《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