Corrosion of One-Step Superhydrophobic Stainless Steel

1 Corrosion of One-Step Superhydrophobic Stainless

2 Steel Thermal Spray Coatings

3 Samantha Michelle Gateman,a Kristopher Page,b Ilias Halimi,†a Alexandre Romão Costa

4 Nascimento,c Sylvio Savoie,d Robert Schulz,d Christian Moreau,c Ivan P. Parkin,b and Janine

5 Mauzerolla*

6 aLaboratory for Electrochemical Reactive Imaging and Detection of Biological Systems, McGill 7 University, Montreal, QC H3A 0B8, Canada

8 bMaterials Chemistry Department, University College London, London, WC1E 6BT, United 9 Kingdom

10 cThermal Spray and Multiphase Flow Laboratory, Concordia University, Montreal, QC H3G 11 1M8, Canada

12 dInstitut de recherche d'Hydro-Québec, Varennes, J3X 1S1, Canada

13 14 KEYWORDS. High-velocity oxygen fuel, thermal spray, coatings, stainless steel, 15 superhydrophobicity, surface chemistry, corrosion

16 ABSTRACT. As most superhydrophobic coatings are made of soft material, the need for harder, 17 more robust films is evident in applications where erosional degradation is of concern. The work 18 herein describes a methodology to produce superhydrophobic stainless steel thermal spray 19 coatings using the high-velocity oxygen fuel technique. Due to the use of kerosene fuel source, a

20 carbon rich film is formed on the surface of the thermal spray coatings, lowering the surface

1 21 energy of the high-energy metallic substrates. The thermal spray process generates a hierarchical 22 micro/sub-micro structure that is needed to sustain superhydrophobicity. The effect of spray 23 parameters such as particle velocity and temperature on the coating’s hydrophobicity state was 24 explored and a high particle velocity was shown to cause superhydrophobic characteristics. The 25 coatings were characterized using scanning electron microscopy, profilometry, X-ray 26 photoelectron spectroscopy, static water contact angle measurements, water droplet roll-off 27 measurements, and water droplet bouncing properties. The corrosion behavior of the coatings 28 was studied using potentiodynamic polarization measurements in order to correlate water 29 repellency with corrosion resistance, however, all coatings demonstrated active corrosion

30 without passivation. This study describes an interesting phenomenon where superhydrophobicity 31 does not guarantee corrosion resistance and discusses alternative applications for such materials.

32 INTRODUCTION. Over the past century, different types of thermal spray coating

33 technologies have emerged including high velocity oxygen spray1,2 plasma spray,3,4 and even cold

34 spray (i.e. near ambient temperatures) technologies,5 all of which are promising or have become

35 essential for the protection of industrial infrastructure.6,7 Various materials such as metals (wire,

36 powder, or rod),8 ceramics,3,9 and polymers10,11 have been thermally sprayed onto many different

37 substrates.12,13 The particles are (semi-) melted and accelerated to high velocities directed at the 38 substrate where they impinge and rapidly solidify to form a thin “splat”. The build up of splats 39 from successive impingement and interbonding results in a deposited coating, generally > 10 μm

40 thick.7 The resulting coatings are complex in microstructure, including oxides, pores/voids, and 41 unmolten particles.

42 Among the spray methods developed, high velocity oxygen fuel (HVOF) spraying, a variation 43 of combustion spraying, utilizes a special torch design that produces particle velocities so high

44 that they achieve supersonic values.7 Thus, the particles spread out upon impact and bond to the 45 substrate and to other neighboring splats to produce a well-bonded, dense coating. Depending on

46 the temperatures/velocities of the particles upon impact, molten and/or semi-molten particles and

2 47 debris can build up across the coating’s surface, resulting in a rough hierarchical micro and sub-

48 micron/nano structure.14,15 This surface texture is one of the two characteristics required to create a 49 superhydrophobic (SH) material, that is, a surface that measures a static water contact angle

50 (WCA) ≥ 150°, as well as a dynamic functional property – water bouncing.16,17 A SH material

51 and/or film must also have inherently low surface energy to minimize interactions with water.18 52 This can enable air to become entrapped between water droplets and the material’s surface, 53 leading to low adhesion. When the drop is resting on top of the asperities of the rough substrate,

54 the material is considered to be in the Cassie-Baxter state.19,20 The apparent (macro) contact angle

55 θapp of a water drop on a partially wetted surface is given by the weighted sum:

56 ��!"" = �� − (1 − �) [ 1 ]

57 Where f is the fractional area of contact of water with the solid and θ is the contact angle on a 58 flat surface of the same material. In competition with the Cassie-Baxter state, the Wenzel state

59 describes when a water droplet impales a material with a rough surface and no air is entrapped.21 60 Although the WCA in the Wenzel state can be almost as high as found in the Cassie-Baxter state, 61 the two differ in the contact angle hysteresis, that is, the difference in advancing and receding

62 WCAs while tilting the specimen.22 Understanding the superhydrophobic state of a material is of

63 interest in this work as such characteristic has been linked to corrosion resistance.23,24

64 The biomimicry of superhydrophobic surfaces is of industrial interest for applications in self-

65 cleaning,18,25,26 drag-reduction,27,28 anti-icing,23 and anti-corrosion.23,29 Due to most polymers’ 66 deformation upon abrasion, it is advantageous to fabricate hard and wear resistant SH materials, 67 thus making thermal spray technologies a desirable hydrophobic coating method. Chen et al. 68 reported a simple route for constructing SH surfaces by flame spray deposition of inorganic 69 materials followed by the post application of a thin polytetrafluoroethylene (PTFE, Teflon®)

70 layer.14 Another example of thermal spray coating technology being utilized for SH applications

71 is the use of the HVOF method to fabricate robust hydrophobic Fe-based amorphous coatings.15

3 72 This method also required a post modification step to decrease the surface energy, which is 73 undesirable in industrial scale-up due to time and financial costs.

74 Recently, the fabrication of ceramic rare earth oxide (REO) thermal spray coatings has been

75 explored as a one-step method for producing SH durable coatings.9,30 By employing the thermal 76 spray technique named suspension precursor plasma spray (SPPS), REO can be applied to create 77 micro/nano scale textured surfaces over various substrates with static water contact angles as

78 high as 163°.31 The origin of REO hydrophobicity was proposed to be due to their unique

79 electronic structure that inhibits hydrogen bonding with interfacial water molecules,32,33 while

80 some recent studies countered with evidence that the adsorption of contaminates (volatile organic

81 compounds, CO2, etc.) is responsible for the apparent hydrophobic nature of the inherently

82 hydrophilic materials.31,34–36 Although the mechanism of wetting is still under debate, this method is 83 extremely advantageous since SPPS is industrially scalable, it eliminates the need for a surface 84 energy decrease post modification step, and the resulting coatings are extremely robust. Despite 85 their success and abundance, REOs are costly due to production difficulties encountered. For 86 instance, the lowest price for a REO in 2018 was 46 US$/kg for holmium oxide and the highest

87 being 123 US$/kg for praseodymium oxide.37 Therefore, more cost-effective materials and 88 methods for producing erosion resistant SH materials must be investigated.

89 This work is proposing a more economical solution by creating a one-step hard 90 superhydrophobic thermal spray coating using a stainless steel (SS), SS 444, precursor and a 91 long carbon chained fuel source. SS 444 is a less expensive alternative to other stainless steels

92 such as SS 316 and 304 as it has low nickel content but portrays similar corrosion resistance.38 To 93 our knowledge, this is the first report of achieving superhydrophobicity on HVOF metallic 94 coatings. Four different coatings were fabricated using varying spray parameters, specifically 95 temperature and velocity. The resulting coatings’ microstructures and surface chemistry were 96 characterized using scanning electron microscopy (SEM), profilometry, and X-ray photoelectron

97 spectroscopy, while the corresponding hydrophobic properties were investigated using static

4 98 WCA, water droplet bouncing experiments, and sliding angle measurements to identify optimal 99 spray parameters for maximum hydrophobicity. The corrosion behavior of the stainless steel 100 thermal spray coatings was observed in 3.5 wt% NaCl, which sparked an interesting discussion 101 regarding the correlation between a material’s corrosion resistance and hydrophobic state the 102 material exhibits.

103 EXPERIMENTAL SECTION

104 Materials and reagents. The feedstock material was water-atomized (by Eutectic Castolin, 105 Granby, Quebec, Canada) AISI grade stainless steel 444 powder produced by ArcelorMittal. The

106 substrates, also produced by ArcelorMittal, were wrought 6 x 10 inch SS 444 plates prepared 107 prior to HVOF spraying via alumina grit blasting (270 μm). The same material was used as the 108 substrate in order to minimize galvanic corrosion between the coating and underlying substrate. 109 Sodium Chloride and petroleum ether were purchased from Sigma-Aldrich. 110 Fabrication of thermal spray coatings. The metallic thermal spray coatings were prepared via 111 the high velocity oxygen fuel spray (HVOF) thermal spray method. The stainless steel powder 112 was first processed using high-energy ball milling (6h) to decrease the as-atomized particle size 113 (>100 μm) and then sieved to a mean particle diameter of ~45 μm, as seen in the supporting 114 information (Figure S1). A Praxair’s JP-8000 HVOF spray gun with a gun barrel length of 4 in 115 was used for spraying all coatings. A powder feed rate of 44 g/min, stand-off distance of 13 in, 116 transversal speed of 0.5 m/s with a pitch step size of 10 mm, and 15 passes were performed to 117 create approximately 500 μm thick coatings (specifics in table S1) on wrought SS 444 plates (6 x 118 10 inch). No substrate cooling system was used. The oxygen and kerosene fuel flow rates were 119 varied to produce four velocity/temperature conditions between 500-800 m/s and 1600-2100°C. 120 Small circular coupons (16 mm in diameter) were water cut from the sprayed plates for further 121 investigation. 122 The parameters used to fabricate the SS 444 HVOF coatings can be found in table 1. The 4

123 coatings were deposited using different oxygen to fuel flow rate ratios and combustion pressures:

5 124 low velocity and low temperature (LVLT), high velocity and high temperature (HVHT), low 125 velocity and high temperature (LVHT), and high velocity and low temperature (LVHT). The 126 particles in-flight characteristics were measured using DPV-evolution system. Similar to what

127 has been shown in previous modeling studies,39,40 the particle velocity and temperature are directly 128 proportional to the combustion pressure and O/F ratio, respectively. 129 130 Table 1. HVOF thermal spray parameters used to deposit four SS 444 coatings using different

131 combustion pressure (psi) and oxygen to fuel (O/F) ratios.1

Sample O2 Flow Rate Kerosene Flow O/F Pressure Velocity Temperature (LPM) Rate (LPM) Ratio (psi) (m/s) (°C)

LVLT 470 18 0.80 54 546 ± 70 1658 ± 300

HVHT 870 23 1.17 100 790 ± 100 2107 ± 250

LVHT 588 15 1.18 60 524 ± 80 2122 ± 200

HVLT 729 28 0.80 95 792 ± 101 1569 ± 320 132 133 Microscopy Characterization. All SEM images were obtained using a Hitachi SU3500 variable 134 pressure scanning electron microscope. Topographic information was obtained by collecting the 135 secondary electron signal produced from the specimen itself while elemental composition 136 information was explored via detection of back-scattered electrons stemming from the incident

137 beam and X-ray emissions. The fraction of coatings microstructural features of SEM cross- 138 section images was determined by Image analysis using the software Image J 1.52A (National 139 Institute of Health, USA). All confocal microscopy data was recorded using a white light 140 interferometry profiler (Zygo Nexview) at 10x magnification using Mx™ Software. All

1 Abbreviations stand for: low velocity and low temperature (LVLT), high velocity and high temperature (HVHT), low velocity and high temperature (LVHT), and high velocity and low temperature (LVHT). The in-flight particle characteristics, velocity and temperature, are the average and standard deviation of 1000 particles measured.

6 141 roughness parameters, i.e., mean height (Sa), Skewness (Ssk), sharpness (sku), and peak spacing (sm), 142 of the coatings were recorded in triplicate and tabulated using Mx™ Software.

143 X-ray photoelectron spectroscopy. Chemical analysis of the carbon rich films on the HVOF 144 coatings surfaces was carried out using an Al K-Alpha X-ray photoelectron spectrometer 145 (ThermoFisher). The spot size was 400 μm and 10 scans were performed per spectrum. 146 Water contact angle, roll-off angle, and bouncing measurements. A 30 μL-sized droplet of 147 distilled water was transferred onto each of the HVOF coatings using an automated syringe 148 pump. The water contact angles were measured at ambient temperature using an optical contact

149 angle meter (FTA 1000) and fit to the Young-Laplace equation. The WCA measurements were 150 carried out 3 times on 2 different coupons to collect enough data for statistical comparison. The 151 same set up was used for the water rolling angle measurements where the stage was tilted at a 152 speed of 0.5° per second until 90° was reached. 153 Water roll-off angle tests were carried out using a 1 mL syringe to dispense 10 μL-sized 154 droplets. The water bouncing measurements were performed at a height of 15 cm from the 155 coating substrate. Both measurements were done at least twice on 2 different coupons for each 156 coating to confirm reproducibility. The videos were taken by a high-speed video camera (FPS 157 1000 HD, Slow Motion Camera Company, United Kingdom) capturing at 1000 frames per 158 second. 159 Potentiodynamic polarization measurements. Electrochemical measurements were performed 160 using a multi-channel VSP-300 potentiostat (BioLogic Science Instruments, USA) with 1 μV 161 resolution. Experimental parameters for PDP measurements were based on the relevant ASTM

162 G-61 test method.41 Briefly, the coatings were immersed at open circuit potential (OCP) for 1h 163 before applying a potential bias. Polarization commenced -250 mV lower than the measured 164 OCP and was linearly polarized to more anodic potential using a scan rate of 0.167 mV/s until a

165 current density of 0.5 mA/cm2 was reached. A standard bench top corrosion cell (K0235 Flat

166 Cell, Princeton Applied Research, AMETEK® Scientific Instruments) with an isolated area of 1

167 cm2 was used as the main corrosion cell for all measurements. A calibrated saturated calomel

7 168 electrode (SCE) and platinum mesh were used as the reference and counter electrodes 169 respectively. A Faraday cage and a vibration isolation table were used during measurements. 170 In order to minimize solution resistance and to simulate harsh corrosion conditions, 3.5 wt% 171 NaCl electrolyte made using deionized water from a Milli-Q® water purification system (18.2 172 MΩ cm resistivity) was chosen as the main test solution for immersion testing and standard 173 electrochemical measurements. All measurements on bulk SS 444 were performed on 174 sandblasted substrates in attempt to introduce surface roughness. PDP measurements on thermal 175 spray coatings were performed on as sprayed, non-polished samples. The true surface areas of 176 each coating, tabulated from the confocal microscopy data, were used to calculate corrosion

177 current densities. All polarization tests were carried out for at least 3 replicates to ensure 178 reproducibility of the material’s corrosion behavior. 179 RESULTS AND DISCUSSION 180 Microstructural and chemical characterization of coatings’ surfaces. The microstructural 181 features of HVOF coatings (Figure 1) are non-molten powdered particles and re-solidified 182 metallic material that protrude from the surface. The semi-molten, molten, and oxidized metal 183 debris from the impact with the substrate is seen scattered across the coatings’ surface in the 184 magnified inset images (Figure 1). The sub-micron structure features created from molten 185 material flying off of the heated powdered particles upon impact complete the hierarchical 186 surface texture that is required to produce a superhydrophobic material.

8 187 188 Figure 1. SEM images revealing the micro and sub-micron (inset) hierarchical structures of the 189 steel HVOF coatings for each set of parameters where the scale bar is 1 mm: low velocity high 190 temperature (LVHT), high velocity high temperature (HVHT), low velocity low temperature 191 (LVLT), and high velocity high temperature (HVLT). The inset images have a scale bar of 10 192 μm.

193 194 Differences in microstructure can be seen between each of the HVOF coatings’ SEM images. 195 The coating fabricated using low velocity and high temperature (LVHT) shows a surface texture 196 that is consistent in size to that of the powdered SS particles used to fabricate the coatings (~45 197 μm, supporting information Figure S1) as seen in Figure 1. The coatings made using HVHT and 198 LVLT show a more molten surface texture, with protrusions of powdered particles more spread 199 out than seen in the LVHT coating. The coating created using HVLT has larger protrusions 200 (>300 μm) and a cauliflower-like surface microstructure. A similar unique microstructure has

201 been documented for some SPPS coating methods due to the deposition of the material having a

9 202 significant velocity component parallel to the substrate.42,43 The submicron structure formed during 203 the HVOF process is random and no apparent differences between the coatings were observed.

204 205 Figure 2. Top panel: Topography images of each SS HVOF coating and their average surface 206 roughness values and associated standard error of the mean for 3 measurements. Bottom panel:

10 207 A 1 mm line scan across HVLT coating revealing its micro textured surface and the inset 208 showing the material’s sub-micron structure. All scale bars represent 200 μm.

209 210 Topographical imaging and analysis were performed for each coating, as surface roughness 211 and parameters are critical for producing a superhydrophobic material. The lowest mean surface 212 roughness (height) was measured for the coating produced at high velocity and high temperature, 213 HVHT, as seen in the topography image presented in the top panel of Figure 2. This may be due 214 to the ability of the molten particles to flatten and pack together when striking the substrate due 215 to the high amount energy and flexibility they posses upon impact. Large cauliflower protrusions 216 greater than 300 μm in diameter are observed on coating HVLT as seen in Figure 2 bottom 217 panel. Although the powder diameter is about 45 μm, some smaller, 1-2 μm sized particles are 218 present in the feedstock. Therefore, the columnar/cauliflower pattern could be due to either

219 horizontal flow of fine, primary powder particles,42 or to perturbation in the deposition of primary

220 particles.44 This coating has the highest surface roughness of 13.7 ± 0.8 μm, which may be caused 221 by the lower particle temperature decreasing deformation capabilities on impact. All coatings 222 showed a sub-micron textured surface at 100x magnification as seen in Figure 2 bottom panel 223 consisting of sub-micron peaks both in height and width protruding from the coating’s surface 224 where the peaks are separated by ~ 2 μm.

225 We also report the skewness of peaks (Rsk), sharpness of peaks (Rku), and the average spacing 226 between peaks (S) for all coatings in table 2. All HVOF coatings portray positive skewness 227 values, which correspond to non-symmetric, predominance of peaks comprising all of their 228 surfaces. The roughest coating, HVLT, showed the lowest degree of symmetry of all the coatings

229 with a large Ssk value of 0.68 ± 0.12. This coating also showed a kurtosis value greater than 3, 230 indicating a perfectly random spikey surface as opposed to the other bumpy HVOF coatings with 231 values less than 3. The average spacing between peaks was also larger for the HVLT coating in 232 comparison to the rest of the HVOF materials. All of these parameters may have an affect on the

233 coatings’ hydrophobicity and will be taken into consider in following sections.

11 234 235 Table 2: Tabulated surface parameters for each coatings fabricated: mean height of the 236 roughened surface, skewness of the peaks, kurtosis of the roughness profile, and the average 237 spacing between peaks and associated standard errors the mean.

Sample Mean Height, Ra Skewness, Rsk Sharpness, Rku Peak Spacing, Sm LVLT 10.6 ± 0.6 0.14 ± 0.02 2.8 ± 0.3 61 ± 3 μm HVHT 9.5 ± 0.1 0.15 ± 0.04 2.8 ± 0.2 61 ± 10 μm LVHT 11.4 ± 1.3 0.40 ± 0.09 2.9 ± 0.4 60 ± 4 μm HVLT 13.7 ± 0.8 0.68 ± 0.12 3.3 ± 0.3 80 ± 7 μm 238 239 X-ray photoelectron spectrometry was used to identify and quantify surface carbon species on 240 all of the HVOF coatings. Using an Al K-Alpha X-ray source with a beam size of 400 μm and 241 the sample surface normal to the detector, C 1s peaks were obtained for each coating (Figure 242 3A). All spectra are convoluted with three peaks assigned to C-C, (284.5 eV), C-O (285.6 eV), 243 and C=O (288.2 eV) and seen in Figure 3B, where all coatings contain mostly graphite structured 244 carbon, probably due to the high amount of carbon residue from the kerosene fuel source. The 245 presence of C-O and C=O peaks indicates the existence of organic contaminants via adsorption 246 from the atmosphere, as previous studies have confirmed that metal and metal oxides can

247 spontaneously adsorb organic materials, lowering their surface energy.45–47 Surface chemical 248 analysis was performed much later than the amount of time needed to plateau the WCA from

249 atmospheric carbon adsorption (>7 days),47 so it is expected to observe such peaks. The amount 250 of oxygen-containing adsorbed carbon from the atmosphere is similar for all coatings, while the 251 C-C peaks seem to vary between coatings. The C-C peaks documented here are much larger than

252 what has been seen for similar studies reported on Fe-based thermal spray coatings,47 indicating 253 that the increase in carbon content is due to the use of the kerosene fuel source. The amount of 254 graphite-like carbon was much larger for the HVHT coating than the rest of the coatings studied,

12 255 while the counts for the LVHT coating is lower. This may be due to the high and low kerosene 256 flow rates used to make each coating, respectively.

257 258 Figure 3: (A) High resolution XPS spectra of C 1s region for each of the stainless steel thermal 259 spray coatings. (B) Peak fitting of binding energies for different carbon bond found on the HVLT 260 coating, where the black dotted line is the fitted spectrum.

261 262 Static Water Contact Angle Measurements and Cross-section Analysis. All coatings 263 demonstrate hydrophobic characteristics with WCAs >90° (Figure 4). In comparison to sand 264 blasted SS 444 coupons with a WCA of 96° ± 9, some HVOF coatings displayed (near) 265 superhydrophobicity with apparent (as seen with the eye or low resolution microscope) WCA >

266 150°.

13 267 268 Figure 4. Image of the HVOF coating fabricated holding a droplet of water on their surfaces. 269 Their hydrophobicity is compared to a sand blasted SS 444 coupon.

270 271 Cross-sectional SEM images of the coatings and their average WCA can be seen in Figure 5. 272 The porosity of each coating was calculated through image analysis and can be found, along with 273 each coating’s thickness, in the supporting information in Table S1. The Cauliflower protrusions 274 described in the previous section produce a very peculiar microstructure across the section of a 275 coating, described as “columnar” or “cone-like”. This can only be seen within one coating: on 276 the left side of the HVLT coating’s cross-section in Figure 5 (red arrows in supporting 277 information, Figure S2), where a line of pores and oxides is seen expanding from the bottom of 278 the coating towards the surface, resulting in the cauliflower surface texture. 279 Coatings made using a low particle velocity, LVLT and LVHT, measured WCAs of 145° ± 4 280 and 136° ± 4, respectively. The lower velocity of the particles may have influenced the 281 material’s micro/nano- structure since the molten material may not have enough kinetic energy 282 parallel to the substrate and therefore is less likely to bond to form the textured surface. Both of 283 these coatings also did not portray any columnar section patterning or cauliflower surface 284 structure, which may be critical for superhydrophobicity. The two coatings fabricated using a 285 high spray velocity, HVHT and HVLT, have WCAs of 153° ± 1 and 157° ± 1, respectively. 286 However, their wetting may originate from different mechanisms. For instance, although the 287 HVHT coating has the lowest surface roughness and other roughness parameters are

288 insignificant, it contains the highest amount of carbon residing on its surface, which would

14 289 decrease adhesion and increase the WCA values measured. The HVLT coating has similar 290 carbon content to the LVLT coating, but also shows a unique surface texture according to the 291 roughness parameters in comparison to the other samples. The cauliflower surface seems to be 292 an ideal texture for supporting water repellency, where the ideal material would have both the 293 highest amount of carbon while portraying similar roughness parameters to the HVLT coating to 294 produce a synergistic effect. Albeit from different origins, it can be seen that HVOF coatings 295 produced using a higher spray velocity is seen to be advantageous as it decreases in terms of its 296 superhydrophobicity nature. 297 Chen et al. reported the development of robust superhydrophobic coatings using a thermal

298 spray process to create a micro/nano roughened surface.14 However, the authors used acetylene as 299 the fuel source, a much shorter hydrocarbon, which did not give rise to any sort of hydrophobic 300 properties as fabricated. This required a post modification step following the thermal spray 301 coating process and immersing the coatings in a PTFE dispersion for 1h. Such secondary 302 immersion step raises costs and requires more time and can facilitate corrosion of the metallic 303 coating, which can degrade its mechanical integrity.

304

15 305 Figure 5. Cross-section SEM images of each coating produced and their average WCAs and 306 associated standard errors of the means.

307 Bouncing water droplet experiment. The superhydrophobicity of a surface can also be 308 indicated by simply counting the number of bounces a water droplet experiences over the

309 materials surface.48 The two coatings that did not reach an apparent WCA greater than 150°, 310 LVLT and LVHT, also did not demonstrate any bouncing characteristics. On the contrary, the 311 coatings fabricated at higher velocities that measured superhydrophobic WCAs showed water 312 bouncing capabilities, as seen for HVLT coating in Figure 6. Such coating with the highest WCA 313 demonstrated 3 bounces of a water droplet. The HVHT coating (WCA 153°) enabled a water 314 droplet to bounce twice before pinning to its surface. High frame rate videos of the water 315 bouncing droplet experiment on each superhydrophobic coating can be found in the supporting 316 information. The number of bounces is directly proportional to the HVOF coatings’ WCA 317 values, which is in agreement with what has been previously reported for coatings with a WCA

318 greater than 151°.48 This implies that the HVOF coatings can prevent water penetration into the 319 nanotextured surface protrusions and demonstrates a temporary Cassie-Baxter state. In order to 320 investigate the coating’s superhydrophobic properties and mechanism further, rolling water 321 contact angle measurements were carried out on superhydrophobic HVHT and HVLT coatings.

322 323 Figure 6. Frames from the water droplet experiment of a 30 μL-sized droplet falling 10 cm from 324 the HVLT HVOF coating’s surface. (A) The droplet approaches the surface (B) the droplet 325 impacts the surface and flattens (C) the water droplet breaks from the surface (D) the first 326 successful bounce (E) the second bounce (F) the final and third bounce.

327 Water rolling measurements. By observing a water droplet while tilting the surface it resides

328 on, two experimental parameters can be extracted to gain more information about a material’s

16 329 superhydrophobic state and capabilities: I) the angle at which the surface is required to be tilted 330 just before the water droplet rolls off, or the roll-off angle, and II) the contact angle hysteresis,

app app 331 ΔΘ, which is the difference in advancing, Θa , and receding, Θr , WCAs while tilting the 332 specimen:

!"" !"" 333 ΔΘ = Θ! − Θ! (2)

334 For self cleaning applications, it is desirable for the roll-off angle to be low (<10°).18,25 The 335 contact angle hysteresis is used to determine the stickiness of the water drop to the surface, an

336 effect in contradiction with water repellency.49 While in a Wenzel state, the contact angle 337 hysteresis is very large (>10°) due to a low receding contact angle since the fraction of water left 338 in the surface textures makes it difficult to remove the liquid. 339 The two superhydrophobic HVOF coatings, HVHT and HVLT, were evaluated for their water 340 droplet sliding characteristics. A 30 μL droplet was placed on the HVOF coating’s surface 341 (Figure 7A), and the stage was tilted at 0.5° per second. The roll-off angles for HVHT and 342 HVLT were 59° and 42°, respectively, values which are in agreement with the coating’s WCA 343 measurements. The water droplet on the HVLT coating moved at a tilt angle of 32° but was 344 pinned to the surface to disable full roll-off as seen in Figure 7B. Furthermore, the contact angle 345 hysteresis was much higher than 10° for both coatings. The high roll-off angle and contact angle 346 hysteresis for the HVOF coatings indicate that the water droplet is in a Wenzel state in which the 347 drop follows the topography of the surface and no air is entrapped.

348 349 Figure 7. Snapshot images of a water droplet on the HVLT HVOF coating’s surface while it’s 350 being tilted. (A) The static water droplet before tilting, (B) movement of the droplet at a tilt angle

17 351 of 32° but it remains pinned to the surface and (C) the moment at which the droplet rolls off the 352 coating at 42°.

353 The classical measurements performed on the HVOF coatings for defining a surface’s state of 354 superhydrophobicity seem to be contradicting; where the coatings enable water droplet bouncing 355 and have an apparent WCA > 150° just like a materials in the Cassie-Baxter state, but the roll-off 356 angle and contact angle hysteresis experiments indicate that the coatings portray a Wenzel state. 357 The coatings may be in an intermediate state where a pure form of either model may not be

358 suitable to describe their behavior, as there is no flat area of contact with the water droplet.50 359 Alternatively, since the density of surface roughness features within the coating are random and 360 the kerosene may not be uniformly distributed, the heterogeneity of the surface may result in a 361 heterogeneous state of (super-) hydrophobicity. This entails that both the roughness factor and

362 the fraction of solid-liquid contact area should be used to report the HVOF coatings surfaces.51

363 It is also possible that the SS HVOF coatings undergo a Cassie-to-Wenzel transition,52 where

364 the water droplet impales the material’s microstructure that can be triggered by vibrations,53

365 squeezing,54 impact,55 rapid deceleration,56 application of an electric field,57 or evaporation.58,59 Over 366 the course of the water droplet tilting experiment, the droplet could be unpinned from the edge of

367 the asperity or the underside of the liquid could sag until it reaches the substrate.22 In both 368 mechanisms, the Cassie-to-Wenzel transition takes place as soon as the lowest point of the 369 meniscus reaches the bottom surface. This could occur within the time frame of the water droplet 370 tilting experiment, giving rise to high roll-off angles and large contact angle hysteresis. 371 Effect of carbon film removal on superhydrophobicity. The stainless steel thermal spray 372 coatings created in this work were fabricated using kerosene as the fuel source in attempt to 373 simultaneously add a low surface energy material to the rough metallic substrate. Furthermore,

374 Fe-based thermal spray coatings have been shown to catalyze CO2 decomposition and build-up a

375 C-based surface layer that provides hydrophobicity within days of atmospheric exposure.47 This 376 adsorption, however, has yet to be enough to transform a hydrophilic surface into a

377 superhydrophobic material47, and hence a roughened surface and kerosene fuel source is required.

18 378 Furthermore, it has been shown in previous sections that while using a specific set of spray 379 parameters, the SS HVOF coatings can portray superhydrophobic characteristics such as high 380 water contact angles and water droplet bouncing behavior, indicating that the coatings possess 381 low surface energy. 382 In order to observe the effect of the kerosene/carbonaceous film on the coating’s 383 superhydrophobicity, it was attempted to remove all carbon from the most water-repelling 384 coating’s surface (HVLT). This was done using two methods. The first consisted of rinsing the 385 coating 10 times with copious amounts of petroleum ether- a volatile organic solvent capable of 386 solubilizing kerosene. The solvent choice was also beneficial in terms of preserving the metallic

387 coating’s chemical integrity, as corrosion of the material was kept to a minimum due to 388 petroleum ether’s low vapor pressure. The coating’s microstructure was maintained after the 389 washing step as observed using SEM imaging before and after in the supporting information 390 Figure S3 to ensure the change in the coating’s behavior was due to the removal of the surface 391 energy lowering material. 392 The elemental composition of the coating’s surface before and after rinsing can be observed in 393 the supporting information in Figure S4. From the high-resolution X-ray photoelectron 394 spectroscopy (XPS) spectra of the C 1s region, no significant difference is observed in both 395 intensity and chemical shift while comparing the as sprayed sample and washed HVOF coating. 396 Furthermore, after washing, the water contact angle of the HVLT coating only slightly decreased 397 to 145° ± 4 indicating that some kerosene/carbonaceous film may still be still present after the 398 washing method. However, once washed, all water droplet-bouncing capabilities were lost. 399 Furthermore, the coating did not show a roll-off angle, but pinned the water droplet to its surface 400 to reveal a large contact angle hysteresis at a 90° tilt in Figure 8A and could even be completely 401 inverted (180°) as seen in Figure 8B. 402

403 404 Figure 8. (A) The washed coating pinned water droplets to its surface at a 90° tilt and (B) 405 completely inverted revealing an interesting Wenzel state. (C) High resolution XPS spectra of C 406 1s as sprayed HVLT HVOF coating (green) and after cleaning with UV radiation for 24h (black 407 dotted).

408 409 The second method for attempting to remove the carbon film from the HVOF coatings was 410 decomposition via ultraviolet radiation. Two HVLT coatings without any previous surface 411 treatments were radiated with UV light (365 nm) for 24h. One cleaned coating was then 412 immediately transferred into a vacuum desiccator and taken to the XPS for chemical analysis 413 while the other was taken to the goniometer for static WCA measurements. The high-resolution 414 XPS C1s spectrum of the cleaned sample shows 86% drop in carbon content in comparison to an 415 as sprayed sample. The amount of carbon-oxygen double bonds on the coating’s surface has also 416 decreased after UV radiation. The left over carbon may just be an adventitious C layer that 417 deposited within the time frame of transferring the sample into the instrument, and has been

418 found in similar studies.35,36 However, unlike what has been seen previously, the UV radiated 419 coating only showed a 30% decrease in WCA to a value of 110 ± 2 after cleaning (time=0),

420 meaning the coating remains hydrophobic after carbon removal. The cleaned coating was left out

20 421 in the lab environment (Montreal in December) for 5 consecutive days for a chance to 422 accumulate carbon contaminants while the WCA was measured periodically. From previous 423 studies, it can be hypothesized that the WCA will plateau, indicating carbon adsorption

424 saturation, within about a week where significant increases are observed within the first 48h.35,47 425 As seen in the supporting information in Figure S5, the WCA values over time do not change 426 significantly. In fact, the hydrophobicity decreases slightly with increasing exposure time. This 427 may be due to natural corrosion occurring on the surface each time a WCA measurement is 428 performed since the material lacks its protective carbonaceous film and the transition from a 429 Cassie-Baxter to Wenzel superhydrophobic state. This brings the material’s durability to

430 question, and will be evaluated extensively in future work. 431 The cleaned coating shows an interesting Wenzel state dissimilar to the coating before 432 washing. Coatings produced using LVHT also demonstrated to be in the Wenzel state, by 433 measuring the lowest static WCA and no water droplet bouncing characteristics. Since this 434 coating has similar roughness parameters to the superhydrophobic HVHT coating, the difference 435 in wettability could be due to lower kerosene content. Therefore, the addition of kerosene can be 436 seen as vital for the promotion to a superhydrophobic state as this carbon film lowers the surface 437 energy of the HVOF coatings. The material remains hydrophobic and possesses a Wenzel state 438 due to its high surface roughness. 439 440 Corrosion Behavior of HVOF Coatings. Of the many applications superhydrophobic materials

441 are found, use in anti- corrosion coatings has become increasing popular.60–63 Water repelling 442 coatings are typically tested for their corrosion resistance using potentiodynamic polarization 443 (PDP) measurements and are compared to uncoated metal. Corrosion parameters of interest

444 include the corrosion potential, Ecorr, and corrosion current density, jcorr, which are tabulated from 445 the potential where the current density decreases to vanishingly low values and the current 446 density at which the linear region of the cathodic and anodic Tafel branches intersect,

447 respectively.64 Since the experiments were performed on as-sprayed coatings without any further

21 448 grinding/polishing, the measured currents were normalized to the true surface areas as opposed 449 to the geometric surface area exposed by the electrochemical cell. The true surface area of each 450 coating was tabulated through the topography data previously acquired. 451 All four SS HVOF coatings’ corrosion behaviors were recorded during PDP measurements in 452 3.5 wt% NaCl, using a slow scan rate as seen in Figure 9. The coating’s corrosion behavior was 453 compared to that of a sand blasted wrought SS 444 coupon to resemble a roughened surface of

454 the material before thermal spraying. The sand blasted sample displayed a noble Ecorr of -130 mV

2 455 vs. SCE and a very low jcorr of 0.02 μA/cm , similar to what has been previously reported for

456 polished wrought SS 444.65 Passivation was achieved until a sharp increase in current occurred

457 due to the formation of a stable pit.66 Unlike the bulk SS substrate, the SS HVOF coatings’

458 corrosion behavior was similar to pure iron actively corroding in 3.5 wt% neutral pH electrolyte,67 459 where the loss of corrosion resistance has been linked to the changes in chemical composition

460 the metal undergoes during powder processing and spraying.68

461 462 Figure 9. Potentiodynamic polarization measurements performed on each of the stainless steel 463 HVOF coatings and a sand blasted wrought SS 444 coupon. All tests were carried out in 3.5 wt% 464 NaCl using a scan rate of 0.167 mV/s.

465 466 No matter which spray parameters were used, the SS HVOF coatings demonstrated active

467 corrosion rather than protective passivation, as seen by the lack of passive region in the anodic

22 468 branch. This is surprising as such material has a Cr content over the threshold of 12 wt%69 (18

469 wt%)68 for what is defined as stainless steel. The lack of passivation also shows that the carbon 470 film is not homogeneously distributed across the coatings’ surfaces, enabling direct contact 471 between the aggressive electrolyte and the vulnerable metallic coating. The average corrosion 472 potential, corrosion current density, and associated standard error of the mean for each sample 473 can be found in table 3. Despite the difference in hydrophobic properties, the HVLT coating’s 474 corrosion potential is not significantly different from the LVLT and HVHT coatings’ values. The 475 superhydrophobic HVHT coating had a lower corrosion current density than the HVLT and 476 LVLT coatings, however, indicating that the material may have a lower rate of degradation. A

477 lower corrosion current density was also found for the LVHT coating. The decrease in corrosion 478 current density could be due to the higher amount of uncompensated porosity that the HVHT and 479 LVHT coatings hold in comparison to the coatings produced with lower particle temperature, or 480 because of the higher amount of carbon content across its surface. 481 It was shown previously that SS 444 HVOF coatings portray a bi-phasic composition of Fe/Cr 482 and are capable of preferential dissolution of Fe due to the oxidation of Cr during powder

483 atomization.68 This was demonstrated by comparing PDP measurements of pure Fe and Cr to the 484 SS HVOF coatings PDP behavior over time. The thermal spray coatings show similar corrosion 485 properties to pure Fe after 1 hr immersion in 3.5 wt% NaCl electrolyte. After days of exposure, 486 the coatings show similar behavior to pure Cr including a passivation region and a lower 487 corrosion potential. The difference in PDP behavior between LVHT coating and the others, 488 specifically the decrease in corrosion potential and change Tafel slopes, could be explained by an 489 increase in Cr oxidation- a consequence of using high particle temperature and low velocity. The 490 particles have more time to oxidize in flight and segregate Cr from the Fe matrix. Consequently, 491 due to its wettability, this coating undergoes Fe dissolution at a faster rate during the 1h OCP rest 492 period prior to the PDP test than the rest of the thermal spray coatings. 493 This work sheds light on a surprising case where superhydrophobicity is not directly

494 proportional to the material’s corrosion resistance. The many successful cases found in the

23 495 literature of increasing corrosion resistance by encouraging superhydrophobicity include 496 methods where the coverage of low surface energy material is higher or complete, fully

497 protecting the underlying metallic substrate.29,60 The lack of corrosion resistance reported here 498 could be due to the discontinuous nature of the carbon film, the change in powder chemistry, and 499 the coatings’ inherit porosity. Yu et al. reported similar behavior for several composite coatings, 500 where they found that superhydrohobicity did not necessarily guarantee better corrosion

501 properties.70 This is important to understand when developing superhydrophobic coatings for 502 aqueous-exposure applications. 503 Albeit the inferior corrosion behaviour to wrought stainless steel, the superhydrophobic SS 444

504 HVOF coatings may still be of industrial value. They could act as physical barriers in 505 atmospheric conditions to keep the substrate within acceptable dimensional tolerances. 506 Alternatively, since they are more active than the underlying substrate, the HVOF coatings could 507 provide cathodic protection similar to the way zinc protects galvanized steels. This procedure 508 could also be explored in future work as a method to protect less nobel substrates such as low 509 carbon steel or magnesium alloys. 510 511 Table 3. Corrosion parameters including corrosion potential and corrosion current density 512 collected from PDP measurements (sample number=n) of each of the HVOF coatings with 513 associated standard errors the mean.

Sample Ecorr (mV) 2 n jcorr (µA/cm ) LVLT -467 ± 4 1.1 ± 0.2 5 HVHT -481 ± 20 0.5± 0.2 4 LVHT -547 ± 35 0.6 ± 0.2 5 HVLT -464 ± 28 1.1 ± 0.3 4 514 515

24 516 CONCLUSIONS. We have provided a method to fabricate superhydrophobic stainless steel 517 coatings in a single step. By utilizing a long carbon chained fuel, the surface energy of the metal 518 was lowered which, when combined with the rough HVOF surface profile, produced a 519 superhydrophobic material. The use of a high particle velocity and low temperature should be 520 encouraged when carrying out this method as such coatings had water contact angles as high as 521 157°. 522 It was shown that the presence of kerosene is crucial for water repellency, as once the kerosene 523 was removed from the coating’s surface, the superhydrophobic characteristics diminished. The 524 material remained hydrophobic, however, due to the surface roughness.

525 526 The corrosion behavior of the HVOF coatings was inferior to the wrought material and 527 actively corroded during potentiodynamic polarization testing. The least water repelling coating 528 portrayed the most active corrosion behavior, possibly due to its wettability. Despite some 529 coating’s superhydrophobicity, they did not portray protectively passivated behavioral and 530 showed similar corrosion properties to the hydrophobic coatings. This could be due to the 531 discontinuation of the carbon film, coating chemistry, and/or porosity. This study highlights a 532 situation where a material’s corrosion behavior is not affected by its inherit superhydrophobic 533 properties, and emphasizes the importance of corrosion testing when proposing a new 534 superhydrophobic material for anti-corrosion applications. 535 536 ASSOCIATED CONTENT

537 The following files are available free of charge.

538 “Supporting Information.pdf”

539 “HVLT.mp4”

540 “HVLT wahsed.mp4”

25 541 “HVHT.mp4”

542 AUTHOR INFORMATION

543 Corresponding Author

544 *e-mail: [email protected], phone: 1-514-398-3898

545 Present Addresses

546 †Department of Chemistry and Biomolecular Science, University of Ottwaw, 10 Marie Curie 547 Street, Ottawa, Ontario, K1N 6N5, Canada

548 Author Contributions

549 S.M.G. performed most experiments and was the major contributor to the data analysis and 550 manuscript preparation. K.P. assisted with WCA and water bouncing measurements. I.H. carried 551 out replicate PDP measurements. A.R.C.N. and S.S. were responsible for the production of the 552 HVOF coatings. All authors provided discussion and contributed to proof reading. All authors 553 have given approval to the final version of the manuscript.

554 Funding Sources

555 The work presented was financially supported NSERC CRD 242399, NSERC-CGS, NSERC

556 CGS-MSFSS, NSERC green-SEAM Strategic network, the Mitacs Globalink program, and

557 Hydro-Québec.

558 ACKNOWLEDGMENT 559 Authors would like to acknowledge Dr. Sanjayan Sathasivam and Dr. Emmanuel Mena Morcillo 560 for assisting with and performing XPS, as well as Frances Heale for insightful discussion.

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