Electrochemical Characterization and Micromachining of Nitinol SMA by WECM Using Citric Acid Mixed H₂SO₄ Electrolyte

The potentiodynamic polarization (PDP) investigation has been carried out on Nitinol shape memory alloy (SMA), and the resulting voltammograms as per the ASTM standard are shown in Fig. 4 as polarization curves created by two different electrolytes and their consequences. For Nitinol, two distinct electrolytes i.e. 0.1 M H₂SO₄ and mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte were used in the corrosion investigations. The electrolyte conductivity plays very important role in all electrochemical experiments, especially in the cyclic polarization technique. The externally polarized current-potential behavior of a metal near its corrosion potential gives an excellent estimate of the rate of corrosion.

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The polarization curve depicts corrosion potential against current and the influence of these electrolytes on Nitinol propensity to localized corrosion has been studied.

It is evident from the ensuing voltammograms in both PDP tests that localized corrosion is not present under either electrolyte state, but uniform corrosion in the transpassive or oxygen evolution zone may occur. These graphs show that Nitinol is more resistant to the start and spread of localized corrosion, assuming that the corrosion potentials of nitinol in both the electrolyte obtained at a fixed scan rate in these tests are similar or having negligible difference. The voltage at which the anodic current grows quickly in this test method indicates the vulnerability to the start of localized corrosion.

The test results reveals that the corrosion potential shifted towards the anodic forward direction with decrease in corrosion current as the surface roughness decreased without significant difference in cathodic currents among both the nitinol samples with different surface roughness in different electrolyte due to the different anodic behavior of the alloy. The specimens in 0.1 M H₂SO₄ electrolyte showed increase in the anodic current compared to mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte which may be responsible for increase in surface roughness due to increase in corrosion rate. The decrease in surface roughness occurs with lower corrosion rate and very high corrosion rate results in pitting resulting in rougher surface. It shows that the higher roughness corresponds to a lower pitting potential. The passive oxide layer can form due to formation hydroxide and gas bubbles. The overall surface-to-electrolyte contact area is greatly reduced by the heterogeneous interface that forms between the solution and the surface, which significantly slows down corrosion. The corrosion current of 1 × 10⁻¹⁰ A is achieved with the acidic H₂SO₄ electrolyte, which is identical to the mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte, but this mixed electrolyte reached the corrosion potential of −0.1 V, which is higher. This test demonstrates the acidic H₂SO₄ electrolyte's excellent active dissolution process. It demonstrates that the controlled dissolution of Nitinol with low current of mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte gives lower corrosion rate, further improving the surface roughness which has been measured using CCI profilometer. Among the selected electrolytes, 0.1 M H₂SO₄ electrolytes have been discovered to be the most corrosive solution. Due to a low passive current and a greater potential, Ni ions present in electrolyte show a lesser tendency to corrode when used in an electrolyte solution containing mixed 0.1 M H₂SO₄ + 0.1 M citric acid. It was discovered that the composition of the electrolyte caused differences in the passive zone of the polarization curves of various electrolytes. The anions present during process have considerable effect on the corrosion behavior of nitinol. The polarization study suggests that mixed 0.1 M H₂SO₄ and citric acid electrolyte solution may effectively machine nitinol. In contrast to other electrolytes, 0.1 M H₂SO₄ in the presence of citric acid electrolyte can be used to generate the smooth surface. The surface topography in the solution of 0.1 M H₂SO₄ electrolyte and mixed 0.1 M H₂SO₄ + 0.1 M citric acid, as shown in Figs. 5a and 5b.

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To achieve high surface quality, the citric acid solution serves as a safe organic acidic electrolyte. According to the findings of the potentiodynamic tests, the surface roughness of the nitinol sample in mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte solution was Sa = 0.5100 μm, which is much less than the surface roughness Sa = 0.7393 μm of 0.1 M H₂SO₄ electrolyte solutions. Thus, it is considered that the electrochemical machinability of the nitinol alloy is better in mixed electrolyte.

The working electrode i.e. nitinol is applied a modest amplitude sinusoidal voltage across a broad frequency range in electrochemical impedance spectroscopy (EIS) using.1 M H₂SO₄ and mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte. Another sinusoidal signal with amplitude (I) and a phase shift in relation to the input signal is produced at each frequency. The imaginary component (Z'') is shown against the real component (Z') for each excitation frequency in an EIS data Nyquist plot as shown in Figs. 6a and 7b. Plots of Bode-magnitude and Bode-phase enable analysis of absolute impedance as shown in Figs. 6b, 6c and 7b, 7c The notion of constant phase element (CPE) may be applied to the modeling of an electrical double layer to correct the capacitor's non-ideal behavior. As illustrated in Figs. 6d and 7d, the surface-deposited double layer capacitor is linked in parallel with the polarization resistor (Rp), which is coupled in series with the bulk resistor (Rs). An electrochemical system analysis starts with this model. The resistance of Rs moves the semicircle's beginning point to higher Z' values in the Nyquist plot of a basic Randles cell. The polarization resistance is also represented by the semicircle's diameter. The equivalent circuit's Rs, Rp, and CPE component is fitted to the semicircle, which represents the rapid oxidation and reduction kinetics. The amount of the current during the polarization of nitinol as working electrode is governed by reaction kinetics. The lower corrosion current results from higher polarization resistance. When an electrode's potential is deviated from its value at corrosion potential, the electrode becomes polarized. Due to electrochemical processes that are triggered at the electrode surface by the polarization of an electrode, current flows. Nitinol alloys with high polarization resistance Rp have good corrosion resistance, whereas those with low Rp have low corrosion resistance. During an electrochemical reaction, the roughness values are low at strong corrosion resistance and a drop in corrosion rate. A smart way to manage corrosion, incidentally, is to increase the solution resistance since a reduction in corrosion current results in a reduction in corrosion rate. In this approach, the solution resistance (Rs) is obtained from the high-frequency intercept of real impedance, and the sum of Rs and Rp is obtained from the low-frequency intercept. When Rp is known, the Stern-Geary equation may be used to calculate the metal corrosion rate.

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To confirm the conclusion drawn in the EIS section, the polarization resistances (Rp) were determined and examined using the Nyquist and Bode plots as can be shown in Figs. 6 and 7. The potentiodynamic tests order of the polarization resistances and the EIS findings order of the resistances accord with one another. A qualitative indicator of corrosion resistance is the capacitive loop's diameter in the Nyquist plot; a greater diameter denotes stronger corrosion resistance for nitinol in respective electrolytes. It shown that for the case of metal under corrosive conditions, the resistance behavior predominates in the low frequency zone and that the Bode impedance values at low frequency may be used as a gauge of the corrosion resistance. The EC system was modeled using comparable electric circuits in order to support this result and quantify the corrosion behavior of the nitinol samples in both electrolytes. The ideal circuit for each sample was chosen using the Nyquist and Bode plots of each circuit. In order to account for the electrolyte behavior and the resistance of the oxide layer, each circuit contains CPE and Rs causes a potential drop between the working electrode i.e. nitinol and reference electrode i.e. Ag/AgCl cell causing errors and Rp behaves like a resistor. The polarization resistance of the nitinol sample in 0.1 M H₂SO₄ electrolyte was 123 Ω is improved to 166 Ω when compared to the nitinol sample in 0.1 M H₂SO₄ with citric acid electrolyte. It is evident from the values of the resistances Rp and Rs that the corrosion resistance of the Nitinol sample dissolute in 0.1 M H₂SO₄ electrolyte is significantly lower than that of the 0.1 M H₂SO₄ and citric acid electrolyte.

The values of the CPE can be dependably utilized to detect the thickness of the oxide film. The sample which shows the highest resistance also has the lowest oxide layer thickness. The interpretation for this inconsistency is the porous oxide film leading to surface area larger than the geometric area. It shows that the nitinol using mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte has lower oxide layer thickness which is responsible for uniform anodic dissolution.

The EIS findings are consistent with the fact that the corrosion resistance of the nitinol alloy is linked to the development of a tough titanium oxide layer on the surface. The Bode plot in the high frequency region showed very low values of phase angle near to 0°. The layer is displaying pure capacitive behavior when the phase angle is near to 90° in the low frequency band. The Bode-phase charts, where the greatest phase angle obtained was around 90°, show a considerable amount of departure from perfect capacitive behavior. The changes in electrolyte conductivity are to blame for variations in the passive oxide layer's resistance. The changes in the surface's roughness are what caused the observed increase in corrosion resistance. The results of frequency response study are connected with the electrochemical process model at the corrosion system interface. The most common way to describe that concept is in terms of an electrical equivalent circuit. The results of the current experiments demonstrate that the nitinol's corrosion resistance is increased by the process-induced surface smoothing as well as the spontaneous creation of a passive layer. The smoothening of the surface and the development of a thin layer of TiO₂ on it are related to the alloy's increased corrosion resistance with uniform corrosion behavior. The corrosion resistance of the nitinol in mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte solution is marginally superior to that of sulfuric acid alone. It is commonly recognized that the EIS measurement's outcome may be used to effectively define a material's surface condition.

Using this electrochemical characterization study for nitinol using 0.1 M H₂SO₄ and mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte, the best and most appropriate electrolyte was identified for further machining during WECM. The concentrations used in the tests and the electrolytic concentrations used in this experimental investigation during WECM were identical. The nitinol has higher electrochemical machinability in terms of machining potential and dissolved surface roughness. Its micro structural compositions are determined by the WECM, which also has a significant impact on how well it resists corrosion as discuss below.

In the WECM process, it is crucial to regulate the groove dimensions (width and depth) by establishing certain input process parameters. These geometrical parameters talk about the process's precision, quality, and controllability. From the previous research work and literature review, it is seen that the pulse voltage is most influencing parameter during machining of nitinol micro grooves using WECM. Under the same set of process settings, the effect of pulse voltage on various groove dimensions i.e. groove width, groove depth and surface roughness have been investigated in order to conduct meaningful research. This investigation's main motivation is to determine how various electrolytic media impact the behavior of electrolysis products at the tool and workpiece. The effect of different pulse voltages ranging from 5 to 8 V on average machined groove width with a maximum tool immersion depth of 100 μm for all experiments at a feed rate of 1.4 μm/s using 0.1 M H₂SO₄ electrolyte and mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolytes has been shown in Fig. 8. As the current density rises due to increase in pulse voltage, the groove's width also increases due to higher dissolution as material removal rate increases with increase in current density. It is well known that H₂SO₄ is strong inorganic acid and citric acid is weak organic tri carboxylic acid. A strong acid will have higher dissociation constant than a weak acid. The dissolution rate of citric acid electrolyte is quite lower as compared to H₂SO₄ electrolyte. An electrolyte with low water content can inhibit the formation of the passive layer since water is thought to play a significant part in the passivation process. Figures 9 and 10 shows the machined microgrooves of nitinol during WECM at pulse voltages ranging from 5 to 7 V in 0.1 M H₂SO₄ electrolyte and mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolytes, respectively. As can be seen, at constant wire feed rate of 1.4 μm/s, 45% duty ratio and other fixed parameters as per Table I, the minimum average machined groove width is 86.20 μm in 0.1 M H₂SO₄+0.1 M citric acid at 5 V pulse voltage as shown in Fig. 9 which is less as compared to 99.125 μm groove width produced using 0.1 M H₂SO₄ electrolyte solution as shown in Fig. 10. When the pulse voltage is 7 V, the groove width is 137. 927 μm which is larger in 0.1 M H₂SO₄+ 0.1 M citric acid which further increases to 145.433 μm in 0.1 M H₂SO₄ electrolyte solution because the dissolution reactivity is greater and passive layer is less stable according to the polarization characteristics. It is thought that high pulse voltage produces more stray currents than tiny, low pulse voltage. At mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte solution, the machining groove width reduces as the electrolyte concentration was most suitable for acquiring proper anodic dissolution of nitinol because the specific electrolyte resistivity is inversely proportional to the electrolyte concentration. The groove width overcut (${{\rm{\Delta }}}_{g}=\displaystyle \frac{{W}_{g}-{d}_{w}}{2}\,,$ where ${{\rm{\Delta }}}_{{\rm{g}}};$ goove width overcut, ${{\rm{W}}}_{{\rm{g}}};$ groove width, ${{\rm{d}}}_{{\rm{w}}};$ diameter of wire) has been greatly reduced with addition of ecofriendly organic citric acid in sulphuric acid due to controlled current density distribution and toxicity of the strong H₂SO₄ electrolyte reduced by taking advantages of eco-friendly citric acid in the electrolyte solution for microgroove fabrication during WECM of nitinol. Citric acid-containing solutions enable for less current to be created than citric acid-free solutions. When citric acid is present in the electrolyte solution, this leads to a lower material removal rate, which also accounts for the decrease in width overcut.

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The production of the insoluble precipitates is decreased when an environmentally friendly citric acid complexing agent is added to the 0.1 M H₂SO₄ aqueous electrolytes. In contrast to acids, these complexing agents are non-toxic and non-corrosive, making them acceptable for use in WECM. The minimum average machined groove width overcut was 18.10 μm in 0.1 M H₂SO₄ + 0.1 M citric acid at 5 V pulse voltage which is less compared to width overcut of 24.85 μm produced at 0.1 M H₂SO₄ electrolyte in same parametric machining condition. The effect of pulse voltages on maximum groove depth are as shown in Fig. 11.

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It shows how the depth of the groove rises as the pulse voltage increases. According to the aforementioned factor, material removal rate increases as pulse voltage increases, increasing the current density at inter electrode gap near machining zone. The maximum depth of groove at 5 V using 0.1 M H₂SO₄ was 73.03 μm which is further increased to 98.9 at 5 V pulse voltage as shown in Fig. 12. The addition of organic citric acid diluted the concentration of strong sulphuric acid to some extent with reduction in toxicity by acting as a complexing agent to the electrolyte solution. However, the depth of the groove also reduced to some extent which can be further enhanced by increasing the molar concentration of citric acid in the solution.

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The deepest machining groove i.e. 93.9 μm has been obtained at 7 V pulse voltage using mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte as shown in Fig. 13. From 5 V to 7 V pulse voltage at constant wire feed rate, the depth of the groove increases. A parabolic form may have been created because, according to a thorough investigation, the entrance of the groove, where the width is measured, was more susceptible to higher machining times than the groove end. However, at low current densities compared to high current densities, the variation in the depth of each groove is greater.

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Further, experimental results reveal that the depth of groove using 0.1 M H₂SO₄ electrolyte was higher as compared to the mixed 0.1 M H₂SO₄ + 0.1 M citric acid solution. It is due to the less current density for machining for same pulse voltage conditions in mixed electrolyte.

The sulphuric acid is strong inorganic acid which has a capability of producing maximum current without producing any dissolved products. However more concentration adversely affects the machining process and produces overcut with poor homogeneity and surface quality. The depth of groove decreases with increasing machining time due to lower wire feed rate. However, when machining time rises at lower wire feed rates, the average groove width inaccuracy grows. As IEG between the machined surface and the wire electrode grows, the depth of the groove decreases with increase in width overcut. Because of this, the electrochemical reaction is more active along the sides of each groove than it is vertically into the material's surface.

The surface properties of the various electrolytes employed also vary, which affects how the machining features work. A slight change in potential during the process may result in a decrease in current, which will inevitably cause a drop in current efficiency. The electrolyte with a very low concentration (0.05 M) resulted in poor machined surface. The ability to produce smooth surface micro-features in mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte solution was particularly useful due to its uniform dissolution in the required concentration. To obtain smooth machined surfaces in WECM, the passive layer function as insulators to prevent corrosion on the machined surfaces. An acid electrolyte composed of sulfuric acid (0.1 M H₂SO₄), citric acid (0.1 M), and distilled water (H₂O) was employed to identify a suitable electrolyte for machining Nitinol. Cit3 ion, a polyhydroxy carboxylic acid group ligand that functions as a chelating agent with significant complexing capacity, is present in citric acid. Numerous donor atoms in the ligand are capable of swiftly transferring electrons into the coordination sphere of metal cations. As a result, the link between the metal and oxygen becomes polarized and gradually weakens, allowing metal ions to dissolve. As a result, in the case of a mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte, Ni is readily dissolved, leading to a uniform dissolution and a reduction in surface roughness. In contrast to mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte solution, the microgroove created while machining Nitinol using 0.1 M H₂SO₄ electrolyte solution was uneven in shape and had poor surface quality. The surface that was achieved in the 0.1 M H₂SO₄ electrolyte was subpar, and the micro grooves that were created had significant electrolytic assault on them in and around the machined surface. Since H₂SO₄ is not environmentally friendly, a mixed electrolyte of H₂SO₄ and citric acid is provided, and its performance is determined to be superior to H₂SO₄. A nitinol sample with an initial surface roughness of 4.076 μm was machined to a roughness of 0.0631 μm using mixed 0.1 M H₂SO₄ + 0.1 M citric acid at 5 V pulse voltage which is lower as compared to 0.09055 μm produced using 0.1 M H₂SO₄ electrolyte for these experiments. The effect of pulse voltage on surface roughness using 0.1 M H₂SO₄ and mixed 0.1 M H₂SO₄ + 0.1 M citric acid is shown in Fig. 14.

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For anode passivity, the voltage is more negative than the other voltage in the cathode, which makes the disparity perceptible with regard to the applied voltage. Once the oxide layer ruptures, the hostile ions are delivered through it to the contact between the oxide and the metal. The aggressive anions go to the interface through oxygen vacancies. Under the passive layer, the metal substrate is attacked and dissolved by the anions.

Then, as part of the oxide layer-thinning mechanism, the aggressive anions adsorb onto the material's passive layer and combine with the oxide to form complexes, which often form in groups. This causes the oxide layer to dissolve locally up to the depth of the metal. As the Ni ions spread into the layer, the defects further break. Since the anode would react with water and oxygen molecules and reoxidize, these faults in the oxide layer cannot penetrate to the anode.

Figures 15 and 16 displays the 3D surface view of the microgrooves made using this the 0.1 M H₂SO₄ and mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte. It is observed that the surface roughness increases with increase in pulse voltage. The machining in 0.1 M H₂SO₄ electrolyte produces an average surface roughness of 0.3943 μm at 7 V and reduced to 0.09055 μm at 5 V as shown in Fig. 15. Because of the poor surface that was formed in the 0.1 M H₂SO₄ electrolyte, the surface roughness increased. Due to poor anodic dissolution and etching, the surface roughness of the 0.1 M H₂SO₄ electrolyte is increased, and the electrolyte has a high current carrying capacity. When mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte were used to machine nitinol, the quality of the microgrooves created with electrolytic attack was superior to that of 0.1 M H₂SO₄ electrolyte in terms of surface integrity and Fig. 16 depicts the surface that was produced during the machining of nitinol using mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte. The micro groove created with 0.1 M H₂SO₄ electrolyte mixed with citric acid is better than that of the 0.1 M H₂SO₄ electrolyte, and the electrolytic attack is very low (i.e., no ions are observed impinging), but the machining speed is slower than with 0.1 M H₂SO₄ electrolyte. Smooth dissolving takes place while machining nitinol with mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte. Ions slowly tear the anodic film, allowing for anodic disintegration, due to the adsorption of the film. The metal ions instead form oxides, which results in a much slower reaction and longer machining times since they do not produce oxygen vacancies. Due to the existence of electrolyte passivating ions, the surface produced in mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte is comparably excellent. With low current density, the passivating ions firmly migrate towards the surface and subtly damage it. This specific electrolyte produced surfaces with an average surface roughness of 0.3193 μm at 7 V and reduced to 0.0631 μm at 5 V. The capacity to smoothly and effectively burst the generated anodic film is excellent. Except at higher voltages, mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte offer the least amount of surface roughness compared to the 0.1 M H₂SO₄ electrolyte. Further, it has been observed that the surface roughness was increased with increase in machined groove depth for 0.1 M H₂SO₄ electrolyte and mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte as shown in Fig. 17. It is due to the fact that the groove depth increases with increase in current density. Formation of sludge, precipitates and generation of bubbles at IEG near the machining zone at higher pulse voltages contributes in an increase in surface roughness.

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In comparison to other neutral electrolytes, there is less oxygen on the Nitinol surface after WECM employing these acid electrolytes. Having a persistent TiO₂ layer on its surface gives it great corrosion resistance. When it comes into touch with an environment that contains oxygen, this oxide layer is created. The nitinol anodic dissolution is hampered by the extremely passive oxide layer that forms. With the molecular formula C₆H₈O₇, citric acid is a type of environmentally friendly complexing agent that has the capacity to interact with most metal ions and produce a soluble complex molecule. H₂SO₄, a passive electrolyte, slows down the electrolysis processes and produces thick TiO₂ layer. A thick TiO₂ layer makes it extremely difficult to machine in the axial direction and results in significant radial overcuts. Therefore, it is essential to keep TiO₂ layer during machining to a minimum so that only stray currents are diminished. Citric acid is a chelating substance that creates soluble complexes with titanium ions, which inhibits the synthesis of TiO₂ when present. This implies that just a thin layer of titanium oxide is generated using citric acid-based solutions, resulting in the machining of micro-grooves with fewer overcuts. Citric acid-containing solutions enable for less current to be created than citric acid-free solutions. When citric acid is present in the electrolyte solution, this leads to a lower material removal rate, which also accounts for the decrease in overcut. The production of the insoluble precipitates is decreased when an environmentally friendly citric acid complexing agent is added to the 0.1 M H₂SO₄ aqueous electrolytes. In contrast to acids, these complexing agents are non-toxic and non-corrosive, making them acceptable for use in WECM.

Further, the micro slit was fabricated at optimized parameter conditions of 5 V pulse voltage, 45% duty ratio and 1.4 μm s⁻¹ shown in Fig. 18.

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The minimum average slit width of 108.325 μm using 0.1 M H₂SO₄ electrolyte with minimum average surface roughness of 0.1217 μm and 92.234 μm using mixed 0.1 M H₂SO₄ + 0.1 M Citric acid electrolyte with minimum average surface roughness of 0.0691 μm shows the capability of WECM for micro-features fabrication without affecting original chemical composition of difficult to cut material i.e. nitinol etc. Although WECM with a neutral electrolyte helped lessen surface roughness, it also made the surface black. However, the mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte forms thin oxide layer that serves as a layer of corrosion resistance forms with uniform dissolution. Because TiO₂ is formed with a negative enthalpy, nitinol undergoes passivation naturally. During WECM, this oxide layer pushes Ni atoms away from the surface, depleting the surface of Ni and also removing the Ni in the form of nickel oxide (NiO). Due to advancements in corrosion resistance, biocompatibility, and surface quality as compared to conventional surface preparation procedures, mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte during WECM is appealing for the preparation of nitinol surfaces. The increase of oxygen value 7.06 in 0.1 M H₂SO₄ and 4.13 in mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte shows formation of oxide layer on nitinol surface after machining. The lower of oxygen in mixed 0.1 M H₂SO₄ + 0.1 M citric acid electrolyte show production of thin oxide layer compared to 0.1 M H₂SO₄ electrolyte which has proven to create uniform dissolution and improvement in homogeneity and surface quality of nitinol during WECM.