Corrosion Protection Properties of Organofunctional Silanes-main

TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 01 /11 pp639-664 Volume 10, Number 6, December 2005

Corrosion Protection Properties of Organofunctional Silanes An Overview

W. J. van Ooij**, D. Zhu, M. Stacy, A. Seth, T. Mugada, J. Gandhi, P. Puomi

Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio, USA;

Salo Business and Health Care College, Karjalankatu 2-6, FIN-24130, Salo, Finland

Abstract: The aim of this paper is to review the development and state of the art in the application of cer-

tain organosilicon compounds known as trialkoxysilanes (or simply silanes) to the problems of the metal

finishing industry. The ultimate goal of this work is the replacement of chromate passivation and paint

pretreatments, as well as overcoming the shortcomings of organic coatings formulated in the modern

environmentally friendly world, thus without chromate or chromated pigments, volatile organic carbon (VOC)

containing compounds or harazardous air pollutants (HAPs).

Key words: organic coating; trialkoxysilane; hydrophobicity

Introduction

Silicon compounds, especially organosilicon chemistries, have been the subject of extensive development for more than fifty years, producing commercial advances in polymeric and composite materials which have fueled industrial innovation in the automotive, aerospace, and electronics markets. A basic review of relevant chemical property characteristics easily reveals the motivation for such experimentation. Silicon has a normal oxidation state like that of carbon; however, silicon is more electropositive, resulting in bond strengths, bond angles, and bond lengths that are quite different from most organic compounds, especially those including electronegative elements, such as oxygen, fluorine, and chlorine[1-3]. Such inorganic compounds often display enhanced properties including thermal endurance, chemical or moisture resistance, increased mechanical strength, and electrical performance over materials with a completely organic nature. One

general class of monomers and oligomers with a large variety of organic functionalization, called alkoxy-silanes, were identified early on by researchers as excellent coupling agents, a material which assists in adhesive bonding between dissimilar surfaces, allowing for better bulk and interface properties[4-6].

Trialkoxysilanes and similar silicon compounds are currently the subject of intensive research due in significant part to the need for green technology in the metal-finishing and the adhesive industries. Chromate and similar hexavalent chromium com-pounds have been reported to be toxic and carcino-genic, and thus their use and waste are regulated heavily by most environmental legislation[7,8]. Volatile organic carbon (VOC) in coatings and other sources have been linked to smog formation[9]. The proposed lower emission levels have forced many companies to look for water-based solutions to their corrosion prevention needs. Isocyanate type cross-linking agents have been identified as being hazardous air pollutants (HAPs)[10] and are making polyurethane and other resin systems more difficult to employ.

Received: 2005-06-09 To whom correspondence should be addressed.

E-mail: [email protected]; Tel: 1-513-556-3194; Fax: 1-513-556-3773

Chromate performs two major applications in corrosion control. The first is conversion sealing

Tsinghua Science and Technology, December 2005, 10(6): 639-665

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pretreatments for metals alloys of iron, aluminum, zinc, copper, and magnesium. The second application is as leachable inhibitive pigments in many primer and general coating formulations. Both of these uses of chromate compounds produce strong corrosion inhibiting effects and it is not readily apparent how to use the barrier and hydrophobic effects of siloxane films to duplicate the overall performance effects.

The exact chromate solution composition and structure of the final conversion coating may vary between substrates, application method or even specific products, but traditionally, chromic acid (H2CrO4) is used as a rinse which reacts with and dissolves the surface and near surface metal layer. Subsequent reduction reactions precipitate Cr(OH)3 on the surface as a coherent layer, with a certain amount of unreacted Cr6+still remaining within the film[11]. This layer provides both adhesion and stability to subsequent coating systems applied on top of it. The inhibition mechanism of the conversion layer was studied by Kendig, who concluded that the Cr6+ still present allows for repair of defects and breaks in the film, otherwise denoted as the so called self-healing effect[12]. It is unclear just how effective this mechanism is beyond cut edge protection as the majority of the chromate treated surface is often encapsulated by an organic primer or topcoat.

The general silicon compound type that Plueddemann first explored as coupling agents[6] has already gained industrial accepted application as adhesion promoting surface treatment for metals[13]. Compounds of this sort, so called mono-silanes, have the general structure X3Si(CH2)nY, where X represents a silicon ester which can transform using a hydrolysis reaction to a silanol group; Y represents an organofunctional group such as chlorine, primary or

secondary amines, or vinyl. Typically, the value of n is around 3, but individual moieties can vary. Both the type of Y group and the value of n have a strong influence on whether a particular monomer is water-soluble, but in general most of them are. The standard application method is solution deposition followed by an immediate condensation reaction upon drying with any available hydroxyl groups present on the inorganic substrate or metal surface to form stable siloxane bonds.

Studies have shown that so called bis-silanes or dipodal silanes display superior protection on metal surfaces, especially when used in conjunction with organofunctional mono-silane treatments for adhesion to other organic layers[14-17]. These compounds have structures X3Si(CH2)nY(CH2)nSiX3 or can exist with no functional group with the following structure X3Si(CH2)mSiX3. They enhance performance by depositing a more pronounced hydrophobic polysiloxane layer (SiOSi linkages) during condensation, but still allow chemical coupling using the functional group present either in the dipodal silane itself or with a mono-silane solution additive. Thus, combinations of silanes (mixtures) usually perform better than films of silanes alone. Table 1 summarizes the silanes that will be discussed in this paper and the abbreviations used for them. Unfortunately, many of the bis-silanes are not water-soluble, which limits their immediate industrial use to those with water stabilizing functional groups such as amines. Regardless, certain optimal mixtures have been obtained, clearly showing the potential of silane deposited films to be able to give equivalent performance to chromate underneath paint systems and even as a stand alone passivation treatment[18,19].

Table 1 The silanes discussed and the abbreviations used for them

Material Structure Abbreviation

Mono-silanes

Vinyltriacetoxy silane CH2 CH Si(OCOCH3)3 VTAS

Bis-silanes

Bis-1,2-(triethoxysilyl) ethane (C2H5O)3Si CH2CH2 Si(OC2H5)3 BTSE

Bis-[trimethoxysilylpropyl]amine (CH3O)3Si (CH2)3 NH (CH2)3 Si(OCH3)3 Bis-amino

Bis-[triethoxysilylpropyl]tetrasulfide (C2H5O)3Si (CH2)3 S4 (CH2)3 Si(OC2H5)3 Bis-sulfur

Universal mixtures Ratio Bis-sulfur/bis-amino 3/1 Bis-amino/VTAS e.g., 5/1

W. J. van Ooij et alCorrosion Protection Properties of Organofunctional 641

One major drawback to the widespread use of silane films as a passivating treatment is that despite their hy-drophobicity, eventually moisture reaches the metal-silane interface. The hydrolysis reaction that allowed the formation of Si O Me bonds is reversible, espe-cially if the substrates metal hydroxide is somewhat soluble[1,6]. This enables a mechanism by which the coatings performance can be undermined. There is evidence to suggest that the superior performance of dipodal silanes can further be enhanced by having higher concentration of Si O Me bonds that are formed during condensation and film formation which can mitigate this reversible effect to varying degrees[20].

Future advancements necessitate formulating a more robust coating; however, thicker films of silane are usually too brittle, and the stability of the solutions used to deposit such layers are relatively poor. Several alternative methods have been attempted, starting with the inclusion of additives in the silane film such as nano-sized filler and inhibitors[21,22]. Research has also progressed into investigating the modification of silane technology with conventional paint coating methodology. This has been accomplished through resin and/or pigment additions to sol-gel processes or direct network formation of siloxane using large silane precursor additions to paint formulations[23-25]. It is hoped that the inorganic and hydrophobic nature of the incorporated siloxane will provide superior barrier protection as well as suggest interesting alternatives to the current crosslinking and cure methods.

1 Experimental 1.1 Silane surface treatment

An important aspect in the interaction of silanes with metallic surfaces is the nature and preparation of the inorganic surfaces. The cleaning of the metal substrates prior to the film deposition is a critical step in the bonding process. Sabata et al.[26] have studied the effect of different cleaning procedures on steel surfaces for the deposition of films. They have concluded that the surface of the substrate has to be cleaned appropriately for the silanes to become effective. According to them, alkaline cleaning seems to be the best pretreatment before the application of silanes. Moreover, from the chemical bonding theory of Plueddemann[6] it can be expected that oxide

surfaces with a high density of hydroxyl groups will be preferred. Child and van Ooij[27] have reported that acid or neutral cleaners are less desirable compared to alkaline cleaners because of their lower hydroxyl group content. Franquet et al.[28] also studied the effect of surface pretreatment but on aluminium. They demonstrated that the amounts of hydroxyl groups at the aluminium surface provided by the different pretreatments strongly influence the initiation and the formation of non-functional silane films.

In summary, the interaction of the silane coupling agents with the metals seems to be dependent on the condition of the metallic surface. The substrates are usually degreased ultrasonically in various solvents such as hexane, ethanol, and acetone for 3-5 min, and then cleaned in a diluted alkaline cleaner (AC1055, provided by Brent P L C, Lakebluff, I L, USA) at 60-70C for 3-5 min, after which they are rinsed with deionized (DI) water and air dried. The cleaned panel surfaces should be completely water-breakfree (i.e., thoroughly wettable by water).

1.2 Silane hydrolysis

Before application on any substrate silanes needs to be hydrolyzed so that they have sufficient silanol Si OH groups to interact with the metal substrate. Usually, silanes are applied from dilute water solutions onto the metal surface. In the solution, the silanes are hydrolyzed and silanol groups are formed according to the following reaction: (CH3CH2O)3Si CH2CH2 Si(OCH2CH3)3 +6 H2O

(OH)3Si CH2CH2 Si(OH)3 + 6 CH3CH2OH (1) After hydrolysis, the hydrolyzed silanol groups can

undergo condensation reactions, resulting in slow po-lymerization and eventual precipitation The factors in-fluencing the kinetics and equilibrium of hydrolysis and condensation of silanes in solutions are the nature of the organofunctional groups, the concentration of si-lanes and water, the value of the solution pH, the tem-perature, and the aging of the solution[27,29,30]. Be-cause both hydrolysis and condensation reactions are acid or base catalyzed, solution pH is the major factor governing the stability of silanes in aqueous solutions[31,32].

Figure 1 illustrates the pH dependency of the hydrolysis and condensation reactions of a typical silane. From Fig. 1, it can be seen that under acidic or


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basic conditions, the rates of both hydrolysis and condensation reactions are high, while at or near neutral pH they are slow. Although acids and bases are catalysts for both hydrolysis and condensation of alkoxysilanes, these two processes have different pH dependences. When the reactions are OH catalyzed, a high rate of condensation is favored with rapid gelation. On the contrary, when reactions are H+ catalyzed, a high rate of hydrolysis is favored with slow gelation. Hydrolysis and condensation will proceed simultaneously unless one of them is limited under specific conditions. For example, preventing precipitation or gelation by using suitable solvents can slow down or even halt condensation[33].

Tesoro and Wu[29] showed that for silanes such as -APS, -MPS, and -GPS the rate of hydrolysis reaches a minimum at pH 7 while the rate of condensation shows a minimum at pH 4.3. Premachandra et al.[34] obtained similar results for -UPS in the water-methanol system. Experimental studies[35,36] have shown that the hydrolysis of alkoxy groups proceeds in a step-wise manner. The first alkoxy group hydrolyzes slowly, due to steric effects, followed by rapid consecutive hydrolyses of the other alkoxy groups.

Fig. 1 Hydrolysis and condensation rate of a typical silane[6].

Osterholtz and Pohl[31] have reviewed the kinetics of hydrolysis and condensation of organofunctional alkoxysilanes. They have concluded that the slower rate of hydrolysis is found at approximately pH 7 (neutral) and that a change of pH by one unit in either the acidic or the basic direction results in an acceleration of the hydrolysis rate. In order to minimize condensation, concentrations of silanes below 1% by weight should be used[31]. Additionally, Arkles et al.[32] have shown that the hydrolysis rate decreases when the carbon chain of the alkoxy groups becomes longer. Moreover, they have also reported

that the hydrolysis rate is enhanced by an increase in the amount of organic substitution.

In general, silanes with multifunctionality such as the bis-(triethoxysilyl)ethane have a very limited stability in water because of hydrolysis and condensation. Therefore, a high amount of organic solvents is needed in the preparation of solutions of multifunctional silanes. The stability, hydrolysis, and condensation kinetics of a typical bis-silane BTSE in water-ethanol-silane solution have been investigated by Pu et al.[33] using Fourier transform infrared (FT-IR) spectroscopy. They showed that at pH of around 4.5, the BTSE solution had the best stability, a high hydrolysis rate, and a low condensation rate.

Silane solutions are usually prepared by adding the silane to a mixture of DI water and solvent. A 5 vol.% bis-silane solution, for example, was prepared by adding 5 mL of the silane to a mixture of 5 mL DI water and 90 mL ethanol. The ratio of bis-silane/DI water/ethanol was 5/5/90 (v/v/v). In order to study the hydrolysis depending upon the silane, acetic acid or potassium hydroxide is used to reduce or raise the pH of the silane solution, respectively. Hydrolysis time is of high importance. A water based silane like bis-UPS hydrolyzes very fast from 15 min to 1 h. However, solvent-based silanes like bis-sulfur take 18 h to 48 h for hydrolysis. Although complete hydrolysis is not necessary, a sufficient number of active SiOH groups should be generated in the solution. In this way, a solid rather than an oily silane film is formed on metal substrates.

1.3 Silane deposition

After hydrolysis, silane coupling agents are generally deposited directly onto inorganic surfaces by means of different techniques. These deposition methods include deposition from solution, vapor, plasma, and the electrodeposition method[15,37,38]. Recently, Gandhi and van Ooij[39] have demonstrated the advantages of the electrodeposition technique, which results in a uniform film and a strong interfacial layer between the silane and metal substrate. Among these various processes, deposition from solution is the most widely used method as it is the simplest and the cheapest one[40]. Deposition from solution is very fast and the thickness of the film remains virtually unchanged, even if the immersion time is varied between 30 s to 30 min.


When the metal is dipped into a dilute silane solution of 2%-5% by volume strength for a few seconds, silanol groups get adsorbed instantaneously on the metal surface through hydrogen bonds, as shown schematically in Fig. 2 (Ref. [22]). Upon drying or curing, the SiOH groups and MeOH groups further condense to form metallo-siloxane (MeOSi), as shown in Fig. 2 and water evaporates during this process:

Me O H + H O Si Me O Si + H2O (2) Figure 2 further illustrates that simultaneously with

reaction (2) silanol groups (SiOH) react with each other by crosslinking and contribute to the formation of an SiOSi network according to the condensation reaction:

Si O H + H O Si Si O Si + H2O (3)

Fig. 2 Adsorption of partly hydrolyzed silane on a clean metal[22].

Several studies have shown that the silane film thickness on metals increases with the concentration of the silane solution. The relationship between silane film thickness and solution concentration is practically linear[19,27,41-43]. It should also be mentioned that in comparison with the conventional chromate layers (about 3 m thick), the typically used silane films, such as the films corresponding to 5% and 2%, are much thinner. For instance, a bis-amino/VTAS mixture silane film corresponding to 5% has the thickness of less than 250 nm, while the one for a 2% silane solution is only around 50 nm thick. Therefore, it is safe to say that the silane films outperform the conventional chromates on a per weight basis[19].

It is well known that silane films on metals inhibit corrosion primarily by functioning as hydrophobic barrier coatings preventing the transport of water/ions to the metal/coating interface[44-46]. Numerous recent

studies[19,41-47] have shown that the cross-linking of the siloxane film can be accelerated by increasing the curing temperature and time. Curing of the silane film also results in reduction of the film thickness, because upon curing a denser film is obtained[19,41-47]. Curing and/or aging of silane films has been investigated in the temperature range of 0-250 and curing time of 0 -180 min[19,41-47]. van Ooij and others typically use a curing temperature of 100 and a curing time of 20-30 min.

Recently, van Schaftighen et al.[48] used a drying method such that steel panels treated with silanes were dried by a rotating spinning method. By this drying method a very homogeneous and reproducible silane coating can be obtained on the metal surface. Their results also indicated that the bis-amino and -APS films are fully cured after 20 min of curing at 100, whereas the BTSE film is fully cured after 40 min of curing at 200.

Very recently, van Ooijs group has found that some of the most promising silane solutions for corrosion protection of metals are unstable, which means that as soon as silanol groups have been formed, condensation to siloxane units will occur. In such solutions, the total amount of hydroxyl groups changes with time. Upon hydrolysis, the number of OH-groups reaches a maximum, and upon further condensation, the hydroxyl content decreases. This solution behavior affects the binding of the siloxane structure to the metal. Upon maximum hydrolysis, a well structured film is formed. With solution aging, the silane coating performance degrades, because of an insufficient number of Si OH groups in the solution. The silane metal interaction decreases resulting in less anchoring via Al O Si bonds, eventually leading to poorer corrosion protection[49].

2 Characterization and Testing 2.1 Characterization techniques

Two major characterization techniques, Fourier-transform infrared reflection-absorption (FTIR-RA) spectroscopy and electrochemical impedance spectros-copy (EIS), have extensively been employed in our work. The FTIR-RA technique is well known as a powerful tool in the field of polymer surface charac-terization. EIS has been known as one of the most


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valuable techniques in the field of corrosion, yet its function as a characterization tool has not been identi-fied until very recently[50,51]. Other surface characteri-zation techniques, such as X-ray photoelectron spec-troscopy (XPS), scanning electron microscopy (SEM), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), have also been used in our work[52]. Very recently, nuclear magnetic resonance (NMR) has proven to be a valuable technique to characterize the hydrolysis degree of silane solutions[49].

Infrared spectroscopic ellipsometry (IRSE) is a technique that combines in one measurement the possibilities of traditional FT-IR methods (molecular information) with those of visible spectroscopic ellipsometry (SE) (morphologicalfilm thickness and opticalrefractive index/extinction coefficient information). Franquet et al.[41-43,47,48] have used EIS, IRSE, and SE along with SEM, Auger electron spectroscopy (AES), and transmission electron microscopy (TEM) to study the effects of silane solution concentration and curing time on the structure and homogeneity of organosilane layers on aluminum and steel. Of these characterization techniques, FTIR-RA and EIS are presented and discussed here. 2.1.1 FTIR-RA spectroscopy Among all surface characterization techniques, FTIR-RA spectroscopy is probably the most powerful tool for observing chemical structure and transformation (or chemical reactions) in coatings/films, because IR absorbance peaks are characteristic of chemical bonds. Since highly reflective metals such as aluminum are usually used as substrates in our work, external

reflection spectroscopy can be used to study the surface species or film structures. The incident light with parallel polarization has sufficient amplitude at the metal surface to interact with the surface species. Such technique is called reflection absorption (RA) spectroscopy.

FTIR-RA has been successfully used to monitor the structural changes in silane films during different processes. An example is given in Fig. 3, which displays FTIR-RA spectra of bis-sulfur silane film deposited on AA 2024-T3, after curing at 100 for various time[44]. The bands centered at 1100 cm1 due to SiOSi become more intensive as the curing time increases. The region becomes broader as compared to that without curing. In addition, the major band for Si O Si has shifted to higher frequencies from 1110 cm1 at 0 min to 1133 cm1 at 1440 min (24 h). All of these changes indicate that the bis-sulfur silane film has experienced extensive crosslinking upon siloxane network formation during curing of the film. It is also notable that the band at 885 cm1 due to hydrogen-bonded Si OH decreases in intensity with curing time, confirming that the siloxanes have formed via the consumption of the silanols. 2.1.2 Electrochemical impedance spectroscopy (EIS) In the field of aqueous corrosion science, EIS is considered as one of the most valuable techniques to investigate the degradation of polymer coated metal systems in corrosive aqueous media. An abundant of literature on this technique exists[53-56], providing information about theories and applications of EIS. Here, a brief introduction of EIS fundamentals is

Fig. 3 FTIR-RA spectra of bis-sulfur silane films on AA 2024-T3 after curing at 100 in air for various times [44].


presented. In EIS measurements, a frequency dependent impedance, Z(f ) (in ), is obtained by applying a sinusoidal alternating potential signal to tested systems (e.g., epoxy-coated steel) in a range of frequencies. The expression for Z(f ) is

Z(f ) = V(t)/I(t) (4) where f is the frequency, t is the time, V(t) is the sinusoidal alternating potential signal, V(t)=V0 sin (2ft); I(t) is the time-dependent current response, I0sin(2ft+); and is the phase angle between V(t) and I(t). EIS measurements are carried out in a conducting solution (i.e., electrolyte). A sodium chloride (NaCl) solution is commonly used for EIS monitoring of an organic coated metal substrate. A schematic of the EIS plots (in the form of Bode plots) for a polymer-coated metal system is shown in Fig. 4a. Two distinct steps (time constant, or RC) are observed in the impedance plot and two maxima in the phase angle plot. The one at high frequencies is believed to contain information about the coating; the other one at low frequencies is related to corrosion occurring at the metal surface under the coating. The associated EIS parameters shown in the Fig. 4 are usually used to describe the coating properties and corrosion kinetics at the metal surface. These EIS parameters can either be roughly read from the graph, as illustrated in Fig. 4a, or can be precisely found by regressing (or fitting) the associated mathematical equation to the experimental EIS data[55]. The fitting process can be done by inputting a proper equivalent electric circuit model (ECM) into commercial-available software. The ECM shown in Fig. 4b is used to fit the data in Fig. 4a. Two EIS parameters related to the coating properties are the coating capacitance (Cc), and its pore resistance (Rpo), the changes in which are used as a measure of the coating performance during exposure to the electrolyte.

Coating capacitance (Cc) in the unit of farad (F) for a non-defect coating is given by

Cc=0 / (A/d) (5) where 0 is the permittivity of free space (8.854 19 1012 F/m); is the dielectric constant of the coating; A is the total area exposed to the electrolyte (m2); and d is coating thickness (m). A typical value for a dried polymer is 3-4, 8.5 for an oxide layer, and 80 for water. It is noted that of water is much greater than that of a dried polymer. Thus, of the polymer changes significantly when water penetrates into the polymer.

Fig. 4 Schematic EIS plots of a polymer coated metal system (a), and equivalent electric circuit model (ECM) for EIS data fitting (b) (Ref. [44]).

For a given coating, the changes in Cc are often taken as a measure of water uptake in the coating[54,57]. If the specific conductivity of a polymeric coating is negligible with respect to that of the electrolyte, Rpo of the coating containing cylindrical defects/pores with the same length is expressed as

Rpo=(d/Ap) (6) where is the specific electrolyte resistivity in the defects/pores (m), which is constant for a given electrolyte system; d is the length of the cylindrical defects (m) or coating thickness; and Ap is the defect/pore area in the coating (m2). For a given coating, Rpo is inversely proportional to Ap. Once the electrolyte (water and ions) penetrates into the coating through defects and pores, conducting shortcuts develop between the electrolyte and the metal substrate. This would cause a decrease in the value of Rpo. Thus, Rpo is another EIS parameter that is used to evaluate the coating performance in an aqueous environment.

It should be noted that the time constant mentioned above is the product of resistance (R) and capacitance (C), in the unit of seconds. The time constant for a polymeric coating is thus given as RpoCc. When a silane film is crosslinked, a majority of the pores in the film would be sealed and water as condensation product would evaporate. This pore sealing would decrease the total pore area (Ap), resulting in an increase in Rpo. In the meantime, water removal


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would cause a reduction in Cc. The other EIS parameters shown in Fig. 4a are: Rso

is the uncompensated solution resistance (), the value of which is small (only several tens of and has no effects on the coated system; Rt is the charge transfer resistance polarization resistance (); and Cdl is the capacitance of the double layer formed at the coating-metal interface (F). The latter two parameters are often observed when corrosion starts on the metal substrate under the coating. It should also be noted that the phase angle plot, in some cases, is more valuable than the impedance plot. It has been reported[57] that some subtle changes in coating structures are clearly reflected in the phase angle plot but not in the impedance plot.

EIS has been first employed in our silane work for characterization purpose[44,58]. The study clearly showed that EIS actually has the ability to monitor chemical structural changes in silane films in different processes. An example is shown in Fig. 5. The Bode plots for the bis-sulfur silane-treated and cured (0.16 h at 80 ) AA 2024 -T3 as a function of immersion time in the 0.5 mol/L K2SO4 solution[58]. It can be seen that the total impedance of the system increase with immersion time throughout most the measured frequency range, with the appearance of a distinctive additional time constant after 4 h of immersion. The additional time constant is shown more clearly in the phase angle plot than in the impedance plot.

At the initial immersion stages (0.16-2 h), only two time constants are observed, one (RC1) located at high frequencies (~104 Hz), and the other (RC2) at low frequencies (~ 0.1 Hz). Since the bis-sulfur silane film was coated uniformly on the AA 2024-T3 surface, these two time constants can be assigned to the silane film (RC1) and the aluminum oxide (RC2), respectively. As the immersion process continues (after 4 h of immersion), the impedance of the silane film (RC1) increases remarkably and, correspondingly, the time constant RC1 is shifted to lower frequencies. It is interesting to notice that an additional time constant (RC3) occurs in the phase angle plot in the medium frequency range between RC1 and RC2. It is also noticed that the low-frequency impedance keeps increasing even after 24 h of immersion (Curve 5 in Fig. 5), but instead of two distinctive time constants, only one broadened time constant appears in the

medium frequency range. This is probably due to the overlap of the two time constants (RC3 and RC1).

Fig. 5 Bode plots for bis-sulfur silane-treated AA 2024-T3 panels during immersion in 0.5 mol/L K2SO4 solution for 0 to 24 h (Ref. [58]).

With the help of FTIR-RA analysis, the above changes in EIS plots can be explained as follows. The significant increase of the total impedance of the system was attributed to: 1) the formation of siloxane structure Si O Si in the existing silane films by condensation of silanol groups SiOH generated from the hydrolysis of residual ester groups

SiOC2H5; 2) the formation and growth of the interfacial layer between the silane film and the aluminum oxide.

2.2 Corrosion testsElectrochemical tests

DC polarization test and EIS are two popular experimental methods in modern corrosion study. Unlike conventional weight loss measurements which require many more days to determine corrosion rate, these two methods need considerably less time. 2.2.1 DC polarization tests Popular polarization methods, such as Tafel extrapolation and polarization resistance, to measure corrosion rates have the following advantages as compared to conventional weight loss measurements: 1) Usually only a few minutes are required to determine corrosion rate; 2) The methods are highly sensitive, and accelerating factors, such as elevated temperature, to increase rates in the laboratory are usually unnecessary. More fundamentals about DC polarization methods appear in Ref. [59].

Figure 6 is an example of DC polarization curves of AA 2024-T3 treated with and without the bis-sulfur


silane, obtained from a 0.6-mol/L NaCl solution (pH 6.5, open to air). The bis-sulfur silane treated AA 2024-T3 panels were immersed in the electrolyte for 5 h before data collection for the reason mentioned earlier. It is seen that both anodic and cathodic current densities of AA 2024-T3 have been reduced by more than 2 decades after the silane treatment (Curve 2) at the applied voltages with respect to that of the bare AA 2024-T3 (Curve 1). The Ecorr (corrosion potential) of AA 2024-T3 and the shape of the curves, however, do not show a substantial variation after the silane deposition. This indicates that the bis-sulfur silane mostly behaves as a physical barrier.

Fig. 6 Representative DC polarization curves of AA 2024-T3 treated with and without bis-sulfur silane, measured in a neutral 0.6-mol/L NaCl solution.

2.2.2 EIS measurements Just as mentioned above, EIS measurements have been proven to be one of the most valuable techniques to investigate the degradation of polymer coated metal systems in corrosive aqueous media. Instead of using noncorrosive electrolyte such as K2SO4, corrosive electrolytes like NaCl or HCl are often used in EIS measurements to simulate real corrosive environments. The changes of EIS parameters, i.e., Cc, Rpo, Cct, and Rp are good indicators for degradation degree of polymeric coatings on metals and corrosion extent at the metal surfaces underneath the coatings.

2.3 Industrial performance tests

2.3.1 ASTM B117 (Salt spray (fog) test, SST) This test was employed to evaluate bare corrosion protection of silane-treated metals without topcoats. According to the specification, 5% salt solution (NaCl) is atomized in a salt spray chamber at 35 with the

solution pH around 7. The tested panels shall be placed at an angle of 45 in the chamber, exposing to the salt fog for a certain period[60]. 2.3.2 ASTM 1654-92 (Corrosion test for painted

or coated metals and alloys) This test provides a method to evaluate the corrosion performance and paint adhesion of painted alloys with silane pretreatment. The testing conditions are similar to that for ASTM B117, except that the painted metal surfaces need to be scribed prior to salt spray testing. Delamination area is measured along the scribe line after exposure, the value of which is often used as a measure for the corrosion performance and paint adhesion of the painted systems[60]. 2.3.3 ASTM B368 (Copper-accelerated acetic

acid-salt spray testing, CASST) This test is derived from ASTM B117. First, 0.25 g/L copper chloride (CuCl2 2H2O) is added into 5% NaCl solution. The pH of the salt copper solution is adjusted to the range of 3.0-3.3 by the addition of glacical acetic acid. The temperature in the salt spray chamber is 49 (Ref. [60]). 2.3.4 A few examples of paint adhesion tests On polymer coated pretreated metal panels, paint adhesion is usually tested with a cross-cut adhesion test, where a cross-hatch is drawn on the painted panel and the adhesion of the coating is tested with adhesive tape. The degree of adhesion is measured according to standard ASTM D3359-93. Dry adhesion of painted pretreated panels is usually very good. Therefore, this test is usually combined with a water immersion or humidity test. After exposing the panels to water immersion or humidity, the wet adhesion of the panels is tested as described in Refs. [61, 62].

van Ooij and Zhu[19] also used the Machu test to evaluate paint adhesion and corrosion performance of painted metals. The painted metal panels are cross-scribed and then immersed into a solution of 5% NaCl + 10% H2O2 at 37 for 1 day. The next day, another 10% H2O2 is added into the original solution. After 2 days of immersion, the panels are taken out and tape is used to pull off the delaminated paints along the scribe lines. The delamination distance is taken as a measure of paint adhesion and corrosion performance of the tested system[63].


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3 Silane Films on Metals 3.1 Different types of corrosion uniform, localized,

and others

Uniform corrosion is defined as the uniform, regular removal of the metal from the surface when the corrosive environment has the same access to all the parts of the surface of the metal[64]. Atmospheric corrosion is probably the most prevalent example of uniform corrosion and it is also visually apparent. Barrier coatings of many sorts, both organic and

inorganic, can be used to inhibit uniform corrosion of metals. Silane surface treatments have been investigated at University of Cincinnati for over 13 years. Table 2 shows where unmodified silane films have been shown to be effective. Table 2 indicates that many forms, including localized forms, of corrosion can be controlled for many metals of engineering interest, ranging from stainless steel to magnesium alloys. In addition to corrosion protection, improved adhesion to paints, adhesives or rubber has also been obtained.

Table 2 Metals and forms of corrosion where films of trialkoxy silanes have been shown to be effective.

Metal Forms of corrosion controlled Bonding Cold-rolled steel Uniform, scribe creep, blistering Powder paints, E-coats, appliance paints, rubber Hot-rolled steel Scribe creep Waterborne paints Stainless steel SCC, pitting, IGC, crevice corrosion Powder paints, rubber, adhesives Electrogalvanized steel Scribe creep E-coats, powder paints, rubber Hot-dip galvanized steel White rusting, scribe creep Powder paints, E-coats, appliance paints Zinc sheets (Ti-zinc) White rusting Galvalume sheet Edge creep, scribe creep, white rust Powder paints, coil paint systems Galvannnealed sheet Scribe creep Automotive E-coats Aluminum alloys 1XXX, 2XXX, 3XXX, 5XXX, 6XXX, 7XXX series

Uniform, pitting, SCC, filiform, CFC Powder paints, solvent-based paints, rubber

Copper, brass Uniform, SCC, dealloying Nickel Galvanic Magnesium alloys Pitting, uniform, galvanic

3.2 Two universal silane mixtures for corrosion protection of various metals

For bare corrosion protection purposes van Ooij et al.[19,44-46] usually use a silane solution concentration of 5% and for silane treatments under paint a concentra-tion of 2%. For bare corrosion protection the hydro-phobic silanes BTSE and bis-sulfur have already dem-onstrated their corrosion protectiveness for many met-als[14,15,52,58,65]. Unfortunately, their hydrophobic nature requires a large amount of organic solvents such as ethanol or methanol in the preparation of the silane solutions.

A solvent-based silane system which has proven to give excellent corrosion resistance in unpainted state is the mixture of the bis-sulfur and bis-amino silanes with the ratio 3:1. This system is able to protect many metals, for instance, aluminum, HDG steel, and Mg alloys. Figures 7 and 8 present the scanned images of

silane-treated AA 2024-T3 and HDG panels after EIS measurements. It is clearly seen that no corrosion is shown on both mixture-treated AA 2024-T3 and HDG panels (Figs. 7c and 8c), indicating that the combination provides good corrosion protection for both metals. This mixture has a relatively short shelf life and high VOC content. Despite these facts, it may find a market nische in applications where VOC is not a major problem.

One of the main objectives in recent years has been to develop water-soluble silane solution mixtures with corrosion protection properties as powerful as the systems based on ethanol-water solutions. In this respect, a universal water-soluble silane system based on bis-amino and vinyltriacetoxy silanes was invented by van Ooij and Zhu[19]. They studied the corrosion resistance and paint adhesion of this mixture on several metals. The silane mixtures and the deposited films were also characterized.


Fig. 7 AA 2024-T3 panels after 32 days of immersion in 0.6 mol/L NaCl solution: (a) untreated; (b) bis-sulfur silane-treated; (c) mixture-treated; and (d) bis-amino silane-treated[46].

Fig. 8 HDG panels after 8 days of immersion in 0.6 mol/L NaCl solution: (a) untreated; (b) bis-sulfur silane-treated; (c) mixture-treated; and (d) bis-amino silane-treated[46].

with polyurethane and polyester powder-paints on The bis-amino/VTAS mixture is, by itself, quite stable and hydrolyzes readily in the aqueous mixture. The mixture works well, because with the addition of a small amount of bis-aminosilane, the VTAS solution becomes less acidic and the condensation of SiOH can be effectively suppressed. A likely mechanism is that the secondary amine groups in the bis-aminosilane form a more stable hydrogen bond with silanol groups than the one between silanols themselves. As a result, condensation of silanols is prevented in the solution. Figure 9 displays the salt spray testing results of AA 6061-T6 treated with the bis-amino/VTAS mixture, as compared with untreated and chromated panels. It is seen that the bis-amino/VTAS mixture treated AA 6061-T6 surface does not show any sign of corrosion after 336 h of SST, while the chromated surface exhibits a certain degree of discoloration. The bare AA 6061-T6 surface on the other hand, has corroded heavily, showing uniform corrosion on the surface. Performance tests also showed that the water-based bis-aminosilane/VTAS mixture provides comparable corrosion protection in the painted state, for example,

Fig. 9 AA 6061-T6 panels after 336 h of SST: (a) untreated (20 h of exposure), (b) chromated (Alodine-series), and (c) silane-treated (bis-aminosilane/VTAS =

[19]

1.5/1, 5%, pH 3.7) .

aluminum alloys, as compared with chromates[19]. If the universal mixtures are, however, compared with each other, it is obvious that the bis-amino/VTAS mixture does not provide as good bare corrosion protection on various metals as the hydrophobic bis-sulfur/bis-amino mixture. This is because the bis-amino/VTAS film is hydrophilic and contains positively charged secondary amino groups which attract water


650

and electrolytes to the siloxane/metal interface[19,46].

olution . On Al alloys th

of these alloys make them thus sli

ing particles (Al2CuMg) and Al-Cu-Fe-M

f corrosion products, l

an silane treatment oattribu

nt w

3.3 Aluminum and aluminum alloys

3.3.1 Character of the Al alloys Numerous studies have shown that several aluminum alloys can be protected by silane coatings[19, 44-46, 66, 67]. Figure 7 demonstrated clearly that all three silane systems: (b) the bis-sulfur silane-treated; (c) the mixture-treated; and (d) the bis-amino silane-treated are able to protect AA 2024-T3 panels after 32 days of immersion in 0.6 mol/L NaCl s [46]

e bis-amino/VTAS system also provides excellent bare corrosion protection[19].

The wide range of aluminum alloys available has emerged from a need of high strength, light weight materials in the aerospace and automobile industries. However, the heat treatments of these aluminun alloys result in dispersion of precipitates and second phases which render the high strength and mechanical properties. Since these precipitates are invariably cathodic or anodic relative to the surrounding matrix, this can lead to local corrosion pairs at phase interfaces. The structures

ghtly vulnerable for local corrosion at phase boundaries.

van Ooijs group has studied silane treatments extensively on AA2024-T3, which is becoming the best known and most widely used aerospace aluminum alloy due to its excellent mechanical properties. AA2024-T3 does, however, not have as good corrosion resistance as most other aluminium alloys and it suffers different forms of corrosion due to its intrinsic electrochemical properties. Copper (Cu) is one major alloying element in AA2024-T3 (3.8 wt.%-4.9 wt.%), which makes it susceptible to corrosion[68]. In general, AA2024-T3 suffers corrosion such as pitting, intergranular, and stress corrosion. The microstructure of AA2024-T3 is characterized by a uniform distribution of second-phase particles such as the Al-Cu-Mg-contain

n-containing particles [Al6(Cu2FeMn)] in the Al matrix[69-72].

Zhu and van Ooij studied the corrosion mechanism of bare AA 2024-T3. They found that the corrosion of this alloy starts from the dealloying of anodic Al-Cu-Mg-containing particles (Al2CuMg), followed by severe cathodic dissolution of the surrounding Al

matrix due to the local alkalization formed around the particles. Such corrosion in AA 2024-T3 can be, however, inhibited efficiently by silanes[73]. The films of the bis-sulfur silane and the mixture of bis-sulfur/bis-amino form a so-called interfacial layer onto aluminium alloys. It has been found that this interfacial layer heavily restricts pit growth underneath the silane films via retarding the transport oas wel as effectively blocks a number of cathodic sites available for cathodic reactions. 3.3.2 Interaction of the silanes with the Al surface Several studies have shown that the treatment of aluminum with silanes is easier th

f other metals, for instance, HDG steel. This can be ted to many factors such as:

The pH treatme indow for aluminum is wider than for zinc[47,19,74]. The covalent Al O Si bond is readily formed between a hydr lyz d silane and alumo e inum, and

h cthe bond formed is less ionic than t e or-responding Zn O Si bond[39,75]. Al is trivalent, and it can crosslink a Si O Si bond by inserting into it. ZnO is divalent and is unable to do the same, i.e., aluminun silicates

o matches the spacing of

(e.g., zeolites) exist in all kinds of ratios and forms but there is no zinc silicate. Aluminum oxide, Al2O3, has an affinity for silicate. The spacing of hydroxyl groups on aluminum alscomparable groups on silane coupling agent oligomers[6].

A few studies[39,75] provide recent, new evidence for the existence of Al O Si bonds at the alumi-num/silane film interface. In both papers, electrodepos-ited and immersion deposited nonfunctional silane films were studied. A BTSE film was depo ited by immersion, immediately rinsed with ethanol and the existence of the instantaneously formed Al

s

O Si monolayer was analyzed by XPS and TOF-SIMS. The latter work provided direct evidence for the presence of O2Al (O Si O) and OAl (O SiO)2 groups at the silane-aluminum oxide interface of the electrodepos-ite e ed film. Such bonds hav be n suggested, but they have largely remained elusive over the years[39,75].

The existence of the Al O Si bonds at the silane film/aluminum interface does not, however, automati-cally guarantee the corrosion protection of the metal.


Although corrosion inhibition of the metal is intimately related to the formation of Al O Si bond, the bond itself is not hydrolytically stable[6]. When exposed to a large amount of water, the Al O Si bond is hydro-lyzed back to reform hydrolphilic AlOH groups and SiOH groups. This obviously destroys the hydropho-bicity of the metal surface. Therefore, suppression of water uptake in a silane film is of virtual importance in terms of maintaining a good adhesion between the si-lane film and the substrate. This can be achieved by fully crosslinking the silane film, i.e., by making the film denser and by enhancing trin ic silane film hy-drophobicity. In this way, water uptake in the films is suppressed

in s

, hydrolysis of Al O Si bond is pre-ve

er

d with

nted, and corrosion protection of the metal is guaran-teed[46].

Zhu and van Ooij[44-46,58] have shown that the film deposited from the bis-sulfur silane solution on alumi-num actually is made up of three different regions as illustrated in Fig. 10, which shows the lay structure from the outside to the inside: a crosslinked outermost bis-sulfur silane film enriche Si O S bo s; an extensively crosslinked or dense bis-sulfur interfa-cial layer dominant with Si

i nd

O Si and Al O Si bonds; and the inner Al oxide layer on the alloy sub-strate. Referring to this structure, the high-frequency time constant centered at 104 Hz in an EIS phase angle plot (as in Fig. 5) is due to the outermost silane film; the mid-frequency one at 10 Hz is the response of the interfacial layer; and the small tail appearing below 0.1 Hz is related to the Al oxide layer. The dense silane film structure described is formed when the silane film is cured at elevated temperatures (100) due to extensive crosslinking of SiOH groups. The thickness of the interfacial layer has estimated to be in the or-der of about 5 nm (Ref. [39]). The bis-sulfur/bis-amino mixture (3:1) shows an EIS behavior similar to that of the bis-sulfur silane[45]. Franquet et al.

confirmed this result by studying these films by surface analy

have also studied silane layers on aluminum and

tical, optical, and electrochemical methods[41-43].

Fig. 10 Schematic of structure of bis-sulfur silane-treated AA 2024-T3 system[46].

3.4 Galvanized steels and zinc

d state in the building and appliance in

he film which highly promotes w

t are po

The bis-sulfur/bis-amino mixture enhances the

Galvanized steel is a generic term used to describe steel, which has been coated with zinc or a zinc alloy. Galvanized steels are widely used both in unpainted and in painte

dustry[76]. Figure 8 clearly demonstrated that the bis-sulfur/bis-

amino mixture gives HDG steel very good bare corro-sion protection while the silanes alone are unable to protect HDG steel. Figure 11 explains schematically the reasons for the failure of the bis-amino and bis-sulfur silanes on HDG steel. The reason for the failure of the bis-amino silane film on HDG steel is due to the hydrophilic nature of t

ater/ion penetration. The hydrophobic bis-sulfur silane performs very

well on AA 2024-T3, but failed on HDG. This is because AA 2024-T3 has a higher surface energy than HDG steel; and therefore, the bis-sulfur silane is able to wet the Al surface but not the HDG surface. This obviously leads to an insufficient wetting of the Zn oxide. As a result, a non-homogeneous bis-sulfur silane layer is formed on HDG (Fig. 11). Local corrosion initiates at those defective sites tha

orly covered by the bis-sulfur silane film[46].

Fig. 11 Schematic structure of bis-amino, bis-sulfur, and mixture treated HDG steel[20].


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corrosion resistance of HDG steel. This improvement is due to the fact that a small portion of bis-amino silane makes the mixture solution hydrophilic enough to wet the Zn oxide on HDG, which facilitates the formation of a homogeneous film on HDG. On the other hand, a large portion of bis-sulfur silane enhances the hydrophobicity of the mixture film, which is the basis for good protective performance of the mixture. However, unlike the mixture-treated AA 2024-T3 system which exhibits two time constants in the EIS phase angle plot, only one time constant is shown for the mixture-treated HDG. This indicates that rather than forming two distinct layers (i.e., outermost silane layer and interfacial layer) as in the case of AA 2024-T3, the mixture only forms one single structure on HDG steel (Fig. 11)[46].

This result is probably due to the different behaviour of the oxides on the metals, discussed in Section 3.3.2. The trivalent aluminum in the Al2O3 oxide surface is able to form covalent Al O Si bonds and thereby crosslink and interpenetrate into the siloxane network upon curing the mixture film. The divalent ZnO is unable to do this. Therefore, an interfacial layer rich in

Me O Si and Si O Si bonds is not formed on HDG steel, even if the film is cured at elevated temperatures. It may, however, be possible to form this type of interfacial layer on HDG steel by modification of the HDG surface before depositing the silane mixture film onto the HDG steel.

The solvent-based bis-sulfur/bis-amino mixture also provides good corrosion protection of galvanized steel under paints. This silane mixture passivates the metal paint interface to such extent that paint thicknesses can be reduced substantially (e.g., from 25 m to 5 m) without loss in performance[67]. Also, the water-based bis-amino/VTAS mixture works well on HDG steel in the painted state but poorer in the unpainted state. The Machu test results of polyester powder painted HDG steel shown in Fig. 12 exhibit an example. The behavior of the bis-amino/VTAS silane mixture and the chromated are comparable, i.e., no paint delamination was observed after testing (Figs. 12b and 12c), while that of the untreated panel (Fig. 12a) shows severe paint delamination an considerable amount of corrosion along the scribe line[19].

Fig. 12 Machu testing results of HDG panels: (a) untreated, (b) silane-treated (bis-amino silane/VTAS = 5/1, 2%, pH 6), and (c) conventional chromate-treated (Granodine 108B)[19].

4 Modified Silane Films on Metals 4.1 Objectives of modified silane films on metals

Silanes have been demonstrated to provide excellent corrosion resistance and paint adhesion in the previous sections. These excellent anticorrosion properties can be further tailored and improved by strengthening the coatings through various methods. The corrosion behaviour and mechanical properties of the films on metals can, however, be further improved if they are modified and strengthened using various methods. The

objective of the modifications is to improve the following drawbacks that silane films on metals have in comparison to chromates:

1) Silane films are very thin (about 200-300 nm). This limits their capability to provide long-term bare corrosion protection.

2) Chromates have defect-healing capabilities, which neither silanes nor any other proposed chromate replacements have. The self-healing capability means that chromates are able to seal small defects in the conversion coating by dissolving hexavalent chromate ions, which then protect the defect by passivating it by


forming trivalent chromium oxide at the damaged site. 3) Silane films are colorless, whereas chromate

films can be detected by their yellow color. This is useful in coil coating applications, because it enables the observer to see the film after running the sheet through the bath.

Our research currently aims at solving the above mentioned problems and in addition we will add some useful properties to the silane films by modifying them. The objective is to completely replace the chromates with the improved silane films. The improvements focus on:

1) Increasing the silane film thickness and thereby improving the corrosion resistance and mechanical properties of the films. This is achieved by adding nanoparticles such as alumina and silica into the systems.

2) Modifying, the silane films with leachable inorganic or organic inhibitors in order to give the films the self-healing effect described above.

3) Giving color to the films by adding dyes to the silane films.

Additionally, the silane group investigates new techniques such as the electrodeposition method for depositing silanes films on metals in order to make films that are very dense and uniform.

4.2 Loading silane films with nano-particles

A primary study on this subject was done on AA5005 treated with the water-based bis-amino/VTAS mixture loaded with a small amount of alumina nano-particles. The 336 h-SST results showed that the corrosion

protection offered by the nano-structured silane film was comparable to that of the chromate control. Recently, a systematic study has also been done on colloidal silica nano-particle loaded bis-sulfur silane films on AA2024-T3 substrates[22,77].

Figure 13 shows the DC polarization test curves of the AA2024-T3 panels coated with the bis-sulfur silane with and without silica nanoparticles. The cathodic shift observed for the bis-sulfur film loaded with 5 ppm of silica (Curve 3 in Fig. 13) indicates that the incorporation of a small amount of silica alters the cathodic kinetics on the alloy surface. It is also observed that the polarization curves of the silica loaded bis-sulfur silane treated AA2024-T3 samples have two cathodic regions while the curves of the bis-sulfur treated and untreated AA2024-T3 only have one. This suggests that a small amount of silica in the silane film also changes the mechanism of the cathodic reactions on the alloy. Instead of one major cathodic reaction, two cathodic reactions reflected by the two cathodic regions dominate over the applied cathodic voltages. The proposed mechanism for this phenomenon is explained in Section 5 (Mechanisms).

In Fig. 14 the values of corrosion potential (Ecorr) and corrosion rate (Icorr) of nano-structured bis-sulfur silane treated AA 2024-T3 panels are plotted as a function of silica content in the silane solution. A reduction in Ecorr is observed for the film obtained from the silane solution containing 5 ppm silica, indicating that the film behaves as a cathodic barrier like cerium compounds[78,79]. A small amount of silica in the silane film suppresses the cathodic reactions. As

Fig. 13 DC polarization curves of AA2024-T3 treated with and without silica loaded bis-sulfur silane films[22].


654

Fig. 14 Ecorr and Icorr of bis-sulfur silane treated AA2024-T3 systems as a function of silica content in the bis-sulfur silane solution[22].

the amount of silica in the films is increased up to 15 ppm such cathodic inhibitive behavior disappears. The Icorr value is lowest for the AA2024-T3 sample treated in the solution containing 15 ppm silica (Fig. 14). As the silica content is further increased the Icorr value gradually rises, showing that the corrosion inhibition of the silane film on AA2024-T3 degrades when loaded with too much of silica (50 ppm).

A similar trend was observed in the EIS results. The low-frequency impedance values (Zlf) of the systems increase with the increase of the silica content until 15 ppm compared with the unloaded panel. The Zlf value, however, drops sharply for the film obtained from the silane solution containing 50 ppm silica. This, again, confirms that a large amount of silica particles is not required from the corrosion protection point of view. The two time-constant behavior observed for the sample corresponding to 50 ppm silica indicated that a double layer had formed at the silane/metal interface. The formation of the double layer in the sample containing 50 ppm silica indicates that an excess of silica particles in the film has a negative effect on the interfacial adhesion, leading to a premature film delamination from the substrate[22].

The hardness of the films was measured using MTS Nanoindenter XP. The hardness results indicated that the interfacial layer of the bis-sulfur silane film is preferentially hardened by the incorporation of a small amount of silica. With a further increase in silica (e.g., 15 ppm), not only the interfacial layer but also the silane surface is hardened by showing an improvement in its hardness value. However, an extra amount of silica (50 ppm) seems only to strengthen the silane surface but not the interfacial layer, as no further increase was seen for the hardness of the interfacial

layer after 15 ppm[22]. Based on the results presented, it can be concluded

that a small amount of silica nano-particles (15 ppm) does improve both the corrosion performance and the mechanical properties of the bis-sulfur silane film on AA2024-T3. Corrosion resistance, however, diminishes when further increasing the amount of silica in the film (15 ppm). Moreover, an extra large amount of silica particles in the film even degrades the corrosion performance of the silane film (> 50 ppm). Therefore, the optimum silica amount in the bis-sulfur coating solution appears to be between 5 ppm to 15 ppm[22,77].

4.3 Loading silane films with inhibitors

4.3.1 Adding inorganic and organic inhibitors to the bis-amino/VTAS film on AA2024-T3

One of the ways to increase the corrosion resistance of the silane films on metals is to add corrosion inhibitors into the films. Such inhibitors can leach out slowly from the film when corrosion initiates in a defect or a scratch on the metal. After establishing the corrosion resistance of various inorganic and organic inhibitors that work for the AA2024-T3 alloy, three inhibitors were selected for further studies. The results for a rare-earth metal salt, cerium nitrate[80], organic inhibitors such as benzotriazole[81], and tolyltriazole[82] are discussed here. The selected inhibitors suppress the corrosion on AA2024-T3 by covering the surface and/or by forming complexes with the alloy[83, 84]. The compounds are also non-toxic and environmentally acceptable as compared with chromate.

Varying concentrations of the above mentioned inhibitors were added to the bis-amino/VTAS silane solution and the silane films were deposited onto Al


2024-T3. The corrosion resistance and the self-healing capabilities of the inhibitor-loaded silane films on AA2024-T3 were examined. The effectiveness of non-chromate inhibitors in the silane film depends on a number of factors such as the solubility of the inhibitors, leachability of the inhibitors, the permeability of the silane coating, and the compatibility of the inhibitors with the silane. The inhibitors were chosen considering the above mentioned factors. 4.3.2 DC polarization test results of the inhibitor

loaded silane film samples Figure 15 shows the DC polarization curves for the AA2024-T3 panels coated with bis-amino/VTAS silane with and without varying concentrations of cerium nitrate and the blank as control. For the panels containing 1000 and 2000 ppm cerium nitrate, a shift in Ecorr in the cathodic direction is observed. A reduction in the cathodic current density values is also detected. This is an electrochemical effect as the rare earth metal salts act by blocking cathodic areas of the

Fig. 15 DC polarization curves for the bare AA2024-T3 panel (1) and panels coated with bis-amino/VTAS silane film (2) and bis-amino/VTAS silane films doped with 100 ppm (3), 1000 ppm (4), and 2000 ppm (5) of cerium nitrate inhibitor on AA2024-T3 (Ref. [80]).

material and consequently reduce the overall corrosion rate. A decrease in both Icorr and Ecorr also indicates that the inorganic inhibitor does not interact with the silane network.

Loading the bis-amino/VTAS film with different amounts of tolyltriazole results in a reduction of the cathodic current density (Icorr), implying a reduction of the cathodic reaction rate and a better corrosion resistance as compared to the silane film without tolyltriazole. The performance of the film increases upon increasing the amount of tolyltriazole. Similar results were obtained for the bis-amino/VTAS film loaded with benzotriazole. The reduction in Icorr indicates a geometric blocking effect of active metal reaction sites. The triazole inhibitor in the silane film can react with the copper phases and form Cu-triazole complexes making the copper unreactive to the corrosive medium. The inhibitor thus gets immobilized at the reactive metal surface and gives a reduction in the cathodic current density. 4.3.3 Self-healing effect and paint adhesion of the

inhibitor loaded silane films The panels coated with the cerium nitrate (8104 g/m2) loaded bis-amino/VTAS silane films were scribed. The width of the scribe was about 100-120 m. The scribed panels were immersed in 0.5 mol/L NaCl electrolyte for 7 days. Figure 16 shows the SEM images of the scribes in the bis-amino/VTAS film (a) with and (b) without cerium nitrate. The scribe in Fig. 16b shows corrosion products along the scribe whereas the scribe in Fig. 16a shows less or no corrosion products indicating that the cerium inserted imparts self-healing capabilities to the film. The scribes of the cerium nitrate containing panel were analyzed by SEM/EDX. Figure 17 shows a part of the EDX

Fig. 16 SEM image of AA2024-T3 coated with bis-amino/VTAS silane mixture (a) containing cerium nitrate and (b) without cerium nitrate, after 7 days immersion in 0.5 mol/L NaCl (Ref. [80]).


656

spectrum obtained. The analysis result confirmed the presence of cerium in the scribe. A Ce peak is observed of the same order of intensity as the copper peak, the major alloying element of AA2024-T3. This indicates that cerium ions have leached out of the silane film and precipitated into the scribed area, where they reduce the rate of the cathodic reaction.

Fig. 17 SEM/EDX analysis of an area on the scribed AA2024-T3 coated with silane containing cerium ni-trate, immersed in 0.5 mol/L NaCl for 7 days[80].

The idea is that the water-soluble inhibitor should be released only on demand, i.e., the inhibitor should not be washed away when exposed to water. Ideally when the electrolyte penetrates the silane film (containing cerium inhibitor), local anodic and cathodic reactions start to occur at the metal surface. The areas where the cathodic reactions occur (at high pH) should trigger the release of cerium (cathodic inhibitors). Therefore, the AA2024-T3 panels coated with the bis-amino/VTAS film containing 100 ppm cerium nitrate were immersed in deionized water at different pH values (2, 7, and 11) for different periods of times; 1 h, 48 h, and 96 h. The same test was conducted by exposing Ce containing panels to 3.5% NaCl for 48 h. After exposure, the immersion solution of each test panel was analyzed for cerium by ICP/MS.

It was found that inorganic Ce(III) is leached out of the hydrophilic bis-amino/VTAS film at pH 2, pH 7, and pH 11, but the amount at pH 11 was 100 times higher than at pH 2 and pH 7. There was no difference in the amounts obtained at pH 2 and pH 7. This indicates that at high pH a cathodic reaction occurs and the cerium nitrate is released into the solution. The generation of hydroxyl ions due to the cathodic reaction swells up the silane film leading to the release of inhibitor. This shows that the cerium is not washed away when exposed to water and is released only on

demand to achieve self-healing. The amount released during salt water exposure was comparable to that released at high pH. The organic inhibitors (tolyltriazole and benzotriazole) were also examined in a similar way but they were not able to impart a self-healing effect to the bis-amino/VTAS film.

AA2024-T3 panels coated with the bis-amino/VTAS mixture containing small amounts of cerium nitrate and natrium chromate were exposed for 336 h to a neutral salt spray test. The amounts of Na2CrO4 in the films were a factor 104 lower than that in a regular chromate conversion layers. The overall protection of the metal was substantially improved by the small chromate addition. Further, the scribes in the panels remained clean and Cr could be detected in the scribes by SEM/EDX. Thus, not only inhibitor cations (Ce3+), but anions as well can be leached out from the silane films.

The AA2024-T3 panels treated with the bis-amino/VTAS silane mixture with and without 1000 ppm of cerium nitrate were coated with a polyester powder paint and tested for paint adhesion. The cross-cut test was made before and after exposure to water. The results indicated that there was no loss of adhesion after doping the silane with cerium nitrate. Thus, the inhibitor in the silane film does not affect its adhesion to the paint.

In summary, cerium nitrate reduces both Icorr and Ecorr and imparts the self healing effect to the silane film studied. Ce nitrate also works well with small amount of chromates. The organic inhibitors added to the bis-amino/VTAS silane films improved the overall corrosion resistance of the AA2024-T3 alloy but did not impart a self-healing effect to the films.

4.4 Adding color to silane films

Two dyes were examined: Basonyl Yellow NB 122 dye and Basonyl Red 482 (xanthene) red powder dye. They were added to a 5% hydrolyzed bis-sulfur silane solution and films were deposited onto AA2024-T3. Mainly, the effect of the dyes on the corrosion and adhesion performance of the bis-sulfur silane films was studied[77].

Both the yellow and the red xanthene dyes were soluble in the bis-sulfur silane solution and the films deposited onto AA2024-T3 were visible to the naked eye. The bare corrosion resistance results investigated


by the DC polarization technique showed that the addition of the dyes did not affect the corrosion resistance of the silane films on AA2024-T3. The dyed and undyed silane treated panels were also immersed in 3.5% NaCl solution for one week. The corrosion performance of all samples was equal and the dyes of the film did not leach out into the salt solution.

Paint adhesion of the dyed silane treated panels was also tested. The panels were coated with a solvent-based polyester paint, and the cross-cut adhesion test with and without exposure to humidity was used for testing. Good results were obtained for the red xanthene dye but the yellow dye made the paint peel off. The reason for the failure of the yellow dye was examined. Both dyes are water and alcohol soluble and do not leach out when exposed to salt solution. This suggested that the solvent solubility of the dye after incorporating it into the silane network could be of importance. Therefore, the dyed silane coated AA2024-T3 panels were exposed to ethanol. In a few days, the yellow dye gradually started to leach out of the film into the ethanol changing the solvent yellow, while the red xanthene dye did not leach out of the film.

Apparently the xanthene dye becomes part of the silane matrix after curing the film and does not affect the structure of the silane film or its corrosion and adhesion properties. Xanthene has two hydroxyl groups at both ends. When the bis-sulfur silane solution containing the xanthene dye is cured on the metal, the dye reacts with the silanol groups that are present in the hydrolyzed bis-sulfur silane solution giving rise to an insoluble complex upon drying the film.

Based on the results obtained certain requirements were set for dyes which are to be added into silane films[77]. They should: 1) be water and alcohol soluble; 2) not react with the silane solution or destabilize it; 3) become incorporated into the silane network, i.e., become insoluble after curing the film; 4) not leach out when exposed to water or solvent.

4.5 Electrodeposition of silanes

4.5.1 The aim with the electrodeposition method Electrodeposition is a process of depositing a coating on a metal substrate by the application of electric current. Hydrolyzed silanes are water-soluble ionized molecules, and so they can be deposited on metals by

this method. Compared with the conventional immersion process, the electrodeposition technique seems to provide a more uniform, highly thickness controlled, and more crosslinked silane coating[85-88]. Various combinations of silane mixtures have been tested in our silane lab at different voltages, pH values, bath concentrations, and exposure times on aluminum and iron. The surface structure of these samples has been characterized by surface analytical methods, ellipsometry and corrosion resistance by the DC polarization technique and EIS (Refs. [39, 75]). 4.5.2 Preparation of the electrodeposited silane films The non-functional bis-silane BTSE was electro-deposited onto mirror polished ferroplate and Al-5052 sheet and unpolished Al-6111. The electrodeposition set up comprised of an electrolytic cell with the silane solution as the electrolyte and the metal substrate as the cathode (negative) and graphite as the anode (positive). A 5-V constant voltage was applied to the cell for 60 min on the Al-6111 substrate, for 30 min on the Al-5052 substrate, and different deposition times were used on mirror polished ferroplate to see the variation of film thickness with time. After electrodeposition, the substrates were dried at 100 for 60 min. The Al-5052 substrate was immediately rinsed in ethanol for 5 min in an ultrasonic cleaner and then dried at 120 for 30 min. The immersed BTSE films were treated and dried as described in the experimental section[39,75]. 4.5.3 Results of the electrodeposited silane films The DC polarization results showed a slight reduction in the current densities for the electrodeposited sample as compared with the immersed sample, indicating that the electrodeposited film slows down corrosion more effectively than the immersed film. The EIS results also showed higher impedance values for electro-deposited films than for the immersed films.

Figure 18 presents the phase angle curves for the immersed and electrodeposited BTSE films on Al-6111. A phase constant at high frequencies and a second time constant at low frequencies can be detected for the electrodeposited sample during the initial hours of testing whereas the immersed film only shows one time constant (Fig. 18). The second time constant is due to the presence of an interfacial layer between the silane film and the metal substrate as verified also by ellipsometry. The EIS results indicated


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that the immersion deposited BTSE film did not form an interfacial layer onto Al-6111.

Fig. 18 Phase angle curves for the immersed and electrodeposited BTSE film on Al-6111 after 8 h of immersion in 3.5% NaCl solution. TC is the time constant, and W is the Warburg impedance[39].

Figure 19 shows the pore resistances of the silane films as a function of time. The pore resistance of the siloxane film on the electrodeposited sample was equivalent to the immersed film but the pore resistance of the interfacial layer in the electrodeposited film was extremely high and it decreased with time as can be seen from Fig. 19. This occurs as the electrolyte penetrates into the film[14,53,55,58,89-91].

Fig. 19 The pore resistance of immersed and electrodeposited BTSE films on Al-6111 as a function of electrolyte immersion time[39].

The thickness of the BTSE film on ferroplate was measured at three different points. The average thickness was 227.5 nm with a non uniformity of 55%. The mean standard error (MSE) for this measurement was 9.8. When deposited by immersion the silane film thickness is virtually independent of immersion time but when the film is electrodeposited the film thickness was found to increase with deposition time. Table 3 summarizes the results of thickness, uniformity, and MSE measured by ellipsometry on the electrodeposited panels.

Table 3 The results of thickness, uniformity, and MSE measured by ellipsometry on the electrodeposited panels

Time (min)

Thickness (nm)

Uniformity (%)

MSE

1 214.1 20 8.4 10 242.0 7 20.8 30 275.4 7 9.2 60 272.7 7 10.5

A graph of the results showed that the film thickness increases almost linearly with electrodeposition time until a deposition time of about 30 min is reached, after which a plateau in film thickness is reached. For the films which were deposited onto Al-5052 and immediately rinsed in ethanol afterwards, the film thickness obtained by immersion was 0 but for the electrodeposited films thicknesses of 4.6 nm and 4.9 nm were measured with mean standard errors of 3.8 and 10.5, respectively. These results, like the EIS results, indicate that a highly cross-linked interfacial layer of about 5 nm in thickness is formed onto the aluminum when the BTSE film is deposited by electrodeposition. The interfacial layer is tightly bound to the substrate and cannot be washed away with ethanol, not even right after deposition. After electrodeposition, two layers, the interfacial layer, and the siloxane layer are present on the metal surface. The total film thickness is about 255 nm including the 5 nm thick interfacial layer.

SEM images of the BTSE films deposited by immersion showed patches on the surface, indicating that the film formed was rather nonuniform. The patches were rich in silicon whereas hardly any silicon could be detected on the rest of the surface by SEM/EDX. The SEM images of the panel electrodeposited for 10 min showed a more uniform film. The film was, however, torn at certain spots. No silicon was detectable on the torn portions whereas on the rest of the surface Si peaks of significant intensity were observed. As the electrodeposition time was increased to 30 min and 60 min, the film got healed up with no torn spots and became highly uniform covering the entire surface. EDX analysis on the whole surface showed silicon peaks of moderate intensity indicating that 60 min of electrodeposition results in a rather uniform film like the ellipsometry results had already indicated.

It can thus be concluded that electrodeposition


results in an organized void-free and uniform film with low porosity on the metal surface. The layer formed at the interface between the silane and the oxide has a very high ohmic resistance and low permeability for the electrolyte.

5 Mechanism of Corrosion Protection of Metals by Silanes

The mechanism by which thin films of organo-functional silanes protect metals from various forms of corrosion appears to be quite simple. There is no evidence that silanes are electrochemically active in solution or in the solid state. They cannot be reduced or oxidized, unless they carry functional groups that have electrochemical activity. Most functional groups are not electrochemically active, so it can be assumed that silane films are primarily barrier coatings. They reduce the rate at which water and electrolyte can reach the interface where they would induce corrosion reactions. Thus, the most important property of silanes films is their hydrophobicity. Figure 20 shows a schematic of a film of a bis-silane without functional groups deposited on aluminum. If sufficiently crosslinked, such a film is fairly hydrophobic. The water contact angle of a film of this type is of the order of 90 and remains at that level during immersion in

water or salt solution for a few weeks. Eventually, however, the contact angle will become lower. This indicates that the Si O Si siloxane groups are not indefinitely stable, but slowly hydrolyze back to silanol groups, which are considerably more hydrophilic. Consequently, silane films cannot protect metals indefinitely. Even the most hydrophobic films will, when immersed continuously, hydrolyze so that water can eventually penetrate and reach the interface. However, the siloxane hydrolysis is a reversible process, so siloxane groups are reformed if the partly hydrolyzed film can dry out again.

Si O Si + H2O Si OH + HO Si In agreement with this simple mechanism, it is commonly found that silane molecules which are more hydrophilic do not provide good corrosion resistance. An example is -APS, which provides good adhesion between coatings and metals, but is not very effective in preventing the metal substrate from general corrosion[92]. This phenomenon illustrates that good adhesion of a coating to a metal does not automatically imply good corrosion resistance. In Ref. [92], the corrosion resistance of an aluminum alloy coated with several different powder coatings was determined using a variety of test methods. The silanes used were -APS, BTSE, a

Fig. 20 Schematic of the structure of a bis-silane film on aluminum.


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mixture of the two, and a two-step process consisting of a BTSE rinse followed by a second rinse by -APS. The two-step system had the best performance in all tests, including adhesion tests. This can be explained on the basis of our simple model as follows. The BTSE treatment provides a hydrophobic interface to the metal which slows down the water diffusion to the metal-silane interface. The second film by -APS, formed on top of the BTSE film, adheres well to the BTSE film via interaction between silanol groups in both films, and at the same time adheres well to many paint systems via its primary amino groups. By itself a film of -APS is too hydrophilic and, as a result, too permeable to water for corrosion protection. Mixtures of BTSE and -APS also worked reasonably well, but were not as hydrophobic as the film formed from BTSE by itself.

The question why bis-silanes seem to perform better as a class than the standard mono-silanes has often been asked. When we consider Fig. 20 and our mechanism of hydrophobicity of silane films being the most important criterion, the answer is as follows. If we would compare the silanes BTSE and methyl trimethoxysilane (MTS), then a 5 wt.% solution of each silane will have the same concentration of silicon atoms, carbon atoms and silanol groups in the solution. Yet, a film of BTSE will work considerably better than a film formed by MTS, assuming equal hydrolysis, deposition, and curing conditions[20]. Inspection of Fig. 20 tells us that only a film of BTSE can be dense and three-dimensionally crosslinked, but a film of MTS cannot, as there are free methyl groups in the film which, due to their mobility, necessarily result in a higher permeability of the film to water and other diffusing species. The film of BTSE is more rigid and has lower diffusion rates for water and ions[20]. However, even for bis-silane films, the condition that the molecules should not contain hydrophilic functional groups remains. As an example, films of bis-[trimethoxy silylpropyl]amine (bis-amino), do not work well by themselves. The secondary amino groups will attract water, become protonated, and will then also attract chloride ions into the film when exposed to a salt solution[20].

Films of bis-[triethoxysilylpropyl]tetrasulfide (bis-

sulfur) form films that provide much better corrosion protection to metals, such as aluminum, zinc, and steel. Such films are considerably more hydrophobic due to the sulfur chain in the molecule[20]. However, a solution of this silane does not wet metals very well. Therefore, films made from bis-sulfur silane solutions are often porous. If mixtures of the bis-amino and the bis-sulfur silane are used, one can tune the wettability and the hydrophilicity accurately and the result is a film that wets perfectly but is still sufficiently hydrophobic to provide excellent corrosion protection. Thus, such a mixture can display a performance far superior to those of the two individual silanes[20]. In addition to this wettability effect, the secondary amino groups have a catalytic effect on the silanol condensation, so films made from mixtures of the bis-amino and the bis-sulfur silane are more completely hydrolyzed and crosslinked than those made from the bis-sulfur silane alone.

Another question that has often been asked is whether the bonds formed between metals and silanes are hydrolytically stable. The answer to that question is that the type of bond that actually exists at the interface is still not well known. In the case of silane films on aluminum, it appears to be a more complex silicate than simply an Al O Si bond[20]. However, whatever the nature of the bond, this bond is not resistant to water. That is the reason why the silane film has to be hydrophobic. If water manages to reach the interface, it will hydrolyze the bonds with formation of Me OH and Si OH groups which are very hydrophilic and more water will be attracted to the interface as a result. Corrosion can then start. It is reasonable to assume that if even Si O Si groups can hydrolyze at the silane surface (see above), bonds of the type SiO Me (shown in this form for simplicity only) will hydrolyze even faster, as such bonds have a higher ionic character resulting in easier attack by polar water molecules than in the case of siloxane bonds. As in the case of Si O Si , the metal-silane bond hydrolysis by water is reversible, as the experiment shown in Fig. 21 convincingly demonstrates. Shown are two panels of cold-rolled


steel coated with the same epoxy paint. Under the paint two different silanes were used. The one on the left was more hydrophilic (porous) than the one on the right. It is seen that the dry adhesion (top of each panel) passed the ASTM D-3339 test for each silane. The wet adhesion (24 h in deionized water) was poor for the hydrophilic silane and excellent for the other silane. However, after allowing a sufficient amount of time for drying (1 week at room temperature), the system on the left had regained the original adhesion, illustrating that the interfacial bond hydrolysis in this system was

reversible. It should be noted that the delaminated area in this system due to hydrolysis of the interfacial bonds revealed a steel surface that was clean and entirely devoid of rust. This delamination was not related to corrosion, but to bond hydrolysis only.

In summary, silane film performance on metals in terms of corrosion protection and adhesion to paints, depends on the hydrophilicity of the film, its crosslink density, wettability of the metal by the silane solution, and film thickness, which will all have an effect on the rate at which water can reach the silane-metal interface.

Fig. 21 Dry and wet adhesion of an epoxy paint to cold-rolled steel treated with a hydrophilic silane (left) and a more hydrophobic silane mixture (right).

6 Conclusions

It has been demonstrated in this paper that thin films of organofunctional silanes can successfully replace chromate or other metal pretreatments. The silane technology is very versatile and works with many metals and paint systems. Good adhesion can be obtained to almost any paint by a judicious choice of the silane system. Often mixtures work better than individual silanes as such mixtures can be fine-tuned for optimum wettability and hydrophobicity properties.

Dipodal silanes, also called bis-silanes, typically are more effective in preventing corrosion of metals than the more widely known mono-silanes. This difference is directly related to the mechanism by which silanes protect metals. Although the actual bonds between

silane film and the metal oxide remain elusive, there is little doubt that with some metals, notably aluminum, covalent bonds are formed. In other cases, e.g., iron, the bond may not be more than a hydrogen bonding. Nevertheless, whatever the nature of the bond, it has been shown that such bonds are not resistant to water. They will hydrolyze back to metal hydroxide and silanol groups. Such groups are very hydrophilic, so that more water will be attracted and corrosion will start. Thus, for good corrosion performance, the silane film needs to be as hydrophobic as possible, so that water ingress is minimized. This poses a dilemma as hydrophobic silane films that perform well in corrosion tests, are typically soluble in organic solvents only. Silanes that are water-soluble form films that are more hydrophilic. Thus, the environmental low-VOC requirement is incompatible with the hydrophobicity


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requirement for good corrosion protection by silanes. Thus, it has been explored whether hydrophobic

silane films can somehow be obtained from hyd

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