A method to obtain zinc / nickel alloy coating by applying direct current for high corrosion resistance in water-free solutions

Abstract

The invention relates to a method of obtaining zinc-nickel alloys in relatively green ionic liquids, which can form a specific phase that can be an alternative to cadmium plating used in the aerospace industry, which in the present art is carried out in toxic and carcinogenic solutions.

Description

[0001] A METHOD TO OBTAIN ZINC / NICKEL ALLOY COATING BY APPLYING DIRECT CURRENT FOR HIGH CORROSION RESISTANCE IN WATER-FREE SOLUTIONS

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for obtaining zinc-nickel alloys in relatively green ionic liquids, which can form a specific phase that can be an alternative to cadmium plating used in the aerospace industry in toxic and carcinogenic solutions.

[0004] BACKGROUND

[0005] Electrochemical plating is less costly in terms of application and cost than almost all currently practiced methods, and in its simplest form, it is a system in which an anode, a cathode and a power source are used as the fundamental components.

[0006] Metals such as cadmium and chromium are known as protective metals in the metal industry and can be coated by electrodeposition on the metal type that forms the base metal skeleton (metal type can be mild or hard steel or metals such as tin, bronze). As a matter of fact, German automobile manufacturers have been coating their parts with chromium for many years and have achieved serious success in the market with the advertisement of 10-year stainless guarantee. Similarly, aerospace parts manufacturers coated their parts with cadmium to produce mechanical parts that could achieve corrosion resistance for years.

[0007] Currently, different alloy ratios for electrochemical zinc-nickel plating (not the zinc system which has 14-18% nickel in the alloy mentioned here) have been obtained by electrochemical plating in various aqueous chemical baths in toxic, caustic, or intense acid environments. Therefore, it is very important to develop such systems without severely acidic and / or non-carcinogenic formulations and to support them with new research.

[0008] Specifically, a significant portion of the work on zinc systems containing between 14-18% nickel is based on the results of Baldwin's research based on DRA (UK Defence Research Agency) data on the corrosion resistance of cadmium and Zn- Ni plating. Baldwin confirmed for the literature that Zn-14-18 wt.% Ni coatings work as well as cadmium by performing these coatings in concentrated acidic solutions. Therefore, ongoing work is focused on obtaining zinc electrochemical coatings containing 14-18 wt.% Ni in the gamma phase. Most of the Zn-Ni coating systems have been studied in various electrochemical baths based on aqueous solutions. Although gamma phase has been reported to be seen in zinc systems containing 25% nickel phase in the literature, it has been reported that the Ni2Znn gamma phase containing 14-18% nickel gives the best corrosion resistance results in corrosion resistance studies after experiments carried out in acidic solutions.

[0009] The liquid medium used in this patent application is known as deep eutectic solvent (DES) in the literature. The literature defines the deep eutectic solvents (DESs) as a sort of ionic medium with a commonly expressed general formula; Cat+X'.zY, and they are classified as Type-I, Type-ll, Type-Ill and Type-IV.

[0010] When electrochemical coatings made in deep eutectic solvents were examined, it was reported that DES systems known as type-ill were mainly used in the electrodeposition of Zn, Ni and Zn-Ni. Type-Ill DES systems are prepared with a mixture of hydrogen bond acceptor and hydrogen bond donor such as choline chloride and ethylene glycol, respectively. In a recent study presented by Lei et al. a zinc-nickel alloy containing 15-19% Ni was reported. In the report, Lei et al. used a pulse current (the type of current that can be generated by sophisticated power sources that allow the direction and intensity of the current to be changed in milliseconds, allowing the direction and intensity of the current to be changed by sophisticated power sources) in an ionic liquid classified as type-ill DES consisting of a mixture of choline chloride and ethylene glycol with propylene glycol and boric acid additives. The cost of the system was high, and bath control and coating conditions are considerably difficult to adjust when compared to traditional direct current applications. The study achieved the closest results to the results put forward by DRA in the literature and patent searches, and baths were used in which system stabilization would be relatively difficult in ongoing coatings due to the difficulty of controlling additive chemicals such as propylene glycol and boric acid. Coating thickness was reported as 10 pm within 120 min. In addition, the coating obtained was obtained using the pulse current technique, which is very costly and difficult to apply on parts larger than laboratory scale.

[0011] Studies on nickel-zinc alloys that could replace cadmium metal electrochemical plating, which is used for its high corrosion resistance but has been banned in Europe, the USA and Japan due to its toxic salts, have been attracting interest for the last 20 years. In fact, such alternative coatings were investigated years ago by various defence industry executives and partners, for example, the DRA stated that equivalent or better corrosion resistance of cadmium surfaces could be systems containing 14- 18% nickel by weight, with nickel nuclei uniformly distributed in the zinc phase. However, defence industry guiding institutions such as DRA, whose equivalent in our country is ASELSAN and ROKETSAN, want this output result to be made in systems that will be more applicable and internationally accepted routes. For example, DRA has obtained the zinc system containing 14-18% nickel, which will be an alternative to cadmium, in two methods; the first of which is electrochemical baths containing concentrated sulfuric acid, and the gases produced by the reaction of sulfuric acid with both vapor pressure and ambient metals in the coating bath result in toxic effects for the user(s) and other living things in nature. The second method is the physical vapor deposition (PVD) method, which is usually used to coat small samples with turbo pumps under vacuum, and its industrial applications require very serious cost and control of system optimization during production. For companies and / or organizations that will apply Zn / Ni alloy deposition commercially and / or industrially, it is very important to develop and improve the coating technique or coating conditions, applicability to avoid any bans on international regulations.

[0012] The biggest disadvantage of aqueous electrochemical baths, which are predominantly used in the literature, is that they cause the formation of hydrogen gas due to water electrolysis on the cathode substrate during electrochemical coating. As a result, an undesirable condition known as hydrogen embrittlement occurs in the final product. Many aqueous electrolyte systems contain complex multi-input component mixtures, and several additive chemicals are used to balance adhesion and surface finish. In addition to all these negative effects, aqueous systems evaporate rapidly due to high vapor pressures, and this results in bath stabilization being difficult to control in continuous production systems.

[0013] Although the goal is to produce a corrosion-resistant and functional product, the content of the electrochemical baths used is also of critical importance. Components of chemical baths are strictly monitored for commercial activities by REACH (Registration, Evaluation, Authorization and Restriction of Chemicals / European Union), OSHA (Occupational Safety and Health Administration / USA) and EPA (United States Environmental Protection Agency). This is to avoid the use of solutions that have proven worldwide to be highly toxic, such as in the case of aqueous electrochemical baths of cadmium and chromium. Research on alternative electrolyte baths is encouraged by international organizations due to the toxicity disadvantages of aqueous solutions prepared with multi-component inputs and evaporating very quickly even at room temperature and entering the atmosphere due to low vapor pressure. For example, it is very important that commercial electrochemical baths to be used industrially in ton scale in electrochemical coating are as non-toxic, nonflammable and do not exhibit high vapor pressure as possible. In the last two decades, relatively green ionic-based deep eutectic solutions (DESs) in electrochemical plating processes have begun to attract attention in the literature because they can provide an environment that can enable the coating of highly electronegative metals such as zinc. DES solutions included in the non-aqueous solvent groups; It has a very low vapor pressure, for example, DES prepared with glycerol and ethylene glycol at a temperature of 50±5 °C, which is considered industrially sustainable, offers a vapor pressure of 2 Pa on average, while water offers an average vapor pressure of 2x104Pa under the same conditions. This difference is on average 103times compared to water. It means negligible evaporation. In addition, DESs offer features such as nonflammability, easy preparation, high conductivity, low cost and not decomposing at high electrical potentials, and features that can be considered relatively green chemistry when compared to the environmental effects of many metal-plated bath chemicals in the literature.

[0014] AIM OF THE INVENTION

[0015] The aim of the invention is to obtain the special Ni2Znn phase containing nickel uniformly dispersed in a zinc phase of 14-18 wt.% on copper with a simple direct current power supply instead of sophisticated and costly systems such as a pulse power supplies.

[0016] Another aim of the invention is to use water-free ionic-based electrochemical baths containing safe ingredients, such as chicken feed choline chloride and ethylene glycol, classified as green, and results in avoiding being subjected to international restrictions.

[0017] Another aim of the invention is to obtain a coating with an average thickness of 28 pm within a coating time of 180 minutes with high coating efficiency.

[0018] Another aim of the invention is to obtain five times harder coatings with Ni2Znn phase on average when compared to currently applied cadmium coating, to minimize the negative effects of the soft nature of cadmium. Another aim of the invention is to obtain high corrosion resistant the Ni2Znn phase coating when compared to currently applied cadmium, and prove its resistance with internationally applied corrosion measurement techniques, such as TAFEL technique (based on potentiodynamic scan), or electrochemical impedance spectroscopy (based on frequency scan at fixed potential).

[0019] LIST OF FIGURES

[0020] Figure 1. Potentiodynamic scan of Pt working electrode in DES solution prepared with a mixture of ZnCl2:EG:ChCI in a molar ratio of 1 :4:0.3 at 20 mVs-1using a 20cm2Pt counter electrode and a pseudo silver wire reference electrode at 55±5 °C.

[0021] Figure 2. Potentiodynamic scan of Pt working electrode in 0.06M NiCh dissolved 1ZnCh: 4EG: 0.3ChCI DES solution at 20 mVs-1using a 20cm2Pt counter electrode and a pseudo silver wire reference electrode at 55±5 °C.

[0022] Figure 3a. Zinc coating on Pt electrode for 120 seconds in ZnCl2:EG:ChCI prepared with molar ratio of 1 :4:0.3.

[0023] Figure 3b. Zinc and nickel plating on Pt electrode for 120 seconds after adding 0.06M NiCh to the DES system containing ZnCl2:EG:ChCI in a molar ratio of 1 :4:0.3.

[0024] Figure 4. High resolution morphological image of the surface.

[0025] Figure 5. Thickness profile of the surface measured with 10 nm precision.

[0026] Figure 6. SEM-EDS analysis of the direct current (DC) Zn / Ni coated shown in Figure 5.

[0027] Figure 7. XRD analysis for the DC coated sample in which it's SEM-EDX analysis shown in Figure 6.

[0028] Figure 8. Following the morphology and weight percentage analysis of Zn / Ni deposited sample via SEM-EDS, and phase verification via XRD, colombic yield calculation during plating was measured.

[0029] Figure 9. Nyquist plots via EIS corrosion tests for both industry provided cadmium coated sample and the developed Ni2Znn coated sample are provided.

[0030] Figure 10a. Electrical equivalent circuit (EEC) model applied for Ni2Znn / Nyquist plot.

[0031] Figure 10b. EEC model applied for Cd / Nyquist plot.

[0032] Figure 11a. High resolution optical microscope image of Ni2Znn coated sample before corrosion. Figure 11 b. High resolution optical microscope image of Ni2Znn coated sample after corrosion.

[0033] Figure 11c. High resolution optical microscope image of Cd coated sample before corrosion.

[0034] Figure 11 d. High resolution optical microscope image after Cd coated sample corrosion.

[0035] Figure 12. The hardness of the copper substrate material, Cd coated sample was measured and found to be similar to the literature and given in the box of the graph. The hardness of the developed Ni2Znn phase was measured with 21 measurements in 3 repetitions at 7 sites.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] Within the scope of the invention the coating solution in which the Ni2Znn phase is electrochemically deposited on the copper surface, which does not contain water molecules, was obtained transparently with a mixture of 1 molar zinc chloride (ZnCI2) and 4 molar ethylene glycol (EG) and 0.3 molar choline chloride (ChCI). This deep eutectic solvent (DES) is called transitional type DES, and can be classified between type-ill and type-IV, which has not been declared before in the literature. The aforementioned DES solution was obtained transparently by stirring on a magnetic heater stirrer at a temperature of 55 °C for 2 hours at a speed of 500 rpm after the addition of the necessary chemicals whose molar ratios are given above paragraph. The electrochemical applicability of the prepared DES solution was tested with the potentiodynamic scan against a reference electrode (potentiostat device was implemented) as shown in Figure 1 , and it was determined that the Zn2+cations in the solution were reduced to the Zn° solid metal state at a value of --0.92 V and oxidised from the metallic Zn° state to the Zn2+ionic state at a value of --0.55 V.

[0038] Following the determination of suitability of the produced Zn2+ containing DES system for Zn plating, 0.06M NiCI2 was dissolved in it to form the nickel source and again analysed by cyclic voltammetry as shown in Figure 2. In the analysis, it was observed that no separate reduction peak was observed for nickel in the system containing Ni+2 cation (this is the first sign that simultaneous nickel zinc plating was achieved), but it was also observed that zinc reduction onset peak value sifted to ~- 0.84 V for Zn2+ reduction. In the reduction region, the simultaneous transition of Ni2+ cations to NiO metal form with zinc was observed, and this biphasic deposition was more clearly supported when the oxidation region was examined. When the oxidation region of the cyclic voltammetry graph of the nickel-plated system presented in Figure 2 is examined, a large peak at --0.38 V (this peak was measured as --0.55 V in the system given in Figure 1 , which contains only Zn2+) and a small oxidation peak at ~- 0.15 V were emerged. These peaks are characterized as the transition from ZnO metal phase to Zn2+ ionic state and from NiO metal phase to Ni2+ ionic state, respectively. Following the 120 seconds electrodeposition of the Pt working electrode in these two DES systems (one contains only Zn2+, and the other one contains Zn2+ and Ni2+), coated surfaces were examined with a high-resolution optical confocal microscope as shown in Figure 3a and Figure 3b and the morphological difference is a clear indicator of simultaneous Zn / Ni deposition. While the coating grain morphology was smaller and more dispersed in the coating made in the main DES system containing only zinc salt, the grain size of the coating made in the system containing 0.06M Ni2+increased, and the grain orientations formed uniform trend resulting in a shiny surface.

[0039] In conclusion, cyclic voltammetry and chronoamperometry are the fundamental analyses for checking the feasibility of an electrochemical system and the simultaneous plating of nickel and zinc have been confirmed. Following the fundamental verification onto 1 .5 mm diameter Pt working electrode surface in cyclic voltammetry, the bulk plating process was initiated. In the bulk plating process, 1 mm thick 99.9% pure copper plates with a diameter of 5 cm x 2.5 cm were cut to prepare the base metals to be coated. Then, each copper base metal was pre-cleaned / polished with 3000 grit sandpaper and then cleaned with alcohol and the bulk electrochemical plating process was carried out in a mixture of ZnCl2:EG:ChCI:NiCl2 in a molar ratio of 1 :4:0.3:0.06 in the two-electrode system as shown in Figure 6.

[0040] In the inventive method, as previously described, the bulk electrochemical process is carried out using a direct current (DC) power supply. In the electrochemical coating system, an inert iridium / ruthenium titanium electrode was used as the counter electrode. The inertness of the counter electrode is important to prevent the formation of resistance in the solution, in other words, not to decrease the conductivity. The electrochemical plating process is carried out on heater and stirrer at a current density of 1 mA.cm-2 current density value to 7 mA.cm-2 current density value; it was increased by increasing 2 mA every 2 minutes in a total of 6 minutes and this value was kept constant during 180 minutes of electrodeposition at 50±5 oC and 350 rpm stirring speed (which means 350 turns in one minute). The importance of the gradual increase in current is very important for stabilizing the cation depletion rate. If the plating is started directly at 7 mA.cm-2, nickel (reduction half-cell potential -0.26V), which is known in the literature to be reduced at earlier potentials in solution, will start to be reduced much faster than zinc (reduction half-cell potential -0.76V), making it impossible to obtain the desired 14-18% nickel content in the final coating.

[0041] The high-resolution morphological image of the surface after the bulk coating is shown in Figure 4 and the thickness profile of the surface measured with 10 nm precision is shown in Figure 5. When it is considered that a minimum thickness of 10 pm is generally required for corrosion protection, the obtained thickness of approximately 30 pm will be sufficient for corrosion protection purposes. In addition, when the morphological image of the surface investigated under high magnification microscope (Figure 4), no randomly formed holes on the surface was encountered as they are commonly known in aqueous solutions due to hydrogen reduction resulting in H2 gas release and makes some spots visible on the substrate.

[0042] Although the coating with the desired thickness as hole-free was obtained, SEM-EDX analysis as shown in Figure 6 was performed to check whether obtained coating was containing 14-25% nickel as it is the first requirement to be within the gamma phase. SEM-EDX analysis showed that the obtained phase contains 16.7% nickel and confirmed that the coating consists of nano-sized nuclei. Following the desired rate of Ni conformation in the alloy, it is important to check whether the exact phase formed with nickel zinc mixture is Ni2Znn, and thus; XRD analysis was performed, and the result is presented in Figure 7. When the XRD analysis was scanned through the library of the software, it was emerged that the entire coating phase is Ni2Znn with different crystal orientation as desired. Also, the reference intensity ratio (RIR) was analysed 97.6%.

[0043] Up to this stage of our inventive method, it has been verified that the ratio of nickel evenly / uniformly dispersed in zinc is within the desired limits and moreover, it has been observed that the obtained phase is gamma-Ni2Znn . As mentioned in the current state of the art, plating efficiency is very important in electrochemical systems. While a 20 pm thick coating can be obtained in 2-3 hours in a system operating with an efficiency above 80%, it is known that the same system efficiency decreases to 50% due to various reasons (for example, the molecules forming the electrolyte bath adhere to the substrate material to be coated and create a partial blocking effect on the material) and in these cases, even 2 hours can not be sufficient to obtain a 10 pm thick coating. In order to determine whether the applied situation is this, an internationally accepted analysis technique; electrochemical microbalance quartz crystal analysis (eQCM) for efficiency analysis was performed and the mass weight of the Ni2Znn alloy amount deposited during the coating was measured, and this mass amount was compared with consumed current in Faraday relation to calculate the efficiency. Since the applied technique, eQCM, has hundreds of pages of theoretical background and has already been universally commercialized, its details are not described in our application for procedural economy. Chronoamperometric measurement with mass of the coated Ni2Znn phase is shown in Figure 8. Within 300 seconds, the deposited mass was measured as 22.59 pg on the quartz crystal, and the consumed current was measured as -74.27 mC as shown in Figure 8 by current time integration.

[0044] As confirmed previously, obtained Zn / Ni alloy is containing 16.3% Ni as confirmed in the EDX analysis. The average molecular weight for Zn containing 16.3% Ni can easily be calculated as 64.38 g.mol-1. The amount of metal / alloy to be coated for the spent electrical charge of -74.27 mC can be calculated by Faraday's law with the equation presented below.

[0045] The expected theoretical amount of Zn / Ni mass for the consumed charge of - 74.27 mC can be calculated as 24.78 pg, but in practise it was measured as 22.59 pg as shown in Figure 8, in other words, the efficiency of the system is 91.1 % (22.59x100 / 24.78).

[0046] Before starting the corrosion resistance tests, it is expected that the obtained gamma phase Ni2Znn should provide better corrosion resistance than cadmium plating based on DRA data. The corrosion resistance was carried out by electrochemical impedance spectroscopy (EIS), an internationally recognised technique for the final verification of the invention. In a 100 ml beaker, 3.5 wt.% was prepared to simulate the corrosion media. EIS analysis was carried out by applying an excitation potential of 10 mV between 10 mHz and 100 kHz to the sample placed in the solution and operates as working electrode. The applied frequency range and the excitation potential are also commonly applied in the literature for corrosion testing. The surface area of the sample was covered with an insulating tape, leaving 0.092 cm2open as electroactive area. A Pt counter electrode with 25 cm2surface area, and Ag / AgCI reference electrode were used to complete the set-up. The experiments were carried out with Gamry 1010e potentiostat in a Gamry brand Faraday cage to prevent deviations at low frequencies. Nyquist plots obtained because of the EIS corrosion test for tested samples are presented in Figure 9. The Nyquist data obtained were fitted to the appropriate electrical equivalent circuit (EEC) models shown in Figure 10a and Figure 10b to obtain the corrosion elements. The deviation observed because of the fitting process of the electrical circuit models was determined as 1.78x1 O'3for Ni2Znn coated sample and 2.11x10'3for Cd coated sample. By fitting the electrical model to the Nyquist data, the corrosion parameters data shown in Table 1 were obtained.

[0047] Table 1. Corrosion elements for Cd coated and Cd and Ni2Znn coated samples were obtained after fitting the the Nyquist data with decided EEC models shown in Figure 10a and Figure 10b.

[0048] If a general evaluation is made for the data obtained for the EIS corrosion test of the Ni2Znn coated sample in corrosion media; the impedance phase angle measured at the intermediate frequency is below -90°, which indicates the stability of the pseudo capacitive structure of the passive film. The presence of the second time constant, which is also seen in the Nyquist plots discussed in the Bode plots, can be interpreted as an indication that the Ni and Zn metals in the alloy composition form a simultaneous independent passivation process. As it is commonly known, the first semicircle or capacitive arc in the Nyquist graph is related to the combined effect of metal dissolution and double-layer capacitance, which is directly related to the polarisation resistance of the passive film formation on the alloys. The polarisation resistance, Rp, reflects the corrosion resistance of the surface passive film and its value can be determined from Nyquist plots. However, the achievement of a relatively perfect semicircle is not typically observed for the most corrosion studies of alloys, which are often flattened, laterally pressurised and may continue with another straight line, induction line, second capacitive arc or semicircle. For these reasons, the direct use of the semicircle or arc diameter alone for polarisation resistance evaluation can lead to inaccuracies and to eliminate this situation, the |Z| value at a fixed frequency of 0.1 Hz is taken from Nyquist graphs and this value is generally accepted as polarisation resistance. When the Nyquist graph data obtained for Ni2Znn, which was corrosion analysed by EIS analysis, is examined, the fitted resistance at 0.1 Hz is 5151Q and the polarisation resistance occurring in the electro active area (0.092 cm2) of the Ni2Znn sample is calculated as 473.89 Q.cm2(5151Q x 0.092 cm2). In general, the frequency distribution of the corrosion phenomenon is analysed by the stationary phase element QCPE for the EEC evaluation.

[0049] Within the scope of the invention, Qdiff is used instead of Warburg impedance to express the mass transfer process as used in many corrosion study analyses, and Qdi is taken as the basis for the indication of surface heterogeneity and compactness level of the film. If the surface heterogeneity increases, the compactness of the film is expected to decrease and this leads to a much easier transfer of the diffusion of corroded metals towards the solution, in other words, the corrosion rate increases. The values of Qdiff and Qdi for the corroding electroactive area were found to be 3.17x10'5Q'1.cm'2.s and 6.17x10-7Q’1.cm’2.s, respectively, as a result of fitting the Nyquist of Ni2Znn coated copper substrate with EEC circuit presented in Figure 10a and Figure 10b. In corrosion systems, the ndi value is generally in the range of 0.5 to 1 and is calculated by the slope obtained from the log (f) versus log |Z| plot on the Bode plot. The phase angle varies between 0° and 90° and in summary, at 0° ndi value 0, that is, the material exhibiting excellent resistance behaviour, and at 90° ndi value 1 , which provides excellent capacitor behaviour. In the analysis of the Ni2Znn coated sample on copper, the ndi value was found to be 0.74, and the general meaning of this value for the cases where the phase angle is between 60°-70° means that the electroactive surface creates capacitance with the applied potential difference, and behaves in a manner similar to the behaviour of a capacitor with leakage in its structure rather than a capacitor behaviour. The electrical charge transfer resistance, Ret, is related to the electrochemical reaction rate occurring at the interface of the passive film and the electrolyte with a series of corrosion-activated zones, in other words, it is also called the passive film dissolution rate. When the Nyquist plot presented in Figure 9 obtained from the EIS analysis of the Ni2Znn coated sample is examined, it is seen that the first flattened semicircle is followed by a half capacitive arc. The resulting second capacitive arc is generally associated with the resistance of the ionic pathways, Rip, through the passive film formed by oxidation and the subsequent adsorption of the released alloy species on the electroactive surface. Accordingly, the first capacitive semicircle, nickel dissolves in 3.5 wt.% NaCI corrosion media by adsorbing to the medium before zinc metal (zinc metal will require higher potential difference / energy than nickel because nickel tends to loose electrons easier than zinc, as mentioned in previous sections), and in this case, the second capacitive arc state can be interpreted as the result of the formation of an ionic pathway from the dissolution of adsorbed nickel. In the analysis of the Ni2Znn coated sample, the Rpvalue was found to be 265.60 Q.cm2. Similar values were found for Cd coated sample on copper and presented in Table 1 .

[0050] When the obtained EIS corrosion data is summarised, the Qdi value of the Ni2Znn coated sample was found to be relatively lower than the Qdi values of the Cd coated sample, and this indicates that the film formed on the Ni2Znn coated sample during corrosion is more compact, that is, protective. In addition, the Ripvalue, which is the ionic transition path resistance, was found to be quite high for Ni2Znn, and this is probably due to the passive film formed on the surface by the nickel species that corrode first and indicates that why Ni2Znn coated sample provides better protection than the Cd coated sample. High resolution optical microscope images of Ni2Znn and Cd coated samples taken from the surface before and after measurement are presented in Figure 11a, Figure 11 b, Figure 11c and Figure 11d. When these figures are examined, it is seen that cadmium is completely separated from the surface, revealing the copper surface, and even pitting corrosion is observed, appearing as black spots on the copper surface. The EIS result and post EIS morphological images of the Cd coated sample support each other. As a matter of fact, the Nyquist graph obtained as a result of the EIS of the Cd coated sample in Figure 9 shows that the sample has formed a high conductivity bridge as a result of going under rapid corrosion resulting with an inductive current curl instead of creating a second capacitive arc as seen in the Nyquist of Ni2Znn. In the Ni2Znn sample, it is seen that the surface remains as it is, but cracks occur on the surface without coating loss. The main reason for the cracks is the stress difference between the base metal and the coated alloy, and the outputs obtained again overlap with the Ni2Znn Nyquist graph, the sample has prevented self-loss by showing a serious resistance with the passive film it forms and no serious damage has occurred as seen in the Cd sample.

[0051] The attractiveness of a coating for industrial applications depends not only on the used chemicals of the plating bath, the implemented method, and the corrosion resistance of the final sample but also the hardness because hardness is one of the significant parameters. Figure 12 exhibits hardness data. The hardness of Cd metal is reported between 40-55 Hv in the literature, and it was measured as 50.4 Hv. Copper hardness is known to be between 70-85 Hv in the literature, and it was measured as 80.2 Hv in the measurements. The hardness of the obtained Ni2Znn coatings was measured as 250 Hv on average and it was revealed that the obtained coatings offer an average of 5 times more hardness compared to cadmium coating.

[0052] As a result, the invention has addressed the problem of partial or complete banning of Cd coatings or Ni2Zni 1 coatings due to the toxicity which they are previously obtained in acidic solutions by defence industry institutions or commercial suppliers. Then, the invention introduced an innovative, relatively green, high efficient, low cost gamma phase Ni2Znn coating recipe with a very easy implementing method, known as DC coating. It is also an advantage of the invention that the produced gamma phase Ni2Znn coatings revealed much better corrosion resistance and higher hardness than the cadmium plating, which is in the plan of termination of the EU and needs alternative environmental replacements. The fact that the chemical bath in which the invention is realised consists entirely of safe chemicals that are allowed to be traded in tonnage quantities in international circulation will also meet the Sustainable Environment Norms of the European Union. It would not be wrong to emphasise that the applied invention is a candidate to be put into practice for pilot trials in the industry branches that make cadmium and chromium equivalent coatings, especially in the defence industry.

Claims

CLAIMS1. A method of obtaining high corrosion-resistant zinc / nickel alloy coating by applying direct current in water-free solutions, characterized by comprising the steps below;- Applying Ni2Znn phase electrochemically to the copper surface, and the main solution, which does not contain any water molecules, will be obtained transparently by mixing 1 molar zinc (Zn) chloride, 4 molar ethylene glycol and 0.3 molar choline chloride (ChCI),- Dissolving 0.06M NiCI2 into the produced DES solution to form the nickel source,- Initiating the bulk electrochemical coating process of copper plates in DES solution.

2. The mixing process according to Claim 1 characterized by being applied on a magnetic heater stirrer at a temperature of 55oC and a speed of 500 rpm for 2 hours.

3. The electrochemical coating process according to Claim 1 characterized by using a direct current (DC) power source and a titanium electrode coated with inert iridium / ruthenium as the counter electrode.

4. The electrochemical coating process according to Claim 1 or 3 characterized by being applied from 1 mA. cm-2current density value to 7 mA.cnr2current density value by increasing 2 mA every 2 minutes within a total of 6 minutes and keeping this value constant and applying it for 180 minutes at a temperature of 50±5 °C and a stirring speed of 350 rpm to prevent the Ni2+and Zn2+ions ready to be reduced in the solution from being reduced onto the cathode metal to be coated and suddenly ending / depleted and not switching from diffusion transfer to mass transfer.

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