Acknowledgement

The project that has led to the present patent application has been carried out in Italy with the collaboration of Department of Environmental Science and Policy - Universita degli Studi di Milano.

Field of the invention

The present invention relates to a process for producing a zinc-plated steel substrate. The process is a valuable alternative to the processes know in the art for recycling zinc from zinc-containing metallurgical wastes, such as steelmaking dust produced by the electric Arc Furnaces (EAF) used in the production of secondary (i.e. recycled) steel, and for producing galvanized steel.

Background of the invention

Zinc is the fourth widely consumed metal in the world, largely employed in the production of galvanized steel. The strong link between zinc and steel continues also at their end-life cycles, in steel mills employing Electric Arc Furnace (EAF) for the production of secondary steel. Several megatons per year of EAF dusts (the waste of this process), being rich in zinc content, represent a valuable source for secondary zinc produced by both pyrometallurgical and hydrometallurgical processes.

Besides EAF dust, various other secondaries and wastes containing zinc are generated in metallurgical industries such as galvanizing, casting, smelting, scrap recycling, etc.. These are mainly zinc ash, zinc dross, flue dusts of smelting operation, automobile scrap, sludges etc. In these secondary sources, zinc is present in the form of metal, oxides and/or alloy and associated with different level of impurities depending on their source.

In order to recover zinc from these secondary sources and wastes both pyrometallurgical and hydrometallurgical processes have been developed. A major drawback of the pyrometallurgical methods is high energy requirement and need of dust collecting/gas cleaning system. The presence of chloride and fluoride salts in the dust causes severe corrosion problems and necessitates use of expensive alloys as materials of construction. The hydrometallurgical processes are more environmentally suitable and economical to treat zinc- containing materials also on a small scale.

One of the currently used hydrometallurgical technologies for the recovery of zinc from these secondary sources and wastes, particularly EAF dust, is the EZINEX® process [9], which involves ammonium salts leaching of dust, cementation-based purification of the leachate solution and zinc separation from the leachate through electrolysis.

In the EZINEX® process, as well as in other Zn- recovery process used in the art, zinc is finally obtained in its metal state with high purity. The recovered metal zinc is subsequently reused in manufacturing processes, for example as zinc source for producing galvanized steel by hot-dip galvanizing.

Summary of the invention

In view of the above state of the art, the Applicant has faced the technical problem of providing an alternative process to recycle zinc contained in secondary sources to produce galvanized steel. Within the present invention, it has been found that it is possible to advantageously recycle Zn-containing wastes by extracting the zinc contained therein by leaching it with an aqueous leaching solution and then applying a zinc coating on an iron-based substrate, e.g. steel, by plasma electrolysis using the leachate solution containing Zn ²⁺ ions, thus avoiding the recovering of zinc in the metal state and its subsequent smelting to produce galvanized steel as commonly necessitated by the hot-dip galvanizing processes of the art.

According to a first aspect, the present invention relates to a process for producing a zinc-plated steel substrate comprising the following steps: a. providing a plasma electrolytic apparatus comprising a liquid electrolyte, at least one steel substrate to be plated acting as a cathode and at least one anode; the cathode being immersed in the liquid electrolyte; the liquid electrolyte containing Zn ²⁺ ions and being obtained by leaching a Zn-containing material with a leaching solution; b. applying an electrical potential to the steel substrate to initiate a plasma electrolytic process at a surface of the steel substrate in contact with the electrolyte to obtain the zinc-plated steel substrate.

According to a second aspect, the present invention relates to a process for producing a zinc-plated steel substrate comprising the following steps: a. leaching a Zn-containing material with a leaching solution to obtain a leachate containing Zn ²⁺ ions; b. providing a plasma electrolytic apparatus comprising: a liquid electrolyte comprising the leachate containing Zn ²⁺ ions obtained in step a; at least one steel substrate to be plated acting as a cathode, the cathode being immersed in the liquid electrolyte; at least one anode; c. applying an electrical potential to the steel substrate to initiate a plasma electrolytic process at a surface of the steel substrate in contact with the electrolyte to obtain the zinc-plated steel substrate.

In the process, the liquid electrolyte containing Zn ²⁺ ions, i.e. the leachate, may contain Zn ²⁺ ions in different forms. For example, the Zn ²⁺ species can be present as such, in hydrated form or in complexed form, depending on the exact composition and pH conditions of the leaching solution used.

Preferably, in the process the leaching solution is an aqueous leaching solution containing chloride ions Cl- and ammonium ions NH ₄ ⁺ .

Preferably, the pH of this leaching solution containing chloride ions Cl- and ammonium ions NH ₄ ⁺ is equal to or higher than 5, preferably within the range of from 7 to 14.5. In one embodiment the pH is within the range of from 9 to 12.

Preferably, chloride ions Cl- and ammonium ions NH ₄ ⁺ are introduced in the aqueous leaching solution as NH ₄ CI. Preferably, the concentration of NH ₄ CI in the leaching solution is within the range of from 0.1 M to 10 M, more preferably within the range of from 0.5 M to 5.0 M.

The pH of a NH ₄ CI aqueous leaching solution can be adjusted, for example, by addition of NaOH to the solution.

According to another embodiment, the leaching solution is an aqueous leaching solution containing sulphate ions.

Preferably, the aqueous leaching solution containing sulphate ions is a sulphuric acid solution.

The leaching step, either using NH ₄ CI or sulfuric acid as lixiviant, can be carried out according to the methods known to the person skilled in the art.

According to an embodiment, the leaching step is carried out as the leaching step of the EZINEX® process as described for example in [9], that is the leachate is obtained by subjecting Zn-containing waste material, such as metallurgical slag and dust, to a process comprising ammonium salts leaching of the Zn-containing waste material and a step of purification of the so obtained leachate solution from impurities (e.g. ions of the following elements: Fe, Ag, Cu, Pb and Cd) through cementation with metallic Zn as precipitating agent.

Preferably, in the process the steel substrate to be plated is pre-treated by a plasma electrolytic process using a pre-treating aqueous solution comprising at least one salt selected from alkaline salt, alkaline- earth salt, ammonium halide salt and mixture thereof, in the substantial absence of metal ions having a redox potential such that they would be plated on the steel substrate during the plasma electrolysis.

Preferably, the above at least one salt is selected from sodium carbonate, sodium bicarbonate and ammonium chloride.

Preferably, the total concentration of the above at least one salt is within the range of from 0.1 M to 10 M, more preferably from 0.5 M to 5.0 M.

In one embodiment, in the process the electrical potential applied in step b is a pulsed on-off electrical potential, preferably with a pulse frequency within the range of from 1 to 10 kHz.

Preferably, in step b the applied electrical potential is within the range of from 50 V to 250 V, preferably within the range of from 70 V to 200 V.

Preferably, in the process the plasma electrolytic pre-treatment is carried out with an applied electrical potential within the range of from 50 V to 250 V, preferably within the range of from 70 V to 200 V.

Preferably, the plasma electrolysis process of the zinc deposition step and/or the pre-treatment step are carried out maintaining the temperature of the liquid electrolyte at a value within the range of from 20°C to 80°C, more preferably from 25°C to 75°C. Preferably, both steps are carried out at ambient pressure.

Preferably, the anode is made of graphite or zinc.

Preferably, in the process the Zn-containing waste material comprises metallurgical slag and dust, preferably electric arc furnace dust from a secondary steelmaking process.

Preferably, in the process the steel substrate to be plated is selected from: rolled steel flat, steel wire rod, drawn and/or rolled steel wire.

According to a further aspect, the present invention relates to a process for producing a zinc- plated steel substrate comprising the following steps: i. melting a Zn-containing scrap steel into an electric arc furnace (EAF) obtaining a molten steel and Zn-containing EAF dust; ii. producing a solid steel substrate using the molten steel; iii. leaching the Zn-containing EAF dust with a leaching solution to obtain a leachate containing Zn ²⁺ ions; iv. providing a plasma electrolytic apparatus comprising: a liquid electrolyte comprising the leachate containing Zn ²⁺ ions obtained in step iii; at least one cathode to be plated comprising the steel substrate obtained in step ii, the cathode being immersed in the liquid electrolyte; at least one anode; v. applying an electrical potential to the steel substrate to initiate a plasma electrolytic process at a surface of the steel substrate in contact with the electrolyte to obtain the zinc-plated steel substrate.

Steps i. and ii. can be carried out according to the methods known in the art.

The steps iii. to v. can be carried out as illustrated in the present description.

The process according to the present description can be carried out using the methods and apparatus known to the person skilled in the art.

Short description of the figures

Figure 1 Polarization curves of

(- )ironINa ₂ CO _3(sol) Igraphite (+) electrolytic cells under direct current potential-control regime at different concentration of Na ₂ CO ₃ ; electrolyte temperature T = 343 K; each point corresponds to the average value of the steady state current density recorded at the specific potential;

Figure 2 Polarization curves of

(- )ironINa ₂ CO _3(sol) Igraphite (+) electrolytic cells under direct current potential-control regime at different temperatures; concentration of Na ₂ CO ₃ = 1.3 M; each point corresponds to the average value of the steady state current density recorded at the specific potential;

Figure 3 - Polarization curve of an iron electrode (geometric area 0.8 cm ² ) in water with 1.3 M NH ₄ CI and 1.3 M NH ₄ OH at 343 K. Each current density value is the average of the stationary currents recorded 5 sec after each potential step;

Figure 4 - Current (line b) flow through a series of three resistors (R _eq = 16.8 Ω) as a function of a square pulsed potential excitation (75 V, 4 kHz, 80% duty cycle; line a). All traces were recorded by an oscilloscope;

Figure 5 - Magnification of a time segment of the current and potential traces recorded during a CPE test for Zn deposition (entry 6 of Table 3). All traces were recorded by an oscilloscope;

Figure 6 SEM micrograph of the zinc deposit on the cathode (entry 6 of Table 3).

Detailed description

The present description addresses a hydrometallurgical process, based on the electrochemical technique called Cathodic Plasma Electrolysis (CPE), for the production of galvanized steel. The process can be advantageously applied to give a new life to the megatons of solid powdery wastes, rich in zinc, resultant from metallurgical slag and waste such as those produced by the Electric Arc Furnaces (EAF) used in the production of secondary (i.e. recycled) steel.

The zinc deposition process and the CPE performance as a pre-treatment stage for both cleaning and surface morphology modification of the metal substrate have been evaluated. Among operational parameters, electrolyte concentration and nature, temperature and applied potential resulted as the most affecting both formation and stability of plasma. A neat increase of the surface roughness of the metal substrate (useful to improve the adhesion of the zinc layer) can be obtained working in both sodium carbonate and ammonium-based solutions. Finally, zinc layer up to 15 μm in thickness can be deposited in few seconds carrying out CPE in Zn ²⁺ containing solutions by applying a pulsed potential stimulus.

Electrolytic Plasma Processes (EPPs) [1] share the same electrode configuration of traditional electrolysis processes but require application of potentials commonly exciding 100 V. The uncommonly high potential drives the ionization of species next to the working electrode resulting in a plasma glow discharge (at atmospheric pressure) within the gaseous sheath made of electrochemically generated products and/or vapor of the solvent.

EPP in cathodic regime (also known as cathodic plasma electrolysis, CPE) has been profitably exploited to clean, pre-treat and even deposit materials on electrodes. In fact, when suitable solutions are employed the high temperature hydrogen plasma combined with the abrasive action of the explosion of bubbles constituting the gaseous sheath give raise to a local surface melting able to eliminate mill scale on steel and/or to flash away organic contaminants like residues of lubricant, grease, etc. [2],[3]. Craters and circular wavelets formed on the surface of the substrate also represent a desirable profile for better anchoring the layer to be deposited [2]-[5]. Concerning deposition, CPE has already been profitably used to deposit coatings made of carbon, titanium, molybdenum, zinc, zinc- aluminum alloy on metal substrates [2]-[7]. CPE offers two main advantages over the traditional electroplating protocol [2]: i) faster deposition rate of the coating; ii) increased adhesion of the coating due to an enhancement of the metallurgical connection resulting from melting of both deposited coating and substrate surface (due to high local temperature of electrogenerated plasma).

As said above, zinc is the fourth widely consumed metal in the world, largely employed in the production of galvanized steel. The strong link between zinc and steel continues also at their end-life cycles, in steel mills employing Electric Arc Furnace (EAF) for the production of secondary steel. Several megatons per year of EAF dusts (the waste of this process), being rich in zinc content, represent a valuable source for secondary zinc produced by both pyrometallurgical and hydrometallurgical processes.

The process according to the present description allows to create a virtuous, integrated cycle that starting from galvanized steel scraps (i.e., the Zn- containing feed for EAF) results directly into new finished galvanized iron-based substrates, such as steel products, employing a CPE-based zinc deposition stage using EAF dusts as zinc source. The process of the present description thus represents an alternative way to the hydrometallurgical electrowinning processes known in the art that result into the recovery of zinc metal by EAF dusts.

Some embodiments of the present invention are provided below solely by way of illustrative example, which must not be considered limiting to the scope of protection defined by the appended claims.

Experimental Section

Sample preparation

The electrodes used in the cathodic plasma electrolysis tests were obtained from a common sheet of iron (Armco®; Fe >99.8%) by cutting samples of 1.5 cm in width, 4 cm in length and 1 mm in thickness. Before using, samples were firstly polished with abrasive papers of 180, 400 and 800 grit, thoroughly rinsed with MilliQ® water, dabbed with absorbent cloth and finally dried in air. Each polished sample was surrounded by a thermo-shrinkable polymeric sheath from which a square (area between 0.25 cm ² and 0.60 cm ² ) was finally cut to define the working area of the resulting electrode.

Besides using thermo-shrinkable insulating sheaths, the working area of the electrode can also be defined by applying a plasma-resistant coating on the surface of the electrode adjacent to the chosen working area, for instance an epoxy-based coating such as the commercially available resin Plasite®.

Cathodic plasma electrolysis protocols and apparatus

CPE-based surface pre-treatment experiments were carried out into two aqueous media: i) solutions of sodium carbonate (99.5%, Sigma Aldrich) at different concentrations, obtained by dissolving the proper amount of the salt in MilliQ® water; ii) solutions of 1.3 M ammonium chloride (≥99.5%; EVS, Italy), prepared in a similar way. For some tests, an equimolar amount of ammonium hydroxide (28-30% solution; Sigma Aldrich) was added to the last solution.

For zinc deposition experiments, electrolyte solutions of zinc sulfate (≥99.5%; Sigma Aldrich) at concentration 9 wt.% and 18 wt.% were employed.

CPE tests were carried out into a thermostated cell filled with the electrolyte solution and equipped with the above described iron electrode, working as cathode, placed at a distance of 4.5-5 cm in front of an anode made of a graphite foil (99.8%, 0.13 mm in thickness; Alfa Aesar). The ratio between the area of the anode and that of the cathode was about 20 in the surface pre- treatment protocol, while it was either 6 or 55 in the zinc deposition setup. For zinc deposition tests, pristine mechanically polished iron electrodes were used (i.e., not pre-treated with the plasma generated into Na ₂ CO ₃ solution) in combination with two types of anode: graphite and zinc (99.999%).

A direct current power supply (600W, HL series; Blind) was used to apply the desired potential difference (up to 150 V) across the two terminals of the cell. In zinc deposition tests, an electronic switch (BC Dynamics, Italy) was connected between the power supply and the cell in order to transform the direct current input signal (from the supply) into an on-off square pulsed output signal (to the cell). The customized equipment for chopping the signal allows to properly set both frequency and duty cycle of the pulses sent to the cell. The resulting current -potential signals were recorded by a 2-channel oscilloscope (SDS1202X-E; Siglent).

Polarization curves for surface pre-treatment tests were obtained by stepping the potential (duration of each stage fixed to 10 sec) and monitoring the resulting current through an amperemeter placed in series in the circuit. The currents reported in the plots of the figures herewith enclosed are the average of the stationary values (commonly in the last 5 seconds of each step).

Conductivity measurements of Na ₂ CO ₃ solutions were carried out with a digital conductimeter (Mod 2131; Amel) coupled with a platinum conductivity cell.

Characterization of the plasma treated samples

The thickness of the deposited zinc layer and its surface roughness, together with that of the iron electrodes after the CPE treatment in Na ₂ CO ₃ solution, were measured using a contact profilometer (DektakXT; Bruker). Morphological and elemental analysis of the zinc deposits were performed with a scanning electron microscope (Hitachi S-2500C) equipped with an X-ray microanalysis system (Noran System SIX, Thermo Fisher Scientific).

Results and Discussion

Cathodic plasma electrolysis for surface pre- treatment

The features of plasma generated at the electrode surface are affected by many experimental parameters. First of all, the elemental composition of the plasma is determined by the nature (i.e. chemical composition) of the electrolytic solution that, in turns, affects the reactions driven by the external applied potential and hence the chemical compounds produced at the electrode - solution interface.

By using solutions prepared with an inert salt such as Na ₂ CO ₃ or NaHCO ₃ acting as simple electrolytes (i.e., to guarantee a good electrical conductivity to the solution, apart from the pH control), the reaction occurring at the cathode interface is water reduction. Therefore, the application of sufficiently high cell potential to this kind of blank electrolytes can start a plasma glow discharge around the cathode, whose colour is related to the wavelength of the light emitted by the excited species (Na ⁺ ions) within gaseous sheath made of electrochemically produced hydrogen molecules.

The plasma glow discharge was studied focusing on three main external parameters: applied potential, temperature and composition of the electrolytic solution. Results of the screening are reported in Figures 1-2 that show the polarization curves of a cell equipped with an iron electrode (geometric area 0.5-0.6 cm ² ) and a graphite one, working as cathode and anode, respectively. The ratio anode/cathode area is around 20. The application of an external potential forces water splitting reaction to occur in the cell, with O ₂ and H ₂ evolution localized at the anode and cathode, respectively.

Generally speaking, the polarization curves evidence two neatly different regimes depending on the applied potential (Figure 1). At low potentials, the cell works in a normal electrolysis regime in which at higher potentials correspond higher current densities. The slope of this near linear region qualitatively resembles the variation of the electric conductivity of the solution, that increases by increasing both temperature and electrolyte concentration (Table 1). By further increasing the potential, almost stagnant values of current densities are obtained until a sudden drop is obtained. This current drop almost invariably corresponded to the appearing of a plasma discharge characterized by a bright orange light emission all around the exposed area of the iron cathode, surrounded by a gaseous sheath of H2 and, possibly, vaporized water. The only significant difference is for the experiment carried out at 303 K, for which a stable plasma was obtained only by apply more extreme potentials (120-130 V, Table 1).

Table 1. Key features from CPE tests on an electrochemical cell made of an iron cathode and a graphite anode. The working medium is an aqueous solution containing different concentrations of Na ₂ CO ₃ .

The width of the current plateau and the value of the so-called breakdown potential, E _break (i.e. the potential at which a plasma is obtained), is mostly influenced by electrolyte concentration than by temperature (Figures 1-2), with a E _break shift of around 30 V and 10 V, respectively. In particular, the critical potential moves toward a lower value (i.e. less energy demanding condition) by increasing the concentration (Table 1).

During polarization tests it has been observed that the simple, but massive, H ₂ evolution occurring in normal electrolysis regime progressively turns from a sparking intermitted plasma to a continuous plasma glow discharge in CPE regime.

As known in the art, ammonium chloride is a valuable leaching agent for zinc recovery by EAF dust and/or its derived dusts (e.g. crude zinc oxide from Waelz process), due to its complexing ability toward zinc, the possibility of buffering solution at alkaline pH (to hamper extraction of iron contained in the dusts) and the lack of problems related to the presence of chloride impurities in the resulting leachate solution (a serious problem during the subsequent electrodeposition step working with sulfuric acid as lixiviant) [8]-[10].

CPE tests were thus performed also in ammonium- based electrolytes to verify the suitability of such solutions because, to the best of the Applicant's knowledge, no literature data are available in this regard. Exploiting the buffer ability of NH ₄ CI, two different solutions were prepared, keeping constant the concentration of ammonium chloride at 1.3 M: NH ₄ CI (pH 6) and NH ₄ CI/NH ₄ OH 1:1 (pH 11). While NH ₄ CI solution at 343 K did not allow plasma generation on the iron cathode, good results were obtained with the more alkaline buffer solution.

The polarization curve obtained in NH ₄ CI/NH ₄ OH 1:1 (Figure 3) qualitatively resembles those obtained with Na ₂ CO ₃ , even if electrode reactions are potentially different (CI ₂ evolution at anode, combined with a partial O ₂ production and, tentatively, oxidation of N- containing species). The main difference in performance between the two media is that a potential neatly higher than the breakdown necessary to obtain a stable plasma (≥ 120 V). The observed bright blue colour of the resulting glow is tentatively attributed to the emission by the excited ammonium cations.

Table 2. Effect of the electrolytic plasma treatment on the surface roughness of iron cathodes. a. Solution temperature: 343 K; b. Root means square roughness, Rrms estimated as Rrms= with y _i means the height of the i-th point of the scan (referred to the linear baseline, y=0).

Cathodic plasma electrolysis for zinc deposition

The employment of electrolyte solutions that contain an active salt (i.e. with cation/anion directly involved into any electrochemical reactions) opens new routes for CPE technique. In particular, Zn-containing salts can be used to deposit a layer of zinc onto a conductive substrate by combining the classical electroreduction process (Zn ²⁺ + 2 e ^_ → Zn) with the electrolytic plasma generation. Differently from the above described CPE-based surface treatment protocol in which a direct current is applied to the terminals of the cell, for Zn electrodeposition a pulsed on-off potential stimulus (in the kHz frequency range) was used. In order to obtain the desired pulsed potential at the cell terminals, the DC output of the power supply was chopped by an electronic switch able to adjust both frequency and duty cycle of the signal (Figures 4-5).

Deposition of a zinc layer on iron samples through cathodic plasma electrolysis was carried out in an aqueous solution of ZnSO ₄ by changing different operative parameters. The total number of tests to be conducted were optimized by adopting a design of experiment strategy based on the Taguchi method. This statistical analysis allows to minimize the number of experiments to be performed to study a given process. In the present case, an orthogonal array matrix OA8(27) was constructed, allowing to consider seven degrees of freedom (i.e •9 the experimental parameters, named factors) by properly designing and performing only eight experiments. According to this design of experiment, each factor can assume two entry z (called levels). The OA8 (27) matrix, coding for the eight experiments by reporting the levels assumed by each factor, is reported in Table 3. The actual duration of each experiment (fixed to be ≥1 second) was calculated in order to match the total "on time" of the pulsed potential for all the eight experiments (reported in Table 3).

Table 3. The orthogonal array matrix OA7(28) used to design the tests for the evaluation of the process aimed to deposit zinc by cathodic plasma electrolysis. a anode nature; ^b "on time" applied potential; c concentration of ZnSO ₄ ; ^d temperature of the solution; e frequency of the on-off square pulsed potential signal; f duty cycle of the potential signal; ^g ratio between anode and cathode area: low = 6; high =56; ^h total duration of the experiment (this is not a factor of the Taguchi matrix)

Each experiment was ranked by simply considering if a blue plasma glow discharge was obtained and a zinc deposit was formed onto the iron sample (in the positive cases, its thickness was estimated by profilometry). The results are summarized in Table 4.

Table 4. Results of the zinc deposition through cathodic plasma electrolysis for each entries of the Taguchi matrix. a After detachment of the possible deposit derived by Zn deposition in normal electrolytic regime (see also the main text); b Root means square roughness, R _rms estimated as with y _i means the height of the i-th point of the scan (referred to the linear baseline, y=0).

In certain cases, plasma either was not generated or it did not last for all the duration of the experiment; in both cases a massive deposition of dendritic-like zinc is observed in normal electrolysis regime. In these conditions, the slightly adherent dendritic zinc was simply detached by the surface and only the remaining underlying compact zinc deposit was analysed, if any (Table 4). In some cases, the adherent deposit suffers for a lack of uniformity, probably due to a preferred localization of the plasma around the borders of the masked area. Nevertheless, the continuum coverage of the electrode was probed by SEM analysis (Figure 6) and profilometry tests (Table 4). Elemental composition by SEM-EDX mapping of both single grains and small zones of the deposited material evidences levels of Zn ranging between 94 and 96 at.%, with traces of iron potentially deriving from a too deep penetration of the X-ray probe.

Based on the data of Table 4, it can be inferred that: i) a higher solution temperature performs better than a colder one; in fact, the best results were obtained working at 343 K; ii) graphite seems to work better than zinc as anode (the last used as sacrificial electrode, dissolving during CPE process); in fact, 3 of 4 positive outputs employed graphite; iii) "on pulse" of 120 V seems better than 150 V (3 of 4 positive outputs); iv) both signal waveform (i.e., frequency and duty cycle) and salt concentration do not seem critical.

The above results support a profitable applicability of cathodic plasma electrolysis (CPE) technique according to the present invention.

Particularly, CPE has been demonstrated to be suitable for both surface pre-treating a metal substrate and for depositing zinc thereon. Advantageously, it is possible to easily switch from the pre-treatment mode to the depositing mode by simply changing the composition of the electrolytic solution, selecting an inert salt or a redox active one, respectively. Under certain conditions, a doubling of surface roughness can be obtained for iron samples, which can advantageously increase the subsequent adhesion of the overlying Zn layer.

The process of the present invention allows to integrate CPE into a circular recovery process for the production of galvanized steel using solid wastes rich in zinc content, given the compatibility between plasma generation and NH ₄ CI solutions demonstrated herein.

The data of the present description confirm the possibility to deposit a compact, high purity zinc layer up to 15 μm thick in few seconds (even just 1 second) using a Zn ²⁺ ions-containing aqueous solution and an iron-based substrate, such as carbon steel, as electrode.

The process of the present invention can be controlled by adjusting the temperature of the electrolyte solution, the chemical composition of the counter electrode as well as by using an on-off square pulsed waveform for the potential applied to the electrolytic cell.

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