Complex material and surface analysis of anterolateral distal tibial plate of 1.4441 steel

  Citation: Hlinka, J.; Dostalova, K.; Dedkova, K.P.; Madeja, R.; Frydrysek, K.; Koutecky, J.; Sova, P.; Douglas, T.E.L. Complex Material and Surface Analysis of Anterolateral Distal Tibial Plate of 1.4441 Steel. Metals 2022,12, 60. https://doi.org/10.3390/ met12010060 Academic Editor: Sina Jamali Received: 4 November 2021 Accepted: 18 December 2021 Published: 27 December 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/). metals Article Complex Material and Surface Analysis of Anterolateral Distal Tibial Plate of 1.4441 Steel Josef Hlinka 1,2,* , Kamila Dostalova 2, Katerina Peterek Dedkova 2, Roman Madeja 3,4, Karel Frydrysek 4, Jan Koutecky 5, Pavel Sova 1,2 and Timothy E. L. Douglas 6,7 1Department of Materials Engineering, Faculty of Materials and Technology, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic; [email protected] 2Centre for Advanced Innovation Technologies, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic; [email protected] (K.D.); [email protected] (K.P.D.) 3Trauma Center, University Hospital Ostrava, 17. Listopadu 1790, 708 52 Ostrava-Poruba, Czech Republic; [email protected] 4Institute of Emengency Medicine, University of Ostrava, Syllabova 19, 703 00 Ostrava, Czech Republic; [email protected] 5Medin a.s., Vlachovicka 619, 592 31 Nove Mesto na Morave, Czech Republic; [email protected] 6Engineering Department, Gillow Avenue, Lancaster University, Lancaster LA1 4YW, UK; [email protected] 7Materials Science Institute (MSI), Lancaster University, Lancaster LA1 4YW, UK *Correspondence: [email protected] Abstract: Nickel-based austenitic stainless steels are still common for manufacture of implants intended for acute hard tissue reinforcement or stabilization, but the risk of negative reactions due to soluble nickel-rich corrosion products must be considered seriously. Corrosion processes may even be accelerated by the evolution of microstructure caused by excessive heat during machining, etc. Therefore, this study also deals with the investigation of microstructure and microhardness changes near the threaded holes of the anterolateral distal tibial plate containing approx. 14wt.% Ni by composition. There were only insignificant changes of microhardness, grain size, or microstructure orientation found close to the area of machining. In addition, wettability measurements of surface energy demonstrated only minor differences for bulk material and areas close to machining. The cyclic potentiodynamic polarization tests were performed in isotonic physiological solution. The first cycle was used for the determination of corrosion characteristics of the implant after chemical passivation, the second cycle was used to simulate real material behavior under the condition of previous surface damage by excessive pitting corrosion occurring during previous polarization. It was found that the damaged and spontaneously repassived surface showed a three-time higher standard corrosion rate than the “as received” chemically passivated surface. One may conclude that previous surface damage may decrease the lifetime of the implant significantly and increase the amount of nickel-based corrosion products distributed into surrounding tissues. Keywords: pitting corrosion; microstructure; implant; traumatology; cytotoxicity; surface contact angle; chemical passivation 1. Introduction Metallic materials are widely used in a large number of implantology applications. Although there are benefits, some complications can occur after the insertion of a metalbased implant into the body. These complications can be classified according to their origin. Some are caused by ill-considered construction design (involving shape and size). Others can be caused by inappropriate material selection. Currently, there is an effort to avoid problems with construction design mainly by a custom-made approach, especially in cases where the surgical treatment can be planned. The topic of inappropriate material selection is more complicated because of the sensitivity of the body to some elements. A typical Metals 2022,12, 60. https://doi.org/10.3390/met12010060 https://www.mdpi.com/journal/metals Metals 2022,12, 60 2 of 14 metallic implant is a solid object that is implanted into a human body during surgery. Even though research in the field of implant production is continuing with the tendency to find materials with properties nearly similar to those of bones, implants are still artificial objects for the body. Therefore, chronic inflammations in the implantation site or allergic reactions can occur. In some cases, the body has a tendency to eliminate the implant. There are a couple of ways to prevent this consequence. The implant surface is the main interface between the material of the implant and the human body environment. Hence, it can be modified in various ways to imitate natural body structure. Generally, the implant surface is covered with a layer that is tolerable for a body and its character is given by its roughness, wettability, chemical composition, etc. There is always a risk of scratching the layer during implant implantation. Moreover, implants commonly used for long-term fixation or reinforcement of damaged hard tissues are typically repetitively stressed by axial and uniaxial forces which may result in premature development of fatigue cracks in their structure [ 1 ]. Moreover, when the implant is in direct continuous contact with other moving parts or tissues, wear damage can appear due to undesirable friction between these parts if any movement or instability occurs [ 2 ]. These mechanisms always have to be considered, even if they do not occur in every case. As the human body contains water-based liquids, all degradation processes related to mechanical actions are synergistically accelerated by corrosion processes [ 3 , 4 ]. The character of an implant surface establishes not only corrosion resistance but also determines the biological response of the tissue [ 5 ]. These aspects make corrosion and technological properties of implants crucial for their proper design, modeling, and determination of their lifecycle. This research focuses on the investigation of the surface properties (microstructure, corrosion, wettability, microhardness, and contact-type surface roughness test) of an implant intended for ankle reparation consisting of AISI 1.4441. As this nickel-rich austenitic steel is widely used for manufacturing mainly short-term hard tissue reinforcements, complications related to ions releasing during specific corrosion processes may result in its terminal failure. The motivation of the research presented here is to primarily evaluate the amount of ions released from the application under conditions simulating its real use. 2. Materials and Methods An AISI 1.4441 steel was used for manufacture of anterolateral distal tibial plates which was further tested by experimental techniques (Figure 1). It is an L-shaped board, anatomically shaped for the left and right sides in lengths of 2 rotations (90 mm) to 16 rotations (300 mm). There are holes in the plate for the angularly stable screws of 3.5 mm, possibly for 3.5/2.7 mm screws and K-wires of Ø 1.5 mm. Three screw holes are divergently directed in the distal part; the other two pairs of screws form their support and fix the fragments in a skew line to the distal screws. The distal part includes a protrusion allowing the fixation of a possibly broken Chaput tubercle. The holes are positioned in the shaft part of the plate, perpendicular to the plate. The anterolateral distal tibial plate is intended for osteosynthesis of complicated fractures of the distal tibia, some storey fractures, or fractures of the distal tibia with a fracture line in the frontal plane. The standardized chemical composition of the studied material is listed in Table 1. The surface of the plate was mechanically polished and pickled in a mixture of hydrofluoric and nitric acid to remove any free particles or to degrade and eliminate any grease and oil residues from previous manufacturing processes. The last step before repeated ultrasonic cleaning in distilled water was surface passivation with nitric acid at a concentration of 30% at a temperature of 50–60 ◦ C for 20 min. The passivation prescription is in accordance with ASTM A967, widely considered as a standard for this type of material [6]. Metals 2022,12, 60 3 of 14 Metals 2022, 11, x FOR PEER REVIEW 3 of 15 Figure 1. Anterolateral distal tibial plate with marked areas of analysis. Table 1. Standardized chemical composition of 1.4441 steel according to [7]. AISI 1.4441, Chemical Composition (wt.%) C Si Mn P S Cr Mo Ni Others <0.03 <1 <2 <0.25 <0.01 17–19 2.5–3.2 13–15 N < 0.1, Cu < 0.5 2.1. Microstructure and Metallography Observation For the evaluation of the corrosive effect and the basic semi-quantitative chemical properties, an analysis of the surface layer was performed using an SEM FEI 450 Quanta FEG (FEI Company, Brno, Czech Republic) equipped with an EDAX EDS detector (AMATEK Company, Tilburg, The Netherlands) in the secondary electron mode. Accelerating the voltage to 15 keV enabled analysis of a wide range of chemical elements from the periodic table. Due to the shape of the analyzed sample, the working distance was 10–15 mm. The samples for metallography and microhardness testing were mounted into bakelite resin (Polyfast) with carbon particles filler supplied by Struers (Roztoky, Czech Republic). This resin stabilizes the samples during mechanical preparation and microhardness testing safely. The metallography observations were performed on samples after mechanical polishing using equipment and diamond suspensions made by Struers (Roztoky, Czech Republic) with chemical etching (22 °C/60 s) in a modified Vilella’s reagent [7] containing 10 parts 35% HCl, 10 parts distilled H2O, and 1 part 65% HNO3. The image capturing and evaluation was performed by an Olympus IX70 inverted metallographic microscope (Olympus, Prague, Czech Republic). 2.2. Corrosion Test Due to a very low corrosion rate of AISI 1.4441 under standard conditions, the corrosion characteristic has to be obtained by accelerated corrosion tests. Potentiodynamic polarization tests were performed in high-density polyethylene and high-density polypropylene corrosion cells with lower exposure hole which exposes 0.49 cm2 of the tested material. A similar corrosion cell setup is often used when complex surfaces are evaluated [8]. A hardware device, Voltalab PGZ 100 with Voltamaster 10 software (Villeurbanne, France), with 3 electrode setups was used. The testing method followed ASTM F 2129, ASTM G 61, and ISO 12,732 with certain temperature and gas bubbling modifications regarding subsequent application in biomedical engineering. This setup allows bubbles formed on the surface during tests to escape freely and not to affect the continuity of the measurements. A three-electrode setup was used for precise measurement. The sample was connected as a working electrode, saturated calomel electrode (SCE, +241 mV vs. Saturated Hydrogen Electrode (SHE))[9] served as a reference electrode, and a high purity carbon rod was connected as an auxiliary electrode. The physiological saline solution (0.9 Figure 1. Anterolateral distal tibial plate with marked areas of analysis. Table 1. Standardized chemical composition of 1.4441 steel according to [7]. AISI 1.4441, Chemical Composition (wt.%) C Si Mn P S Cr Mo Ni Others <0.03 <1 <2 <0.25 <0.01 17–19 2.5–3.2 13–15 N < 0.1, Cu < 0.5 2.1. Microstructure and Metallography Observation For the evaluation of the corrosive effect and the basic semi-quantitative chemical properties, an analysis of the surface layer was performed using an SEM FEI 450 Quanta FEG (FEI Company, Brno, Czech Republic) equipped with an EDAX EDS detector (AMATEK Company, Tilburg, The Netherlands) in the secondary electron mode. Accelerating the voltage to 15 keV enabled analysis of a wide range of chemical elements from the periodic table. Due to the shape of the analyzed sample, the working distance was 10–15 mm. The samples for metallography and microhardness testing were mounted into bakelite resin (Polyfast) with carbon particles filler supplied by Struers (Roztoky, Czech Republic). This resin stabilizes the samples during mechanical preparation and microhardness testing safely. The metallography observations were performed on samples after mechanical polishing using equipment and diamond suspensions made by Struers (Roztoky, Czech Republic) with chemical etching (22 ◦ C/60 s) in a modified Vilella’s reagent [ 7 ] containing 10 parts 35% HCl, 10 parts distilled H 2 O, and 1 part 65% HNO 3 . The image capturing and evaluation was performed by an Olympus IX70 inverted metallographic microscope (Olympus, Prague, Czech Republic). 2.2. Corrosion Test Due to a very low corrosion rate of AISI 1.4441 under standard conditions, the corrosion characteristic has to be obtained by accelerated corrosion tests. Potentiodynamic polarization tests were performed in high-density polyethylene and high-density polypropylene corrosion cells with lower exposure hole which exposes 0.49 cm 2 of the tested material. A similar corrosion cell setup is often used when complex surfaces are evaluated [ 8 ]. A hardware device, Voltalab PGZ 100 with Voltamaster 10 software (Villeurbanne, France), with 3 electrode setups was used. The testing method followed ASTM F 2129, ASTM G 61, and ISO 12,732 with certain temperature and gas bubbling modifications regarding subsequent application in biomedical engineering. This setup allows bubbles formed on the surface during tests to escape freely and not to affect the continuity of the measurements. A three-electrode setup was used for precise measurement. The sample was connected as Metals 2022,12, 60 4 of 14 a working electrode, saturated calomel electrode (SCE, +241 mV vs. Saturated Hydrogen Electrode (SHE)) [ 9 ] served as a reference electrode, and a high purity carbon rod was connected as an auxiliary electrode. The physiological saline solution (0.9 wt.% NaCl in distilled H 2 O) was used as a corrosion solution for potentiodynamic polarization to intentionally simulate the environment of living tissue. The testing temperature was standard: 25 ◦C. There was a 60 min time gap applied after filling corrosion cells with a physiological solution to stabilize partial corrosion processes. Before starting the potentiodynamic polarization, the initial potential value was set to − 80 mV vs. the potential after stabilization of the corrosion equilibrium (OCP), with the polarization rate set to 60 mV · min −1 [ 10 ]. The dependence of the current flowing through the potential applied to the test sample was recorded during the measurement. The potential was gradually applied to the tested sample, which increased over time with the value of the polarization rate. There were two polarization curves measured in this experiment. The first curve represents the corrosion behavior of the chemically passivated surface and the second curve was recorded to illustrate materials’ self-passivation abilities in physiological solution. Once more, a 60 min time gap was applied between the two polarizations. Each polarization test was terminated when the value of corrosion current density reached 2 × 10 −3 A/cm −2 , which ensured that the material was located in a transpassive state and the surface was actively corroded [ 11 ]. 2.3. Wettability Test The surface angle between the sample and water was evaluated by the sessile drop method. The surface contact angle was found by the SEE (surface energy evaluation) system and free surface energy was calculated by Advex Instrument software (Brno, Czech Republic). We applied 3 µ L droplets of high purity water to the tested surface and the contact angle θ was determined by the tangent to the drop profile at the point of contact of the three phases (liquid, solid, gas) with the plane of the sample surface [ 12 ]. The free surface energy of the solid sample is determined by Young’s Equation (1), where γSV , γLV , and γSL represent the interfacial tensions per unit length of the solid-vapor, liquid-vapor, and solid-liquid contact line, respectively [13]. γSV −γSL =γLV ×cos θ(1) 2.4. Microhardness Testing Metallographic samples were tested for microhardness repeatedly. The smooth surface after diamond paste polishing allows low loading force hardness testing to be performed, so only HV 0.1 (1 N, approx. 0.1 kg) could be used to determine hardness parameters of the implant microstructure. By this method, the hardness of separate grains in microstructure could be measured easily. The test was performed according to ASTM E92 using a LECO AMH 2000 (Plzen, Czech Republic) equipped with a diamond Vicker’s indentor and a high-resolution camera. The Vickers microhardness can be calculated by Equation (2), where F is the value of applied loading force in N and d 1 and d 2 are diagonals of studied indent [14]. HV (F) = 0.189 ×F/[((d1+ d2)/2)]2(2) 2.5. Contact-Type Surface Roughness Measurement The surface roughness of the machined workpieces was measured using the conventional stylus instrument. A Taylor-Hobson Talysurf Intra 50 profilometer was used to measure the average roughness value (Ra) and average maximal height of profile (Rz) [ 15 ]. The device was equipped with a floating arm with a diamond tip of a radius of 2 µ m. A 1 mm/s canning rate was set to follow ASTM D7127 standard. The measured length was set to 5 mm due to the complicated geometrical shape of the surface to be characterized. Metals 2022,12, 60 5 of 14 3. Results 3.1. Microstructure and Metallography Observation Figure 2A shows a longitudinal cut near a threaded hole. The microstructure near the thread contains equiaxial austenitic grains with no signs of carbide and oxide particles, neither inside of grains nor at the grain boundaries, and fully corresponds with the microstructure of the bulk material presented in Figure 2B. The microstructure near the threaded hole also shows no changes in texture, grain shape, or size. This area is crucial due to its role as a stress concentrator during force loading and transmitting into the rest of the application so any microstructural changes may cause cracks formation leading to premature failure. The machining cut was precisely driven through the grains which are indicated by the smooth interface between the threaded hole and mounting resin with no significant deformation zone visible in the materials’ microstructure [16]. Metals 2022, 11, x FOR PEER REVIEW 5 of 15 3. Results 3.1. Microstructure and Metallography Observation Figure 2A shows a longitudinal cut near a threaded hole. The microstructure near the thread contains equiaxial austenitic grains with no signs of carbide and oxide particles, neither inside of grains nor at the grain boundaries, and fully corresponds with the microstructure of the bulk material presented in Figure 2B. The microstructure near the threaded hole also shows no changes in texture, grain shape, or size. This area is crucial due to its role as a stress concentrator during force loading and transmitting into the rest of the application so any microstructural changes may cause cracks formation leading to premature failure. The machining cut was precisely driven through the grains which are indicated by the smooth interface between the threaded hole and mounting resin with no significant deformation zone visible in the materials’ microstructure [16]. Figure 2. (A) Microstructure of material near the threaded hole, (B) microstructure of bulk material. 3.2. Microhardness Testing The microhardness of the samples was tested and evaluated after polishing so the diagonals of indents could be measured precisely. There was a line testing set into device software to evaluate microhardness changes from the surface into the bulk material. The zero position was set at a distance of 100 µm from the interface between the threaded hole and mounting resin and the step between each indent was set to 200 µm for both longitudinal transversal cuts. The values of each measurement are listed in Table 2 together with average and standard deviation values. Table 2. Values of microhardness HV 0.1 for transversal and longitudinal cut direction. Longitudinal Direction Transversal Direction Distance from the Thread (mm) HV 0.1 Distance from the Thread (mm) HV 0.1 0.1 346 0.1 321 0.3 327 0.3 304 0.5 302 0.5 346 0.7 338 0.7 304 0.9 338 0.9 295 1.1 332 1.1 331 1.3 308 1.3 361 1.5 327 1.5 314 1.7 327 1.7 294 1.9 333 1.9 301 Average HV 0.1 328 Average HV 0.1 317 Standard deviation 14 Standard deviation 20 Figure 2. ( A ) Microstructure of material near the threaded hole, ( B ) microstructure of bulk material. 3.2. Microhardness Testing The microhardness of the samples was tested and evaluated after polishing so the diagonals of indents could be measured precisely. There was a line testing set into device software to evaluate microhardness changes from the surface into the bulk material. The zero position was set at a distance of 100 µ m from the interface between the threaded hole and mounting resin and the step between each indent was set to 200 µ m for both longitudinal transversal cuts. The values of each measurement are listed in Table 2together with average and standard deviation values. The average microhardness is increased in comparison to previously published values [ 17 ]. As the difference between each value is significant and standard deviations are approximately 4% for the longitudinal and 6% for the transversal direction compared to average microhardness, the microstructure near the indents was further investigated. Figure 3A illustrates indents in a longitudinal cut after polishing and Figure 3B after etching where the lower magnification of microscope was used to capture more indents. Significant differences in microstructure were observed for each indent. The smallest indents (highest microhardness) were measured in grains showing a high level of deformation indicated by the presence of deformation twins [ 18 ]. Some high values also indicate the possibility of initiation of εmartensite transformation [19]. Metals 2022,12, 60 6 of 14 Table 2. Values of microhardness HV 0.1 for transversal and longitudinal cut direction. Longitudinal Direction Transversal Direction Distance from the Thread (mm) HV 0.1 Distance from the Thread (mm) HV 0.1 0.1 346 0.1 321 0.3 327 0.3 304 0.5 302 0.5 346 0.7 338 0.7 304 0.9 338 0.9 295 1.1 332 1.1 331 1.3 308 1.3 361 1.5 327 1.5 314 1.7 327 1.7 294 1.9 333 1.9 301 Average HV 0.1 328 Average HV 0.1 317 Standard deviation 14 Standard deviation 20 Metals 2022, 11, x FOR PEER REVIEW 6 of 15 The average microhardness is increased in comparison to previously published values [17]. As the difference between each value is significant and standard deviations are approximately 4% for the longitudinal and 6% for the transversal direction compared to average microhardness, the microstructure near the indents was further investigated. Figure 3A illustrates indents in a longitudinal cut after polishing and Figure 3B after etching where the lower magnification of microscope was used to capture more indents. Significant differences in microstructure were observed for each indent. The smallest indents (highest microhardness) were measured in grains showing a high level of deformation indicated by the presence of deformation twins [18]. Some high values also indicate the possibility of initiation of ε martensite transformation [19]. Figure 3. (A) Indents after polishing, (B) identic indents in the revealed microstructure. 3.3. Wettability Test The polished surface was used for the wettability test to avoid any effect of local roughness or unevenness on contact angle values [20]. Before the test, the sample was cleaned in an ultrasonic bath firstly in acetone, then in double distilled water with the testing surface facing up to avoid being scratched. Average values of contact angle and calculated surface energy and their standard deviations are presented in Table 3. The standard deviation of the presented surface energy is not symmetrical due to the cosine function used for its calculation. Representative images of droplets on tested surfaces are shown in Figure 4. Table 3. Value of measured contact angle and calculated surface energy. Sample Contact Angle (°) Surface Energy (mJ·m−2) Close thread 50 ± 4 53.5 + 2.6; −2.3 Between threads 52 ± 2 53.6 + 1.2; −1.2 Figure 3. (A) Indents after polishing, (B) identic indents in the revealed microstructure. 3.3. Wettability Test The polished surface was used for the wettability test to avoid any effect of local roughness or unevenness on contact angle values [ 20 ]. Before the test, the sample was cleaned in an ultrasonic bath firstly in acetone, then in double distilled water with the testing surface facing up to avoid being scratched. Average values of contact angle and calculated surface energy and their standard deviations are presented in Table 3. The standard deviation of the presented surface energy is not symmetrical due to the cosine function used for its calculation. Representative images of droplets on tested surfaces are shown in Figure 4. Table 3. Value of measured contact angle and calculated surface energy. Sample Contact Angle (◦) Surface Energy (mJ·m−2) Close thread 50 ±4 53.5 + 2.6; −2.3 Between threads 52 ±2 53.6 + 1.2; −1.2 3.4. Corrosion Testing Methods There were two polarization curves collected for the purpose of precise investigation of the tested application corrosion properties. The testing was repeated at the same location with no change of testing instrument. The first polarization curve represents the electrochemical properties after surface finishing procedures (machining, grinding, Metals 2022,12, 60 7 of 14 polishing, degreasing, chemical passivation, sterilization, etc.). The second curve illustrates the behavior of the same material as the previous polarization actively removed more thermodynamically active system elements (thick passive layer, secondary phases particles, oxide layers, deformed material layer, etc.) after the spontaneous formation of the passive layer in physiological solution on the previously corroded surface. Values of current density (Y-axis) and potential (X-axis) were continuously recorded during polarization. After the polarization procedure, the semilogarithmic polarization curve was drawn up from these points and is illustrated in Figure 5. Finally, the corrosion properties (corrosion potentials, polarization resistance corrosion current density, and a corrosion rate) were calculated from the initial part of the polarization curves with the characteristic “V-shape” by Tafel extrapolation automatically using Volta Master 10 software [ 21 ]. These are listed in Table 4. There was an exchange of two electrons (Fe 0→ Fe 2+ , Ni 0→ Ni 2+ , Cr 0→ Cr 3+ ), and an average material molar mass of 56.2 g/mol and density of 7.8 g/cm 3 was considered for calculation of corrosion rate by Faraday laws [22]. Metals 2022, 11, x FOR PEER REVIEW 7 of 15 Figure 4. Water droplets on the surface of the tested sample. The value of contact angle evaluated is given. 3.4. Corrosion Testing Methods There were two polarization curves collected for the purpose of precise investigation of the tested application corrosion properties. The testing was repeated at the same location with no change of testing instrument. The first polarization curve represents the electrochemical properties after surface finishing procedures (machining, grinding, polishing, degreasing, chemical passivation, sterilization, etc.). The second curve illustrates the behavior of the same material as the previous polarization actively removed more thermodynamically active system elements (thick passive layer, secondary phases particles, oxide layers, deformed material layer, etc.) after the spontaneous formation of the passive layer in physiological solution on the previously corroded surface. Values of current density (Y-axis) and potential (X-axis) were continuously recorded during polarization. After the polarization procedure, the semilogarithmic polarization curve was drawn up from these points and is illustrated in Figure 5. Finally, the corrosion properties (corrosion potentials, polarization resistance corrosion current density, and a corrosion rate) were calculated from the initial part of the polarization curves with the characteristic “V-shape” by Tafel extrapolation automatically using Volta Master 10 software [21]. These are listed in Table 4. There was an exchange of two electrons (Fe0→Fe2+, Ni0→Ni2+, Cr0→Cr3+), and an average material molar mass of 56.2 g/mol and density of 7.8 g/cm3 was considered for calculation of corrosion rate by Faraday laws [22]. Table 4. Corrosion properties obtained by Tafel extrapolation. Curve NO. Corrosion Potential Ecor (mV vs. SCE) Corrosion Rate CR (μm/Year) Polarization Resistance Rp (kΩ·cm2) Corrosion Current Density JC (μA/cm2) 1 134 0.76 107 0.065 2 414 2.01 84 0.172 Figure 4. Water droplets on the surface of the tested sample. The value of contact angle evaluated is given. Metals 2022, 11, x FOR PEER REVIEW 8 of 15 Figure 5. Polarization curves of 1.4441 for first and second polarization in physiological solution. As the anodic part of the curve exhibits no inflection point, the pitting potentials represented by orange and blue arrows were determined from the critical value of current density 10 µA/cm2, which is given by the dotted green line in Figure 5. This potential is formally defined by destabilization of the passive layer or formation and spontaneous growth of corrosion pits, respectively. The values for each curve are marked on the potential axis. As there are metal ions released from the material during the spontaneous corrosion process, the weight of released ions during the time interval from defined surfaces can be theoretically calculated from the chemical composition of the steel and values of corrosion current density by modification of Faraday’s law of electrolysis (1). The following calculation is shown for calculation of the mass of released ions during a year period. mz + = (JC × t × M × X)/(F × z) (3) where: • m is the mass of ions released during corrosion in grams, • JC is corrosion current density, • t is time in seconds (31536000 s/year), • M is the molar mass of the substance in grams per mol, • X is the atomic volume fraction of metal in steel composition (e.g., 0.15 for Ni in 1.4441 steel), • F is the Faraday constant (96.485 Coulomb per mol), andS • z is the valency number of ions (electrons transferred per ion during the reaction). Equation (1) was used for calculation of theoretical ion release from 1 cm2 of material during its 1-year period of exposure under conditions used for corrosion test. Table 1 provides the maximal atomic volume fraction of manganese, chromium, molybdenum, and nickel which were considered for the calculations. Calculated results are given in Table 5, which also shows the results after recalculation to the amount of substance of released ions. Figure 5. Polarization curves of 1.4441 for first and second polarization in physiological solution. Metals 2022,12, 60 8 of 14 Table 4. Corrosion properties obtained by Tafel extrapolation. Curve NO. Corrosion Potential Ecor (mV vs. SCE) Corrosion Rate CR(µm/Year) Polarization Resistance Rp (kΩ·cm2) Corrosion Current Density JC (µA/cm2) 1 134 0.76 107 0.065 2 414 2.01 84 0.172 As the anodic part of the curve exhibits no inflection point, the pitting potentials represented by orange and blue arrows were determined from the critical value of current density 10 µ A/cm 2 , which is given by the dotted green line in Figure 5. This potential is formally defined by destabilization of the passive layer or formation and spontaneous growth of corrosion pits, respectively. The values for each curve are marked on the potential axis. As there are metal ions released from the material during the spontaneous corrosion process, the weight of released ions during the time interval from defined surfaces can be theoretically calculated from the chemical composition of the steel and values of corrosion current density by modification of Faraday’s law of electrolysis (1). The following calculation is shown for calculation of the mass of released ions during a year period. mz+ = (JC×t×M×X)/(F ×z) (3) where: •m is the mass of ions released during corrosion in grams, •JCis corrosion current density, •t is time in seconds (31,536,000 s/year), •M is the molar mass of the substance in grams per mol, • X is the atomic volume fraction of metal in steel composition (e.g., 0.15 for Ni in 1.4441 steel), •F is the Faraday constant (96.485 Coulomb per mol), andS •z is the valency number of ions (electrons transferred per ion during the reaction). Equation (1) was used for calculation of theoretical ion release from 1 cm 2 of material during its 1-year period of exposure under conditions used for corrosion test. Table 1 provides the maximal atomic volume fraction of manganese, chromium, molybdenum, and nickel which were considered for the calculations. Calculated results are given in Table 5, which also shows the results after recalculation to the amount of substance of released ions. Table 5. Theoretical mass and amount of ions released from 1 cm2during the 1-year exposition. Polarization µg/(Year·cm2) Mn Cr Mo Ni 1st polarization 12.5 118.3 19.9 99.6 2nd polarization 32.9 312.6 52.6 263.2 Polarization µmol/(Year·cm2) Mn Cr Mo Ni 1st polarization 0.2 2.3 0.2 1.7 2nd polarization 0.6 6.0 0.5 4.5 After the corrosion test was terminated, the area of interest was observed using a scanning electron microscope (SEM, FEI QANTA FEG 450, Brno, Czechia), in a regime of secondary (SE) and backscattered (BSE) electrons. The semiquantitative chemical analysis of marked areas was performed by energy-dispersive X-ray spectroscopy analysis (EDX). The result of the observation is illustrated in Figure 6. There was no significant difference Metals 2022,12, 60 9 of 14 in the chemical composition of analyzed areas found by EDX, which indicates significant solubility of corrosion products [23]. Metals 2022, 11, x FOR PEER REVIEW 9 of 15 Table 5. Theoretical mass and amount of ions released from 1 cm2 during the 1-year exposition. Polarization μg/(Year·cm2) Mn Cr Mo Ni 1st polarization 12.5 118.3 19.9 99.6 2nd polarization 32.9 312.6 52.6 263.2 Polarization μmol/(Year·cm2) Mn Cr Mo Ni 1st polarization 0.2 2.3 0.2 1.7 2nd polarization 0.6 6.0 0.5 4.5 After the corrosion test was terminated, the area of interest was observed using a scanning electron microscope (SEM, FEI QANTA FEG 450, Brno, Czechia), in a regime of secondary (SE) and backscattered (BSE) electrons. The semiquantitative chemical analysis of marked areas was performed by energy-dispersive X-ray spectroscopy analysis (EDX). The result of the observation is illustrated in Figure 6. There was no significant difference in the chemical composition of analyzed areas found by EDX, which indicates significant solubility of corrosion products [23]. Figure 6. (A) Surface after corrosion test in SE, (B) surface after corrosion test in BSE with marked areas of EDX analysis, (C) EDX spectrum for area No.1, (D) EDX spectrum for area No.2. Figure 6. ( A ) Surface after corrosion test in SE, ( B ) surface after corrosion test in BSE with marked areas of EDX analysis, (C) EDX spectrum for area No.1, (D) EDX spectrum for area No.2. 4. Discussion The metallic implant can release various elements into the surrounding tissues during their lifetime. If the concentration of released elements reaches critical doses, these elements can induce adverse effects around the area of implantation or in the whole body. Therefore, it is necessary to determine the potential toxicity of metal ions and metal-based particulates that may be released from medical implants. Medical implants are exposed to diverse conditions related to pH and the internal environment of the human body and consequently undergo various types of corrosion leading to the release of metal ions or wear particulates emission [24–26]. Although there are various methods of surface treatment of implants of stainless steel, the chemical passivation is both very simple and also one of the most effective, therefore it had previously been standardized according to ASTM or ISO for specific application in implantology [ 27 ]. During this procedure, a bath of oxidizing acid is used. In this study, a bath of 30% nitric acid was used. During the process of passivation, the chromium contained in the materials is primarily oxidized into the form of chromium trioxide which acts as a barrier on the free surface [ 28 ]. The composition of a passive layer formed on the