Advanced chrome-free organic-inorganic hybrid pretreatments for aerospace aluminum alloy 2024-T3--application of novel bis-ureasil sol-gel precursors.

Abstract Effective pretreatment of aluminum alloys is very critical to success of protective coating systems for aerospace applications. While chromate-based pretreatments have been very successful for corrosion protection, they have been a target for replacement due to the increasingly stricter regulatory requirements arising from toxicity and carcinogenic nature of Cr(VI) used in such pretreatments. Among many approaches to develop alternative systems, the organic-inorganic hybrid (OIH) coatings based on sol-gel technology has advanced rapidly. We have successfully developed OIH coating systems by using suitably tailored organo-silane precursors and sol-gel processing conditions. A series of novel bis-ureasil precursors have been developed and employed as organic precursor of OIH systems. Statistical design of experimental methodology (DoE) has been used to study and optimize compositional and process parameters using multifactor analysis of variance (ANOVA) analysis method. The corrosion resistance study (Potentiodynamic polarization, salt-spray corrosion test) shows that by proper choice of sol-gel precursors, cross-linkers, and reaction conditions, very dense, adherent and protective hybrid coatings, comparable in performance to chromate-based ones, can be obtained for aerospace aluminum alloy 2024-T3.

Keywords Organic-inorganic hybrid, Sol-gel, Pretreatments

Introduction

Aluminum alloys, such as AA2024-T3, are materials of choice for aerospace applications due to their excellent mechanical properties combined with high strength-to-weight ratio. These materials, however, are very susceptible to oxidation and corrosion under aggressive environments. For protection of metal substrates from corrosion, application of multilayer coating systems is among the most successful ones. Each coating layer in such a system is carefully designed for a specific role in the overall corrosion protection mechanism. Metal surfaces are required to be pretreated to improve adhesion of organic coating and for corrosion resistance, which is critical to the success of overall coating systems. Chromium-based pretreatments are among the most efficient and successful systems for Al and alloys. However, due to their carcinogenic nature ([Cr.sup.+6]), toxicity hazards, and associated costs, chromium-based compounds pose serious challenges in their usage and are a target for replacement. The new OSHA PEL for [Cr.sup.+6] and growing restrictions on the use of chromates around the world are driving manufacturers to find less harmful alternatives. (1)

During the past several years, a number of alternatives have been explored with the goal of developing environmentally benign conversion coatings with comparable or better performance to chromate-based ones.(2) Many pretreatment products and techniques are now commercially available. The success of any such new approach/technology is primarily dictated by its corrosion performance, ease of application, compatibility with broad range of coating materials and substrates, and cost.

In recent years, the sol-gel approach has emerged as a versatile method for preparing a host of oxide materials that are candidates for use in protective coating applications, as it is possible to form highly adherent, chemically inert, and dense films that are covalently bonded on metal substrates at temperatures at or near ambient. (3) The sol-gel method consists of simultaneous hydrolysis and condensation reactions, originating with alkoxide precursors, to form glassy polymer networks. Such inorganic sol-gel coatings are derived from lower alkoxides of such metals as Si, Zr. Ti, and Al. and have been extensively studied as barriers for corrosion protection, on Al alloy, (4), (5) steel (6) and galvanized steel, (7) in addition to others. The principal shortcomings of such inorganic oxide films arc their poor mechanical properties (flexibility, toughness), compatibility with organic coatings, and limitations in application of higher film thickness.

Organic-inorganic hybrid (OIH) thin film by sol-gel processing is a very promising alternative to chromatc surface treatments because it offers potential to incorporate chemically tailored features for enhancement of corrosion resistance. (8) Such hybrids, also called Ormosils or Ceramers, are formed by hydrolysis and condensation of organosilanes with traditional alkoxide precursors.(11), (12) It has been found that they provide good corrosion resistance because they combine the mechanical and chemical characteristics of the comprising organic-inorganic networks, producing films that are durable, scratch resistant, adherent to metal substrates, flexible, dense, and functionally compatible with organic coaling systems. (13)

Our approach

The primary objectives of the present study are: (1) to derive OIH coalings with significantly improved barrier properties, (2) to maximize covalent bonding of sol-gel matrix with alloy substrate through Si-O-Al linkages, which is known to be an important factor for adhesion and hence corrosion performance of the system,(14), (15) and (3) to study and optimize the key compositional and sol-gel process conditions for maximum corrosion protection.

In order to improve barrier performance we have designed novel bis-ureasil compounds as organic sol-gel precursors. It has consistently been observed that bis-silanes, with six hydrolysable alkoxy groups, perform much better in corrosion protection performance than mono-silanes on various metals and alloys.(10) We believe that incorporation of functionality, such as urea (-NH-CO-NH-), onto the bis-silane structure that allows for strong intermolecular H-bonding would improve the barrier properties of the films while maintaining good mechanical performance. Furthermore, such groups would also improve adhesion with epoxy or urethane primer through similar interactions. Therefore, we designed bis-ureasil precursors (Fig. 1), capable of intermolecular H-bonding, as primary component of sol-gel systems.

[FIGURE 1 OMITTED]

Van Ooij ct al. (10) have shown that it is important to have highest silanol group (-Si-OH) concentration in the sols (hydrolysis step) prior to their application on to the substrate. This would ensure maximum metal-losiloxane bond (-Si-O-Al-) formation, which is a key factor governing corrosion performance of the pre-treatment. Since hydrolysis rates of bis-ureasil and Tetraethoxyorthosilicate (TEOS). the two precursors in our proposed system, are significantly different under given processing conditions, we have tailored sol-gel processing to maximize silanol concentration prior to their application, as described in the experimental section. Figure 2 is a schematic representation of OIH pretrcatment on aluminum alloy substrate.

[FIGURE 2 OMITTED]

As is well known, corrosion performance of sol-gel OIH system depends upon a number of compositional and processing factors. We have attempted to study and optimize such factors using statistical design of experimental methodology (DoE).

Thus, in the present work, a novel family of organosilanes--bis-ureasil compounds--has been designed and synthesized for use as organic precursor of OIH coating systems. OIH coatings (pretreatments) based on TEOS and bis-ureasil. in varying proportions, have been derived onto the aluminum alloy AA-2024T3 surfaces, under varying conditions. Commercially available non-chromate pretreatment systems have also been studied as reference, along with a chromate-based pretreatment as control. Corrosion resistance performance of these systems have been studied and compared. A statistical design of experiment (DoE) has been used to systematically vary important compositional and process parameters to study their effects on corrosion performance of pre-treatments. Process optimization has been done using multifactor analysis of variance (ANOVA) analysis method.

Materials and methods

(3-Aminopropyl) trimethoxysilane (APTMS), (3-aminoproyl) triethoxysilane (APTES), (3-glycidyloxypropyl) tri-methoxysilane (GPTMS), thethylenetriamine (DETA), tetraethyl orthosilicate (TEOS), ethyl alcohol, tetrahydrofuran (THF), isopropyl alcohol (IPA), and glacial acetic acid (GAA) were reagent grade chemicals (Sigma Aldrich). Isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), and hexamethylene diisocyanate (HDI) were commercial samples (Bayer Material Science), All materials were used as received. AC 131[R] (Advanced Chemistry & Technology Inc.) and X-It PreKote[R] (Fortune Chemical Company) are commercially available non-chromate pretreatments systems. Brulin 815 GD[R] (alkali detergent) for cleaning and scrubbing of panels was acquired from Brulin and Company Inc.

Synthesis of his-ureasils

Three bis-ureasil compounds, differing in chemical structures, were synthesized by reaction of amine functional silane, APTMS, with different diisocyanate compounds, namely IPDI, HDI, and TDI, using 2:1 molar ratio. Figure 3 shows a general reaction scheme for bis-ureasil synthesis. The following general procedure was used for synthesis of bis-ureasils. Into a three-necked reaction flask, provided with a mechanical stirrer, thermometer, and an inlet for nitrogen gas, calculated amounts of dry THF and APTMS were charged and the mixture was stirred for 10 min until clear solution was obtained. was then slowly added from an addition funnel, under stirring at room temperature. After complete addition of diiso-cyanate, the mixture was stirred for 2.0 h. Completion of reaction was followed by FTIR spectroscopy for disappearance of-NCO absorption band at 2,270 [cm.sup.-1]. THF was then removed under rotary vacuum evaporator at 50[degrees]C. The product bis-ureasil was then dissolved in dry ethanol (30-70% solids) and stored in plastic bottles under desiccator. In synthesis of TDI-based bis-ureasil, IPA was used as solvent in place of THF, for the reasons of solubility and stability of the product. Based on the type of diisocyanate used, the bis-ureasils are designated by letters C (1PDI), I (HDI), L (TDI). A bis-ureasil was also synthesized from APTES and IPDI, following the stated general procedure and is designated as T.

[FIGURE 3 OMITTED]

Preparation of sols and application

The preparation of sols and their application were carried out in the following stepwise manner:

Step-1: Pre-hydrolysis of TEOS: TEOS was added to the mixture of ethanol and double distilled water, and GAA (acid catalyst). The pH of the solution was varied in the range of 2.6-2.9. The mixture was stirred under magnetic stirrer, at room temperature for 24-72 h.

Step-2: Post-addition of bis-ureasil: Appropriate quantity of bis-ureasil compound was then added into pre-hydrolyzed TEOS solution (step-1) and solid contents were adjusted (~25% by wt.) using deionized water and ethanol. The pH of the solution was varied between 3.25 and 5. Bis-ureasil/TEOS ratio was varied from 0.5 to 2.0. For all the samples studied, the ratio of water/ethanol and solid contents was maintained constant.

Step-3: Panels preparation and coating application: AA 2024 T3 panels (2" x 3") were prepared, prior to application of sol-gel pretreatment, as follows. First, panels were hand scrubbed using a maroon scotch-brite[R] with 10% solution of Brulin 815 GD[R] detergent in DI water and then rinsed with DI water. They were then immersed in 10% solution of Brulin 815 GD[R] detergent in DI water at minimum 142 F followed by washing with DI water. Then the panels were deoxidized for 2 min in phosphoric acid-based deoxidizing solution, followed by DI water rinse and drying, and were then stacked for 24 h before being used for application. Before coating application all the panels were washed with DI water for removal of any loose dirt on the surface.

Coatings were applied at room temperature (25-30[degrees]C) by dip-coating method at standard withdrawal speed of 15--17 cm/min, with residence time being 15-20 s. After application, panels were placed vertically in a panel stacker, for air drying. The typical dry-film thickness of coating was about 2-5 microns. Commercial pretreatment materials were applied as per the directions in their technical data sheets. For comparison purpose, the commercially available non-chromate AC-131, X-It PreKote pretreatments were applied as per suppliers' directions. Chromate-based control, Alodine 1200S-coated panels, were obtained from Henkel Corporation and used as received.

Study and optimization of process parameters using statistical design of experiment (DoE) methodology

Sol-gel process is very complex and involves a number of chemical reactions occurring at different rates under given conditions. These reactions are significantly affected by process conditions such as pH, temperature, hydrolysis time, solvent content, cure time, and temperature, among others. In order to study the effects of the key parameters as a function of the corrosion resistance performance of pretreatments, the DoE experimental methodology has been used. DoE allows for the multiple factor selection in designing at different levels, and the response for its interaction can be used as the outcome to determine the statistically significant factors. (16), (17) A 3-factor-3-level design methodology has been used to study the effects of bath hydrolysis lime, pH, and cure time as input factors at three different levels. The corrosion resistance performance rating as per salt-spray test has been used as response and the optimizations of factors have been worked out using STATGRAPHICS[R] Centurion XV (Statpoint Inc.) and Minitab 15 (Minitab Inc.) software.

Characterization

Characterization of bis-ureasils: Bis-ureasils have been characterized for functional group assignment using Fourier Transform Infrared spectroscopy (FTIR). FTIR spectra were recorded at room temperature, on Tensor 27, using OPUS 6.0 software (Bruker Optics Inc.). Bis-ureasils diluted with THF were coated at room temperature on NaCJ plates and dried at 40[degrees]C for 5-7 min. Multiple scans were operated in 4,000 [cm.sup.-1] to 400 [cm.sup.-1] range.

Characterization of coatings: Sol-gel pretreatmcnts, along with other commercial chromate and non-chromate pretreatments, have been characterized for accelerated corrosion performance by neutral salt-fog test--ASTM B117 salt-spray test--and were monitored for varying lengths of time. The performance rating of samples was done as per ASTM D 610. PRA-IT-004, SSPC-VIS2 (Publication no. 00-08, ISBN-1-889060-48-8). Corrosion resistance performance was also studied by DC potentiodynamic polarization, using Gamry Instruments (Warminster PA), Series G 300 Potentiostat/Galvanostat/ZRA coupled with PTC1 Paint Test Cell consisting of three electrodes, reference-saturated calomel, auxiliary-graphite and coated 2024-T3 aluminum alloy as working electrode. Aluminum alloy panel had exposed area of 15 [cm.sup.2]. For all the coatings, actual measurements were taken after 30 min of open circuit potential equilibration time. For correlation with salt-spray test, 5% NaCl solution in DI water was used as electrolyte (pH-6.6-6.8). The acquisition of polarization curve was started from this open circuit potential, with a constant sweep of 1 mV/sec. CorrView Electrochemical Analysis software (Scribner Associates, Inc., Southern Pines, NC) was used to interpret and illustrate all DC data. Samples have also been tested for hydrolytic stability using 4-h boiling water immersion test. (18)

Results and discussion

Bis-urcasil synthesis: Bis-ureasil synthesis and final products were monitored by FTIR spectroscopy. The completion reaction was followed by disappearance of peak at 2,270 [cm.sup.-1] characteristic of asymmetric stretching -NCO, and emergence of new band at 1,700-1,550 [cm.sup.-1]. The presence of strong band at 1,080 [cm.sup.-1] confirmed the presence of silicon methoxide (Si-OMe) in the bis-ureasils. The absence of strong bands at 1,750 also indicated the absence of free isocyanate or free amine in the product. Figure 4 is a representative FTIR spectrum of bis-ureasil based on HDI.

[FIGURE 4 OMITTED]

Corrosion resistance performance tests

Table 1 shows the results of water boiling test and salt-spray test for various ureasil-based pretreatments and also for commercial systems. Samples C, I. L, and T were prepared by addition of corresponding bis-ureasils into TEOS solution that was pre-hydrolyzed for 24 h. After addition of bis-ureasil, the mixture was further hydrolyzed for 30 min. (12 h for sample T, since it contains ethoxy groups, which undergo much slower hydrolysis). Samples were then dip-coated and cured at ambient temperature. The pH of all solutions was between 3.5 and 4.5. Two accelerated corrosion tests, water boiling, test and salt-spray lest, were conducted to characterize performance of ureasilbased pretreatment in comparison with commercial samples and control.

Table 1: Pretreatment compositions and their corrosion resistance
performance

C         IPDI         1/1.5             No               9       7
                                         discoloration

I         HDI          1/1.5             No             Failed (24 h)
                                         discoloration

L         TDI          1/1.5             No.              5       4
                                         discoloration

T         IPDI         1/1.5             No.              7       5
                                         discoloration

Controls  Supplier     Chemistry         4 h of water    Salt spray
                                         boiling test    test rating
                                                        500 h  1000 h

Alodine   Henkel       Chromic acid,     No              10      10
1200S     Surface      complex           discoloration
          Tech         fluorides, and
                       accelerators

AC 131    Adv. Chem.   Organosiloxanes,  Discoloration  Failed (24 h)
          and Tech.    Zirconates
          Inc.

X-It      Fortune      DB and NMP        Discoloration  Failed (24 h)
PreKote   Chemical     solvent*
          Co.

Q**       Lab          GPTMS and TEOS    No             Failed (336 H)
          Synthesized                    discoloration

*DB-diethyleneglycol monobutylether, NMP-n-methyl pyrrolidone

**System Q is based on sol preparation from GPTMS and TEOS followed by
cross-linking of epoxy groups with aliphatic amine-based hardener (19)

The water boiling test primarily provides information about hydrolytic stability and permeability of films to water.(18) Water boiling test can be used for rapid screening of pretreatments on 2024-T3 substrate. ASTM B117 salt-spray test is a standard lest to determine resistance of films to corrosion under accelerated conditions.

As shown in Table 1, all the bis-ureasil-based samples showed good resistance to discoloration, indicating their much better film integrity, comparable with chromate-based pretreatment, Alodine 1200S. Sample Q, a sol-gel pretreatment based on TEOS and oxirane functional silane (amine cross-linked), also showed resistance to discoloration. AC-131 [R] and X-It Prekote [R] showed marked discoloration.

Salt-spray test results showed very interesting trend. Within the bis-ureasil family, noticeable difference in corrosion performance was found, with IPDI-based bis-ureasil performing the best, and comparable (slightly inferior) to the control Alodine 1200S. The cause of failure for these systems was the formation of pits, and their density varied with change in their chemical composition. In general, superior performance of bis-ureasil-based pretreatments can be attributed to their covalent binding with substrate and also to their unique chemical structure.

Among other factors, ultimate corrosion performance also depends on the adhesion of primer onto the pretreatment surface. In order to assess this we prepared pretreated samples of epoxy primer (M Mil-P-23377 J, Class C2. chromated epoxy primer [Deft Inc.]). The cured films were tested for wet and dry Crosshatch tape adhesion as per ASTM D 3359 Method B (Crosshatch). As shown in Table 2, all the sol-gel coatings showed excellent adhesion. Their methyl ethyl ketone (MEK) and water double rub resistance were also found to be excellent.

Table 2: Adhesion and solvent resistance performance of
pretreatment/epoxy primer systems

Sr. No  Coating identity  Dry tape
              code        adhesion  Wet tape adhesion
                                    24 h  48 h  108 h

1       AC-1 3 1             5B      5B    5B     5B
2       Prekote              5B      5B    5B     5B
3       Alodine 1200S        5B      5B    5B     5B
4       Q                    5B      5B    5B     5B
5       C                    5B      5B    5B     5B
6       I                    5B      5B    5B     5B
7       L                    5B      5B    5B     5B
8       T                    5B      5B    5B     5B

Sr. No     Coating       MEK double     Water double
        identity code  rub resistance  rub resistance

1       AC-1 3 1            NA              NA
2       Prekote             NA              NA
3       Alodine 1200S       > 200           > 200
4       Q                   > 200           > 200
5       C                   > 200           > 200
6       I                   > 200           > 200
7       L                   > 200           > 200
8       T                   >.200           > 200

Potentiodynamic polarization study

Potentiodynamic polarization curves were acquired in the Eoc vs -2.0 V. All of the coatings were scanned in triplicates and the most representative scans were used for comparison purpose. E corr and corresponding I corr values were obtained by using Tafel analysis method and CorrView software. E corr and I corr values are tabulated in the inset of Fig. 5. The corrosion current density I corr value of sample C (Fig. 5) is found very low in comparison to all other samples. In general, lower I corr values for all sol-gel coatings compared to bare substrate indicates that sol-gel coatings indeed can provide a physical barrier for blocking the electrochemical process. Alodine 1200 S showed very well-defined passivation in the range of -1.0 V to -600 mV and also showed fading/loss of color in the salt-spray test. This can be attributed to the release of chromium (VI) compounds due to their solubility in water.

[FIGURE 5 OMITTED]

Effect of TEOS hydrolysis time on corrosion resistance performance

For all of the coatings based on bis-ureasils so far studied, we used 24 h of TEOS hydrolysis time, during the sol preparation step. Concentration of silanol in a sol is a function of hydrolysis time, besides such factors as water/silicon ratio, pH, and type and amount of catalyst. (3) In order to study the effect of hydrolysis time, we varied TEOS hydrolysis time, while keeping all other factors constant (Water/Ethanol/GAA as 1.390.735:0.058 molar ratios). Solutions were kept in closed plastic containers for stirring at room temperature. The coatings were formulated with bis-ureasil-IPDI (B-IPDI) (bis-ureasil/TEOS ratio of 1.5/1.0) at intervals of 3,12, 24, 48h, and the coatings were coded as Sol-3, Sol-12, Sol-24, Sol-48, respectively. After application onto AA 2024 T3, panels were cured at room temperature for 7 days and tested for field corrosion performance in salt-spray test for 3 weeks. As shown in Fig. 6, with increase in hydrolysis time, there was an increase in corrosion resistance, with the best performance at 24 h (Sol-24). We attribute these results to the silanol concentration of the sols. It appears that under the given processing conditions, maximum silanol concentration is achieved at around 24 h of TEOS hydrolysis. Sols with higher silanol concentration would provide for increased metallosiloxane bonds (Si-O-AI), which would enhance corrosion resistance. (11) At higher hydrolysis time above 24 h, the decrease in corrosion resistance may be due to the lower silanol concentration as a result of increased condensation of silanol groups.

[FIGURE 6 OMITTED]

Effect of bis-ureasil/TEOS ratio on corrosion resistance performance

In OIH coating systems, the balance between organic and inorganic component is important in achieving good barrier properties while maintaining desirable mechanical performance. Organic component provides desirable resilience, hydrophobicity, and reduced stresses in the coating matrix, depending upon its chemical structure. Bis-ureasils used in his study have unique chemical structure that afford H-bonding and high functionality, and it was through interesting to study and optimize the ratio of bis-Ureasil/TEOS in these coating. Coatings with bis-unreasil- IPDI/TEOS molar ratio varying from 0.5 to 2.0 (with all other factors being constant) were derived, cured at room temperature for 7 days, and evaluated for their corrosion resistance performance. Figure 7 shows corrosion resistance performance of samples after 3 weeks of salt-sprat test. Corrosion resistance performance improved with increase in bis-ureasil/TEOS ratio up 1.5/1.00 and then dropped at 2.0/1.0. This indicates that there is an optimum organic/inorganic ratio for maximum performance.

[FIGURE 7 OMITTED]

Statistical design of experiment for optimization of process parameters: pH, cure time, an bath hydrolysis time

The application of DoE strategy entails the selection of critical parameters to be optimized. In sol-gel process, there are a number of factors that affect the performance of coatings. For DoE, it is important to select the right factors and response criteria to obtain a meaningful outcome. In this study, selection of process factors has been done to optimize the corrosion performance of room temperature cured OIH systems. Factors that significantly influence formation of dense coatings have been selected for optimization. Which include bath hydrolysis time. pH, and cure time. The factors selected are closely related to the condensation (water condensation) Kinetics of sol-gel films. Since pH of solution controls the basic kineties of hydrolysis and condensation, it was considered as an important factor and its levels were selected with acid pH range, which is a prerequisite for dense film formation. Bath hydrolysis time (which is the length of time for which the solution is stirred after addition of bis-ureasil into the TEOS sol followed by dilution with water) has been considered as an important factor since this determines silanol group formation and also the stability of the bath. Since the film formation in this process is solely regulated by evaporation of water/alcohol and sol--gel coatings are known for vitrification, the number of hours of curing was selected as the third important factor. The selected process parameters and their levels are reported in Table 3. Basic design grid for experimentation was obtained from the Statgraphic software (Table 4). All the coatings are based on IPDI based bis-ureasil and cured at temperature and relative humidity of 17-20 C/78--80. After curing for 12, 24, and 168 h, the panels were subjected to salt-spray test for 3 weeks and then evaluated performance, and their ratings were used as responses for further analysis.

Table 3: Experimental factors and their levels

                                   Factors  Low (-1)    0    High (1)

Bath hydrolysis time (min)            A      30       240       480
pH of bath                            B       3.25      4.2       5
Cure time at room temperature (h)     C      12        24       168

Table 4: Basic grid design

Runs   A   B   C   A    B     C   Response/Results

1      1   0  -1  480  4.2    12          4
2     -1   1  -1   30  5      12          4
3      1   0   0  480  4.2    24          4
4      1   0   1  480  4.2   168          6
5     -1   0   0   30  4.2    24          4
6      1   1   0  480  5      24          3
7     -1  -1  -1   30  3.25   12          3
8     -1   1   0   30  5      24          4
9      0  -1   1  240  3.25  168          5
10    -1   0  -1   30  4.2    12          3
11    -1   0   1   30  4.2   168          6
12    -1  -1   0   30  3.25   24          4
13     0   1   0  240  5      24          4
14     1   1  -1  480  5      12          3
15     0   1   1  240  5     168          5
16     0   0  -1  240  4.2    12          4
17     1   1   1  480  5     168          6
18     0   0   1  240  4.2   168          6
19    -1  -1   1   30  3.25  168          5
20     0  -1  -1  240  3.25   12          4
21     0   1  -1  240  5      12          3
22     1  -1  -1  480  3.25   12          4
23     0   0   0  240  4.2    24          4
24     0  -1   0  240  3.25   24          4
25     1   1   0  480  3.25   24          5
26     1  -1   1  480  3.25  168          6
27    -1   1   1   30  5     168          6

A--Bath hydrolysis time; B--Bath; pH C--Cure time (under
ambient conditions)

The multifactor ANOVA was conducted by feeding the factor levels and response to Statgrpahic(R) software. Results are reported in Table 5 and their plots are shown in Fig. 8. In order to identify significant factors and their interactions, four ANOVA, the have been carried out. In the first ANOVA, the factors and their responses have been analyzed for the F ratio and P value, which give information about the significance of the factors on coating performance, If the p value is less than 0.05 or F value is greater than 3.0, the factor is considered statistically significant. From the first ANOVA, where the effect to interactions between the factors was not considered, factor C, the cure time, has been found to be the most significant factor governing corrosion resistance performance of coatings. In the subsequent ANOVA (Table 5), the responses were analyzed for threefold (2nd ANOVA) and twofold interactions (3rd ANOVA). As expected, the analysis shows that there exists statistically significant interaction between bath hydrolysis time an pH. No other interactions between the factors studied have been found to be significant.

[FIGURE 8 OMITTED]

Table 5: Multifactor ANOVA analysis

Source                            Sum of squares  Df  Mean square

(1) Analysis of variance for
result--Type III Main effects

  A: Hydrolysis time of solution      0.296296     2    0.148148

  B: pH of solution                   0.518519     2    0.259259

  C: Curing condition                22.2963       2   11.1481

Residual                              5.40741     20    0.27037

  Total (Corrected)                  28.5185      26

(2) Analysis of variance for
result--Type III Main effects

  A: Hydrolysis time of solution      0.296296     2    0.148148

  B: pH of solution                   0.518519     2    0.259259

  C: Curing condition                22.2963       2   11.1481

Interactions

  AB                                  2.37073      4    0.592593

  AC                                  0.592593     4    0.148148

  BC                                  1.03704      4    0.259259

  ABC                                 1.40741      8    0.175928

Residual                              0            0

  Total (Corrected)                  28.5185      26

(3) Analysis of variance for
result--Type III Main effects

  A: Hydrolysis time of solution      0.296296     2    0.148148

  B: pH of solution                   0.518519     2    0.259259

  C: Curing condition                22.2963       2   11.1481

Interactions

  AB                                  2.37073      4    0.592593

  AC                                  0.592593     4    0.148148

  BC                                  1.03704      4    0.259259

Residual                              1.40741      8    0.175926

  Total (Corrected)                  28.5185      26

(4) Analysis of variance for
result--Type III Main effects

  A: Hydrolysis time of solution      0.296296     2    0.148148

  B: pH of solution                   0.518519     2    0.259259

  C: Curing condition                22.2963       2   11.1481

Interactions

  AC                                  0.592593     4    0.148148

  BC                                  1.03704      4    0.259259

Residual                              3.77778     12    0.314815

  Total (Corrected)                  28.5185      26

Source                                         F-ratio  p-Value

(1) Analysis of variance for result--Type III
Main effects

  A: Hydrolysis time of solution                 0.55    0.5866

  B: pH of solution                              0.96    0.4002

  C: Curing condition                           41.23    0.0000
Residual

  Total (Corrected)

(2) Analysis of variance for result--Type III
Main effects

  A: Hydrolysis time of solution                 0.55    0.5866

  B: pH of solution                              0.96    0.4002

  C: Curing condition                           41.23    0.0000

Interactions

  AB

  AC

  BC

  ABC

Residual

  Total (Corrected)

(3) Analysis of variance for result--Type III
Main effects

  A: Hydrolysis time of solution                 0.55    0.5866

  B: pH of solution                              0.96    0.4002

  C: Curing condition                           41.23    0.0000

Interactions

  AB                                             3.37    0.0676

  AC                                             0.84    0.5359

  BC                                             1.47    0.2964

Residual

  Total (Corrected)

(4) Analysis of variance for result--Type III
Main effects

  A: Hydrolysis time of solution                 0.55    0.5866

  B: pH of solution                              0.96    0.4002

  C: Curing condition                           41.23    0.0000

Interactions

  AC                                             0.47    0.7566

  BC                                             0.82    0.5348

Residual

  Total (Corrected)

All F-ratios are based on the residual mean square error

Minitab 15 R software was used to work out global solution by response surface regression for further optimization of process conditions (data and plot not shown). The results of this analysis clearly showed that for the organic/inorganic hybrid system studied, the organic/inorganic hybrid system studied, the optimum conditions for maximum corrosion resistance performance are; 30 min of bath hydrolysis time. pH of 5.0. and 138 h of curing at room temperature.

Conclusion

The study demonstrates synthesis of a new class of organosilanes, bis-ureasils, and their usefulness in deriving chromate -free OIH pretreatments on 2024-T3 aluminum alloy, using a modified two-step sol-gel process. In general, OIH coatings derived from TEOS and bis-ureasils of varying chemical structure show much improved corrosion resistance performance compared to a few selected commercial non-chromate pretreatments. Potentiodynamic polarization study clearly show much lower corrosion current densities (reduced corrosion rate), indicating the formation of dense coatings, with IPDI-based bis-ureasil having the best performance. Accelerated corrosion test (salt-spray) results supplement the results of electrochemical tests and clearly identify factors affecting corrosion resistance performance of these coatings. The factors such as chemical structure of bis-ureasil/ TEOS ratio and silane hydrolysis time are the key factors controlling the corrosion performance of the coatings. DoE methodology has been found to be a very effective tool in the optimization of key process parameters. The results show that among other factors, cure time at room temperature is the most significant one, and hence gives directions for further studying these systems at elevated temperatures that might show substantial improvement in their corrosion resistance performance.

Acknowledgments Authors acknowledge the financial support of US Air Force Research Laboratory, (UTC Prime Contract FA8650-05-D-5807, Task Order BH Subcontract Agreement 06-S568-BH-Cl--Advanced Coating Systems for Aluminum Surfaces) and wish to thank N.N.Voevodin (University of Dayton--Air Force Research Laboratoru, Wright Patterson Air Force Base) and A. N. Khramov (Universal Technology Corporation. Dayton, OH) for useful technical discussion and help.

References

(1.) Metal Finishing Workshop--Chromate Alternatives for Metal Treatment & Sealing. SERDP/ESTCP & DoD Merit Sponsored, Layton, UT, May 15-17.2007

(2.) Jeffcoate, CS, Voevodin, NN, Khobaib, M. Reynolds. LB, Kuhn, WK, Donley, MS, "Future Trends in Non-Chromate Corrosion Inhibitors." Proceedings of 43rd International SAMPE Symposium, Anaheim, CA, 31 May-4 June 1998

(3.) Brinker, CJ, Scherer, GW, Sol-Gel Science. Academic Press. San Diego. CA (1990)

(4.) Mackenzie, JD, Bescher, EP, "Structures, Properties and Potential Applications of Ormosils." J. Sol-Gel Sci. Technol., 12 371 (1998). doi: 10. 1023/A: 1008600723220

(5.) Metroke, TL, Parkhill, RL, Knobbe, ET, "Passivation of Metal Alloys Using Sol-Gel-Derived Materials--A Review." Prog. Org. Coat., 41 233 (2001). doi: 10. 1016/S0300-9440(01)00134-5

(6.) G.P.Sundarajan, W. J. VanOoij, "Silance Based Pretreatments for Automotive Steels." Surface Engg. 16 (2000) 3315.

(7.) Montemor, MF, Simoes, AM, Ferreria, MGS, Williams, B, Edwards, H, "The Corrosion Performance of Organosilane-Based pretreatments for Coatings on Galvanized Steel." Prog. Org. Coat., 38 17 (2000). doi: 10.1016/S0300-9440 (99) 00080-6

(8.) Zheludkevich, ML, Serra, R, Montemor, MF, Yasakau, KA, Miranda Salvado, IM, Ferreira, MGS, "Nanostructured Sol-Gel Coatings Doped with Cerium Nitratc as Pre-Treatments for AA2024-T3 Corrosion Protection Performance." Electrochim. Acta, 51 208-217 (2005). doi: 10.1016/j.electacta. 2005.04.021

(9.) Lamaka, SV, Zheludkevich, ML, Yasakau, KA, Mokntemor, MF, Ceci'lio, P, Ferriera, MGS, "TiOx Self-Assembled Networks Prepared by Templating Approach as Nano-structured Reservoirs for Self-Healing Anticorrosion Pre-Treatments." Electrochem. Commun., 8 421-428 (2006). doi: 10.1016/j.elecom. 2005.12.019

(10.) van Ooij, WJ, Zhu, D, Palanivel, V, Anna Lamar, J, Stacy, M, "Overview: The Potential of Silanes for Chromate Replacement in Metal Finishing Industries." Silicon Chem, 3 11-30(2006). doi: 10.1007/s11201-005-4407-6

(11.) Parkhill, RL, knobbe, ET, Donley, MS, "Application and Evaluation of Environmentlly Compliant Sprayk-Coated Ormosil Films as Corrosion Resistant Treatments for Aluminum 2024-T3." Prog. Org. Coal., 41 261 (2001). doi: 10.1016/S0300-9440 (01) 00138-2

(12.) Kasemann, R, Schmidt, H, First European Workshop on Hybrid Organic-Inorganic Materials, Chateau de Bierville, France, 8-10, November, 1993, p. 171

(13.) Zandik-zand, R, Ershad-langroudi, A, Rahimi, A, "Silica Based Organic-Inorganic Hybrid Nanocomposite Corrosion Protection." Prog. Org. Coat., 53 286-291(2005). doi: 10.1016/j.porgcoat.2005.03.009

(14.) Sugama, T, Taylor, C, "Pyrolysis-Induced Polymetallosil-oxane Coatings for Aluminium Substrates." J. Mater. Sci, Lett., 27 1723 (1992)

(15.) Metroke, TL, Parkhill, RL, Knobbe, ET, "Passivation of Metal Alloys Using Sol-gel-drived Materials--A Review. Progress in Organic Coatings." Prog. Org.Coat., 41 233-238(2001). doi: 10.1016/S0300-9440 (01) 00134-5

(16.) Roy, SK, Dey, R, Mitra, A, Mukherjee, S, Mitra, MK, Das, GC, "Optimization of Process Parameters for the Synthesis of Silica Gel-WC Nanocomposite by Design of Experiment." Mater. Sci. Eng. C, 27 725-728 (2007). doi: 10.1016/j. msec. 2006.07.001

(17.) Soucek, MD, Johnson, AH, Wegner, JM, "Ternary Evaluation of UV-Curable Seed Oil Inorganic/Organic Hybrid Coatings Using Experimental Design." Prog. Org. Coat., 51 300-311 (2004). doi: 10.1016/j.porgcoat.2004.07.019

(18.) Donlely, MS, Balbyshev, VN, Voevodin, NN, "Self-Assem-bled NAnophase Particle (SNAP) Surface Treatments for Corrosion Protection of AA2024-T3." Prog. Org. Coat., 52 34-38 (2005).doi: 10.1016/j.porgcoat.2004.05.007

(19.) Voevodin, NN, Kurdcziel, JW, Mantz, R, "Corrosion Protection for Aerospace Aluminum Alloys by Modified Self-Assembled NAnophase Particle (MSNAP) sol-gel." Surf. Coat. Technol., 201 1080-1084 (2006). doi: 10.1016/j.surf-coat.200s6.01.028

This paper was awarded the Southern Society A.L. Hendry Award for best student paper. The paper was presented at FutureCoat! 2008, sponsored by Federation of Societies for Coatings Technology, held October 14-16. 2008, in Chicago, IL.

V. Kakde, V. Mannari

Coatings Research Institute, Eastern Michigan University, Ypsilanti, MI, USA

e-mail: [e-mail]

[c] FSCT and OCCA 2008