REVIEW Sol Gel Coatings on Metal for Corrosion Protection

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  • Progress in Organic Coatings 64 (2009) 327338

    Contents lists available at ScienceDirect

    Progress in Organic Coatings

    journa l homepage: www.e lsev ier .com

    Review

    Solge io

    Duhua WDepartment o

    a r t i c

    Article historReceived 31Received in rAccepted 12

    Keywords:SolgelCorrosion reProtective co

    Contents

    1. Intro2. Gen

    2.1.2.2.

    3. Corr3.1.

    3.2.

    3.3.4. Chal

    4.1.4.2.4.3.

    5. ConcAcknRefe

    1. Introdu

    Metalstheir alloy

    CorrespE-mail ad

    0300-9440/$doi:10.1016/jf Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58105, USA

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    y:October 2007evised form 12 August 2008August 2008

    sistanceatings

    a b s t r a c t

    Solgel protective coatingshave shownexcellent chemical stability, oxidation control andenhancedcorro-sion resistance for metal substrates. Further, the solgel method is an environmentally friendly techniqueof 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 coversthe recent developments and applications of solgel protective coatings on different metal substrates,such as steel, aluminum, copper, magnesium and their alloys. The challenges for industrial productionsand future research on solgel corrosion protective coatings are also briey discussed.

    2008 Published by Elsevier B.V.

    duction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327eral background of solgel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

    Brief history of solgel chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328Preparation of solgel coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

    osion protective solgel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329Steel substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3293.1.1. Metal oxide coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3293.1.2. Organicinorganic hybrid solgel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3313.1.3. Inhibitor doped solgel coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3323.1.4. Inorganic zinc-rich coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332Aluminum substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3333.2.1. Metal oxide coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3333.2.2. Organicinorganic hybrid solgel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3343.2.3. Hybrid solgel magnesium-rich coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Copper and magnesium substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

    lenges and future studies of solgel corrosion protective coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337Basic theory studies of solgel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337Optimization and new synthesis routes of solgel coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337New raw materials and multiple component systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

    lusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337owledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337rences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

    ction

    , such as iron, aluminum, copper and magnesium ands are used in amyriad of structural,marine, aircraft appli-

    onding author.dress: [email protected] (Gordon.P. Bierwagen).

    cations and cultural heritage, etc. While these metals are usefulbecause of their physical characteristics, such as stiffness and highstrength toweight ratios, they are highly susceptible to corrosion inaggressive environments. Corrosion is always the major reason ofenergy andmaterial loss. It was reported that 1/5 of energy globallyand average 4.2% of gross national product (GNP) is lost each yeardue 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

    see front matter 2008 Published by Elsevier B.V..porgcoat.2008.08.010l coatings on metals for corrosion protect

    ang, Gordon. P. Bierwagen / locate /porgcoat

    n

  • 328 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338

    States alone [2]. This cost includes the application of protectivecoatings (paint, surface treatment, etc.), inspection and repair ofcorroded surfaces and structures, and disposal of hazardous wastematerials. A generic way to protect metals from corrosion is toapply protective lms or coatings, which also permit the desiredproperties of the substrate to be coated through the chemicalmod-ication of the coatings [3,4], such as mechanical strength, opticalappearance, bioactivity, etc.

    There are several techniques for the deposition of coatingson metals, including physical vapor deposition (PVD), chemicalvapor deposition (CVD), electrochemical deposition, plasma spray-ing and solgel process. There are many advantages using solgelcoatings, several most important features are listed as follows[5,6]:

    (A) Solgel processing temperature generally is low, frequentlyclose to room temperature. Thus thermal volatilization anddegradation of entrapped species, such as organic inhibitors,is minimized.

    (B) Since liquid precursors are used it is possible to cast coatings incomplex shapes and to produce thin lms without the need formachining or melting.

    (C) The solgel lms are formed by green coating technologies:It uses compounds that do not introduce impurities into theend product as initial substances, thismethod iswaste-free andexcludes the stage of washing.

    Ten year ago, Guglielmi [7] has already discussed the potentialof solgel coatings as a corrosion inhibiting system for metal sub-strates. Since then, a great deal of work has been done to makevarious solgel based protective coatings. This review will intro-duce the basic chemistry involved in solgel processes, then theprogress and development of solgel protective coatings on metalsubstrate, such as steel, aluminum, etc. Finally some problems andfuture work on solgel coatings will be summarized briey.

    2. General background of solgel coatings

    2.1. Brief history of solgel chemistry

    The solgel process is a chemical synthesis method initiallyused for the preparation of inorganic materials such as glasses andceramics [8]. And this process can be traced back to 1842, whenFrench chemist, J.J. Ebelmen reported the synthesis of uraniumoxide by heating the hydroxide, but the aging and heating processlast almost a year to avoid crackingwhichmade it difcult forwiderapplication and did not catch many eyes that time [9]. It was notuntil 1950s, when R. Roy and his colleague changed the traditionalsolgel process into the synthesis of new ceramic oxides, makingthe solgel silicate powders quite popular in the market [1012].In 1971, the production process of so-called low-bulk densitysilica involving the hydrolysis of tetraethoxysilane (TEOS) in thepresence of cationic surfactants was patented [13]. In the middle

    solgFig. 1. Hydrolysis and condensation involved in making el derived silica materials.

  • D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338 329

    1980s, many material scientists and chemists, represented by H.Schmidt and G.L. Wilkes started to synthesis organicinorganichybrid materials (OIHMs) by solgel process and published aseries of pitechnologythe elds ochemistry,novel OIHM

    2.2. Prepara

    The solnetwork bycursors in aprepare solmethod. Ththrough theand gelation1100nm)most widelystarts withprecursorsorganic solvsuch as Si,(CxH2x+1).

    Generallhydrolysis,form chainseration of tnetworks ththickening,condensatioreactionhascondensatiosuch as alcodriven off aoccur. Thestion conditisolvent comreviews forprocess [6

    A solgevarious techthe two moand electrobe the majBut whatevis a substantion due toCracks aremation conand heat treing on diffeapplication

    The forminorganic anium, tin othan alkoxyLewis acidiand mild mformation oincluding teicates (Ormthe most whybridmate

    Table 1Abbreviation, chemical name and functional group of some commonly usedalkoxysilane precursors for solgel protective coating

    tion

    moss are

    rosio

    el su

    l anlds ber, the coepos], as

    etal2, ZrOy and2 canals und chonas ced Si, O and Fe elements and formed a transition layern steel substrate and SiO2 layer. The obtained solgel silicas were homogeneous, free of cracks. Samples were testedl/LH2SO4 solution and 3.5% NaCl solution, both corrosion

    ial increased and corrosion current density decreased, indi-this 100nm thin SiO2 layer improved the anti-corrosionance of stainless steel substrate.

    2 has a high expansion coefcient very close to many bulk, which can reduce the formation of cracks during high tem-re curing process [26,36]. ZrO2 also shows good chemicaly and high hardness [35] which makes it a good protectiveals. Perdomo et al. [31] made ZrO2 coatings on 304 stainlessy solgel method using zirconium propoxide as precursornsied in air and in oxygen-free (argon or nitrogen) atmo-s. The corrosion behavior of the stainless steel substrate wasby potentiodynamic polarization curves. It was found that2 coatings extended the lifetime of the material by a factor

    ost eight in a very aggressive environment, independentlypreparation procedure. In order to improve the adhesionn protective organic coating and metal substrate, Fedrizzioneering research articles [1417]. Since then, solgelhas attracted a great deal of attention, especially inf ceramics, polymer chemistry, organic and inorganicphysics and played an indispensable role in preparings [5,18,19].

    tion of solgel coatings

    gel process can be described as the creation of an oxideprogressive condensation reactions of molecular pre-liquid medium [18]. Basically, there are two ways to

    gel coatings: the inorganic method and the organice inorganic method involves the evolution of networksformation of a colloidal suspension (usually oxides)of the sol (colloidal suspension of very small particles,

    to form a network in continuous liquid phase. But theusedmethod is the organic approach, which generallya solution of monomeric metal or metalloid alkoxideM(OR)n in an alcohol or other low-molecular weightent. Here, M represents a network-forming element,Ti, Zr, Al, Fe, B, etc.; and R is typically an alkyl group

    y, the solgel formation occurs in four stages: (a)(b) condensation and polymerization of monomers toand particles, (c) growth of the particles, (d) agglom-he polymer structures followed by the formation ofat extend throughout the liquid medium resulting inwhich forms a gel. In fact, both the hydrolysis andn reactions occur simultaneously once the hydrolysisbeen initiated. As seen in Fig. 1, both the hydrolysis andn steps generate low-molecular weight by-productshol and water. Upon drying, these small molecules arend the network shrinks as further condensation maye processes are basically affected by the initial reac-ons, such as pH, temperature, molar ratios of reactants,position, etc. Readers may refer to other studies anda more complete understanding of the entire solgel8,18,19].l coating can be applied to a metal substrate throughniques, such as dip-coating and spin-coating,which arest commonly used coating methods. Spraying [20,21]deposition [2224] also emerged recently and couldor solgel coating application methods in the future.er technique is used, after the coating deposition, theretial volume contraction and internal stress accumula-the large amount of evaporation of solvents and water.easy to form due to this internal stress if the lm for-ditions are not carefully controlled. Usually the curingatment of solgel coatings vary substantially depend-

    rent microstructures, quality requirement and practical.ation of silica solgels also holds true for non-silicatelkoxides. In fact, metal alkoxides of titanium, zirco-r aluminum are much more reactive towards watersilanes due to the lower electronegativity and higherty [8,25]. But it is that the reaction is quite gentleakes the alkoxysilanes studied most extensively in thef solgel materials, especially OIHMs. Alkoxysilanes,traoxy silicate (Si(OR)4) and organically modied sil-osils, Rn Si(OR)4n or (RO)3Si RSi(OR)3) have beenidely used metal-organic precursors for preparation ofrials by solgel processing. Table 1 and Fig. 2 lists some

    Abbrevia

    TEOSTMOSMTESMTMSVTMSPTMSPHS

    APS

    AEAPS

    GPTMS

    MAPTS

    MPTMS

    BTSTS

    of thecoating

    3. Cor

    3.1. Ste

    Steetrial eHowevions. Tlms d[2645

    3.1.1. MSiO

    stabilitSiO

    of mettance acoating(TEOS)containbetweecoatingin 1mopotentcatingperform

    ZrOmetalsperatustabilitmateristeel band despherestudiedthe ZrOof almof thebetweeChemical name Functional group

    Tetraethyl orthosilicateTetramethyl orthosilicateMethyl triethoxysilane Methyl-Methyl trimethoxysilane Methyl-Vinyl trimethoxysilane Vinyl-Phenyl trimethoxysilane Phenyl-Diethylphosphonatoethyltriethoxysilane

    Phosphonato-

    3-Aminopropyltrimethoxysilane

    Amino-

    3-(2-Aminoethyl)aminopropyltrimethoxysilane

    Amino-

    3-Glycidoxypropyltrimethoxysilane

    Glycido-

    -Methacryloxypropyltrimethoxysilane

    Methacryloxy-

    -Mercaptopropyltrimethoxysilane

    Mercapto-

    Bis-[3-(triethoxysilyl)-propyl]tetrasulde

    Sulde-

    t commonly used alkoxysilanes in solgel protectivea.

    n protective solgel coatings

    bstrates

    d stainless steel are widely used in different indus-ecause of their mechanical and corrosion properties.

    hey still tend to corrode in the presence of haliderrosion resistance behavior of solgel coatings or thinited onto steel substrate has been extensively studiedsummarized inTable2 following the timeofpublication.

    oxide coatings2,Al2O3, TiO2 andCeO2, etc. all haveverygoodchemicalcan provide effective protection to metal substrate.improve the oxidation and acidic corrosion resistancender different temperatures due to its high heat resis-emical resistance [29,34]. Vasconcelos et al. made SiO2AISI 304 stainless steel using tetraethyl orthosilicatehemical precursor [34]. It was found that the coating

  • 330 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338

    et al. [35] prthen applieing to adheshowed prochemical trphate pretrlm on milat 400 C anthe corrosiorespectively

    Al2O3 istivity for tcoatings. MAl2O3 coatilocal anti-crent densittemperaturrent densityauthor belie(less resista(corundumFig. 2. Chemical structure of some commonly used alkoxysilane precu

    epared ZrO2 solgel coating on low carbon steel sheets,d polyester organic coating onto the ZrO2 layer. Accord-sion testing, the samples pretreated with ZrO2 layermising performance, in comparison with commercialeatments, such as tricationic phosphate and iron phos-eatment. Li et al. [36] also reported on thin ZrO2 solgeld steel sheets, and found that ZrO2 layers heat-treatedd 800 C were homogeneous, crack-free and increasedn resistance of the mild steel by a factor of 6.3 and 2.3,.a well-known insulator and has very low conduc-

    ransmitting electrons, which is ideal for protectiveasalski et al. [33] prepared two-, four- and six-layerngs on AISI 316 stainless steel in order to improve itsorrosion ability. It was found that the cathode cur-y varied with sintering temperature: higher sinteringe (within the range 500850 C), the lower cathode cur-values, but also the lower breakdown potentials. Theved that at higher temperatures conversion of -Al2O3nt to aggressive agents) into the -Al2O3 modication, more resistant to aggressive agents) proceeds more

    readily. Howtemperaturof the polarin the coati

    TiO2 haselectron corial. But pufew TiO2 lsubstrate [2in optics, casors, ceriuminhibitors [

    Two andlimitation ocation areaof steel sub70SiO2-30Tcorrosion pous NaCl aincrease thin 3% NaClrsors for solgel protective coating.

    ever, on the other hand, an increase in the sinteringe resulted in a marked increased on the anodic branchization curve and thus increased the number of defectsng.excellent chemical stability, heat resistance and low

    nductivity, making it an excellent anti-corrosion mate-re TiO2 lm is mostly used in catalyst chemistry. Veryms have been reported as protective coatings on steel8]. CeO2 is in the similar situation, althoughwidelyusedtalyst chemistry, pigments, superconductors and sen-ismore popular in hybrid solgel coatings as corrosion

    41,44], which will be discussed later.multiple-component oxide coatings can overcome thef single-component oxide layers, broaden their appli-s and improve the comprehensive protective abilitystrates. Early works, such as Atik et al. [26] reportediO2 and 75SiO2-25A12O3 acting very efciently asrotectors of 316L stainless steel substrates in aque-nd acid media at room temperature. The lms coulde lifetime of the substrate by a factor of up to 10and 5 in 15% H2SO4 solutions. In order to improve

  • D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338 331

    Table 2Corrosion protective solgel coatings on steel substrates

    Composition and precursors Steel substrate Coating method Thickness (m) Reference and year

    ZrO2TiO2-SiO2Al2O3-SiO2

    ZrO2-PMMA

    CeO2TiO2

    SiO2SiO2-CaO-P2O

    CH3-SiO2B2O3-SiO2MgO-SiO2

    ZrO2ZrO2-PMMAAl2O3SiO2ZrO2ZrO2TEOS-MAPTS

    TEOS-MAPTS

    SiO2-Na2O

    APSAEAPSGPTMSMAPTS

    SiO2-PMMASiO2-PVB

    Cerium-APSTEOS-MAPTSTEOS-MTESCerium-TEOS-CaO-P2O5

    the bioactivVijayalakshration of Clm had corosion resiused as a pSiO2-CaO-Psion resista[29].

    3.1.2. OrganFromthe

    good protecdrawbacks otant layers:are difcultatures (400[8].

    To overcsuch as brihas been dganic solgeThese mateof coatings

    Thoughbeen succesferent syntapproaches316L SS Dip-coating

    316L SS Dip-coating

    304 SS Dip-coating

    316L SS Dip-coating5

    304 SS, 430 SS Dip-coating

    304 SS Dip-coating316L SS Dip-coating316L SS Dip-coating304 SS Dip-coatingCarbon steel Dip-coatingMild steel Dip-coating304 SS Dip-coating

    304 SSDip-coating

    316L SS

    Zinc-plated steel Electrodepositing

    Iron plate Dip-coating

    304 SS,Dip-coating

    Zinc-plated steelCarbon steel Dip-coatingCarbon steel BrushingGalvanized steel Dip-coating

    MTES 304 SS Spin-coating316L SS Spin-coating

    ity and corrosion resistance of an implant material,mi and Rajeswari [45] recently reported the prepa-aO-P2O5 coating on 316L stainless steel. The solgelmbined effects of good adherence with higher cor-stance acting as a diffusion barrier and could beotential material for implantation purposes. Similar

    2O5 coating was also studied to improve the corro-nce and bioactivity of stainless steel implant material

    icinorganic hybrid solgel coatingsstudies above, the inorganic oxide coatings canprovidetion on metal substrates. But there are still some majorf these coatings, from the standpoint of corrosion resis-(1) oxide lms are brittle and thicker coatings (>1m)to achievewithout cracking; (2) relatively high temper-800 C) are often required to achieve good properties

    ome the limitation of pure inorganic solgel coatings,ttleness and high temperature treatment, much workone to introduce organic component into the inor-l to form theorganicinorganic hybrid solgel coatings.rials turned out to be among the most interesting areasscience in last decade [27,32,3944].many organic (polymeric/oligomeric) species havesfully incorporated within inorganic networks by dif-hetic methods, they are classied into three majoraccording to the chemical bond between inorganic

    and organithe inorgature, and tinorganic cgroups witthe hydrolical bondingas the soleR being aried out bysolgel reacFig. 2).

    Atik et a(PMMA) anbehavior wdynamic poact as geomincrease theet al. [32] ascanning elfound thatcontinuousresistance oing 17vol.%but tendedscale and ththe breakdochemical te0.40.6 [26] 1995

    0.2 [27] 1997

    0.5 [28] 1997

    0.41.4 [29] 1998

    0.22 [30] 1998

    0.7 [31] 19980.21.0 [32] 19992.03.0 [33] 19990.15 [34] 20000.30.6 [35] 2001

    [36] 20010.2 [37] 2001

    0.2 [38] 2003

    1.0 [22] 2003

    1012 [39] 2003

    1.0 [40] 20042.12.5 [41] 2005N/A [42] 20064.0 [43] 20061.92.0 [44] 20061.0 [45] 2007

    c phases: (1) mix organic component directly intonic solgel system, the product is a simple mix-here is no chemical bonding between organic andomponents; (2) utilize already existing functionalhin the polymeric/oligomeric species to react withzed of inorganic precursors, thus introducing chemi-between them; (3) use alkoxysilanes Rn Si(OR)4n

    or one of the precursors of the solgel process withsecond-stage polymerizable organic group often car-either a photochemical or thermal curing following thetion, e.g.methacryloxy group inMAPTS (see Table 1 and

    l. [27]made hybrid coatings of polymethylmethacrylated ZrO2 onto 316L stainless steel. Coatings anticorrosionas analyzed in 0.5M H2SO4 solution through potentio-larization curves at room temperature. The coatingsetric blocking layers against the corrosive media andlifetime of the substrate up to a factor 30. Messaddeq

    nalyzed the microstructure of ZrO2-PMMA coating byectron (SEM) and atomic force microscopy (AFM) andzirconium concentrated domains were surrounded byPMMA secondary phase domains. Maximum corrosionf the substrate was observed for the coating contain-PMMA. Higher PMMA volume made thicker coatingsto form a single-phase structure at the micrometereir adhesion to the substrate was worse resulting inwn and the peeling of the coating during the electro-sting. Similarly, a SiO2-PVB (polyvinyl butyral) hybrid

  • 332 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338

    Fig. 3. AFM images of 90% TEOS10% MAPTS solgel coating on a stainless steelsubstrate [37].

    coating was also deposited on Zn-plated steel substrate [40]. Theresults of salt spray testing of substrates coatedwith hybrid solgellms indicated that SiO2-1wt% PVB coating was the relatively bestone against corrosion and crack-free even its thickness was close to1m.

    Chou et al. [37,38], prepared hybrid sols by copolymerizing TEOSandMAPTSwitha two-stepacid-catalyst process. Thenhybrid coat-ingswere dip-coated on304 stainless steel substrates and annealedat 300 C founiform anprotectionseparated t

    Ref. [42]wereuniforTEOS had aworkers [43coating (ththat the corchange with

    vided good protection for the zinc coating underneath and steelsubstrate at the same time.

    3.1.3. Inhibitor doped solgel coatingsBesides the organic component, other additives, such as

    inhibitors, can also be incorporated into the solgel system toincrease the corrosion resistance of the metal substrates. Sugama[41] made hybrid coatings to protect steel and aluminum againstcorrosion by adding about 20wt% of Ce acetate as corrosioninhibitor into 3-aminopropyl trimethoxysilane (APS) sol. Thecerium compound could minimize the content of non-reactedwater-soluble APS and form a passive Ce3+ oxide lm insen-sitive to Cl dissolution over the metal surface. The coatingthickness was about 2.5m and extended the useful lifetime ofsteel exposed in a salt-fog chamber from only 10h to 768h,and aluminum panels from 40h to 1440h. Duran and co-workers [44] also studied cerium ions loaded hybrid silica solgelcoatings deposited on AISI 304 stainless steel substrates. It wasfound that coatings prepared with the Ce (III) salt enhancethe corrosion resistance through a barrier effect, but devel-oped defects later with immersion time, showing no apparentinhibiting effects. At the same time, coatings obtained from Ce(IV) chemicals enhanced the coating performance, probably dueto the formation of Ce(OH)3 on the surface through a chemi-cal/electrochemicalmechanisminvolvinga redox reactionbetweenCr and Ce ions.

    Through these studies of solgel coatings on steel substrates(stainless steel, carbon steel and zinc steel), it was clear that solgel

    can provide effective protection against corrosive media inal sewas

    norgdditsucho bediffete isr 30min. The resultant coatings were relatively dense,d defect free (Fig. 3) and provided excellent corrosionprobably because the dense physical barrier coatingshe anode from the cathode.also reported that the TEOS-MAPTS hybrid coatings

    mandcrack-freewhile thepure inorganic coatings frompparent cracks on the surface (Fig. 4). Duran and co-] used TEOS and MTES to form a thick hybrid solgel

    ickness around 4m) under basic catalyst and foundrosionmechanisms for solgel galvanized steels did notrespect to the uncoated steel. The solgel coating pro-

    coatingpracticstrates

    3.1.4. IIn a

    mentscan alsto thesubstration.Fig. 4. Surface morphologies solgel coatings from (a) TEOS; (b) TEOS (enlarged); (crvice conditions. The corrosion resistance of steel sub-substantially improved.

    anic zinc-rich coatingsion to the corrosion inhibitors, sacricial metal pig-as zinc, aluminum,magnesium and their alloy particlesincluded into the solgel coating formula accordingrent metal substrate to be protected. Generally, steeloften a suitable target under this cathodic protec-

    ) TEOS-MAPTS; (d) TEOS-MAPTS (enlarged) [42].

  • D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338 333

    Inorganic zinc-rich coatings are a widely used and unique classof coatings that provide cathodic protection to ferrous and steelsubstrates. The coatings have been categorized by Steel StructuresPainting Council (SSPC) into three major groups: (1) post-cured,water-bornealkalimetal silicates; (2) self-cured,water-bornealkalimetal silicates; (3) self-cured, solvent-borne alkyl silicates. Of thethree types, the self-cured, solvent-borne alkyl silicates have thegreatest commercial usage by far [46]. The most common binderprecursors are TEOS and its oligomers derived from it by controlledpartial hydrolysis with a small amount of water. Ethyl or isopropylalcohol is used as theprincipal solvent, since an alcohol helpsmain-tain package stability. After application, the alcohol evaporates, andwater from the air completes the hydrolysis of the oligomer to yielda lm of polysilicic acid partially converted to zinc salts. TEOS canundergo hydration and condensation processes and form a com-plex polysiloxane network in atmosphere, and its nal hydrationproducts are SiO2 and water [4749].

    Unlike the general solgel protective coatings, the protectionability of the inorganic zinc-rich coatings is more relied on thecathodic protection of zinc pigments, rather than the barrier prop-erties of the solgel binder. And the current research of thesecoatings is mainly focused on the effects of pigment volume con-centration [46,5052], size and shape of zinc pigments [51], zincalloy pigments [53,54] and extenders [55]. The studies on usingdifferent alkoxysilane precursors or hybrid binder materials havenot 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 lowcost, lightweight, high thermal and electrical conductivity grantaluminum a remarkable industrial and economical importance.Many of its applications are practicable due to its natural tendencyto formapassivatingAl2O3 oxide layer,which can also be articiallygenerated by anodizing the substrate. However, this passivatinglayer deteriorates in aggressive media, such as chloride, whichresults in pitting corrosion [5862]. Although current chromateconversion coatings function very well in corrosion protection,the US Environmental Protection Agency is planning to totallyban the use of chromates in coating materials in the near futurebecause of their extremely toxic effect. A broad range of research isbeing actively pursued to develop less toxic and environmentallybenign organic coatings for corrosion protection. One relativelynew but very promising approach is solgel coating and has beenextensively studied in the last ten years [20,23,3941,6385], assummarized in Table 3 following the time of publication.

    3.2.1. Metal oxide coatingsSimilar 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 3Corrosion protective solgel 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] 1998ZrO2-TEOS-GPTMS

    SiO2 Al plate Dip-coating N/A [64] 2000SiO2-ZrO2 Al 2024-T3 Dip-coating 0.1 [65] 2001ZrO2-TiO2-soybean oil Al plate Blade-casting 4595 [66] 2001ZrO2-TEOS-GPTMS Al 2024-T3 Dip-coating 34 [67] 2001SiO2-vinylpolymer Al 2024-T3 Dip-coating 34 [68] 2001Cerium-SiO2-epoxy Al 2024-T3 Dip-coating 2-3 [69] 2001Cerium-ZrO2-GPTMS Al 2024-T3 Dip-coating 23 [70] 2001

    Aminosilane-epoxy Al 2024-T3Spraying 3050 [71] 2001Epoxysilane-epoxy Al 7075-T6

    TEOS-GPTMS Al 2024-T3 Spraying 2.2 [20] 2001

    TMOS-GPTMScross-linkers

    APSAEAPSGPTMSMAPTS

    TEOSMTESPTMS

    SiO2-BTSTS

    SiO2-PMMASiO2-PVB

    TMOS-GPTMS

    Bis-silane inhiCerium-APSZrO2-TEOS-MAZrO2-GPTMS i

    TEOS-GPTMS--amineAl 2024-T3 Dip-coating

    Al plate Dip-coating

    Al electrode Electrodeposition

    Al 2024-T3

    Dip-coatingAl 7075-T6Al 6061-T6Al 5005

    Al alloy (ADC12) Dip-coating

    -organic inhibitor Al 2024-T3 Dip-coating

    bitor Al 2024-T3 Dip-coatingAl 3003 Dip-coating

    PTS Al disk Spin-coatingnhibitor Al 2024-T3 Dip-coating

    PDMSAl 2024-T3

    Spin-coatingAl 6061-T61[72] 2003[73] 2003

    1012 [39] 2003

    0.160.18 [23] 2003

    0.40.6 [74] 2003

    0.10.3 [40] 2004

    1[78] 2004[79] 2005

    0.3 [80] 20052.12.5 [41] 20051.97.5 [81] 20051.82.0 [84] 2007

    N/A [85] 2007

  • 334 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338

    dry chemical control agent (DCCA) to obtain SiO2 lms. But itsadhesion was quite limited and coating developed defects duringthe heat treatment process. Then anodic polarization pretreatmentwas 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 solgel coatingon Al 2024-T3 substrate under immersion in dilute Harrisonssolution [3.5 g/L (NH4)2SO4, 0.5 g/L NaCl]. SEM, AFM, EIS (electro-chemical impedance spectroscopy) and XPS (X-ray photoelectronspectroscopy)were usedduring the evolution of the coating systemunder immersion. It was found that pitting corrosion and degrada-tion products on the solgel coating surface developed after 2 daysof immersion, the impedance increased, and itwas conjectured thataluminum oxide and silicon oxide may form a stable mixed oxidebarrier layer at the interface after initial corrosion, which prohibitsfurther pitting corrosion.

    3.2.2. Organicinorganic hybrid solgel coatingsHybrid solgel coatings are much more popular than pure inor-

    ganic oxide layers in terms of the corrosion protection of metalsubstrates for two main reasons. First, hybrid coatings can eas-ily form a thicker coat in micrometer scale without cracks andmuch lower curing temperature is needed (usually

  • D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338 335

    NaCl solution. It was found that the organic inhibitor tolytriazoleimproved the overall corrosion resistance of the Al 2024-T3 alloybut did not impart a self-healing effect to the lms, while the inor-ganic ceriuexposed me

    ZrO2-TEminumdiskfunction ofcompositiomolar ratioon the coatpositive inbrittlenessthe GPTMSrosion resiscoatings haR=4.

    TheAir Finvestigatinaluminum senvironmenmethod [20

    Voevodifrom ZrO2-TA two-stagsolgel coatassociated wthe coatingcurrent stagsurface regeven delamThese studiof corrosioncracks or dextend or te

    Rare earimprove coaphous struKasten et alhybrid soltion in alum

    Voevodicoatings baapproach inface treatmformation inanoparticlelastic thinsubstrate suquency impof 107 cmstill had 1thickness oproperties wterm corros

    Khramovhybrid solTMOS andthe sol was-cyclodexadded sequT3 substratthe hybridorganic inharea, provid

    IS bode plots for SNAP lms as a function of immersion time in dilutes solution [72].

    osiveeas

    rnal icomibitoov ege ste -

    Hybrnaloof so beierwh coa[868s weirstlyon, tally cd ont55m thick and crack-free. Fig. 7 shows the SEM imagehybrid Mg-rich coating. The pigment volume concentration

    Fig. 7. SEM image of hybrid solgel Mg-rich primer (35% PVC).m inhibitor showed the property of protecting freshtal surface.OS-MAPTS hybrid coating was deposited onto alu-s by spin-coating technique [81]. Coating thickness as athe spin-coating rotational speed and the chemical sol-n was investigated. It was found that the ZrO2/MAPTShad signicant inuence on the sol viscosity and thusing thickness. Higher ZrO2 contents seemed to have auence on the machinability but at the same time theof the coatings increased. Wu et al. [85] also reported/TEOS molar ratio R had a crucial inuence to the cor-tance of hybrid TEOS-GPTMS-PDMS solgel coatings,d the relatively best anti-corrosion performance when

    orceResearch Laboratory of theUnited States havebeeng solgel derived anti-corrosion coatings on aerospaceubstrates for long time, and has developed high quality,tal benign corrosion protective coatings by a solgel,6773,78,79].n et al. formulated and tested hybrid solgel coatingsEOS-GPTMS [67] and SiO2-vinylpolymer [68] systems.

    e mechanism in pit development was observed for aed Al 2024-T3 samples. An initial low current stagewasith electrolyte penetration and pit initiation through

    to the aluminum substrate surface, and the second highe was associated with active pit growth in the metalions. The growth of pits caused coating cracking andination by the pressure of corrosion products and gases.es can help to identify the ways for the improvementprotection through solgel coatings: elimination of

    efects in the coating or use of corrosion inhibitors torminate the initial stage of pitting development.th elements, such as cerium, yttrium, hafnium, canting properties by unifying crystal size, reducing amor-ctures and brittleness upon heating treatment [28].. [69,70] added small amount of cerium (13wt%) intogel systems. Cerium showed effective cathodic inhibi-inum when good dispersion was achieved.n et al. [72,73] developed solgel surface treatmentsed on the Self-assembled Nanophase Particle (SNAP)order to replace the traditional chromate-based sur-

    ents on aircraft aluminum alloys. Through the in situn an aqueous solgel process, the functionalized silicaes could cross-link with each other to form a dense,lm well covered and adhered onto aluminum alloyrface. From the EIS bode plots of Fig. 6, the low fre-edance modulus, |Z|, initially as high as on the order2 and maintained that value for almost 2 months, and106 cm2 after 80 days of immersion. Considering thef the SNAP lms was only around 1m, their barrieras quite amazing and showed good potential as long-

    ion protection coatings.et al. [78] proposed to add corrosion inhibitors into

    gel systems to improve coatings anti-corrosion ability.GPTMS were rst hydrolyzed under acidic conditions,diluted and aged for 3 days, then the organic inhibitor,trin and cross-linker (diethylenetriamine DETA) wereentially. Finally, coating was deposited onto Al 2024-e by dip-coating technique. It was found that not onlycoating had very good barrier properties, but also theibitors could gently release in the region of damageding self-healing ability of the localized corrosion attack

    Fig. 6. EHarrison

    in corrple andof inteto formthe inhKhramexchanwith th

    3.2.3.By a

    tectioncan alsalloy. BMg-riclishedprimertem. Fconditisol, nsprayeare 45of thismedia. Though the way of adding inhibitors is sim-y to achieve, it is very difcult to control the releasenhibitors to the surface. The author use -cyclodextrinplexes with inhibitors and insured the slow release ofr and its continuing delivery to corrosion sites. Later,t al. [79] also tried to deliver inhibitors by the ion-rategy, but resulted in poorer performance comparedcyclodextrin complexation strategy.

    id solgel magnesium-rich coatingsgy to the formulation of zinc-rich coatings for the pro-teel, magnesium-rich, magnesium pigmented coatingsformulated for the corrosion protection of aluminumagen and the corrosion group at NDSU are leading thistings research and a series of articles have been pub-8] on their properties. Recently, hybrid solgelMg-richre developed from MAPTS-TEOS-PVB-Mg particles sys-, MAPTS and TEOS were hydrolyzed under the acidichen small amount of PVB powder was added into theertain amount ofmagnesiumpigmentsweremixed ando the aluminumalloy substrates. The resultant coatings

  • 336 D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338

    Table 4Corrosion protective solgel 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 1012 [93] 1997SiO2-ZrO2 Copper Dip-coating 13 [94] 1999GPTMS-MTMS Bronze copper Brushing 510 [95] 2003SiO2-MAPTS-MPTMS Mg AZ91D Spraying 2123 [21] 2005TEOS-PHS Mg AZ31B Dip-coating 0.60.7 [98] 2006

    (PVC) was 35%, which was a little lower than the critical volumepigment concentration (42% in this formulation), making a roughbut still non-porous surface.

    Coatings were evaluated by ASTM standard tests and foundthat this hybrid coating had excellent chemical resistance, abra-sion resistance and high corrosion resistant (Fig. 8) which was dueto the combination of barrier properties from solgel binder andcathodic protection by magnesium particles.

    Currently, solgel coatings have been used on aerospace alu-minum alloy substrates for corrosion protection [8991]. Thecorrosion events from etching electrolytes, temperature gradientsor mechanical stresses can be controlled and limited substantially.The traditional chromate pretreatment on aluminum alloys can bereplaced by this environmentally benign solgel protective coat-ings.

    3.3. Copper

    Solgel cper and main Table 4.

    Copper aand kitchenchemical rewill be acceSiO2 solge[7,92,93], buature up toand Cu areganic SiO2-and stabilit

    thermal expansion coefcient ZrO2 component. SEM showed thecoating surface was uniform, defects-free and well adhered to thesubstrate surface. Bescher et al. [95] reported using GPTMS+MTMSas precursors to form hybrid solgel coatings on copper and bronzesurfaces. The coatings had a strong adhesion on the substrates andcan be applied as thicker layers (510m). SEM showed almostno corrosion products appeared after two years exposure to highsulfur/humidity conditions. The anti-corrosion ability was furtherimproved after top-coated with a uoropolymer layer. The authorbelieved this is because of the high degradation resistance of theuoropolymer.

    Magnesium and its alloys have many useful properties, suchas high strength-to-weight ratio, good thermal conductivity, highdamping characteristics, goodmachinability and attracted a revivalof interest in industrial applications recently [96].However,magne-sium and its alloys are highly susceptible to corrosion, which limits

    ractic-2%Mwaswas

    cellen98]hyl trore(Fig

    g tophosl coatatiochemte.

    Fig. 8. Suand magnesium substrates

    oating were also investigated for the protection of cop-gnesium surfaces, and several studies are summarized

    nd bronze are popular metal materials for sculpturesutensils owing to their beautiful appearance and low

    activity. But inwet environment, their corrosionprocesslerated by forming hydroxides and harmful complexes.l coatings have been reported as barrier layer on coppert the coatings tended to peel off when raising temper-400 C since the thermal expansion coefcients of SiO2quite different. Boysen et al. [94] formulated the inor-ZrO2 coating on copper surface. The interface adhesiony problems were expected to be solved by using high

    their pMAPTStiatorcoatinging exet al. [natoethave mgroupsbindintion ofsolgecombinstrongsubstrarface images of hybrid solgel Mg-rich primer (35% PVC) after 700h of Prohesion exposal application [97]. Tan et al. [21] reported using 68wt%PTMS-30% SiO2 as hybrid coatingmaterials, photoini-

    also added for later UV curing process. The resulteddefects-free and could be as thick as 23m, provid-t physical barrier against corrosive attack. Khramov

    used phosphonate functional silane (diethylphospho-iethoxysilane, PHS) and found its phosphonate groupsafnity on the magnesium surface than silane head-. 9), thus forming a solgel coating with phosphonatemagnesium surface. The improved corrosion protec-phonate-containing coatings as compared to pure silicaings has been observed and explained by the favorablenofbarrierpropertiesof theorgano-silicatematrixwithical bonding of phosphonate groups to themagnesiumure (left) and salt spray test (right), the scribe length was 5 cm.

  • D. Wang, Gordon.P. Bierwagen / Progress in Organic Coatings 64 (2009) 327338 337

    Fig. 9. Schemational silane [9

    4. Challengprotective

    With theings, especiinvestigatedmercial appstill in the ilarge-scale

    4.1. Basic th

    The inteand delamiings, but thand stabilitmore studiecursors andanti-corrosieters to sosolvents, sogelation antion duringoverall undings and fuproperties.

    4.2. Optimi

    Currentprocessing a few days,ration is ano

    post treatment, especially in hybrid coating synthesis. So the opti-mization of current synthesis routes anddesignof newgelation and

    eatment methods are the keys to promote the developmentplicatrocl coan pHredwthodllableositectroqueoree aeathmoredynultedpro

    l lmn thn apage o

    w ra

    tradive asorsusedgel pot. Mtingthiczinctiestic of solgel processing of hybrid coatings with phosphonate func-8].

    es and future studies of solgel corrosioncoatings

    extensive studies in the last decade or so, solgel coat-

    heat trand ap

    ElecsolgesolutioCompaing mecontrobe depthe elefrom acrack-fundernmuchthermothe reslable insolgebetweepositioadvant

    4.3. Ne

    Theexpensprecurcan beof solties a lthe coacoatingtainingproperally hybrid systems for corrosion protection are being

    and developed rapidly and already have some com-lications. But on the whole, this solgel technique isnitial stage, facing many difculties and challenges forindustrial production.

    eory studies of solgel coatings

    rface properties of solgel coatings, such as adhesionnation, are the crucial factors to the quality of the coat-e systematic theory of how to evaluate the feasibilityy of solgel coatings has not been established. So muchs shouldbe focusedon the effects of addingvariouspre-functional additives to the lm formation process andon performance of the coating. In addition, the param-l formation (temperature, pH, H2O/-OR molar ratios,l aging, etc.), kinetics of hydrolysis and condensation,d curing process, cracks formation and crystal transi-the heat post treatment, are all very important for theerstanding of the lm formation theory of solgel coat-rther design and control of coating compositions and

    zation and new synthesis routes of solgel coatings

    synthesis route of solgel coatings requires long timeow, e.g. the sol aging alone often needs several hours to

    which is not favorable to plant production. Phase sepa-ther common phenomenon during the curing and heat

    particles, arareas.

    5. Conclus

    Solgelimprove thand practicresistant, echromatesexpected insolgel coaresistant, anies of solgsolgel proapplication

    Acknowled

    The authResearch (GtoDr. Scott P

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    ions

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    Sol-gel coatings on metals for corrosion protectionIntroductionGeneral background of sol-gel coatingsBrief history of sol-gel chemistryPreparation of sol-gel coatings

    Corrosion protective sol-gel coatingsSteel substratesMetal oxide coatingsOrganic-inorganic hybrid sol-gel coatingsInhibitor doped sol-gel coatingsInorganic zinc-rich coatings

    Aluminum substratesMetal oxide coatingsOrganic-inorganic hybrid sol-gel coatingsHybrid sol-gel magnesium-rich coatings

    Copper and magnesium substrates

    Challenges and future studies of sol-gel corrosion protective coatingsBasic theory studies of sol-gel coatingsOptimization and new synthesis routes of sol-gel coatingsNew raw materials and multiple component systems

    ConclusionsAcknowledgmentsReferences