Comparison of Electropolishing of Aluminum in a Deep Eutectic Medium and Acidic Electrolyte

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Article Comparison of Electropolishing of Aluminum in a Deep Eutectic Medium and Acidic Electrolyte

Tarek M. Abdel-Fattah 1,* and J. Derek Loftis 1,2 1 Applied Research Center at Thomas Jeﬀerson National Accelerator Facility and Department of Molecular Biology and Chemistry at Christopher Newport University, Newport News, VA 23606, USA; [email protected] 2 Department of Physical Sciences, Virginia Institute of Marine Science, College of William & Mary, Williamsburg, VA 23187, USA * Correspondence: [email protected]; Tel.: +1-(757)-594-7606; Fax: +1-(757)-594-7356

Academic Editor: Paul Nancarrow Received: 20 October 2020; Accepted: 24 November 2020; Published: 3 December 2020

Abstract: Research advances in electropolishing, with respect to the field of metalworking, have afforded significant improvements in the surface roughness and conductivity properties of aluminum polished surfaces in ways that machine polishing and simple chemical polishing cannot. The effects of a deep eutectic medium as an acid-free electrolyte were tested to determine the potential energy thresholds during electropolishing treatments based upon temperature, experiment duration, current, and voltage. Using voltammetry and chronoamperometry tests during electropolishing to supplement representative recordings via atomic force microscopy (AFM), surface morphology comparisons were performed regarding the electropolishing efficiency of phosphoric acid and acid-free ionic liquid treatments for aluminum. This eco-friendly solution produced polished surfaces superior to those surfaces treated with industry standard acid electrochemistry treatments of 1 M phosphoric acid. The roughness average of the as-received sample became 6.11 times smoother, improving from 159 nm to 26 nm when electropolished with the deep eutectic solvent. This result was accompanied by a mass loss of 0.039 g and a 7.2 µm change in step height along the edge of the electropolishing interface, whereas the acid treatment resulted in a slight improvement in surface roughness, becoming 1.63 times smoother with an average post-electropolishing roughness of 97.7 nm, yielding a mass loss of 0.0458 g and a step height of 8.1 µm.

Keywords: ionic liquid; electrochemical polishing; choline chloride; phosphoric acid; surface characterization; atomic force microscopy

1. Introduction Electrochemical polishing (electropolishing) is the controlled corrosion of metal surfaces [1–3]. The concept behind this mechanism of corroding metals with liquids is to yield a reduction in the surface roughness of the polished metals [4,5]. Another major benefit of electropolishing over surface buffing alternatives is the practical application of reducing surface roughness and impurities to nearly negligible quantities on polished surfaces [6,7]. Currently, large quantities of surface-polished products are being treated with hazardous chemical solutions [8–10]. Phosphoric and sulfuric acid mixtures account for a plurality of these acid electropolishing treatments in pure metals and alloys [11,12]. The benefits of electrochemical polishing have gained notoriety, being determined to be an ideal method for improving a metal’s optimum roughness while also greatly improving electrical conductivity [6,7,12]. Many acid treatments currently utilized for electropolishing metal surfaces provide an ideal mirror finish by removing the exposed surface layer of the sheet metal. However, acid solutions provide this clean electropolished finish to the metal at the expense of hydrogen

Molecules 2020, 25, 5712; doi:10.3390/molecules25235712 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 8

Molecules 2020, 25, 5712 2 of 8 provide an ideal mirror finish by removing the exposed surface layer of the sheet metal. However, acid solutions provide this clean electropolished finish to the metal at the expense of hydrogen contaminationcontamination [[4,9,12].4,9,12]. The The removal removal of ofhydrogen hydrogen contamination contamination generally generally entails entails the use the of use high- of high-temperaturetemperature treatments treatments in excess in excess of 800 of 800°C for◦C several for several hours hours in vacuo in vacuo [13]. [ 13 ]. High-purityHigh-purity metal metal samples samples of aluminumof aluminum were were tested tested to determine to determine the effectiveness the effectiveness of an industry of an standardindustry acidstandard solution acid of solution 1 M phosphoric of 1 M phosphor acid andic an acid ionic and liquid an ionic medium liquid composed medium composed of ethylene of glycolethylene and glycol choline and chloride choline mixed chloride in a 2:1mixed molar in ratio.a 2:1 Previousmolar ratio. studies Previous demonstrated studies demonstrated the effectiveness the ofeffectiveness various conformities of various of conformities this ionic liquid of this with ionic stainless liquid steel with and stainless other alloyssteel and [1,5 other,10]. It alloys was the [1,5,10]. aim ofItthis was investigation the aim of this to investigation optimize the to electrochemistry optimize the electrochemistry tests to produce tests a superior to produce polish a superior of this ionicpolish liquidof this and ionic to weighliquid theand benefits to weigh of thisthe ionicbenefits liquid of this against ionic an liquid industry against standard an industry 1 M phosphoric standard acid 1 M electropolishingphosphoric acid treatment. electropolishing treatment.

2.2. Results Results TheThe general general mechanism mechanism of of the the electropolishing electropolishing of of metal metal surfaces surfaces in in this this study study using using the the ionic ionic liquidliquid deep deep eutectic eutectic solvent solvent electrolyte electrolyte is is represented represented in in Figure Figure1. 1. The The optimum optimum conditions conditions for for the the steadysteady electropolishing electropolishing of of aluminum aluminum metals metals with with the the ionic ionic liquid liquid were were revealed revealed to be to 2 be V for2 V 900 for s900 via s 1 linearvia linear sweep sweep voltammetry, voltammetry, as shown as shown in Figure in Figure2. Set 2. to Set progress to progress at a scan at a ratescan ofrate 20 of mVs 20 mVs− from−1 from 0 to 0 8to V, 8 the V, e fftheects effects of electropolishing of electropolishing became became erratic anderratic irreproducible and irreproducible beyond 4beyond V. A point 4 V. of A inflection point of presentinflection at 0.098present A/cm at2 0.098in the A/cm linear2 sweepin the voltammogramlinear sweep voltammogram indicated the optimum indicated conditions the optimum for chronoamperometryconditions for chronoamperometry for ionic liquid treatments for ionic liquid in aluminum treatments samples in aluminum with a current samples range with of a 0.092 current to 0.11range A/cm of 20.092(Figure to 0.112A). A/cm Chronoamperograms2 (Figure 2A). Chronoamperograms of the high-purity aluminumof the high-purity samples aluminum were run duringsamples thewere polishing run during process the polishing using phosphoric process using acid atphosphoric an average acid current at an ofaverage 3.51 A current/cm2 and of 3.51 at 1.08 A/cm A/cm2 and2 withat 1.08 the ionicA/cm liquid2 with eutecticthe ionic (Figure liquid2 B).eutectic Mass comparison(Figure 2B). ofMass the originalcomparison weight of ofthe aluminum original weight prior to of electropolishingaluminum prior reported to electropolishing a difference reported of 0.039 a g diff whenerence compared of 0.039 with g when the post-polishing compared with weight the post- of 5.79polishing g for aluminum weight of polished5.79 g for via aluminum the ionic polished liquid method via the (Table ionic1 liquid). method (Table 1).

FigureFigure 1. 1. SchematicSchematic conceptualizationconceptualization ofof anodicanodic levelingleveling ofof aluminumaluminum surfacessurfaces via via atomic atomic force force microscopymicroscopy (AFM)(AFM) atat di differentﬀerent phases phases of of the the study: study: (A(A)) beforebefore electropolishing, electropolishing, andand (B(B)) after after electropolishingelectropolishing treatments treatments withwith thethe ionic liquid, liquid, with with representati representativeve 2D 2D surface surface profiles proﬁles depicted depicted (C) (Cbefore,) before, (D ()D during,) during, and and (E ()E after) after experiments. experiments.

Table 1. Average electropolishing rate (µg/s) calculations for each electrolyte for each metal sample over a 900 s treatment.

Medium Mass Before (g) Mass After (g) Mass Diﬀerential (g) Surface Degradation Rate (µg/s) Deep Eutectic 5.83 5.79 0.039 43.77 Phosphoric Acid 5.85 5.80 0.045 50.88 Molecules 2020, 25, x FOR PEER REVIEW 3 of 8

Table 1. Average electropolishing rate (µg/s) calculations for each electrolyte for each metal sample over a 900 s treatment.

Mass Mass Mass Surface Degradation Medium Before (g) After (g) Differential (g) Rate (µg/s)

Deep Eutectic 5.83 5.79 0.039 43.77

Phosphoric Molecules 2020, 25, 5712 5.85 5.80 0.045 50.88 3 of 8 Acid

FigureFigure 2.2.Linear Linear sweep sweep voltammograms voltammograms (A) ( forA) bothfor polishingboth polishing agents agents with aluminum with aluminum samples steppedsamples 1 −1 fromstepped 0 to from 4 V at 0 ato constant 4 V at a scanconstant rate scan of 20 rate mVs of− 20. The mVs dashed. The line dashed indicates line indicates the ideal the voltage ideal utilized voltage forutilized chronoamperometry. for chronoamperometry. (B) Chronoamperometry (B) Chronoamperometry for phosphoric for ph acidosphoric (3.51 A acid/cm2 )(3.51 and A/cm the ionic2) and liquid the eutecticionic liquid (1.08 eutectic A/cm2) (1.08 with A/cm high-purity2) with aluminumhigh-purity samples. aluminum samples.

PriorPrior to electropolishing, electropolishing, the the aluminum aluminum measured measured a root a root mean mean square square roughness roughness average average of 159 of± 9159 nm, 9while nm, whileafter polishing after polishing treatments treatments with the with ethylene the ethylene glycol and glycol choline and chloride choline chloridesolution ± solutionyielded a yielded roughness a roughness of 26 ± 2 nm of 26via atomic2 nm viaforce atomic microscopy force microscopy (AFM) (Table (AFM) 2, Figure (Table 3).2 The, Figure relative3). ± Thesmoothing relative efficiency smoothing with effi ciencythe ionic with liquid the ionicrevealed liquid a roughness revealed a difference roughness of di ff132erence nm for of 132aluminum nm for aluminumacid-free polishing acid-free polishingfor an overall for an overallelectropolishing electropolishing efficiency efficiency of 83.2% of 83.2% (Table (Table 2).2). Step heightheight measurementsmeasurements discloseddisclosed thethe divisiondivision betweenbetween thethe polishedpolished andand unpolishedunpolished regionsregions toto bebe comparablecomparable withwith aa weightweight didifferencefference of of 7.2 7.2µ µmm (Table (Table3 ).3).

TableTable 2. 2.Roughness Roughness averageaverage (Ra)(Ra) inin nmnm forfor eacheach metalmetal postpost andand priorprior toto treatmenttreatment withwith thethe respectiverespective solutionssolutions notednoted atat 7070 ◦°CC forfor 900900 s.s. Calculated didiffeﬀerencesrences determineddetermined %% smoothingsmoothing eefficiencyﬃciency (SE)(SE) forfor eacheach sample.sample.

Medium Ra Before (nm) Ra After (nm) Ra Diﬀerence (nm) % Ra SE * Deep EutecticMedium Solvent Ra Before 159.3 (nm) Ra After 26.6 (nm) Ra Difference 132.6 (nm) % Ra 83.2 SE * Phosphoric Acid 159.1 97.7 61.3 38.5 Deep Eutectic Solvent* All Smoothing159.3 Eﬃciency measurements26.6 reported as a %.132.6 83.2 Molecules 2020, 25, x FOR PEER REVIEW 4 of 8

Phosphoric Acid 159.1 97.7 61.3 38.5

* All Smoothing Efficiency measurements reported as a %.

Figure 3.FigureAtomic 3. Atomic force force microscopy microscopy of of aluminum aluminum post-electropolishingpost-electropolishing with with the ionic the ionicliquid liquidin both in both (A) two( andA) two (B and) three (B) three dimensions—recording dimensions—recording an an averageaverage roughness roughness of 26 of ± 26 2 nm2 by nm utilizing by utilizing the root the root mean square method for calculation. A 10 × 10 µm recording region was utilized.± mean square method for calculation. A 10 10 µm recording region was utilized. × Table 3. Average recorded step heights in µm for each metal after treatment with the respective electrolytes at 70 °C for 900 s.

Medium Step Height (µm)

Deep Eutectic Solvent 7.2

Phosphoric Acid 8.1

Phosphoric acid polishing treatments for the aluminum samples demonstrated a peak value of 0.286 A/cm2 at 1 V with a local minimum of 0.23 A/cm2 immediately following at 1.5 V according to the linear sweep voltammetry analysis (Figure 2A). This yielded a current range of 0.23 to 0.25 A/cm2 for chronoamperometry (Figure 2B) as the linear sweep progressed from 0 to 8 V at a scan rate of 20 mVs−1 (Figure 2). Upon completion of this cursory scan, the optimum conditions for chronoamperometry could be ascertained to be similar to those for the ionic liquid (at 2 V), while the current density range was greater for the acid treatments. With a composite mass of 5.85 g prior to electrochemical treatment, a mass loss of 0.045 g was calculated (p < 0.001; 3.405 × 10−6) after the post- electropolishing weight yielded a mass of 5.80 g (Table 1). Post-treatment roughness measurements reported a root mean square roughness of 97 ± 6 nm (Figure 4), indicating that a low smoothing efficiency rating of 38.5% was achieved (Table 3). Upon comparative observation between aluminum 3D topography and AFM roughness statistics, a focused smoothness of 6 nm was achieved for a region 1/16th of the 10 × 10 µm region sampled via AFM (Figure 4). This revealed that the overall smoothness of the sample was marred by intermittent peaks and the bubbling that often occurs with pitting at low current densities during electropolishing. A representative example contrasted the two aluminum samples shown in the AFM console view, where additional evidence of this bubbling greatly affected electropolishing efficiency when surface roughness was concerned (Figure 5). Step height measurement indicated that phosphoric acid electropolishing etched an average of 8.1 µm of aluminum from the surfaces of the treated samples (Table 3).

Molecules 2020, 25, 5712 4 of 8

Table 3. Average recorded step heights in µm for each metal after treatment with the respective

electrolytes at 70 ◦C for 900 s.

Medium Step Height (µm) Deep Eutectic Solvent 7.2 Phosphoric Acid 8.1

Phosphoric acid polishing treatments for the aluminum samples demonstrated a peak value of 0.286 A/cm2 at 1 V with a local minimum of 0.23 A/cm2 immediately following at 1.5 V according to the linear sweep voltammetry analysis (Figure2A). This yielded a current range of 0.23 to 0.25 A /cm2 for 1 chronoamperometry (Figure2B) as the linear sweep progressed from 0 to 8 V at a scan rate of 20 mVs − (Figure2). Upon completion of this cursory scan, the optimum conditions for chronoamperometry could be ascertained to be similar to those for the ionic liquid (at 2 V), while the current density range was greater for the acid treatments. With a composite mass of 5.85 g prior to electrochemical treatment, a mass loss of 0.045 g was calculated (p < 0.001; 3.405 10 6) after the post-electropolishing weight × − yielded a mass of 5.80 g (Table1). Post-treatment roughness measurements reported a root mean square roughness of 97 6 nm ± (Figure4), indicating that a low smoothing e fficiency rating of 38.5% was achieved (Table3). Upon comparative observation between aluminum 3D topography and AFM roughness statistics, a focused smoothness of 6 nm was achieved for a region 1/16th of the 10 10 µm region sampled × via AFM (Figure4). This revealed that the overall smoothness of the sample was marred by intermittent peaks and the bubbling that often occurs with pitting at low current densities during electropolishing. A representative example contrasted the two aluminum samples shown in the AFM console view, where additional evidence of this bubbling greatly affected electropolishing efficiency when surface roughness was concerned (Figure5). Step height measurement indicated that phosphoric acid electropolishing etched an average of 8.1 µm of aluminum from the surfaces of the treated samples (Table3). Molecules 2020, 25, x FOR PEER REVIEW 5 of 8

Figure 4. AtomicAtomic force force microscopy microscopy of of aluminum aluminum post-e post-electropolishinglectropolishing with phosphoric acid in ( A) 2D and (B) 3D—recording an average roughness of 97 6 nm by utilizing the root mean square method and (B) 3D—recording an average roughness of 97 ± 6 nm by utilizing the root mean square method for calculation. A 10 10 µm recording region was utilized. for calculation. A 10 × 10 µm recording region was utilized.

Figure 5. AFM console window view of aluminum samples electropolished with (A) an ionic liquid and (B) 1 M phosphoric acid, both on a 20 µm scale.

3. Discussion The phosphoric acid-facilitated aluminum polishing produced an electropolishing rate of 50.8 µg/s, which is a notably faster rate of electropolishing than the ionic liquid rate of 43.7 µg/s (Table 1). While this evidence appears favorable for the acid-polishing electrolyte, surface roughness—the primary purpose of electropolishing—was not nearly as effective for phosphoric acid, which produced an aluminum metal roughness of 97 ± 6 nm after electropolishing (Figure 4). This was calculated to an average difference of 61 nm etched from working electrode surfaces (Table 2). Step height measurements revealed that an average of 8.1 µm of aluminum material was etched away from the working electrode surface during electropolishing procedures with phosphoric acid (Table 3). When contrasted with the smaller figure of 7.2 µm returned from the ionic liquid electrolyte treatments (Table 3), it became apparent that treatments with phosphoric acid on aluminum were indeed more efficient at etching based on experiment duration and the calculated electropolishing rates from Table 1. The aluminum treatments with choline chloride and ethylene glycol had an electropolishing rate of 43.7 µg/s, whereas the phosphoric acid treatments had the superior rate of 50.8 µg/s (Table 2) [14]. This faster rate of polishing comes with the caveat of a poor surface polish in terms of overall roughness [15]. With the ionic liquid treatment of aluminum demonstrating a visibly superior roughness, with an average roughness of 26 ± 2 nm (Figure 3), when compared to the roughness of 97 ± 6 nm for phosphoric acid treatments (Figure 4), the roughness average for the acid solution has a faster polishing rate at the detriment of surface polishing efficiency (Table 2). As this smoothing

Molecules 2020, 25, x FOR PEER REVIEW 5 of 8

Figure 4. Atomic force microscopy of aluminum post-electropolishing with phosphoric acid in (A) 2D Moleculesand2020 (B), 3D—recording25, 5712 an average roughness of 97 ± 6 nm by utilizing the root mean square method 5 of 8 for calculation. A 10 × 10 µm recording region was utilized.

Figure 5.5. AFM console window view of aluminumaluminum samples electropolished with (A)) anan ionicionic liquidliquid and ((B)) 11 MM phosphoricphosphoric acid,acid, bothboth onon aa 2020 µµmm scale.scale.

3. Discussion The phosphoricphosphoric acid-facilitated acid-facilitated aluminum aluminum polishing polishin producedg produced an electropolishingan electropolishing rate ofrate 50.8 of µ50.8g/s, whichµg/s, which is a notably is a notably faster rate faster of electropolishingrate of electropolishing than the than ionic the liquid ionic rate liquid of 43.7 rateµg /ofs (Table43.7 µg/s1). While (Table this 1). evidenceWhile this appears evidence favorable appears for thefavorable acid-polishing for the electrolyte,acid-polishing surface electrolyte, roughness—the surface primaryroughness—the purpose ofprimary electropolishing—was purpose of electropolis not nearlyhing—was as effective not for nearly phosphoric as effect acid,ive which for phosphoric produced an acid, aluminum which metal roughness of 97 6 nm after electropolishing (Figure4). This was calculated to an average produced an aluminum± metal roughness of 97 ± 6 nm after electropolishing (Figure 4). This was dicalculatedfference ofto 61an nmaverage etched difference from working of 61 nm electrode etched from surfaces working (Table electrode2). Step su heightrfaces measurements (Table 2). Step revealedheight measurements that an average revealed of 8.1 µ mthat of aluminuman average material of 8.1 µm was of etched aluminum away frommaterial the workingwas etched electrode away surfacefrom the during working electropolishing electrode surface procedures during withelectropol phosphoricishing acidprocedures (Table3 ).with When phosphoric contrasted acid with (Table the smaller3). When figure contrasted of 7.2 µm with returned the smaller from the figure ionic liquid of 7.2 electrolyte µm returned treatments from (Tablethe ionic3), it liquid became electrolyte apparent thattreatments treatments (Table with 3), phosphoricit became apparent acid on aluminumthat treatments were indeedwith phosphoric more efficient acid aton etching aluminum based were on experimentindeed more duration efficient and at theetching calculated based electropolishingon experiment duration rates from and Table the1 .calculated The aluminum electropolishing treatments withrates cholinefrom Table chloride 1. The and aluminum ethylene treatments glycol had with an electropolishing choline chloride rate and of ethylene 43.7 µg/ s,glycol whereas had thean phosphoricelectropolishing acid treatmentsrate of 43.7 had µg/s, the whereas superior the rate phos of 50.8phoricµg/ sacid (Table treatments2)[14]. had the superior rate of 50.8 µg/sThis (Table faster rate2) [14]. of polishing comes with the caveat of a poor surface polish in terms of overall roughnessThis faster [15]. rate With of polishing the ionic liquidcomes treatmentwith the cave of aluminumat of a poor demonstrating surface polish in a visiblyterms of superior overall roughness, with an average roughness of 26 2 nm (Figure3), when compared to the roughness of roughness [15]. With the ionic liquid treatment± of aluminum demonstrating a visibly superior 97 6 nm for phosphoric acid treatments (Figure4), the roughness average for the acid solution has roughness,± with an average roughness of 26 ± 2 nm (Figure 3), when compared to the roughness of a97 faster ± 6 nm polishing for phosphoric rate at theacid detriment treatments of (Figure surface 4), polishing the roughness efficiency average (Table for2). the As acid this solution smoothing has eaffi fasterciency polishing is the desired rate at design the detriment for electropolishing, of surface polishing the acid solutionefficiency is (Table clearly 2). less As capable this smoothing than the ionic liquid mixture for creating a smooth surface (Figure5)[14,16]. For the 900 s experiment duration, a chemical reaction occured at the anode immersed in the ionic liquid due to the oxidation of Al and the formation of AlCl3 (Equation (1)).

3+ 3Cl− Al 3e− + Al AlCl (1) → → 3 We observed the presence of trimethylamine, ethanol, ethylene glycol, and other products, with the incidence of trimethylamine being accounted for by Hoﬀman elimination of the choline base as choline hydroxide (Equation (2)):

(CH ) NCH CH OH + −OH N(CH ) + H O + HOCH = CH O = CH CH (2) 3 3 2 2 → 3 3 2 2 2 ↔ 2 3 The reduction reaction at the cathode (counter electrode) involves the decomposition of choline by formation of a choline radical via the acceptance of an electron:

+e (CH ) NCH CH OH − [(CH ) NCH CH OH] N(CH ) + CH CH OH (3) 3 3 2 2 → 3 3 2 2 → 3 3 2 2 Molecules 2020, 25, 5712 6 of 8

Thus, the transient choline radical, depicted in parentheses (Equation (3)), results from the addition of an electron from the anode at the cathode, and quickly decomposes into trimethylamine [2]. A review of the literature also indicates that the residual pitting on the surface of the metal not only affects surface reflectivity, but is also likely to affect conductivity due to increased surface area [12,17]. The plentiful abrasions and bumps generated during the electropolishing procedure (clearly visible in the AFM imagery) are a result of bubbles that formed on the metal’s surface during electropolishing. With significant proportions of the metal surface being deteriorated per second, the bubbling that occurs at the cathode can often leave these marks as they pop on the metal’s surface, sometimes marring the newly treated surface (Figure5B) [12,18]. This exchange is heavily recorded in the electropolishing literature, and hydrogen evolution at the cathode (counter electrode) is a potential driver for this type of pitting. The bubbles tended to form at points in the range when low current densities occurred in the electropolishing procedure. This tended to happen towards the end of the trial, when most of the originally protruding surfaces to be polished had been removed [19]. When this occurred, the associated chronoamperogram reported a slow and steady decline, as the remaining surface area available to be electropolished slowly decreased at the rate recorded in Table1. Pitting tended to occur at low current densities, or when the current applied through the 2 cm2 exposed surface of the metal was flowing over a decreasing surface area as the sample was being polished. This pitting is relatively inevitable, as the objective of electropolishing is to always have less surface area than prior to treatment [17]. We expect that this decreased surface area will result in an overall smoother surface. For this to occur, however, the protruding peaks must electropolish at a faster rate than the average surface upon which the roughness average calculation is based, while polishing significantly faster than any recesses in the working electrode surface. Provided that all of these interactions occur appropriately at the metal’s surface and at the proper prescribed rates, an efficient electropolishing treatment can be achieved [19].

4. Materials and Methods Electrochemical tests made use of an industry standard 1 M phosphoric acid mixture for acid treatments. Acid-free treatments were carried out using an ionic liquid composed of choline chloride (Acros Organics 99%, Pittsburgh, PA, USA) and ethylene glycol (Sigma-Aldrich 99.8%, St. Louis, MO, USA); both chemicals were used as received. The ionic liquid mixture was created by stirring the two components together at a component ratio of 2 ethylene glycol: 1 choline chloride at 70 ◦C until a homogeneous colorless liquid emerged. This ionic liquid’s effects on electropolishing the metal surfaces of interest were analyzed using the necessary machines. Voltammetry and chronoamperometry tests were conducted using a Gamry PCI4-G750 potentiostat (Gamry Instruments, Warminster, PA, USA) and controlled using the accompanying framework and e-chem Analyst (v. 5.5) software (Gamry Instruments, Warminster, PA, USA). The electropolishing procedure made use of a platinum cathode (counter electrode) plate with a silver wire reference for the experimental setup. Both electrodes were degreased using deionized water and acetone to preserve the purity of samples during testing. The working electrode was abraded with 150 grit glass paper, rinsed, and dried prior to each recorded measurement to ensure reproducible voltammetric effects. Electrochemical measurements were performed at 70 ◦C with a 1 constant scan rate of 20 mVs− used in voltammetric experiments. Surface analysis was carried out using a Dimension 3100 Nanoscope IV Scanning Probe Microscope, by Bruker Scientific Instruments (Billerica, MA, USA) manufactured by Digital Instruments with software version 6.12 in tapping mode. Step height measurements were recorded in µm via the Alpha Step 200 by Tencor Instruments (Milpitas, CA, USA). A KH-1300 HIROX digital microscope (Tokyo, Japan) was utilized for optical comparison to produce representative images scaled to 1600 1200 pixels. After completion of each experiment, × weight was recorded for calculations of mass loss due to electrochemical etching. Molecules 2020, 25, 5712 7 of 8

Samples of high-purity aluminum (99.98%) were bored from supplied sheets (3 mm thickness) and labeled for use. In each test, samples of each respective metal were taped with polyimide ﬁlm tape to restrict electrochemical activity to polish a 1 cm2 region on both the front and back faces of each metal sample. This resulted in a 2 cm2 region of polishing for each sample when calculating current density. Metal samples were then immersed in the electropolishing solution of choice, such that the regions for polishing were completely immersed. Aluminum samples were submitted to a cursory linear sweep voltammetry test to determine optimum voltage conditions to set for running chronoamperometry over a 900 s polishing sequence. Aluminum samples were tested with both the 1 M phosphoric acid and acid-free electropolishing solutions, upon which resulting data were managed via spreadsheet.

5. Conclusions It was determined that the phosphoric acid electrolyte mixture etched at a faster rate than the ionic liquid electropolishing treatments. This distinction is likely to be the reason that the global industry has made it and other acid solutions the standard for electropolishing. The disadvantage of this fast rate of polishing is that this acid-based method of electropolishing facilitates extensive hydrogen evolution at the working metal cathode (counter electrode), causing pitting at low current densities, or bubbling that ultimately mars the treated surfaces of acid-polished samples. The occurrence of overpotentials causes the roughness of samples to yield more favorable results in terms of average roughness with the utilization of the deep eutectic medium composed of ethylene glycol and choline chloride as compared to the acid treatment components. Regarding replacement of the industry standard 1 M phosphoric acid mixture, it is likely that the ionic liquid mixture could replace phosphoric acid as an efficient electrolyte for polishing, on the grounds that smoother surfaces are generated. The ionic liquid mixture provides the added benefits of recyclability without the loss of electropolishing efficiency to the nonhazardous components of which the chemical is composed. Thus, it represents an ecologically friendly supplement for the electropolishing of aluminum metals.

Author Contributions: T.M.A.-F. suggested the original idea, facilitated the resources, advised and guided the study, and edit the paper; J.D.L. conducted the research, documented the results, and wrote the paper draft. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors acknowledge appreciation for T.M.A.-F.’s Lawrence J. Sacks Professorship in Chemistry. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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