Electrochemical Corrosion Behavior of Pure Mg Processed by Powder Metallurgy

coatings Article Electrochemical Corrosion Behavior of Pure Mg Processed by Powder Metallurgy Jozef Minda 1,*, Stanislava Fintová1,2 , Branislav Hadzima 1,3, Pavel Doležal 1,4 , Michaela Hasoˇnová1, Leoš Doskoˇcil 1and Jaromír Wasserbauer 1   Citation: Minda, J.; Fintová, S.; Hadzima, B.; Doležal, P.; Hasoˇnová, M.; Doskoˇcil, L.; Wasserbauer, J. Electrochemical Corrosion Behavior of Pure Mg Processed by Powder Metallurgy. Coatings 2021,11, 986. https://doi.org/10.3390/ coatings11080986 Received: 3 June 2021 Accepted: 16 August 2021 Published: 19 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Institute of Materials Science, Faculty of Chemistry, Brno University of Technology, Purkyˇnova 464/118, 612 00 Brno, Czech Republic; [email protected] (S.F.); branislav[email protected] (B.H.); [email protected].cz (P.D.); [email protected] (M.H.); [email protected] (L.D.); [email protected] (J.W.) 2Institute of Physics of Materials, Czech Academy of Sciences, Žižkova 22, 616 00 Brno, Czech Republic 3University of Žilina Research Centre, Univerzitná8215/1, 010 08 Žilina, Slovakia 4Institute of Materials Science and Engineering, Faculty of Mechanical Engineering, Brno University of Technology, Technická2896/2, 616 69 Brno, Czech Republic *Correspondence: [email protected]; Tel.: +421-944-496-222 Abstract: Pure Mg samples were prepared by powder metallurgy using the cold and hot compacting methods. Cold compacted pure Mg (500 MPa/RT) was characterized by 5% porosity and the mechanical bonding of powder particles. Hot compacted samples (100 MPa/400 ◦ C and 500 MPa/400 ◦ C) exhibited porosity below 0.5%, and diffusion bonding combined with mechanical bonding played a role in material compaction. The prepared pure Mg samples and wrought pure Mg were subjected to corrosion tests using electrochemical impedance spectroscopy. Similar material corrosion behavior was observed for the samples compacted at 500 MPa/RT and 100 MPa/400 ◦ C; however, hot compacted samples processed at 500 MPa/400 ◦ C exhibited longer corrosion resistance in 0.9% NaCl solution. The difference in corrosion behavior was mainly related to the different binding mechanisms of the powder particles. Cold compacted samples were characterized by a more pronounced corrosion attack and the creation of a porous layer of corrosion products. Hot compacted samples prepared at 500 MPa/400 ◦ C were characterized by uniform corrosion and the absence of a layer of corrosion products on the specimen surface. Powder-based cold compacted samples exhibited lower corrosion resistance compared to the wrought pure Mg, while the corrosion behavior of the hot compacted samples prepared at 500 MPa/400 ◦C was similar to that of wrought material. Keywords: magnesium; corrosion; cold pressing; hot pressing; powder metallurgy 1. Introduction The Mg-based alloys used in the automotive and aerospace industries exhibit good specific strength [ 1 ], and studies on them have focused on the reduction of the weight of the components and the subsequent economic and ecological savings. On the other hand, due to their biocompatibility, biodegradability, and nontoxicity, the Mg-based materials are often used also for biomedical applications. In comparison with other metallic materials used for biomedical applications (stainless steel and Ti alloys) is the biggest advantage of Mg the mechanical properties very similar to the properties of human bones [ 1 – 4 ]. While the mechanical properties of a material are mostly predetermined by its nature and structure, influencing the material structure by its processing and by mechanical and thermal treatment are the most useful methods for their tailoring [ 3 , 5 ]. One of the methods quite easily used for the production of components with specific structures and thanks to the possibility of processing different materials also with specific chemical composition is powder metallurgy (PM). The PM methods allow the tailoring of component mechanical properties due to the large range of applicable pressures, temperatures, distribution of pores and their sizes or shapes, and finally processing times it employs [1,6]. Coatings 2021,11, 986. https://doi.org/10.3390/coatings11080986 https://www.mdpi.com/journal/coatings Coatings 2021,11, 986 2 of 15 PM has already been proven to be a suitable method for the preparation of Mg-based materials [ 7 – 9 ]. Several approaches of powder production, powder consolidation, and sintering techniques were already published in the literature. In the case of Mg, the gas atomized powder is usually used, while homogeneous blending with alloying elements can be adopted [ 7 ]. The powder is usually compacted at high pressure and the green compacts are sintered in a specific (mostly inert) atmosphere under a temperature high enough for diffusion between particles but below the melting point [ 7 ] of the used materials. Processing methods such as compression followed by sintering, extrusion, severe plastic deformation (SPD), or spark plasma sintering (SPS) methods often appear in the literature [1,10,11] for Mg-based materials preparation. Mg’s high reactivity is a factor strongly influencing its processing via PM. Since the powder particles react with the atmosphere, a thick layer of oxides is usually present on the surface of the particles (even thicker in the case of the powder prepared by grinding) acting as a barrier against diffusion. The layer is usually damaged during powder compaction and during the sintering, the diffusion is allowed, at least partially [ 7 ]. The compacting pressure has to be sufficient to damage the oxide layer created on the particles and to provide plastic deformation of the powder particles ensuring their interlocking. However, it must not be too high to avoid the deformation of the spacer particles, ensuring the requested porosity, if they are used. The sintering time must be short enough to prevent unwanted chemical changes and at the same time long enough to successfully bond the particles and stabilize the system against stresses in the materials [ 6 , 12 ]. Another method of powder compaction is extrusion. In the case of Mg, the elevated temperature has to be used during extrusion [ 13 ]. In such a case, the strong texture resulting in final component anisotropy was shown to be suppressed by powder milling before compaction. The temperature has also an effect on the reached structure while higher temperatures can be resulting in the structure coarsening. Thus the choice of processing parameters has a significant effect on the product properties and porosity, and they have to be set precisely according to the demanded properties of the product. The porosity of the final product depends on the PM processing conditions and can even be enhanced by the addition of pore-forming agents [ 1 ]. Porosity can be beneficial in the case of implants with mechanical properties close to those of human bone [ 6 ]. Moreover, the introduced porosity enables fluid transport for healing, and new bone tissue can be incorporated into the implant [ 6 ]. Porous PM materials are usually prepared with a density of 25–85% of the theoretical mean density [ 7 ]. At the same time, porosity has to be considered from the point of view of the mechanical and corrosion properties of the component. The porosity of PM-processed Mg and Mg-based materials decreases mechanical properties and corrosion resistance and increases the corrosion rate by allowing the corrosion environment to penetrate the pores [6,14–17]. As mentioned above, the PM processing of magnesium is limited by its high affinity to oxygen and high corrosion rate, especially in Cl − containing environments [ 3 ]. Mg dissolves many more oxygen atoms compared to other metals, such as titanium; on the other hand, it creates a thermodynamically stable MgO layer [ 18 ]. The layer created on Mg powder particles before processing usually contains not only MgO but also Mg(OH) 2 . The naturally created oxide layer is loose and cannot provide sufficient protection for Mg against corrosion [ 4 ]. This layer also has a negative influence on the diffusion process required for the densification of the material during powder sintering (diffusion bonding) [ 18 ]. The formation and dissolution of a surface layer of corrosion products are primarily determined by the electrochemical potential of the material. The magnitude of the difference between the electrochemical potential of the material and the corrosive environment expresses the thermodynamic susceptibility to the anodic dissolution of the layer of corrosion products. The anodic polarization of Mg can alter the integrity and stability of the surface layer, making Mg more active or passive. It has been reported that the continuity/integrity of the surface layer can determine the negative difference effect (NDE) or “anodic hydrogen evolution” (AHE) behavior of anodically polarized Mg [19]. Coatings 2021,11, 986 3 of 15 Properties of Mg and Mg-based materials can be improved by the preparation of ultrapure Mg and by the alloying of Mg, respectively [ 3 , 4 , 7 , 13 , 20 , 21 ]. It has already been shown that the zinc content in Mg-based materials and PM-processed materials has a positive effect on their mechanical properties due to grain refinement and the improvement of material corrosion properties [ 22 – 24 ]. Mechanical properties can also be improved by the addition of 2 wt.% ytterbium. Likewise, additional mechanical treatment such as hot rolling can lead to improved material mechanical properties [ 20 ]. Composites of Mg/Ca prepared from pure magnesium and calcium (1 wt.%) powders using PM had a positive influence on the tensile strength and the corrosion resistance of the material; however, higher Ca content (5 wt.% and 10 wt.%) resulted in a deterioration of the examined material properties [ 21 ]. Even though the properties of PM-processed Mg-based materials are improved by the addition of an alloying element, the biocompatibility, biodegradability, and nontoxicity of the final material have to be taken into account for biomedical applications. Thus, the alloying elements have to be chosen carefully. Besides the alloying, also coatings can be used to improve Mg corrosion resistance in the case of bioapplications. Since the MgF 2 -based coatings were shown to be promising in several works [ 2 , 4 , 22 , 25 – 29 ], PM was already used for the preparation of magnesium Mg-MgF 2 composite [ 30 ]. Preparation of magnesium-based composites by powder metallurgy could be therefore a suitable option to significantly improve the corrosion properties of magnesium materials [31]. To study and describe the corrosion mechanism of alloyed and coated materials, the influence of the processing parameters—specifically, compacting pressure and temperature— on pure PM-prepared Mg needs to be understood. The goal of this work is to investigate the corrosion behavior of pure Mg materials prepared by PM through cold and hot pressing in an environment of 0.9 wt.% NaCl, this environment being the simplest saline solution to simulate body fluid. Such an approach allows a better understanding of the role of chlorides in the corrosion process and observation of the different corrosion behaviors of materials prepared by PM. The corrosion resistance of cold-compacted samples processed at 500 MPa at room temperature and that of hot-compacted samples processed at 100 MPa at 400 ◦ C and at 500 MPa at 400 ◦ C were analyzed in 0.9 wt.% NaCl solution using electrochemical impedance spectroscopy. The processing parameters were chosen based on the previous studies [ 16 ], focused on material porosity and mechanical properties of pure Mg prepared via PM. One sample prepared at RT and one at elevated temperature were chosen for the corrosion properties characterization. The corrosion behavior of these samples was compared to the behavior of wrought pure Mg, considered as reference material with known corrosion behavior and resistance in NaCl solutions. 2. Materials and Methods Compacted samples (CS) of pure Mg prepared via PM were used as the experimental material. Scanning electron microscopy (SEM; ZEISS EVO LS 10, Oberkochen, Germany) was used to characterize the powder particles. Irregular shaped Mg powder (99.8% purity declared by the producer, Goodfellow (Huntingdon, UK)) with an average particle size of approximately 30 µ m (Figure 1—observed by SEM) was processed by cold compaction (compacted at room temperature (RT)) and hot compaction (400 ◦ C). Due to the contact of the powder with air during powder preparation and transport, the Mg powder particles used for CS production were covered by a thin layer of corrosion products. This layer protected the powder against further reactions with oxygen. Coatings 2021,11, 986 4 of 15 Coatings 2021, 11, x FOR PEER REVIEW 4 of 15 using a Zwick Z250 Allround-Line machine (Zwick GmbH&Co. KG in Germany, Ulm, Germany) equipped with a MultiXtens sensitive extensometer (Zwick GmbH&Co. KG in Germany, Ulm, Germany) set between the pressing plates. The die filled with Mg powder was pressed for 20 s at RT under a pressure of 500 MPa, or for 1 h at 400 °C under pressures of 100 MPa or 500 MPa. In the case of Mg pressed at an elevated temperature, a heating chamber was arranged on the machine. After processing, the CS was removed from the steel die. The final CS was 20 mm in diameter and 5 mm in height. Figure 1. Magnesium powder particles; SEM. Metallographic analysis of the CS was performed using light microscopy (LM, Zeiss Axio Observer Z1m, Zeiss, Oberkochen, Germany) and SEM (ZEISS, Oberkochen, Germany). LM was used for microstructural observation of the prepared samples, to estimate material porosity and its distribution and reveal the corrosion attack and corrosion process mechanism. SEM was used for CS microstructure analysis to specify the powder particle bonding mechanism. Analysis of the layer of corrosion products created on the impacted surface during electrochemical testing was performed using a ZEISS Stemi 2000C stereomicroscope (SM, Zeiss, Oberkochen, Germany). The samples used for microscopical observations were prepared by the standard metallographic procedure involving grinding and polishing using isopropyl alcohol as a cooling medium to prevent Mg CS corrosion during preparation. Nital etchant (5% nitric acid in ethanol) was used to reveal the material microstructure for SEM analysis. The exact value of porosity of the CS was calculated according to the dimensions and mass of the sample and the density of pure Mg. Electrochemical corrosion testing was conducted using a VSP-300 potentiostat (Biologic, Seyssinet-Pariset, France) three-electrode cell was used for the electrochemical measurement, using a Pt electrode as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the compacted sample (1 cm2 exposed area) as the working electrode. Electrochemical impedance spectroscopy (EIS) was used for the characterization of material electrochemical corrosion behavior. For corrosion experiments, the surfaces of samples were ground with 4000 grit SiC paper; then, the samples were rinsed with distilled water and isopropanol. Hot air was used for drying. Immediately after sample preparation, electrochemical experiments were conducted in a corrosive environment. Measurements were carried out in 0.9% NaCl solution at RT. All measurements were performed in potentiostatic mode at an open circuit potential (OCP), and the used frequency range was from 100 kHz to 10 mHz with 6 points per decade, with a sinusoidal signal input of amplitude 10 mV. EIS measurements were performed at the following time points after the beginning of exposure to the corrosion environment: 0, 1, 2, 4, 8, 12, 24, 48, 72, and 96 h; before each EIS measurement, an OCP step was included, and the measurement started after 5 min of system stabilization. EIS measurements were performed on two CS for each processing method. The models described by King et al. [32] were applied to the analysis of the obtained electrochemical data. Energy-dispersive Xray spectroscopy (EDS, model OXFORD INSTRUMENTS X-MAX 80 mm2) was used to examine the surface of the samples after electrochemical testing to specify the character of Figure 1. Magnesium powder particles; SEM. A steel die was employed for the transfer and compaction of Mg powder (2.7 g) used for sample preparation. The steel die was filled with Mg powder in an inert atmosphere of N 2 (5.0 purity); then the die was transported to the machine for double-action compaction and pressed with a velocity of 2 mm · min −1 . CS (experimental materials) was prepared using a Zwick Z250 Allround-Line machine (Zwick GmbH&Co. KG in Germany, Ulm, Germany) equipped with a MultiXtens sensitive extensometer (Zwick GmbH&Co. KG in Germany, Ulm, Germany) set between the pressing plates. The die filled with Mg powder was pressed for 20 s at RT under a pressure of 500 MPa, or for 1 h at 400 ◦ C under pressures of 100 MPa or 500 MPa. In the case of Mg pressed at an elevated temperature, a heating chamber was arranged on the machine. After processing, the CS was removed from the steel die. The final CS was 20 mm in diameter and 5 mm in height. Metallographic analysis of the CS was performed using light microscopy (LM, Zeiss Axio Observer Z1m, Zeiss, Oberkochen, Germany) and SEM (ZEISS, Oberkochen, Germany). LM was used for microstructural observation of the prepared samples, to estimate material porosity and its distribution and reveal the corrosion attack and corrosion process mechanism. SEM was used for CS microstructure analysis to specify the powder particle bonding mechanism. Analysis of the layer of corrosion products created on the impacted surface during electrochemical testing was performed using a ZEISS Stemi 2000C stereomicroscope (SM, Zeiss, Oberkochen, Germany). The samples used for microscopical observations were prepared by the standard metallographic procedure involving grinding and polishing using isopropyl alcohol as a cooling medium to prevent Mg CS corrosion during preparation. Nital etchant (5% nitric acid in ethanol) was used to reveal the material microstructure for SEM analysis. The exact value of porosity of the CS was calculated according to the dimensions and mass of the sample and the density of pure Mg. Electrochemical corrosion testing was conducted using a VSP-300 potentiostat (Biologic, Seyssinet-Pariset, France) three-electrode cell was used for the electrochemical measurement, using a Pt electrode as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the compacted sample (1 cm 2 exposed area) as the working electrode. Electrochemical impedance spectroscopy (EIS) was used for the characterization of material electrochemical corrosion behavior. For corrosion experiments, the surfaces of samples were ground with 4000 grit SiC paper; then, the samples were rinsed with distilled water and isopropanol. Hot air was used for drying. Immediately after sample preparation, electrochemical experiments were conducted in a corrosive environment. Measurements were carried out in 0.9% NaCl solution at RT. All measurements were performed in potentiostatic mode at an open circuit potential (OCP), and the used frequency range was from 100 kHz to 10 mHz with 6 points per decade, with a sinusoidal signal input of amplitude 10 mV. EIS measurements were performed at the following time points after the beginning of exposure to the corrosion environment: 0, 1, 2, 4, 8, 12, 24, 48, 72, and 96 h; before each EIS measurement, an OCP step was included, and the measurement started after 5 min of system stabilization. EIS measurements were performed on two CS for each processing method. The models described by King et al. [ 32 ] were applied to the Coatings 2021,11, 986 5 of 15 analysis of the obtained electrochemical data. Energy-dispersive X-ray spectroscopy (EDS, model OXFORD INSTRUMENTS X-MAX 80 mm 2 ) was used to examine the surface of the samples after electrochemical testing to specify the character of the corrosion products. The crosscuts of the samples after the testing were analyzed in terms of SEM to reveal the corrosion progress within the material bulk. All the obtained results were compared to the data obtained by analysis of wrought pure Mg (Goodfellow, UK) delivered in a form of an as-drawn rod with a diameter of 50 mm, length of 500 mm with a chemical composition of 99.9 % Mg (provided by the producer). 3. Results 3.1. Microstructural Observation The analyzed microstructures of wrought magnesium and CS are shown in Figure 2. The inserts in Figure 2provide information about powder particles boundaries showing the present oxide layer and closed porosity. The wrought Mg microstructure was characterized by polyhedral grains with an average grain size of 55 ± 9 µ m (Figure 2a). As a result of material preparation, deformation twins were observed in some grains of wrought pure Mg. Coatings 2021, 11, x FOR PEER REVIEW 5 of 15 the corrosion products. The crosscuts of the samples after the testing were analyzed in terms of SEM to reveal the corrosion progress within the material bulk. All the obtained results were compared to the data obtained by analysis of wrought pure Mg (Goodfellow, UK) delivered in a form of an as-drawn rod with a diameter of 50 mm, length of 500 mm with a chemical composition of 99.9 % Mg (provided by the producer). 3. Results 3.1. Microstructural Observation The analyzed microstructures of wrought magnesium and CS are shown in Figure 2. The inserts in Figure 2 provide information about powder particles boundaries showing the present oxide layer and closed porosity. The wrought Mg microstructure was characterized by polyhedral grains with an average grain size of 55 ± 9 μm (Figure 2a). As a result of material preparation, deformation twins were observed in some grains of wrought pure Mg. Figure 2. Microstructure of wrought Mg and Mg CS; LM and SEM detail; etched by 5% Nital: (a) wrought pure Mg; (b) 500 MPa/RT; (c) 100 MPa/400 °C; (d) 500 MPa/400 °C; (e) detail of 500 MPa/RT; (f) detail of 100 MPa/400 °C; (g) detail of 500 MPa/400 °C. Some differences in the microstructures of the CS can be seen in Figure 2. Increasing the compaction temperature resulted in more pronounced deformation of the powder particles during processing, this effect even more obvious in the case of the higher compaction pressure used. The details in Figure 2 reveal a change in the powder particle bonding mechanism from mechanical bonding to a combination of mechanical bonding and solid-state diffusion between particles (100 and 500 MPa at 400 °C, Figure 2c,d). In the case of elevated temperature processing, a lower contribution of the diffusion mechanism to Mg powder particle bonding is assumed at the lower compaction pressure (100 MPa) compared to the higher pressure (500 MPa). The change in bonding mechanism at elevated temperature results in better compaction of the material and lower material porosity, Figure 2c,d (SEM details). The porosity of the cold CS was 4% ± 1%, and the porosity of the hot CS was less than 1.0% ± 0.5%. All of the prepared samples were characterized by closed porosity with a Figure 2. Microstructure of wrought Mg and Mg CS; LM and SEM detail; etched by 5% Nital: ( a ) wrought pure Mg; ( b ) 500 MPa/RT; ( c ) 100 MPa/400 ◦ C; ( d ) 500 MPa/400 ◦ C; ( e ) detail of 500 MPa/RT; (f) detail of 100 MPa/400 ◦C; (g) detail of 500 MPa/400 ◦C. Some differences in the microstructures of the CS can be seen in Figure 2. Increasing the compaction temperature resulted in more pronounced deformation of the powder particles during processing, this effect even more obvious in the case of the higher compaction pressure used. The details in Figure 2reveal a change in the powder particle bonding mechanism from mechanical bonding to a combination of mechanical bonding and solid-state diffusion between particles (100 and 500 MPa at 400 ◦ C, Figure 2c,d). In the case of elevated temperature processing, a lower contribution of the diffusion mechanism to Mg powder particle bonding is assumed at the lower compaction pressure (100 MPa) compared to the higher pressure (500 MPa). The change in bonding mechanism at elevated temperature results in better compaction of the material and lower material porosity, Figure 2c,d (SEM details). Coatings 2021,11, 986 6 of 15 The porosity of the cold CS was 4% ± 1%, and the porosity of the hot CS was less than 1.0% ± 0.5%. All of the prepared samples were characterized by closed porosity with a pore size lower than 10 µ m. The pores present between compacted Mg powder particles can be seen in SEM details of the microstructures of the prepared samples, shown in Figure 2. 3.2. Electrochemical Corrosion Characteristics The results of EIS measurements represented by Nyquist plots for individual CS performed for the time series from 0 h (measured after 5 min of stabilization of the system) to 96 h of exposure to 0.9% NaCl solution are shown in Figure 3. Coatings 2021, 11, x FOR PEER REVIEW 6 of 15 pore size lower than 10 µm. The pores present between compacted Mg powder particles can be seen in SEM details of the microstructures of the prepared samples, shown in Figure 2. 3.2. Electrochemical Corrosion Characteristics The results of EIS measurements represented by Nyquist plots for individual CS performed for the time series from 0 h (measured after 5 min of stabilization of the system) to 96 h of exposure to 0.9% NaCl solution are shown in Figure 3. Figure 3. Nyquist plots for pure wrought Mg and Mg CS in 0.9% NaCl: (a) wrought pure Mg (different axis scale); (b) 500 MPa/RT; (c) 100 MPa/400 °C; (d) 500 MPa/400 °C. A total of three equivalent circuits (Figure 4) were used to fit the obtained Nyquist plot curves. The values determined for the electrochemical corrosion characteristics of wrought Mg and CS are given in Tables 1–4. The equivalent circuits used to describe the corrosion behavior of the analyzed samples consisted of different elements used for the description of different chemical processes. If no inductive response of the material was observed on the polarization curve, a simplified Randles circuit was used for data analysis, as shown in Figure 4a. The equivalent circuit presented in Figure 4a consists of elements representing the solution resistance Rs, the constant phase element (CPE) of the present corrosion layer Q1, and the polarization resistance Rp. The equivalent circuit shown in Figure 4b consists of components representing the solution resistance Rs, the constant phase element (CPE) of the present corrosion layer Q2, the resistance of the capacitance part R2, the inductance L3 (the time-independent part of the circuit characterizing the inductive loop of the curve), and the resistance R3 of the inductive loop. The resulting polarization resistance Rp is given by Equation (1): 1 𝑅p =1 𝑅2 +1 𝑅3 (1) The equivalent circuit shown in Figure 4b describes two corrosion processes simultaneously running on the surface of the sample. R2 and Q2 values characterize the layer of corrosion products created on the sample surface consisting of MgO and/or Mg(OH)2. R3 Figure 3. Nyquist plots for pure wrought Mg and Mg CS in 0.9% NaCl: ( a ) wrought pure Mg (different axis scale); (b) 500 MPa/RT; (c) 100 MPa/400 ◦C; (d) 500 MPa/400 ◦C. A total of three equivalent circuits (Figure 4) were used to fit the obtained Nyquist plot curves. The values determined for the electrochemical corrosion characteristics of wrought Mg and CS are given in Tables 1–4. Coatings 2021, 11, x FOR PEER REVIEW 7 of 15 and L3 values describe the adsorption of intermediate Mg+ or other species occurring in the solution. This can be observed in the Nyquist plot curve as the low-frequency inductive loop. Intermediate Mg+ has its origin in the corrosion reaction of magnesium and the solution and is connected with the phenomenon of the so-called negative difference effect for Mg corrosion (NDE) [33,34]. The equivalent circuit shown in Figure 4c consists of three time-independent loops. There are two capacitance loops (represents by CPE) for high and intermediate frequencies and one low-frequency inductive loop. The element Rs characterizes the solution resistance, and Q2 and R2 CPEs are used in the equivalent circuit and represent the capacitance and resistance of the layer of corrosion products, respectively. Q3 and R3 are CPEs describing the resistance of the inner porous layer. Elements of the low-frequency inductive loop are inductance L4 and the resistance of the inductor R4. The resulting polarization resistance of the system is given by Equation (2): 1 𝑅p =1 𝑅2+ 𝑅3 +1 𝑅4 (2) The third used equivalent circuit described the simultaneous responses of the layer of corrosion products and the base material to the used corrosion environment solution, as seen in Figure 4c. In such a situation, the corrosive medium penetrates through the porous layer of corrosion products, and the sample surface is exposed to the medium. Also, in this case, the inductive element of the circuit describes chemical reactions connected with the NDE. Besides the capacitive loop, there is, on the Nyquist plots, probably a capacitive response between the sample surface and the layer of corrosion products created on it. The two capacitance loops observed on the Nyquist plots could be explained by the creation of a partially stable layer of corrosion products over the inner porous structure of PM-processed Mg CS. The fraction coefficient n indicates the level of corrosion layer stability expressed as a measure of the non-ideality of the capacity response (n = 1 for an ideal capacity response). That is, a value of n below 1 indicates the presence of an imperfect layer of corrosion products, e.g., a layer containing pores filled with electrolyte, or that the surface is not fully covered [35]. Figure 4. Equivalent circuits used for the Nyquist plot evaluation: (a) simplified Randles circuit; (b) equivalent circuit with one capacitance and one inductive loop; (c) equivalent circuit with two capacitances and one inductive loop. The electrochemical corrosion behavior of wrought pure Mg samples was characterized by three types of Nyquist plot curves, as shown in Figure 3a. At the beginning and after 1 h of exposure, the corrosion behavior is described by the model of the circuit in Figure 4c. The Nyquist curve characterizing sample behavior from 2 up to 8 h of exposure to the 0.9% NaCl solution was evaluated using a simplified Randles circuit, shown in Figure 4a. The evaluation showed that no inductive response of the material was observed on the curve. The equivalent circuit shown in Figure 4b was used for the evaluation of the rest of the measured data (16–96 h). The determined electrochemical characteristics are given in Table 1. Figure 4. Equivalent circuits used for the Nyquist plot evaluation: ( a ) simplified Randles circuit; ( b ) equivalent circuit with one capacitance and one inductive loop; ( c ) equivalent circuit with two capacitances and one inductive loop. Coatings 2021,11, 986 7 of 15 Table 1. Polarization resistance of wrought pure Mg. Time Rs [Ω·cm2] R2 [Ω·cm2] R3 [Ω·cm2] R4 [Ω·cm2] Rp [Ω·cm2] Q2 [µF·sn–1] Q3 [µF·sn–1]n2n3L[H] 0 82 394 223 NA 142 18.4 NA 0.88 NA 1068 1 88 49 1 38 35 145.1 11.26 0.99 0.99 87 2 84 81 NA NA 81 366.8 NA 0.93 NA NA 4 86 150 NA NA 140 219.1 NA 0.95 NA NA 8 89 240 NA NA 276 167.6 NA 0.94 NA NA 12 91 282 122 NA 404 157.3 NA 0.92 NA 1708 24 84 401 54 NA 252 123.7 NA 0.91 NA 2001 48 86 223 315 NA 402 84.9 NA 0.91 NA 6926 72 91 438 270 NA 443 83.3 NA 0.86 NA 2501 96 95 739 NA NA 730 78.3 NA 0.83 NA 1068 NA—not applicable. Lis the expression of the values of L3or L4, depending on the used equivalent circuit (Figure 4). Table 2. Polarization resistance of pure Mg compacted at 500 MPa/RT. Time Rs [Ω·cm2] R2 [Ω·cm2] R3 [Ω·cm2] R4 [Ω·cm2] Rp [Ω·cm2] Q2 [µF·sn–1] Q3 [µF·sn–1]n2n3L[H] 0 59 58 21 96 43 0.0 0.0 1.00 0.66 343 1 60 284 88 NA 45 439.0 NA 0.65 NA 922 2 61 81 87 NA 42 514.0 NA 0.68 NA 1107 4 64 931 116 NA 49 677.0 NA 0.62 NA 1888 8 66 62 204 212 118 327.0 0.5 0.24 0.97 727 12 74 132 849 178 151 49.4 204.0 0.55 0.67 0 NA—not applicable. Lis the expression of the values of L3or L4, depending on the used equivalent circuit (Figure 4). Table 3. Polarization resistance of pure Mg compacted at 100 MPa/400 ◦C. Time Rs[Ω·cm2]R2[Ω·cm2]R3[Ω·cm2]Rp[Ω·cm2]Q2[µF·sn–1]n2L3[H] 0 117 298 107 79 36.11 0.83 2026 1 120 247 126 84 61.99 0.86 2201 2 121 227 165 96 90.66 0.86 3001 4 118 187 213 99 180.1 0.78 3856 8 116 141 317 98 252.0 0.73 5957 12 116 141 523 111 236.6 0.73 8440 Table 4. Polarization resistance of pure Mg compacted at 500 MPa/400 ◦C. Time Rs [Ω·cm2] R2 [Ω·cm2] R3 [Ω·cm2] R4 [Ω·cm2] Rp [Ω·cm2] Q2 [µF·sn–1] Q3 [µF·sn–1]n2n3L[H] 0 61 6 40 306 40 0.0 22.2 1.00 0.88 1801 1 64 399 1617 174 160 28.9 1230.0 0.89 1.00 2242 2 64 375 167 NA 115 33.0 NA 0.89 NA 5244 4 63 325 154 NA 105 36.1 NA 0.90 NA 3950 8 60 290 157 NA 102 34.3 NA 0.92 NA 2590 12 59 1877 367 NA 307 33.6 NA 0.92 NA 0 24 65 323 182 NA 116 37.3 NA 0.91 NA 5233 48 63 272 203 NA 116 41.7 NA 0.88 NA 4774 72 68 250 239 NA 122 59.8 NA 0.84 NA 4854 96 68 204 163 NA 91 82.9 NA 0.81 NA 3781 NA—not applicable. *—the values are in order 10 −6 .Lis the expression of the values of L 3 or L 4 , depending on the used equivalent circuit (Figure 4). The equivalent circuits used to describe the corrosion behavior of the analyzed samples consisted of different elements used for the description of different chemical processes. If no inductive response of the material was observed on the polarization curve, a simplified Coatings 2021,11, 986 8 of 15 Randles circuit was used for data analysis, as shown in Figure 4a. The equivalent circuit presented in Figure 4a consists of elements representing the solution resistance R s , the constant phase element (CPE) of the present corrosion layer Q 1 , and the polarization resistance Rp. The equivalent circuit shown in Figure 4b consists of components representing the solution resistance R s , the constant phase element (CPE) of the present corrosion layer Q 2 , the resistance of the capacitance part R 2 , the inductance L 3 (the time-independent part of the circuit characterizing the inductive loop of the curve), and the resistance R 3 of the inductive loop. The resulting polarization resistance Rpis given by Equation (1): 1 Rp =1 R2 +1 R3(1) The equivalent circuit shown in Figure 4b describes two corrosion processes simultaneously running on the surface of the sample. R 2 and Q 2 values characterize the layer of corrosion products created on the sample surface consisting of MgO and/or Mg(OH) 2 .R 3 and L 3 values describe the adsorption of intermediate Mg + or other species occurring in the solution. This can be observed in the Nyquist plot curve as the low-frequency inductive loop. Intermediate Mg + has its origin in the corrosion reaction of magnesium and the solution and is connected with the phenomenon of the so-called negative difference effect for Mg corrosion (NDE) [33,34]. The equivalent circuit shown in Figure 4c consists of three time-independent loops. There are two capacitance loops (represents by CPE) for high and intermediate frequencies and one low-frequency inductive loop. The element R s characterizes the solution resistance, and Q 2 and R 2 CPEs are used in the equivalent circuit and represent the capacitance and resistance of the layer of corrosion products, respectively. Q 3 and R 3 are CPEs describing the resistance of the inner porous layer. Elements of the low-frequency inductive loop are inductance L 4 and the resistance of the inductor R 4 . The resulting polarization resistance of the system is given by Equation (2): 1 Rp =1 R2+R3 +1 R4(2) The third used equivalent circuit described the simultaneous responses of the layer of corrosion products and the base material to the used corrosion environment solution, as seen in Figure 4c. In such a situation, the corrosive medium penetrates through the porous layer of corrosion products, and the sample surface is exposed to the medium. Also, in this case, the inductive element of the circuit describes chemical reactions connected with the NDE. Besides the capacitive loop, there is, on the Nyquist plots, probably a capacitive response between the sample surface and the layer of corrosion products created on it. The two capacitance loops observed on the Nyquist plots could be explained by the creation of a partially stable layer of corrosion products over the inner porous structure of PM-processed Mg CS. The fraction coefficient nindicates the level of corrosion layer stability expressed as a measure of the non-ideality of the capacity response (n= 1 for an ideal capacity response). That is, a value of nbelow 1 indicates the presence of an imperfect layer of corrosion products, e.g., a layer containing pores filled with electrolyte, or that the surface is not fully covered [35]. The electrochemical corrosion behavior of wrought pure Mg samples was characterized by three types of Nyquist plot curves, as shown in Figure 3a. At the beginning and after 1 h of exposure, the corrosion behavior is described by the model of the circuit in Figure 4c. The Nyquist curve characterizing sample behavior from 2 up to 8 h of exposure to the 0.9% NaCl solution was evaluated using a simplified Randles circuit, shown in Figure 4a. The evaluation showed that no inductive response of the material was observed on the curve. The equivalent circuit shown in Figure 4b was used for the evaluation of the Coatings 2021,11, 986 9 of 15 rest of the measured data (16–96 h). The determined electrochemical characteristics are given in Table 1. The electrochemical corrosion behavior of PM-prepared pure Mg samples compacted at 500 MPa/RT was characterized by two types of Nyquist plot curves, as seen in Figure 3b. At the beginning of the exposure of the samples to 0.9% NaCl solution, three independent loops were observed on the curve, as seen in Figure 3b. The period of exposure between 1 and 4 h was characterized by two measured loops, and from 8 h three loops were present on the measured curve again. The measurement was stopped after 17 h of exposure due to the rapid corrosion attack and its damage. Values for the material polarization resistance, R p , determined using appropriate equivalent circuits characterizing the obtained curves, are given in Table 2. The Nyquist plots obtained for exposure times of 0, 8, and 12 h were evaluated using the equivalent circuit shown in Figure 4c. The rest of the obtained curves (1, 2, and 4 h) were evaluated using the equivalent circuit shown in Figure 4b. The electrochemical corrosion behavior of pure Mg CS prepared at 100 MPa/400 ◦ C was characterized by one type of Nyquist plot curve, which can be seen in Figure 3c. From the beginning of the exposure until the end of exposure, the equivalent circuit shown in Figure 4b was used to evaluate the obtained Nyquist plots. Values for the polarization resistance, Rp, determined using the appropriate equivalent circuit are given in Table 3. Nyquist plots characterizing the electrochemical corrosion behavior of pure Mg CS prepared at 500 MPa/400 ◦ C exhibited two types of curves, as shown in Figure 3d. At the beginning of exposure (0–1 h) to the corrosion environment, three loops were obtained on the curves, and the equivalent circuit shown in Figure 4c was used for data evaluation. The rest of the obtained curves were evaluated using the equivalent circuit shown in Figure 4b. The polarization resistance of the hot compacted Mg samples (500 MPa/400 ◦ C) determined using the appropriate equivalent circuits are given in Table 4. Wrought pure Mg was characterized by a polarization resistance (R p ) of 142 Ω· cm 2 at the beginning of the measurement, as seen in Table 1. This value rapidly decreased to 35 Ω· cm 2 after 1 h of exposure to the NaCl solution. Further exposure resulted in an increase to 404 Ω· cm 2 after 12 h and a subsequent decrease to 252 Ω· cm 2 after 24 h. After this decrease, continuing exposure to the NaCl solution resulted in a further gradual increase in the polarization resistance, with a maximum value of 730 Ω· cm 2 obtained after 96 h. The layer of corrosion products created on the material surface (except at 72 and 96 h of exposure) was assumed to exhibit high stability based on high corresponding n values (Table 1). The obtained electrochemical characteristics and the character of the plots indicate the creation of a layer of corrosion products at the beginning of exposure (0 h), with the layer gradually becoming damaged (1 h) and then gradually being recreated (2–8 h) . After 12 h of exposure, the layer of corrosion products was created on the material surface, and its protection can be assumed up to 72 h with NDE an accompanying process of the corrosion process. After 96 h of exposure, the plot character changed. This indicates that the layer was damaged and that the revealed surface of the sample was exposed to the environment, causing a new layer of corrosion products to begin to grow. Cold compacted Mg samples (500 MPa/RT) were characterized by a polarization resistance (R p ) of 43 Ω· cm 2 at the beginning of the measurement, as seen in Table 2. The resistance of the samples remained almost unchanged up to 4 h of exposure to 0.9% NaCl solution. From 8 to 12 h of exposure, the polarization resistance increased from 118 to 151 Ω· cm 2 . As can be seen in Table 2, quite low values of n 2 for the layer of corrosion products and n 3 for the inner porous layer suggest that these layers created on the PM CS surface were not very stable. The obtained data also indicate the growth of a porous layer of corrosion products on the sample surface at the beginning of the exposure, accompanied by NDE, with a quite compact layer of corrosion products created after 1 h of exposure. Further exposure to the corrosion environment resulted in layer damage, detected after 8 h of exposure. The layer did not grow significantly with increasing exposure time. After 12 h of exposure, a porous layer of corrosion products can be assumed to have been present on