion of y-Al2O3 ous Nacl and acid media at room temperature. The films could (less resistant to aggressive agents)into the a-Al2O3 modification increase the lifetime of the substrate by a factor of up to 10 (corundum, more resistant to aggressive agents) proceeds more in 3% Nacl and 5 in 15% H2S04 solutions. In order to improve Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi:10.1016/ porgcoat.200808010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 4 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 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. Accord￾ing to adhesion testing, the samples pretreated with ZrO2 layer showed promising performance, in comparison with commercial chemical treatments, such as tricationic phosphate and iron phos￾phate 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. Al2O3 is a well-known insulator and has very low conduc￾tivity for transmitting electrons, which is ideal for protective coatings. Masalski et al. [33] prepared two-, four- and six-layer Al2O3 coatings on AISI 316 stainless steel in order to improve its local anti-corrosion ability. It was found that the cathode cur￾rent density varied with sintering temperature: higher sintering temperature (within the range 500–850 ◦C), the lower cathode cur￾rent density values, but also the lower breakdown potentials. The author believed that at higher temperatures conversion of -Al2O3 (less resistant to aggressive agents) into the -Al2O3 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 mate￾rial. 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 sen￾sors, 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 appli￾cation areas and improve the comprehensive protective ability of steel substrates. Early works, such as Atik et al. [26] reported 70SiO2-30TiO2 and 75SiO2-25A12O3 acting very efficiently as corrosion protectors of 316L stainless steel substrates in aque￾ous 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% H2SO4 solutions. In order to improve

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS D Wang. Gordon. P. Bierwagen/ Progress in Organic Coatings xxx(2008)xxx-xXx Corrosion protective sol-gel coatings on steel substrates Composition and precursors Steel substrate oating method Thickness (um) Reference and year TiOz-SiO2 316L SS Dip-coating 04-06 26】1995 Al2O3-SiOz Zroz-PMMA 71997 Dip-coating Oz-Ca0-P2O5 316L SS Dip-coating 04-14 29]1998 CH3-SiOg B,O3-SiO2 Dip-coating 0.2-2 1301998 304sS O,-PMMA Dip-coating 2.0-3.0 0.3-06 TEOS-MAPTS 04 SS Dip-coating 0.2 1371200 Dip-coating SiO2-Na20 Zinc-plated steel Electrodepositing 10 2003 Iron 10-1 139】2003 Sioz-PMMA 304SS SiO2-PVB Zinc-plated steel Dip-coating 4012004 Dip-coat 2.1-2.5 141120 TEOS-MTES Galvanized steel Cerium-TEOS-MTES pin-coating 19-20 442006 CaO-P2Os 316Ls Spin-coating the bioactivity and corrosion resistance of an implant material, and organic phases: (1) mix organic component directly into Vijayalakshmi and Rajeswari[45 recently reported the prepa- the inorganic sol-gel system, the product is a simple mix ration of Cao-P2O5 coating on 316L stainless steel. The sol-gel ture, and there is no chemical bonding between organic and film had combined effects of good adherence with higher cor- inorganic components: (2) utilize already existing functional psion resistance acting as a diffusion barrier and could be groups within the polymeric/ oligomeric species to react with used as a potential material for implantation purposes. Similar the hydrolized of inorganic precursors, thus introducing chemi- SiO2-CaO-P2O5 coating was also studied to improve the corro- cal bonding between them; (3)use alkoxysilanes r'n Si(or)4-nm sion resistance and bioactivity of stainless steel implant material as the sole or one of the precursors of the sol-gel process with Rbeing a second-stage polymerizable organic group often car- ried out by either a photochemical or thermal curing following the 3.1.2. Organic-inorganic hybrid sol-gel coatings sol-gel reaction, e.g. methacryloxy group in MAPS(see Table 1 and From the studies above the inorganic oxide coatings can provide Fig. 2). good protection on metal substrates. But there are still some major Atik et al. [27 ] made hybrid coatings of polymethylmethacrylate drawbacks of these coatings, from the standpoint of corrosion resis- (PMMA)and ZrOz onto 316L stainless steel. Coatings'anticorrosion tant layers: (1)oxide films are brittle and thicker coatings(>1 um) behavior was analyzed in 0.5 M H2SO4 solution through potentia- are difficult to achieve without cracking: (2)relatively high temper- dynamic polarization curves at room temperature. The coatings atures(400-800 C)are often required to achieve good properties act as geometric blocking layers against the corrosive media and ncrease the lifetime of the substrate up to a factor 30. Messaddeq To overcome the limitation of pure inorganic sol-gel coatings, et al. [32] analyzed the microstructure of ZrO2-PMMA coating by as brittleness and high temperature treatment, much work scanning electron (SEM)and atomic force microscopy(AFM)and been done to introduce organic component into the ino found that zirconium concentrated domains were surrounded by ganic sol-gel to form the organic-inorganic hybrid sol-gel coatings. continuous PMMA secondary phase domains. Maximum corrosion These materials turned out to be among the most interesting areas resistance of the substrate was observed for the coating contain of coatings science in last decade[27, 32, 39-44]. ing 17 voL% PMMA. Higher PMMA volume made thicker coatings Though many organic(polymeric/oligomeric) species have but tended to form a single-phase structure at the micrometer een successfully incorporated within inorganic networks by dif- scale and their adhesion to the substrate was worse resulting in ferent synthetic methods, they are classified into three major the breakdown and the peeling of the coating during the electro- approaches according to the chemical bond between inorganic chemical testing. Similarly, a Sio2-PVB(polyvinyl butyral) hybrid Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi: 10.1016/j-porgcoat 2008. 08.010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 5 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 316L SS Dip-coating 0.4–0.6 [26] 1995 Al2O3-SiO2 ZrO2-PMMA 316L SS Dip-coating 0.2 [27] 1997 CeO2 304 SS Dip-coating 0.5 [28] 1997 TiO2 SiO2 316L SS Dip-coating 0.4–1.4 [29] 1998 SiO2-CaO-P2O5 CH3-SiO2 B2O3-SiO2 304 SS, 430 SS Dip-coating 0.2–2 [30] 1998 MgO-SiO2 ZrO2 304 SS Dip-coating 0.7 [31] 1998 ZrO2-PMMA 316L SS Dip-coating 0.2–1.0 [32] 1999 Al2O3 316L SS Dip-coating 2.0–3.0 [33] 1999 SiO2 304 SS Dip-coating 0.15 [34] 2000 ZrO2 Carbon steel Dip-coating 0.3–0.6 [35] 2001 ZrO2 Mild steel Dip-coating [36] 2001 TEOS-MAPTS 304 SS Dip-coating 0.2 [37] 2001 TEOS￾MAPTS 304 SS Dip-coating 0.2 [38] 2003 316L SS SiO2-Na2O Zinc-plated steel Electrodepositing 1.0 [22] 2003 APS Iron plate Dip-coating 10–12 [39] 2003 AEAPS GPTMS MAPTS SiO2-PMMA 304 SS, Dip-coating 1.0 [40] 2004 SiO2-PVB Zinc-plated steel Cerium-APS Carbon steel Dip-coating 2.1–2.5 [41] 2005 TEOS-MAPTS Carbon steel Brushing N/A [42] 2006 TEOS-MTES Galvanized steel Dip-coating 4.0 [43] 2006 Cerium-TEOS-MTES 304 SS Spin-coating 1.9–2.0 [44] 2006 CaO-P2O5 316L SS Spin-coating 1.0 [45] 2007 the bioactivity and corrosion resistance of an implant material, Vijayalakshmi and Rajeswari [45] recently reported the prepa￾ration of CaO-P2O5 coating on 316L stainless steel. The sol–gel film had combined effects of good adherence with higher cor￾rosion resistance acting as a diffusion barrier and could be used as a potential material for implantation purposes. Similar SiO2-CaO-P2O5 coating was also studied to improve the corro￾sion 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 resis￾tant layers: (1) oxide films are brittle and thicker coatings (>1 m) are difficult to achieve without cracking; (2) relatively high temper￾atures (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 inor￾ganic 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 dif￾ferent 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 mix￾ture, 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 chemi￾cal 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 car￾ried 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 H2SO4 solution through potentio￾dynamic 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 contain￾ing 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 electro￾chemical testing. Similarly, a SiO2-PVB (polyvinyl butyral) hybrid

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS 1.00= vided good protection for the zinc coating underneath and steel sawples substrate at the same time 3.1.3. Inhibitor doped sol-gel coatings 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 Ce+ oxide film insen- sitive to Cl- dissolution over the metal surface. The coating thickness was about 2.5 um and extended the useful lifetime of steel exposed in a salt-fog chamber from only 10 h to -768h, and aluminum panels from 40h to 1440h. Duran and Co- Fig 3. AFM images of 90% TEOS-10% MAPTS sol-gel coating on a stainless steel workers [44] also studied cerium ions loaded hybrid silica sol-gel substrate [37- coatings deposited on AlSI 304 stainless steel substrates. It was found that coatings prepared with the Ce (lln)salt enhance the corrosion resistance through a barrier effect, but devel- ting was also deposited on Zn-plated steel substrate [40]. The oped defects later with immersion time, showing no apparent aIts of salt spray testing of substrates coated with hybrid sol-gel inhibiting effects. At the same time, coatings obtained from Ce Ims indicated that SiO2-1 wt% PVB coating was the relatively best(iv) chemicals enhanced the coating performance, probably due one against corrosion and crack-free even its thickness was close toto the formation of Ce(oH)3 on the surface through a chemi- callelectrochemical mecl involving a redox reaction between Chouet al. [37,38]. prepared hybrid sols by copolymerizing TEos Cr and Ce ions and MAPTS with a two-step acid-catalyst process Then hybrid coat Through these studies of sol-gel coatings on steel substrates gs were dip-coated on 304 stainless steel substrates and annealed (stainless steel, carbon steel and zinc steel). it was clear that sol-gel at 300 C for 30 min. The resultant coatings were relatively dense coating can provide effective protection against corrosive media in niform and defect free( Fig. 3)and provided excellent corrosion practical service conditions. The corrosion resistance of steel sub- protection probably because the dense physical barrier coating strates was substantially improved eparated the anode from the cathode. Ref [ 42] also reported that the tEoS-MAPTS hybrid coatings 3. 1.4. Inorganic zinc-rich coatings were uniform and crack-free while the pure inorganic coatings from In addition to the corrosion inhibitors, sacrificial metal pig- TEOS had apparent cracks on the surface( Fig. 4). Duran and co- ments such as zinc, aluminum, magnesium and their alloy particles orkers [43 used TEOS and MTES to form a thick hybrid sol-gel can also be included into the sol-gel coating formula according coating(thickness around 4 um) under basic catalyst and found to the different metal substrate to be protected. Generally, steel that the corrosion mechanisms for sol-gel galvanized steels did not substrate is often a suitable target under this cathodic protec change with respect to the uncoated steel. The sol-gel coating pro-tion 20 um 20 um Fig 4. Surface morphologies sol-gel coatings from(a)TEOS: ( b)TEOS (enlarged): (c)TEOS-MAPTS: (d) TEOS-MAPTS(enlarged)[42). Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi:10.1016/ porgcoat.200808010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 6 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 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 andMAPTS with a two-step acid-catalyst process. Then hybrid coat￾ings 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 co￾workers [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￾vided good protection for the zinc coating underneath and steel substrate at the same time. 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 insen￾sitive 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 co￾workers [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 devel￾oped 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 chemi￾cal/electrochemicalmechanism 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 sub￾strates was substantially improved. 3.1.4. Inorganic zinc-rich coatings In addition to the corrosion inhibitors, sacrificial metal pig￾ments 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 protec￾tion. Fig. 4. Surface morphologies sol–gel coatings from (a) TEOS; (b) TEOS (enlarged); (c) TEOS-MAPTS; (d) TEOS-MAPTS (enlarged) [42]

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS Inorganic zinc-rich coatings are a widely used and unique class 3. 2. Aluminum substrates of coatings that provide cathodic protection to ferrous and stee substrates. The coatings have been categorized by Steel Structures Aluminum and its alloys are obvious target substrates for corro- Painting Council(SSPC) into three major groups: (1)post-cured, sion studies due to their widespread applications 56, 57. The low vater-borne alkali metal silicates: (2)self-cured water-borne alkali cost, lightweight, high thermal and electrical conductivity grant etal silicates; (3)self-cured, solvent-borne alkyl silicates Of the aluminum a remarkable industrial and economical importance. three types, the self-cured, solvent-borne alkyl silicates have the Many of its applications are practicable due to its natural tendency greatest commercial usage by far [46]. The most common binder to form a passivating Al2O3 oxide layer, which can also be artificially recursors are tEOS and its oligomers derived from it by controlled generated by anodizing the substrate. However, this passivating partial hydrolysis with a small amount of water. Ethyl or isopropyl layer deteriorates in aggressive media, such as chloride, which alcohol is used as the principal solvent, since an alcohol helps main- results in pitting corrosion [58-62. Although current chromate ain package stability After application, the alcohol evaporates, and conversion coatings function very well in corrosion protection, water from the air completes the hydrolysis of the oligomer to yield the US Environmental Protection Agency is planning to totally a film of polysilicic acid partially converted to zinc salts. TEOS can ban the use of chromates in coating materials in the near future undergo hydration and condensation processes and form a com- because of their extremely toxic effect. a broad range of research plex polysiloxane network in atmosphere, and its final hydration being actively pursued to develop less toxic and environmentally products are Sio2 and water[47-49]. benign organic coatings for corrosion protection. One relatively Unlike the general sol-gel protective coatings, the protection new but very promising approach is sol-gel coating and has been ability of the inorganic zinc-rich coatings is more relied on the extensively studied in the last ten years[ 20, 23, 39-41,63-85, as cathodic protection of zinc pigments, rather than the barrier prop- summarized in Table 3 following the time of publication erties of the sol-gel binder. And the current research of these coatings is mainly focused on the effects of pigment volume con- 3.2.1. Metal oxide coatings centration [46,50-52], size and shape of zinc pigments 51, zinc Similar to steel substrates there have been some studies focus alloy pigments (53, 54] and extenders [55]. The studies on using ing on using pure inorganic oxide coatings, such as Sioz, ZrO2 different alkoxysilane precursors or hybrid binder materials have on aluminum substrates. Thim et al. [64] dipped aluminum(98% not been reported in the last decad AD) surfaces into silicic acid aqueous solution containing urea as Corrosion protective sol-gel coatings on aluminum substrates Composition and precursors Coating method ce and year IO3-TEOS-GPTMS Zroz-TEOS-GPTMS 叫叫 8 Dip-coating O2-zro2 Al202413 erium-SiO2-epoxy 2001 Cerium-ZrO2-GPTMS AI2024 Aminosilane-epoxy A202413 Epoxysilane-epoxy Spraying 30-50 1712001 TEOS-GPTMS Spraying TMOS-GPTM AS-amine crOss-linkers l2024T3 Dip-coating AEA5 GPTMS Dip-coating 9]200 MAPTS TEOS rodeposition 016-0.1 SiOz-BTST 04-06 SiO -PMMA Al alloy (ADC12) Dip-coating TMOS-GPTMS-organic inhibitor Dip-coating Zroz-TEOS 5N5 Zroz-GPTM Dip-coating 18-20 TEOS-GI AI20241 Spin-coating 85]2007 Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi: 10.1016/j-porgcoat 2008. 08.010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 7 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 main￾tain 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 com￾plex 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 prop￾erties of the sol–gel binder. And the current research of these coatings is mainly focused on the effects of pigment volume con￾centration [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 corro￾sion 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 Al2O3 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 focus￾ing 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 Al2O3-TEOS-GPTMS Al plate Spin-coating 7 [63] 1998 ZrO2-TEOS-GPTMS SiO2 Al plate Dip-coating N/A [64] 2000 SiO2-ZrO2 Al 2024-T3 Dip-coating 0.1 [65] 2001 ZrO2-TiO2-soybean oil Al plate Blade-casting 45–95 [66] 2001 ZrO2-TEOS-GPTMS Al 2024-T3 Dip-coating 3–4 [67] 2001 SiO2-vinylpolymer Al 2024-T3 Dip-coating 3–4 [68] 2001 Cerium-SiO2-epoxy Al 2024-T3 Dip-coating 2-3 [69] 2001 Cerium-ZrO2-GPTMS Al 2024-T3 Dip-coating 2–3 [70] 2001 Aminosilane-epoxy Al 2024-T3 Spraying 30–50 [71] 2001 Epoxysilane-epoxy Al 7075-T6 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 Al plate Dip-coating 10–12 [39] 2003 AEAPS GPTMS MAPTS TEOS MTES Al electrode Electrodeposition 0.16–0.18 [23] 2003 PTMS SiO2-BTSTS Al 2024-T3 Dip-coating 0.4–0.6 [74] 2003 Al 7075-T6 Al 6061-T6 Al 5005 SiO2-PMMA Al alloy (ADC12) Dip-coating 0.1–0.3 [40] 2004 SiO2-PVB TMOS-GPTMS-organic inhibitor Al 2024-T3 Dip-coating 1 [78] 2004 [79] 2005 Bis-silane inhibitor Al 2024-T3 Dip-coating 0.3 [80] 2005 Cerium-APS Al 3003 Dip-coating 2.1–2.5 [41] 2005 ZrO2-TEOS-MAPTS Al disk Spin-coating 1.9–7.5 [81] 2005 ZrO2-GPTMS inhibitor Al 2024-T3 Dip-coating 1.8–2.0 [84] 2007 TEOS-GPTMS-PDMS Al 2024-T3 Spin-coating N/A [85] 2007 Al 6061-T6

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS dry chemical control agent(DCCA)to obtain Sio2 films. But its alkoxysilane precursors, such as APS, AEAPS, GPTMS and MAPTS. adhesion was quite limited and coating developed defects during Coatings were sprayed onto aluminum substrates, then cured by UV ne heat treatment process. Then anodic polarization pretreatment or thermal source. Standard ASTM tests showed the coatings had vas used on aluminum surfaces before depositing Sio, coatings, very good adhesion on surface as well as high mechanical, chemical, nd corrosion resistance was i ed. Yang et al. [65 inve and thermal stability. tigated the corrosion behavior of 3.4SiO2-1ZrOz sol-gel coating Soucek and co-workers [66] developed the ZrO2-TiO2-soybear on Al 2024-T3 substrate under immersion in dilute Harrison s oil hybrid coatings. The beneficial synergistic effects of using solution 3.5g/L(NH4)2SO4. 0.5g/L NaCi]. SEM, AFM, EIS (electro- mixed metal oxides as the inorganic phase of hybrid coatings were chemical impedance spectroscopy )and XPS(X-ray photoelectron observed in tensile modulus and fracture toughness, but the adhe- spectroscopy )were used during the evolution of the coating system sion decreased when double sol-gel precursors were used. Also, a under immersion. It was found that pitting corrosion and degrada UV-curable hybrid coating based on epoxynorbornene linseed oil tion products on the sol-gel coating surface developed after 2 days (ENLO) was prepared using the in situ method of TEOS oligomers of immersion, the impedance increased, and it was conjectured that [ 82]. It was found that the formulations including -10 wt% TEOS aluminum oxide and silicon oxide may form a stable mixed oxide oligomers provided the optimum balance of the properties such barrier layer at the interface after initial corrosion, which prohibits as Youngs modulus, tensile strength, thermo-mechanical proper urther pitting corrosion. ties, fracture toughness, and protective properties. Soucek et al. [83 indicated later that both the inorganic and organic phases must 3.2.2. Organic-inorganic hybrid sol-gel coatings have fast curing kinetics in order for the hybrid coatings having the Hybrid sol-gel coatings are much more popular than pure inor- inorganic particles to be uniformly dispersed in the organic phase ganic oxide layers in terms of the corrosion protection of metal at nanoscale level. ily form a thicker coat in micrometer scale without cracks and sol-gel protective coatings on metal substrates [74-77. The bis- luch lower curing temperature is needed(usually 100C)than sulfur silane(bis-3-(triethoxysilyl)-propyll-tetrasulfide, BTSTS)as 00-800C for sintering of oxide layers. Second, the hybrid sol-gel a sol-gel precursor with later addition of silica nanoparticles to system has much more flexibility in adaptation of anti-corrosion make hybrid protective coatings on aluminum alloy substrates was dditives, such as inhibitors, pigments, etc so the overall corro- reported [74, 75]. It was found that the bis-sulfur silane film was on protection ability of the sol-gel system can be substantially thickened and strengthened by loading silica nanoparticles into the film up to 15 ppm in the corresponding silane solution. But Early studies, such as Chen et al. [63] made hybrid nanocom- extra amount of silica only further hardens the surface but not posite protective coatings on aluminum substrates by the the interfacial layer and may form a porous film, which promotes corporation of nanosized particles(Al203, ZrO2, Sioz)into hybrid film delamination. According to his studies, a general accepted sol-gel matrices. The obtained coatings were thick(7 um), dense, bonding mechanism of silanes to metal surfaces is illustrated in smooth and inhibited corrosion Sol-gel/epoxy resin hybrid coat- Fig. 5. Silanols(SiOH) spontaneous adsorption onto the metal sur- ngs were formulated and studied at Boeing Company [71. The face through hydrogen bonds before condensation and upon drying. hybrid coating showed enhanced mechanical strength such as forming covalent metallo-siloxane bonds(MeOSi)at the coating hardness and abrasion resistance and passed wet adhesion testing metal interface hen cured at elevated temperatures(80C). However, water Later, Van Ooij and co-workers[80] added corrosion inhibitors sensitivity remains for most of the room temperature cured hybrid (tolytriazole, benzotriazole molecules and inorganic cerium salts) coatings. Sayilkanet al [39] developed hybrid sol-gel coatings from to the sol-gel films and studied their corrosion properties in 0.5 M H -8/s|-/sH 0-SI0-0-0-si-o linear siloxane Metal oxide Me Immediately after adsorption After curing at RT or highe echanism between silane molecules and metal surface hydroxide layer: (a) before condensation: hydrogen-bonded interface; (b) face[74]- Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi:10.1016/ porgcoat.200808010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 8 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 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] inves￾tigated 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)2SO4, 0.5 g/L NaCl]. SEM, AFM, EIS (electro￾chemical 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 degrada￾tion 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 inor￾ganic oxide layers in terms of the corrosion protection of metal substrates for two main reasons. First, hybrid coatings can eas￾ily 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 corro￾sion protection ability of the sol–gel system can be substantially improved. Early studies, such as Chen et al. [63] made hybrid nanocom￾posite protective coatings on aluminum substrates by the incorporation of nanosized particles (Al2O3, ZrO2, SiO2) into hybrid sol–gel matrices. The obtained coatings were thick (7 m), dense, smooth and inhibited corrosion. Sol–gel/epoxy resin hybrid coat￾ings 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, water￾sensitivity 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 adhe￾sion 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 proper￾ties, 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 bis￾sulfur 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 sur￾face 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]

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS D Wang. Gordon. P. Bierwagen/ Progress in Organic Coatings xxx(2008)xxx-xXx NaCl solution. It was found that the organic inhibitor tolytriazole proved the overall corrosion resistance of the Al 2024-T3 alloy Immersion time: but did not impart a self-healing effect to the films, while the inor- ganic cerium inhibitor showed the property of protecting fresh exposed metal surface. ZrO2-TEOS-MAPTs hybrid coating was deposited onto alu minum disks by spin-coating technique[ 81. Coating thickness as a function of the spin-coating rotational speed and the chemical sol- malposition was investigated. It was found that the ZrO2/MAPTS molar ratio had significant influence on the sol viscosity and thus 下 n the coating thickness. Higher Zroz contents seemed to have a ositive influence on the machinability but at the same time the rittleness of the coatings increased. Wu et al. [85 also reported le GPTmS/TEOS molar ratio R had a crucial influence to the cor- rosion resistance of hybrid TEOS-GPTMS-PDMS sol-gel coatings, coatings had the relatively best anti-corrosion performance when The Air Force Research Laboratory of the United States have been 010102103104105 investigating sol-gel derived anti-corrosion coatings on aerospace aluminum substrates for long time, and has developed high quany: Fig. 6. EIS bode plots for SNAP films as a function of immersion time in dilute 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 167] and Sio2-vinylpolymer [68] systems. in corrosive media. Though the way of adding inhibitors is sim- a two-stage mechanism in pit development was observed for a ple and easy to achieve, it is very difficult to control the release sol-gel coated Al 2024-T3 samples. An initial low current stage was of internal inhibitors to the surface. The author use B-cyclodextrin associated with electrolyte penetration and pit initiation through to form complexes with inhibitors and insured the slow release of the coating to the al ubstrate surface and the second high the inhibitor and its continuing delivery to corrosion sites. Later, current stage was associated with active pit growth in the metal Khramov et al. [79] also tried to deliver inhibitors by the ion surface regions. The growth of pits caused coating cracking and exchange strategy, but resulted in poorer performance compare even delamination by the pressure of corrosion products and gases. with the B-cyclodextrin complexation strategy These studies can help to identify the ways for the improvement of corrosion protection through sol-gel coatings: elimination of 3. 2.3. Hybrid sol-gel magnesium-rich coatings cracks or defects in the coating or use of corrosion inhibitors to By analogy to the formulation of zinc-rich coatings for the pro extend or terminate the initial stage of pitting developmen tection of steel, magnesium-rich, magnesium pigmented coatings Rare earth elements, such as cerium, yttrium, hafnium, can can also be formulated for the corrosion protection of aluminum improve coating properties by unifying crystal size, reducing amor- alloy. Bierwagen and the corrosion group at NDSU are leading this phous structures and brittleness upon heating treatment [28. Mg-rich coatings research and a series of articles have been pub- Kasten et al. [69, 70] added small amount of cerium(1-3 wt%)into lished [86-88]on their properties. Recently, hybrid sol-gel Mg-rich hybrid sol-gel systems. Cerium showed effective cathodic inhibi- primers were developed from MAPTS-TEOS-PVB-Mg particles sys- tion in aluminum when good dispersion was achieved. tem. Firstly, MAPTS and tEoS were hydrolyzed under the acidic Voevodin et al. [72, 73 developed sol-gel surface treatment condition, then small amount of Pvb powder was added into the coatings based on the Self-assembled Nanophase Particle(SNAP) sol, finally certain amount of magnesium pigments were mixed and approach in order to replace the traditional chromate-based sur- sprayed onto the aluminum alloy substrates. The resultant coatings face treatments on aircraft aluminum alloys. Through the in situ are 45-55 um thick and crack-free. Fig. 7 shows the SEM image formation in an aqueous sol-gel process, the functionalized silica of this hybrid Mg-rich coating. The pigment volume concentration nanoparticles could cross-link with each other to form a dense, elastic thin film well covered and adhered onto aluminum al substrate surface. From the Eis bode plots of Fig. 6. the low fre quency impedance modulus, IZ initially as high as on the order of 107 s cm2 and maintained that value for almost 2 months and still had 1 x 10692 cm2 after 80 days of immersion. Considering the thickness of the SNAP films was only around 1 um, their barrier properties was quite amazing and showed good potential as lon erm 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 B-cyclodextrin and cross-linker(diethylenetriamine DETA)were added sequentially. Finally, coating was deposited onto Al 2024 T3 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 Fig. 7. SEM image of hybrid sol-gel Mg-rich primer(35% PvC). Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi: 10.1016/j-porgcoat 2008. 08.010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx 9 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 inor￾ganic cerium inhibitor showed the property of protecting fresh exposed metal surface. ZrO2-TEOS-MAPTS hybrid coating was deposited onto alu￾minum disks by spin-coating technique [81]. Coating thickness as a function of the spin-coating rotational speed and the chemical sol￾composition 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 cor￾rosion 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 amor￾phous 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 inhibi￾tion 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 sur￾face 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 fre￾quency 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 long￾term 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 2024- T3 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 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 sim￾ple 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 ion￾exchange 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 pro￾tection 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 pub￾lished [86–88] on their properties. Recently, hybrid sol–gel Mg-rich primers were developed from MAPTS-TEOS-PVB-Mg particles sys￾tem. 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).

G Model POC-2114: No of Pages 12 ARTICLE IN PRESS Corrosion protective sol-gel coatings on Cu and Mg substrates Composition and metal-organic precursors Cu and Mg substrate Coating method Reference and year Bronze Dip-coating PTMS-MTMS SiOz-MAPTS-MPTMS TEOS-PHS Mg AZ31B Dip-coating 98]2006 (PVC) was 35%, which was a little lower than the critical volume thermal expansion coefficient Zro component. SEM showed the pigment concentration(42% in this formulation), making a rough coating surface was uniform, defects-free and well adhered to the but still non-porous surface. substrate surface. Bescheret al [95]reported using GPTMS+ MTMS Coatings were evaluated by AStM standard tests an as precursors to form hybrid sol-gel coatings on copper and bronze hat this hybrid coating had excellent chemical resistand surfaces. The coatings had a strong adhesion on the substrates and to the combination of barrier properties from sol-gel binder and no corrosion products appeared after two years exposure to high cathodic protection by magnesium particl sulfur/ humidity conditions. The anti-corrosion ability was further Currently, sol-gel coatings have been used on aerospace alu- improved after top-coated with a lymer laver. the minum alloy substrates for corrosion protection [ 89-91. The believed this is because of the high degradation resistance of the corrosion events from etching electrolytes, temperature gradients fluoropolymer. or mechanical stresses can be controlled and limited substantially Magnesium and its alloys have many useful properties, such The traditional chromate pretreatment on aluminum alloys can be as high strength-to-weight ratio, good thermal conductivity high replaced by this environmentally benign sol-gel protective coat- damping characteristics, good machinability and attracted a revival of interest in industrial applications recently (96 However, magne- sium and its alloys are highly susceptible to corrosion, which limits MAPTS-2% MPTMS-30% SiO, as hybrid coating materials, photoini ol-gel coating were also investigated for the protection of cop- tiator was also added for later UV curing process. The resulted per and magnesium surfaces, and several studies are summarized coating was defects-free and could be as thick as 23 um, provid in Table 4 ing excellent physical barrier against corrosive attack. Khramov Copper and bronze are popular metal materials for sculptures et al. [98] used phosphonate functional silane(diethylphospho- and kitchen utensils owing to their beautiful appearance and low natoethyl triethoxysilane, PHS) and found its phosphonate groups hemical reactivity. But in wet environment, their corrosion ss have more affinity on the magnesium surface than silane head- will be accelerated by forming hydroxides and harmful complexes. groups( Fig 9), thus forming a sol-gel coating with phosphonate Sio, sol-gel coatings have been reported as barrier layer on copper binding to magi surface. The improved corrosion protec [7, 92.93]. but the coatings tended to peel off when raising temper- tion of phosphonate-containing coatings as compared to pure silica ature up to 400 C since the thermal expansion coefficients of SiO2 sol-gel coatings has been observed and explained by the favorable Ind Cu are quite different. Boysen et al. [94] formulated the inor- combination of barrier properties of the organo-silicate matrix with ganic Sio2-Zro2 coating on copper surface. The interface adhesion strong chemical bonding of phosphonate groups to the magnesium and stability problems were expected to be solved by using high 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 5cm. Please cite this article in press as: D. Wang, Gordon. P. Bierwagen, Sol-gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008). doi:10.1016/ porgcoat.200808010

Please cite this article in press as: D. Wang, Gordon.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. (2008), doi:10.1016/j.porgcoat.2008.08.010 ARTICLE IN PRESS G Model POC-2114; No. of Pages 12 10 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings xxx (2008) xxx–xxx Table 4 Corrosion protective sol–gel coatings on Cu and Mg substrates Composition and metal-organic precursors Cu and Mg substrate Coating method Thickness (m) Reference and year GPTMS, MAPTS Bronze Spraying 10–12 [93] 1997 SiO2-ZrO2 Copper Dip-coating 1–3 [94] 1999 GPTMS-MTMS Bronze copper Brushing 5–10 [95] 2003 SiO2-MAPTS-MPTMS Mg AZ91D Spraying 21–23 [21] 2005 TEOS-PHS Mg AZ31B Dip-coating 0.6–0.7 [98] 2006 (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, abra￾sion 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 alu￾minum 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 coat￾ings. 3.3. Copper and magnesium substrates Sol–gel coating were also investigated for the protection of cop￾per 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 temper￾ature up to 400 ◦C since the thermal expansion coefficients of SiO2 and Cu are quite different. Boysen et al. [94] formulated the inor￾ganic SiO2-ZrO2 coating on copper surface. The interface adhesion and stability problems were expected to be solved by using high 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,magne￾sium 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, photoini￾tiator was also added for later UV curing process. The resulted coating was defects-free and could be as thick as 23 m, provid￾ing excellent physical barrier against corrosive attack. Khramov et al. [98] used phosphonate functional silane (diethylphospho￾natoethyl triethoxysilane, PHS) and found its phosphonate groups have more affinity on the magnesium surface than silane head￾groups (Fig. 9), thus forming a sol–gel coating with phosphonate binding to magnesium surface. The improved corrosion protec￾tion 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-silicatematrix 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