The Effect of Corrosion on Conventional and Nanomaterial Copper Cold Spray Surfaces for Antimicrobial Applications

The U.S. spends over $125 billion a year in prevention of touch surface infections [1-5]. Copper cold spray coatings have been identified as having greater antimicrobial effectiveness than other additive methods [1]. This paper builds off of prior work from the paper “Effectiveness of Nanomaterial Copper Cold Spray Surfaces on Inactivation of Influenza A Virus “, where copper cold spray antimicrobial properties are improved with the use of nanoagglomerate Cu powder [2]. There is a need to further qualify consolidated nanomaterial Cu material properties in relation to conventional Cu to better understand Cu kill-mechanisms. The primary killing mechanism of copper is contact killing through deformation of microbe membranes by copper ions leading to cell cytoplasm release and subsequent internal damage to the cell [68]. Further research is needed to determine the main mechanisms for copper ion uptake into the cell [4,9-11]. Research is being performed in the biology field to better understand microbial copper ion defense and uptake mechanisms, including mapping of internal signal pathways and external cell interactions that lead to selective ligand favorability [4,10,12-14].

open structure than that of atomic grains, making the barrier for diffusion much less in grain boundaries than in the material lattice [3]. In nanomaterial Cu, smaller grain size allows for a greater number of grain boundaries and an increase in ion diffusion as compared to conventional Cu, making microstructure a key factor in nanomaterial Cu's increased antimicrobial effectiveness [18][19][20].
Nanomaterial Cu alsohas a much greater surface roughness than that of conventional Cu at the nanoscale, which is the same scale for Influenza A Virus. This may also be a contributing factor in Cu ion release [21]. Environmental factors also affect corrosion rate, including changes in temperature, pH, and humidity.
Water can become de-aerated with increasing temperature, where this decrease in oxygen may decrease corrosion rate [22]. This is not a concern for this work since testing is performed at room temperature. As pH decreases, Cu-alloys and protective films are unable to develop, causing an increase in corrosion rate [22,23]. In a study performed by Feng et al, copper oxide thickness in relation to H2O pH was measured where pH below 4 caused oxide dissolution, pH higher than 4 formed Cu 2 O, and pH above 10 formed CuO [24]. For this paper 3% (weight) NaCl solution is used in corrosion testing. NaCl is formed from HCl and NaOH, which are a strong acid and base, respectively. Salts comprised of both a strong acid and base do not hydrolyze, instead they dissociate in water into Na + and Clions [25]. The dipole nature of H 2 O allows for this to occur, where Na+ is attracted to the electronegative oxygen and Cl-is attracted to the electropositive hydrogen [15,26]. The result is a neutral pH solution. It follows that the predominant oxide to form at this pH is Cu 2 O. However, humidity also plays a role in oxide species presence, where Cu(i), also known as Cu 2 O, is more stable in dry environments and Cu(ii), also known as CuO, is more stable in aqueous environments [27].
Other factors determining the favorability of copper oxide state include Cu geometry, bonding, and hard-soft acid-base (HSAB) theory [10,14,17,27,28,29]. Cu(i) prefers a tetrahedral geometry, favors back-bonding, and is a soft acid that associates with soft bases, whereas Cu(ii) prefers octahedral geometry, has a low degree of back bond donation, and is a borderline hard acid that associates with harder bases. All of the above listed factors may be affected by the environment and the microbes in it. Microbes can select for certain kinds of geometries and bonds using specific ligands and bonding sites, they can also excrete material that changes local pH, and can change their own internal pH [4,10,12,14,15,27,30]. So even if a certain kind of Cu species manifests on the surface of a consolidated Cu coating as a result of environmental factors, this does not mean that the same species will be received or internalized by the microbe. When considering lifecycle and maintenance of Cu coatings, it is important to note that oxides/tarnishing of the Cu surface does not affect the antimicrobial efficacy of the coating [31,32].
Post-processing heat treatment is not recommended for this application as it could negate the antimicrobial effects of the nanoparticles used in the nanomaterial Cu cold spray coating [3].
It is possible that for other applications the need for residual stress relief outweighs the need for antimicrobial efficiency, in those cases heat treatment may be used. Heat treatment work is included in this paper, but most data is placed in the supplemental section as this work is out of scope for the given antimicrobial application. The  Since NaCl solution in water is neutral, the cathodic reaction would be: O 2 +2H 2 O+4e-= 4OH-. Considering that neutral pH is 7 and that standard electron potential of Cu is 0.34, the pourbaix diagram can be used to plot these values, as indicated by the red dashed lines in (Figure 1). An estimate of the resulting corrosion products is determined to be right on the line between CuCl*3 Cu(OH) 2 , and Cu 2 O. However, the Pourbaix diagram serves only as an estimate, since it does not take into consideration corrosion rate which can be changed with voltage [23,33,34]. The pourbaix estimate can be confirmed through X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS). Both methods can determine the Cu species present on the sample post-corrosion testing, however, XPS is able to more accurately measure the top-most atomic layers (~5nm depth) of the sample. Linear polarization (LP) and electrochemical impedance (EIS) corrosion tests can be performed using an electrochemical cell. LP measures corrosion rate and EIS measures impedance response at a fixed frequency [23].
Ion concentration can also be measured through an ion release assay using an inductively coupled plasma mass spectrometer (ICP-MS). Corrosion results are comparable to efficacy of nanomaterial vs. conventional Cu, where an increase in corrosion rate or ion concentration in the environment indicates an increase in Cu ion release from the substrate [23]. Since Cu's kill mechanism is through Cu ion release through contact with the surface, it follows that an increase in Cu ion release will be proportional to the material's antimicrobial effectiveness. Measuring changes in corrosion and composition will allow for a better understanding of why nanomaterial Cu is more efficient in the contact killing of Influenza A Virus than conventional Cu and will aid future research in better understanding the contact killing mechanism.

Materials
Cold Spray: Cold spray samples from the first paper,      smaller the diameter or Rp value, the higher the corrosion rate of the material. Ohm's law states that resistance is the ratio between voltage and current. Capacitance is the ratio between charge stored on the capacitor and electrical potential, making capacitance inversely proportional to resistance and directly proportional to corrosion rate [38][39][40]. For LP data, all samples fit to a graph of electron potential vs. current density, as seen in Figure 8.

XRD: WPI's PANalytical Empyrean X-ray Diffraction machine
was used with a Cu tube and Ni filter from 20 to 140 2theta at 45Kv and 40Ma, with a ½ degree divergence slit, 1-degree antiscatter slit, 0.04 radian soller slit, and 10mm mask. Due to the thin Cu cold spray coating thickness (50um or less), the time per step was increased to 100 seconds per step. Prior to running, the depth was checked with a goniometer and a depth of 1 was confirmed.

Results were collected and analyzed using Data Viewer and High
Score Plus software. Results were compared against reference PDF4database, with PDFs 00-004-0836, 00-005-0061, 00-005-0667 for Cu, CuO, and Cu2O, respectively. Data was collected for top-down measurement of the corroded area of the samples.

XPS: Due to the high amounts of carbon contamination, left
by cutting fluids and corrosion testing, attempts were made to delicately clean the copper surfaces. Prior to analysis by XPS, all samples were sequentially sonicated in acetone then isopropanol for 5 minutes each. Cleaning with water and detergents was avoided as to not affect the surface chemistry being probed. A PHI5600 XPS system with a third-party data acquisition system (RBD Instruments, Bend Oregon) acquired all photoelectron spectra as detailed previously [42]. Analysis chamber base pressures were <1×10 − 9 Torr. A hemispherical energy analyzer that was positioned at 90° with respect to the incoming monochromated Al Kα X-ray flux and 45° with respect to standard sample positioningcollected the photoelectrons. Survey spectra utilized a 117 eV pass energy, a 0.5 eV step size, and a 50-ms-per-step dwell time. High-resolution XP spectra employed a 23.5 eV pass energy, 0.025 eV step size, and a 50 ms dwell time per step. Spectra were acquired for the Al 2s, Al 2p, C 1s, Cu 2p, N 1s, and O 1s photoelectron regions as well as the Cu LMM Auger region. Post-acquisition data fitting employed Shirley-style baselines to all spectra based on a qualitative visual assessment of baseline shape. For a given oxidation state, fits that employ multiple peaks within a spectral region utilized identical fwhm (full width and half max) values for each peak to minimize mathematically optimized but possibly chemically unrealistic fits [43]. All areas were fit with GL (30) pseudo-Voigt peak functions except for features in the Cu 2p region that employed GL (70) functions. Lastly, Cu 2p features ascribed to Cu0 and to Cu1+ utilized an asymmetric tail function.

Results and Discussion
Corrosion Figure 9: EIS Data in impedance vs. resistance.
EIS data, in (Figure 9), shows that nanomaterial Cu cold spray has a greater corrosion rate than that of conventional Cu cold spray. And that bulk Cu has a corrosion rate that is greater than both conventional and nanomaterial CuThe data is ( Figure 9) is manifested as lines rather than semi-circles due to the presence of  (Table 1). As resistance decreases there is an increase in capacitance, where capacitance is directly proportional to corrosion rate [38][39][40]. Bulk Cu has the lowest resistance and highest capacitance, followed by nanomaterial Cu and conventional Cu, respectively. LP data, in (Figure 10), supports EIS resultsAt first glance, the above data appears, with the exception of Bulk Cu as an outlier to the right (blue line on the graph). Upon further analysis, differences are seen between the samples as detailed in (Table 2 and passivation values. It also has the lowest passivation potential range. Nanomaterial Cu also has a higher corrosion rate than conventional Cu, which is expected as the nanoparticles present in the coating provide a greater percentage of grain boundaries for ion diffusion [2]. Bulk Cu performed worse than nanomaterial Cu, in that it had a higher corrosion rate. This may be due to bulk Cu not being manufactured using cold spray technology, which provides denser coatings with minimal oxides and inclusions [2].  Microstructure and hardness analysis in the following section can confirm this. Additionally, it is possible that the nanomaterial Cu was able to form a passive layer more rapidly than that of bulk Cu, for which the Ipass values show this to be true [45]. Ion release assay data supports both EIS and LP test results, as seen in Figure   11. Cu performing better than conventional Cu in the contact killing of Influenza A Virus [2]. The next section will look the differences in microstructure between the two coatings to further explain why nanomaterial Cu is able to have a faster ion release rate.    XPS: XPS data for as made and corroded conventional, nanomaterial, and bulk Cu is in (Figure 15). The As made bulk Cu sample spectra was limited due to sample contamination. Figure 15 represents spectra for the Auger Cu LMM and XP Cu 2p regions for both conventional and nanomaterial copper surfaces.

Conclusion
In conclusion, 1. Nanomaterial Cu has a higher corrosion rate than conventional Cu.

2.
Nanomaterial Cu has smaller grain size and consequently greater percent grain boundaries than conventional Cu, expected to lead to increased ion release which contributes to increased corrosion rate.

3.
Since the main antimicrobial mechanism of Cu is through contact killing, it follows that increased ion release and corrosion rate of nanomaterial Cu contributes directly to its increased antimicrobial effectiveness in the contact killing of Influenza A Virus as compared to conventional Cu [2].

4.
XRD and XPS show sample composition to be of pure Cu and Cu oxides, with XPS determining the dominant Cu oxide present to be Cu 2 0.

5.
Follow-on work is needed to determine if the presence of different oxide species varies Cu kill-rate.

6.
Additional work should also be done to quantify the energetics of nanomaterial versus conventional Cu cold spray for grains, grain boundaries, and subsequent transport phenomena.