Protective films of stearic and octadecylphosphonic acid formed by spray coating

15 Spray coating formation of stearic and octadecylphosphonic acid films for corrosion 16 protection of cupronickel alloy was studied in this work as a more practical alternative to 17 widely studied dip-coating method. Protective properties of organic films formed under 18 various experimental conditions were examined by electrochemical studies in 3% NaCl 19 solution as a corrosive medium. Polarization resistance measurements as well as electro20 chemical impedance spectroscopy were employed to follow in time the corrosion 21 behaviour of cupronickel alloy modified by studied organic acids. It was found that among 22 examined experimental parameters, time elapsed between two sprays and number of 23 sprays have the strongest influence on the film stability and its protective properties. This 24 study confirmed that it is possible to form by spray coating the films of stearic and 25 octadecylphosphonic acid with protective properties that resemble to those of the films 26 prepared by dip-coating method. Differences in corrosion behaviour of samples protected 27 with stearic and octadecylphosphonic acid were attributed to difference in the bond 28 strength between substrate and each organic acid. Studied samples were also examined 29 by the scanning electron microscopy. Fourier transformed infrared spectroscopy studies 30 showed that crystalline structure dominates in studied films, while contact angle measure31 ments confirmed that modified cupronickel alloy surface exhibits hydrophobic properties. 32


Introduction 37
Modification of surface properties of metallic or semiconductor substrates by long-chain 38 organic molecules has been thoroughly studied in recent years due to interest for its application in 39 U n c o r r e c t e d P r o o f various fields such as organic electronics, sensors or corrosion protection [1][2][3][4][5][6][7]. Molecules with 40 sufficiently long alkyl chain usually possess the self-assembling property, i.e. they spontaneously 41 form ordered structures in a form of self-assembled monolayers or multilayers [8,9]. Such thin 42 films present a barrier between the substrate and environment. If the film has a low number of 43 defects and is strongly attached to metallic substrate it can provide good corrosion protection to a 44 metal in various corrosive environments. It has been shown that long-chain organic acids such as 45 carboxylic, phosphonic or hydroxamic acids, can form such barrier layers on different metals 46 [6,7,[10][11][12][13][14][15]. In most of cases, self-assembled monolayers/multilayers were prepared by dip-coating 47 method. Although this method is characterized by good reproducibility, its drawback is that it is 48 time consuming and requires high amounts of solution. Spraying method is often used for organic 49 coatings as it enables their quick application on metallic objects of different dimensions. Despite 50 advantages over dip-coating method, there are only few works where spraying method was used 51 for formation of self-assembled monolayers [10,11,13,16,17]. Studies on films formed by dip-52 coating method showed that many different parameters should be taken into account in obtaining 53 a layer with small number of defects and high crystallinity [15]. Among the most important 54 parameters is the adsorption time, as a certain time is always needed for molecules to reach the 55 metal surface, adsorb on it and organize in well-ordered structures [18]. Studies performed on 56 nickel oxide modification by alkylphosphonic acids self-assembled monolayers also showed that 57 well-ordered monolayers can be formed by both immersion and aerosol spraying methods, if 58 adequate conditions for monolayer formation were selected [11]. In our previous studies it was 59 shown that multilayer films of stearic acid (SA) [14,15] or octadecylphosphonic acid (ODPA) [7] 60 formed by immersion method provide high and durable corrosion protection to copper-nickel alloy 61 (CuNI) in chloride medium. The aim of this work is to determine under which conditions SA films 62 that provide good corrosion protection to underlying CuNi substrate, can be formed by spraying 63 method. Then, similar experimental conditions were applied for preparation of ODPA films. 64

Sample preparation 66
Investigations were performed on cupronickel alloy (70Cu30Ni) obtained from Goodfellow Inc., 67 UK. The specimens were cut-out from a cupronickel rod with 1.3 cm diameter and 0.5 cm in 68 thickness. In order to prepare working electrodes for electrochemical measurements on the back-69 side of these plates, a copper wire was soldered and then, the electrodes were embedded into 70 epoxy resin. The exposed surface area of the working electrodes was 1.33 cm 2 . Prior to all investi-71 gations and surface modifications, the electrodes were abraded with emery paper grade 800, 72 1200, 2500, and polished with α-Al2O3 particle size 0. Croatia. Solutions for electrochemical measurements were prepared with deionized water. 77 Ethanolic solutions of 0.01 mol dm -3 stearic and octadecylphosphonic acid were used for spray 78 application. The first step was to optimize the method of preparing the films with stearic acid, and 79 after specific preparation procedure was chosen, it was applied for preparation of ODPA films. The 80 film formation was conducted according to the experimental procedures presented in Table 1. 81 They consisted of three steps: substrate oxidation which was conducted in order to obtain 82 reproducible oxide layer for 24 h at 80 °C, followed by acid adsorption from EtOH solution of SA or 83 U n c o r r e c t e d P r o o f ODPA by spraying method, and final drying step, 5 h at 50 o C (SA) or 80 o C (ODPA) [7,14]. Initially, 84 all the samples were sprayed 5 times while the influence of temperature (A) and time (B) between 85 two consecutive spraying on the protective properties of SA film were studied. In the next step, 86 the influence of the number of sprays (C) on the protective properties of the formed film was 87 examined. For comparison, a blank sample (CuNi) was prepared, on which native oxide layer was 88 formed during 24 hours at 80 °C. The oxidation and drying were conducted in an air convection 89 oven with temperature control accuracy of 0.5 °C. 90 impedance spectra were interpreted on the basis of electrical equivalent circuits using ZSimpWin 100 software. In general, χ 2 value was always below 5·10 -4 . The electrochemical measurements were 101 performed using a Bio-Logic SP-300 potentiostat. All measurements were conducted in triplicate. 102

Surface studies 103
Contact angle (CA) measurements, Fourier transform infrared spectroscopy (FTIR) and scanning 104 electron microscopy (SEM) were performed with the aim of surface layer characterization. The 105 contact angle measurements on bare cupronickel and SA and ODPA treated CuNi samples were 106 conducted using a goniometer DataPhysics Contact Angle System OCA 20, with a drop of 2 μL 107 water under the ambient atmospheric conditions. All measurements were conducted in at least 108 ten points. SEM morphology analysis was performed with VEGA 3 SEM TESCAN at an acceleration 109 voltage of 10 kV. FTIR measurements were carried out by attenuated total reflectance Fourier 110 transform infrared spectroscopy (ATR FTIR), using a Spectrum One FTIR spectrometer from Perkin 111 Elmer, with the scan range from 4000 -650 cm -1 , having a resolution of 0.5 cm -1 , and the results 112 shown in this paper were averages of 25 scans. 113

Results and discussion 114
In order to determine the experimental conditions at which SA film with excellent barrier 115 properties is formed, protective properties of differently formed samples were tested by 116 polarization measurements in 3 % NaCl solution. 117 Since our previous studies on samples prepared by immersion method showed that adsorption of 120 SA and ODPA is enhanced at elevated temperature [7,14,15], there was a need to investigate the 121 influence of temperature on the adsorption step in the spraying method. According to the literatu-122 re [19], lower activation energy is required for a molecular gas or liquid precursor dispersed in small 123 droplets at room temperature to bind by the chemisorption on a heated surface of the substrate, 124 than for the heated solution and cold substrate. Accordingly, it is expected that heating of the sub-125 strate during spraying could improve adsorption of molecules on the substrate. Moreover, by spray-126 ing droplets of a room temperature in comparison to that of elevated temperature, the solvent eva-127 poration during spraying is reduced. The solvent evaporation changes the droplet size, which affects 128 the concentration and spatial arrangement of a drop, resulting in uneven spraying of droplets and 129 disordered layer on the substrate surface [20,21]. For above mentioned reasons, SA solution was not 130 heated as in the case of immersion method [7,14]. Instead of that, between two consecutive sprays, 131 T40 sample was heated at 40 °C for 1 h, after which it was sprayed with a solution of room tempe-132 rature. The sample thus prepared was compared to T25 sample that was left at room temperature. 133 The

152
From the polarization curves presented in Figure 1a and corrosion parameters given in Table 2,  153 it is clear that corrosion current density of both SA treated samples is significantly lower compared 154 to that of the blank sample. The protective films on T25 and T40 samples acted as barrier toward 155 oxygen diffusion, thus slowing down cathodic corrosion reaction, while anodic corrosion reaction 156 decreased to a lesser extent. This effect was observed in most of papers investigating use of self-157 assembled monolayers (SAMs) in corrosion protection [22,23]. 158 For application of SAMs in corrosion protection, it is not sufficient to determine only the initial 159 protection level but also to verify if the protection remains satisfactory with passing of time. For that 160 reason, polarization DC measurements in narrow potential range were conducted over longer 161 period. The polarization resistance (Rp) values determined from polarization measurements are 162 given in Figure 1b. After examining the substrate temperature effect, the next parameter tested was the time 179 elapsed between two consecutive sprays. For that purpose, samples were sprayed at room 180 temperature five times, but between two consecutive sprays samples were left in air for 60 181 minutes (t60 sample), 30 minutes (t30 sample), or 10 minutes (t10 sample). In this way, it was 182 attempted to determine the minimum time required for the preparation of stable and protective 183 layers. Their protective efficiency was examined by polarization measurements as presented in 184 Figure 2, while corrosion parameters obtained from polarization curves are presented in Table 3.   Ecorr The largest decrease can be observed for t30 sample which indicates formation of an effective 199 barrier to oxygen diffusion towards the metal surface, probably due to the formation of a layer 200 with less defects and pores. From corrosion parameters shown in Table 3, it can also be noted that 201 the lowest value of corrosion current density and hence the lowest corrosion rate and the highest 202 corrosion inhibition efficiency was obtained for t30 sample. Similar results were obtained by 203 testing the durability of protection by polarization resistance measurements in time, as can be 204 seen in Figure 2b. From the obtained results it can be seen that 10 minutes was not sufficient time 205 between two sprays, while time longer than 30 minutes was even not required, as it didn't 206 contribute to improvement of protective properties of SA film. 207 Influence of the number of sprays 208 The next step in optimizing the spraying method was to examine how many sprays are needed 209 to obtain good corrosion protection. As aforementioned, it is expected that thicker SA layers 210 would provide better corrosion protection. However, if such layers are porous and with high 211 number of defects, their protective effect is not very high. Therefore, our aim was to prepare 212 sufficiently thick protective layer, but with low degree of porosity in order to achieve good 213 corrosion protection. Therefore, the prepared samples were sprayed at room temperature two (2x 214 sample), five (5x sample) and ten times (10x sample). Between the individual spraying, samples 215 were left for 30 minutes at room temperature. The results of the polarization measurements are 216 shown in Figure 3, and the parameters obtained by Tafel extrapolation method from polarization 217 curves are shown in Table 4.  227 Figure 3a shows that the cathodic current density was reduced for all treated samples, while 228 the slightest reduction could be observed for 2x sample, which is in accordance with the fact that 229 by increasing the number of sprays, a larger amount of SA is deposited on the surface thus 230 resulting in a thicker protective layer. It is interesting to note that 10x sample showed the greatest 231 anodic inhibition, probably because thicker multilayer structure caused more difficult penetration 232 of water and chloride ions towards the substrate surface. From the values of corrosion parameters 233 (Table 4), decrease of corrosion current density with an increase of the number of sprays can be 234 observed. From the dependence of the polarization resistance on corrosive media exposure time 235 shown in Figure 3b, it can be seen that although 10x sample initially shows the best results, on the 236 third day of immersion, its Rp value is already equated with 2x sample. The possible reason of such 237 result is that at the beginning, a very thick layer on 10x sample was formed due to the large 238 number of sprays, but as the molecules in the multilayer structure were not tightly bounded, the 239 outer layers were removed with time. Therefore, it can be concluded that the best results were 240 achieved when the samples were sprayed 5x. electrodes is represented by Rel. For EIS spectra of SA treated samples measured on the first day of 270 exposure to 3 % NaCl, the same model (Figure 5a) was used. In this case, however, Rf represented 271 resistance of the pores in stearic acid film and Qf was related to the capacitance of the surface film. 272 After 14 days of exposure to corrosive medium (3 % NaCl), some changes in EIS spectra were 273 observed (Figure 4c and 4d). Firstly, the difference between impedance modulus of blank and 274 protected samples was not as high as at the beginning of the experiment. Secondly, the third 275 maximum in the phase angle plot appeared at high frequencies. In our previous work on stearic 276 acid films [14], appearance of this time constant was attributed to transformation of the organic 277 film into porous outer layer and more compact inner layer. For that reason, impedance spectra 278 were modelled with the equivalent circuit presented in Figure 5b,  is reversely proportional to film thickness, its values should be the same for these three samples. 293 As this is not the case, the probable explanation is that SA layer on t10 sample was more porous, 294 which enabled penetration of higher amounts of water into the film and resulted in increase of its 295 dielectric constant. This is also in accordance with higher Qdl and lower Rct values obtained for t10 296 compared to t30 and t60 samples. When examining the number of sprays, it is evident that the 297 highest resistance values (both Rf and Rct) and the lowest capacitance values (Qf and Qdl) were 298 obtained for the sample with the highest number of sprays, 10x. 299 From data presented in Table 6, it can be observed that two weeks (14 days) exposure of 300 samples to 3 % NaCl solution resulted in decrease of almost all resistive elements values and 301 increase of capacitive elements values when compared to data in Table 5 for the first day of 302 exposure. Such changes can be attributed to ingress of water into the SA film and dissolution of 303 outer film layers. 304

307
For most of the samples, however, both resistance values (Rf,i and Rct) were still higher than 308 those obtained for the blank sample, thus indicating that SA films provided corrosion protection 309 even after two weeks of exposure to corrosive medium. The most significant decrease of Rf,i and 310 Rct values was observed for 10x sample that exhibited the highest resistive values for the first day 311 of exposure 3 % NaCl (Table 5). The possible explanation for such behaviour would be that by 312 increase of the film thickness, the number of defects in the film is also increased, especially in the 313 outer layers, enabling thus their easier desorption from the surface. Obtained results confirm that 314 the most protective SA film was formed on the sample t30/5x. 315

Comparison of stearic and octadecylphosphonic acids films 316
After examining the parameters of spray deposition of SA, ODPA films were prepared in the 317 same way (t30/5x procedure), except at the final drying step that was conducted at 80 o C as 318 applied in immersion deposition of ODPA [7]. Such prepared samples were immersed in 3 % NaCl 319 solution and electrochemical measurements were conducted. The results of the polarization 320 measurements are shown in Figure 6, and corrosion parameters obtained by Tafel extrapolation 321 method from polarization curves are shown in Table 7.

U n c o r r e c t e d P r o o f
From polarization curves given in Figure 6a it can be observed that while the protective films of 329 SA significantly decreased cathodic current densities and only slightly anodic current densities, 330 ODPA films reduced both anodic and cathodic current densities. Apart from acting as a barrier 331 film, it may be assumed that ODPA is adsorbed on anodic sites of alloy thus preventing their 332 dissolution. Protective efficiency of ODPA film was slightly lower than that of SA film. Also, on the 333 1 st day of exposure to 3% NaCl, SA film showed higher polarization resistance values compared to 334 ODPA film. However, contrary to a continuous decrease in Rp values of SA films in time, Rp values 335 of ODPA films, increased on the third day of immersion, probably due to the reorganization of 336 molecules in the film into the more stable configuration. Later on, Rp values of ODPA samples 337 decreased in time but remained always higher than those obtained for SA samples. 338 Studies were also conducted by EIS in order to better understand corrosion behaviour of ODPA 339 samples (Figure 7). 340  347 EIS spectra given in Figure 7 confirm findings from polarization measurements that ODPA films 348 initially provided lower corrosion protection than SA films (Figure 4), but this protection was much 349 more stable in time. The shapes of phase angle -log frequency curves of Bode plots in Figure 7b  350 reveal that it was necessary to use the electrical equivalent circuit with three time constants to 351 adequately fit EIS spectra measured for both the first and the last day of exposure to 3% NaCl. For 352 this purpose, the electrical equivalent circuit given in Figure 5b was used and the obtained 353 impedance parameter values are presented in

U n c o r r e c t e d P r o o f
that Rct value for ODPA sample was twice higher than for SA sample, while Qdl values of ODPA 360 sample were five times lower than for SA sample. Taking into account that polarization measure-361 ments (Figure 6a) showed that unlike SA, ODPA exhibited strong anodic current inhibition, and 362 also that phosphonic acids are stronger than carboxylic acids, it may be proposed that ODPA 363 molecules were more strongly bounded to CuNi surface than SA molecules. Thus, despite the 364 formation of more porous surface layer, dissolution of strongly bounded ODPA molecules was 365 more difficult than dissolution of SA molecules. 366 Results obtained in this work are similar to results obtained in our previous study [7], where 367 ODPA films were prepared by dip-coating method, except for the fact that for dip-coating method, 368 presence of porous outer layer was not clearly observed for the first day of exposure to 3 % NaCl. 369 However, the Rp values observed in this work for ODPA films formed by spraying are only slightly 370 higher than those observed for ODPA films formed by dip-coating [7].  Comparing the morphology of stearic and octadecylphosphonic acid films, it can be seen that 426 after preparation of the sample, the stearic acid films appeared more uniform. However, after 14 427 days of exposure to corrosive media, SA films exhibited large number of cracks in the film, while 428 ODPA films visually didn't change as much. 429

Conclusion 430
Studies conducted in this work confirmed that spraying method can be used to prepare 431 protective films of SA and ODPA on CuNi alloy and that adequately prepared films exhibit similar 432 properties to those observed for films formed by dip-coating method. Several parameters of 433 spraying deposition method were examined, among which time elapsed between two sprays and 434 number of sprays have the strongest influence on the film stability and its protective properties. 435 Comparison of similarly prepared SA and ODPA films showed that initially, SA films provide better 436 corrosion protection than ODPA films. Prolonged exposure to corrosive medium, however, 437 showed faster decline of SA film protective properties than that of ODPA film, what is related to 438 stronger attachment of phosphonic acids on CuNi surface. It was also shown that spray formed 439 protective films provide corrosion protection to the underlying CuNi alloy even after 14 days of 440 continuous exposure to corrosive medium. 441