Z.Shhri, S.R.Allhkrm,?, R.Soltni, H.Jfri

a School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O.Box: 11155-4563, Tehran, Iran

b Materials Engineering Department, Faculty of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran

Abstract Plasma electrolyte oxidation (PEO) is a surface treatment method with high dependency on the process parameters.This paper focuses on maximizing the corrosion resistance of PEO coatings applied on Mg-5Zn-0.4Ca (ZX504) alloy by optimizing the process parameters.For this purpose, the Taguchi method based on L18 orthogonal array with mixed level design was used for optimization and determining effective parameters.Main process factors including electrolyte concentration, current density, frequency and duty cycle were considered at different levels.The corrosion resistance, as the performance indicator, was obtained using electrochemical impedance spectroscopy technique.Surface characteristics were also evaluated using SEM(scanning electron microscopy),EDS(energy dispersive spectroscopy),profilomete and contact angle goniometer.The statistical analysis showed that the optimum condition could be obtained at a current density of 200mA/cm2, frequency of 500Hz and at a duty cycle of 30%, in an electrolyte containing 15g/L Na3PO4·12H2O and 10g/L KF.

Keywords: Plasma electrolyte oxidation; Optimization; Taguchi; Corrosion; ZX504 alloy.

1.Introduction

PEO (plasma electrolytic oxidation) method is a new surface treatment technique capable of forming well adhered nano structured ceramic coatings on valve metals such as Mg alloys, in order to improve their corrosion and wear resistances [1-9].This electrochemical technique involves the creation of an oxide layers analogous to anodized film with incorporation of species originating from both the substrate and the solution.However, alkaline electrolytes and special current regimes are used to work at potentials higher than the breakdown potentials of growing oxide films Plasma discharges created during this process could result in high temperature and conversion of the growing coating into crystalline phases [7,10].In this method, coatings may be formed under various current regimes, including direct DC, periodic DC(unipolar and bipolar)and alternating AC current.In PEO technology, AC and periodic DC electrical regimes supply higher control over plasma chemical processes compared to DC regimes, thus creating a more uniform coating with less porosity.Yerokhin and co-workers [11] found that the pulse unipolar DC (PUP) regime for magnesium provided superior quality coatings as compared to the pulsed bipolar (PBP).This can be attributed to H2liberation at the coating-substrate interface throughout the negative biasing cycles.A two layer structure is usually ascertained with a comparatively thinner and more compact barrier layer at the substrate-coating interface together with an outer porous region that is resulted due to the eruption of reaction products via the discharge channels[6].In the plasma electrolyte oxidation process, parameters including current density, voltage, frequency, duty cycle and electrolyte composition play significan roles on the coating’s properties such as corrosion resistance [8-12].Therefore,investigating the optimization of these parameters is crucial step to obtain better coating properties.For this purpose,experimental design techniques (DOE) can be employed to fin the proper parameters to achieve required properties.

Taguchi method is a statistical and effectual technique of design of experiment (DoE) widely utilized in engineering analysis for optimization, process characterization and modeling, based on orthogonal array experiments.It consists of a special plan of experiments using an orthogonal array (OA)that helps in reducing variations in a process and determining the most significan function parameters [13-18].By means of this method, the optimum process condition can also be recognized to get rid of the traditional approach of changing one factor as holding the other factors constant (one factor at a time).In that old methodology, valuable information on combined effect of two or more factors may not be recognized.Taguchi employs a minimum possible matrix of combinations to improve quality of a system at minimum time and cost with more extensive results and information on the performance of a specifie process.The results interpretation is based on a statistical quantify of performance via the signal to noise ratio(S/Nor SNR) and the averages analysis.S/N analysis is a measure of the desired signal (mean) to the level of noise (standard deviation) which considers effectively the mean and variability encountered into account.According to the objective of experiments, theS/Nratio character can be separated into three categories for the response performance measuring; (i) smaller is better, (ii) larger is better and (iii)nominal is better [19].

Therefore, the aim of the present study is to achieve the optimum PEO process parameters condition on magnesium alloy for suitable corrosion performance, using Taguchi approach.Taguchi L18 orthogonal array technique with mixed level design with four parameters at three levels and one factor at six levels was carried out.

2.Materials and method

2.1.Preparation of materials

The Mg-5wt%Zn-0.4wt%Ca (ZX504) alloy was prepared from Mg, Zn and Ca block of metals (with high purity of 99.9%) by melting at 750°C in a mild steel crucible underneath the protecting atmosphere of inert gas (argon) using an electrical resistance chamber.The melted metal was poured into a crucible that was pre-heated at 200°C and then the billets were homogenized at the temperature of 350°C for 24h.Afterwards, magnesium discs with 4mm thickness and 30mm diameter were cut out from the center section of the cast billets.

2.2.Preparation of PEO coating

Prior to the oxidation process, surfaces of the metal alloy discs were successively ground with abrasive SiC papers from 100 up to 2500 grades, followed by degreasing ultrasonically in dissolving agent and removal in water.A periodical DC gavlanostatic power supply was utilized for the PEO treatment.The magnesium alloy and stainless steel samples instrumentality were used as the anode and the cathode, respectively.A recirculation cooling system was employed to keep the temperature of solution at room level.The base alkaline electrolyte employed in this experiment was sodium phosphate(Na3PO4·12H2O)and potassium fluorid (KF)with different levels of concentration in 2L of distilled water(Table 1).A pulsed unipolar current mode was utilized for coating procedure using the design of experiments by Taguchi approach.

Table 1Electrolyte composition and operating conditions.

2.3.Experimental design

PEO coatings were applied on ZX504 specimens based on Taguchi orthogonal design technique (from Minitab software.).The Taguchi experimental design was considered to investigate the influenc of coating parameters such as electrolyte composition, current density, frequency and duty cycle on corrosion performance and identifying optimal process parameters to obtain maximum corrosion resistance.The limits for process variables and their levels are given in Table 2.In this research, a special mixed style of orthogonal arrays for the experimental arrange with different levels(L18 orthogonal array) was used and consequently 18 different mixed experiments were meted out (Tables 2 and 3 provide the Supplementary information).Each test was repeated for three times.The current study tries to maximize the corrosion resistance of PEO coatings which was ascertained with electrochemical impedance spectroscopy (EIS) technique.The individual effects of the coating parameters on the process responses will be studied by theS/Nratio.Table 3 shows the orthogonal array with factors of design and levels allotted.

2.4.Coating characteristics

The characteristics of optimum coating such as surface morphology, surface roughness and wettability were investigated.The SEM (scanning electron microscopy) and EDS(energy-dispersive X-ray qualitative analysis) were used for morphology imaging along with elemental maps analysis.Density and size of the pores on the surface layer were calculated by analyzing SEM images using MIPCloud software.Surface roughness was evaluated via a Hommelwerke T800 stylus profilomete.Further, the wettability was investigated by measuring the contact angle average of 6μL PBS droplet deposited at different locations, using Jikan CAG-10 goniometer and SPPF (Sub-Pixel Polynomial Function)method for high accurate contact angle measurement.

Electrochemical Impedance Spectroscopy (EIS) could be a powerful methodology widely used for corrosion studies.Corrosion behavior of the coatings was evaluated by EIS; Solartron 1260A Impedance Analyzer in a phosphate buffered saline solution(PBS)at pH～7.3.All electrochemical measurements were conducted employing a standard three electrodes electrochemical cell with the working electrode(sample), reference electrode (saturated calomel electrode)and the counter conductor (a noble metal plate).A wave with±10mV amplitude was applied to the cell at the ambient temperature; within the frequency limit of 0.01-100,000Hz.All the EIS information was analyzed using ZSim software.

Table 2Experimental factors and their levels.

Table 3Experimental results and S/N ratio.

3.Results and discussion

3.1.Analysis of results

The result of Taguchi designed experiments (polarization resistance) is shown in Table 3.Prior to statistical analysis,it is better to use an unscaled data.The relative percentage deviation (RPD) formula is one of the unscaling methods,which is applied on the fina results of polarization resistance in this study before analysis, as follow [20]:

Where “Methodsol” is the observed value and “Bestsol” is the best obtained value.

Since corrosion resistance is considered as the coating performance (the maximum polarization resistance ～3.68E+06 as the best value), therefore, the lower RPD values are desired.TheS/Nquantitative relation was chosen for findin optimum process conditions and was calculated by the following equation as:

Whereyis the value of design table responses and n is the number of repeated tests [19].

To analyses the influenc of each process factors affecting corrosion resistance; theS/Nquantitative relation values for all experiments were calculated.The correspondingS/Nratios of experimental results are given in Table 3.In view of the fact that the experimental design is orthogonal, the separation of each coating parameters effect at special levels is possible.Namely, the mean S/N quantitative relation for parameter A at levels one and two and parameter B at level three will be calculated via averaging the S/N ratios of 1-3,4-6 and 3,6,9,12,15,18 experiments,respectively.The averageS/Nquantitative relation for all levels of factors is computed and summarized in Table 4 and Fig.1.Moreover, this statistical response table includes the values of rank and Delta for each factor that compare the relative magnitude of the effects on corrosion resistance of the coatings.The delta statistics for each factor were derived by subtracting the maximum average value ofS/Nratio from the minimum average value.Basically, the rank of 1 is considered for the highest value of Delta, the rank of 2 for the second highest Delta value, and the rest.According to Table 4, the sodium phosphate concentration (parameter A) plays the most prominent role in controlling the corrosion characteristics of PEO coating.The potassium fluorid concentration is the second parameter affecting the corrosion behavior.The best condition was obtained at the concentration of 15g/L of sodium phosphate and there was an increase in corrosion resistance with an increase in the potassium fluorid concentration.The important sequence of parameters affecting the corrosion behavior was in the following order: Na3PO4·12H2O concentration>KF concentration>frequency>current density>duty cycle.

Fig.1.Main effect plots for Means and S/N ratios; (A) sodium phosphate concentration, (B) potassium fluorid concentration, (C) current density, (D)frequency and (E) duty cycle.

Table 4Response table for S/N ratios.

To illustrate the optimal combination of coating parameters for maximum corrosion resistance, the resultant main effect plots (means andS/Nratio) for each process factors is shown in Fig.1.The lower the RPD value (based on the higher polarization resistance),the better is the quality characteristic for optimization.Therefore, the optimal coating conditions were determined from the lowest mean value or the highestS/Nratio in each factor.It was observed that a moderate level of Na3PO4·12H2O (15g/L), high level of KF (10g/L) and current density (200mA/cm2), low level of frequency (500Hz)and duty cycle (10%) are preferred to acquire the most suitable corrosion behavior.So,the optimum combination of PEO coating process factors to achieve the maximum corrosion resistance is found to be A2B3C3D1E1.Moreover, as it can be seen in Table 3,all the coated samples have a higher corrosion resistance than the bare alloy (～500Ωcm2), which indicates the advanced effect of PEO coatings on corrosion protection properties.

The meanS/Nratio variation with phosphate concentration is shown in Fig.1(a).As it can be seen, this curve has ascending and descending trends.At first theS/Nratio increases to its highest value as the concentration of Na3PO4increases from 10 to 15g/L.Then, it decreases as the concentration of Na3PO4increases.Therefore, the corrosion resistance decreases after it reaches a peak value at level 2.The thickness and composition of coatings are dependent on the phosphate concentration in the electrolyte.It has been reported that the thickness and phosphate content in the coatings became higher when the proportion of phosphate in the electrolyte was increased [21], which cause to the corrosion improvement.However, above a certain point (>15g/L),the arc discharge phenomenon becomes so intense that it may cause destructive effects on the oxide layer such as thermal cracking.On the other hand, further increasing of the coating thickness does not necessarily improve the corrosion performance.When the coating thickens, the diameter of discharge channels increases since a higher energy is required for the current to pass through the thicker coating[22].Therefore, there is a phosphate concentration of 15g/L in which coating composition, surface morphology as well as thickness is optimum.Fig.1(b) shows response of theS/Nratio to the KF concentration.The meanS/Nratio increases with the concentration of KF.This may be due to an increase in the electrolyte conductivity and decrease in the fina voltage in the PEO process that influence on the morphology of coatings [23].Previous studies [23,24] have suggested that KF addition results in significan changes in the voltage-time response.By increasing KF concentration, the large spark discharges are conversed to fin spark discharges and the number of spark discharges increases which results in the formation of the fin grain structure and homogeneous coatings.It has been experimentally observed that, due to the slight decrease of the voltages, the surface roughness of PEO coatings are decreased by the addition of KF, while the coating compactness is increased, which would be beneficia for corrosion resistance of the coatings.Huang et al.[25] found that fluorid were useful to improve the corrosion protection of inner layer of PEO film on magnesium alloys.

The response of theS/Nratio to current densities is shown in Fig.1(c).The meanS/Nratio increases with increasing current density.This may result from the variation of surface morphology [26-34].According to the literatures[26-29], at higher applied currents in the PEO process due to the higher energy a large number of micro pores sinter together and leads to smaller dimension in discharge channels, which in turn lower the surface roughness.In fact, the plasma discharge and solidificatio process works in such a way that the formed melt could fl w back to partially fil the pores.So, the pore size decreases and the maximum pore size do not increase further as the current density increases.Moreover, at this condition the discharge product can be transformed completely into the phase and the content of MgO increases.Consequently, the corrosion resistance is improved by the protection afforded by MgO phase.At the same time, the growth rate of the coatings increases as a result of stronger plasma-chemical reactions.The coatings thickening obstructed the movement of corrosive ions towards the magnesium substrate; thus retarding the corrosion deterioration of oxide coating.Based on the observations of Yue and Hua [28], with increasing of current density the sparking behavior changes from extensive and large to small and short sparks.Although, it has been suggested by Yerokhin et al.[30], that higher current densities result in relatively increased surface roughness and reduced compactness of the coatings.Ezhilselvi et al.[29] observed that the coating processed at low current density has uniform surface appearance and by increasing the current density, the pores diameters are increased, though the density of pores is decreased.Rapheal et al.[26] discovered that higher current densities exhibited more defective barrier layer and increased porosity in porous layer.However, higher current densities provide an increased amount of crystalline phase (MgO), which resisted dissolution, counterbalanced the negative effects of defective barrier layer and increased porosity resulting in a relatively lower rate of degradation of the corrosion resistance.It also noted that the coating produced at higher current density exhibited better corrosion resistance owing to the lower surface roughness and porosity [29].A similar observation has been reported by Khan et al.[33] that the coating surface becomes more even with an increase in current density.In this research, the oxide coating prepared at the current density of 200mA/cm2exhibited a superior corrosion resistance that can be attributed to its morphology, roughness and phosphate content in the coatings as discussed above.

It can be seen from Fig.1(d) and (e) that the corrosion resistance increases as the frequency and duty cycle decrease.This observation is due to the variation in the surface morphology and thickness of the coatings.Dehnavi et al.[35] discussed the relationship between the duty cycle and coating characteristics.It was observed that duty cycle strongly affects the micro arc discharge characteristics in the coating growth process.The PEO coatings produced at higher duty cycle contains pores with increased size and micro cracks on the surface in comparison to the lower duty cycle.So, the surface roughness increases with duty cycle and lower duty cycles create smoother films Generally, an increase in duty cycle (long on-times) increases pulse energy and leads to more intense and longer lasting micro discharges with less number either on the surface or deep inside the layers [36], which reduces the corrosion resistance of the coating.Whereas at low duty cycles, micro discharges with lower intensity and higher density created and the radius of the spark craters decreases [37].Moreover, the strong electric fiel at this condition increases the incorporation of anions into the coating and the coating growth rate increases gradually with decreasing duty cycle.The response of theS/Nratio to the frequency(Fig.1(d))is similar to that for the duty cycle and with increasing of frequency the corrosion resistance is decreased.The coatings produced at 500Hz had a better corrosion resistance, which is attributed to the higher thickness,phase composition and more compact microstructure with relatively less pore density.Higher energy per pulse due to the longer on-time at low frequency leads to a higher growth rate and a better sintering of the coating.It was reported that the higher degree of plasma-chemical reactions/sintering at this condition also influence the phase composition and promotes the formation of phases that cannot be created at higher frequencies [36,38].It was observed that the phosphorous content was increased with decreasing of pulse frequency in magnesium alloy.Furthermore, the morphology of the coatings formed at lower frequencies appeared smooth and dense with fin pores, fewer porosities and defects.

These plots also give an idea regarding the relative importance of parameters on the system response.If a particular parameter in the main effect plot possesses a greater slope,that parameter would be a more significan effect.Alternatively, the near horizontal line for a parameter in the plot has no significanc [17].As observed in Fig.1, the highest inclination was seen for parameter A (phosphate concentration) in the range of 10-20g/L, indicating its highest influence Parameters of B (KF concentration) and D (frequency) are also quite significan for corrosion characteristics of PEO coatings.Furthermore, the current density (factor C) and duty cycle(factor E) showed less sensitive variations than other factors over the selected levels.

Fig.2.SEM images (surface and cross section) of the optimized coating.

3.2.Characteristic of the optimized coating

The SEM images of the optimized coating are shown in Fig.2.As it can be seen, the porous coating surface, created from gas evolution by micro sparks during the discharge reactions,has a pancake appearance formation.It shows a relatively low rough surface morphology (Ra=0.34, Rz=2.27)as a result of small pores (with the average size of 0.6μm)that homogeneously cover up about 4% of the surface coating area (Fig.3).The cross sectional image (Fig.2) shows an inner dense barrier layer next to the substrate and an outer layer with pores and discharge channels that is thicker than the inner one.According to the performed EDS analysis (Fig.4),Mg (～65at%) and P (～20at%) are the main elements in the coating.The element of F and Zn is additionally detected on the surface of the coating.Presence of these elements on the coating surface implies the interaction of the solution parts with that of the substrate during the coating process.The contact angle droplet images are shown in Fig.5.As it can be seen, the PBS droplet is not quickly penetrating the coating and the sample exhibits relatively hydrophobic behavior with an equilibrium contact angle of 108±4°.By expanding the drop volume (advancing contact angle), equilibrium contact angle does not change.In contrast, contracting the drop volume (receding contact angle) leads to its decreasing.Actually, contact angle hysteresis (the difference between advancing and receding contact angle) occurs due to the surface morphology and roughness.

The corrosion behavior of the PEO coatings and the Mg substrate was studied by the analysis of electrochemical impedance spectroscopy (EIS) testing technique.The Bode and phase plots of EIS for the coating obtained in the optimum condition and the magnesium alloy for comparison,with adequate fittin (χ2<0.01),are shown in Fig.6.As it can be seen, for the coated sample two time constants delineated by two peaks in the phase plot is discerned.This can be the results of two layer structure of PEO-coating, which is consistent with the SEM analysis shown in Fig.2.The peak at high frequencies correspond to the results of the outer porous part of the coating and the clear presence of a second peak at medium frequencies are attributed to the inner compact region of the PEO coating formed on the electrode surface.The primary one characterizes the geometric capacity of oxide layer that was formed due to the plasma interaction between the substrate and solution.The second time constant is responsible for the nonporous sub-layer that was formed due to the applied voltage over the metal and protects the substrate by retarding down the diffusion process of solution.The phase plot of bare Mg substrate shows three relaxation time constants represented by two peaks in the high and medium frequency regions, originating from a surface oxide (MgO) or hydroxide fil (Mg(OH)2) formed quickly on the surface of bare Mg adjacent to the aqueous solution and charge transfer process as well as one peak at low frequency regions due to the pitting corrosion process (inductive behavior).

Moreover, in the impedance plot (Bode), the whole curve related to the bare alloy was below that of PEO coating; considering that the passive fil on bare Mg was much thinner than the fabricated coatings.The higher |Z| suggests a better anticorrosive property of the PEO coated sample in corrosive medium compared to that of the bare alloy.

An electrical equivalent circuit model, as shown in Fig.7,was employed for analyzing the EIS spectrum.This circuit consists of a solution resistance (Rs) between the reference and the working electrode,Routand QPEoutat high frequency;used to represent the resistance and capacitance behavior of the outer porous layer,Rinand QPEin; in the medium frequency range,concerning the inner barrier of the coating sample,Rpand QPEprepresents the resistance and capacitance behavior of oxide layer formed immediately after immersion on bare Mg,Rcand QPEcfor charge transfer process of the magnesium alloy,R3andLfor interpreting the inductive behavior of the Mg substrate at low frequencies.

Fig.3.3D surface topography and corresponding line scan profil for the optimum PEO coating.

Fig.4.The EDS spectrum and relative contents of chemical elements.

Fig.5.Wettability behavior of the optimized coating.

Fig.6.Bode-Phase plots of the coated sample at optimum condition (a) and the bare Mg substrate (b).

Fig.7.An equivalent circuit model employed to analyze the obtained Impedance plot for (a) the PEO coating and (b) the Mg substrate.

The total impedance of the corrosion system for the coated sample (Eqs.(4) and (6)) and the bare Mg (Eqs.(5) and (7))can be expressed with the subsequent equations:

While the frequency is adequately low (f→0), the impedance of capacitive components tends to be infinit and thus the polarization resistance of the corrosion system{define as the difference between zero frequency impedance (|Z|f→0) and the electrolyte resistance (|Z|f→∞)} can be obtained as:

4.Conclusion

The Taguchi method (L18 mixed design) based onS/Nanalysis was applied to optimize the PEO coating parameters for maximum corrosion resistance in ZX504 alloy.The related analysis showed that electrolyte composition (concentration of Na3PO4followed by KF concentration) was the principle controlling factor.The other process factors affecting the corrosion behavior were in the following order: frequency>current density>duty cycle.The recommended optimal parametric condition for maximum polarization resistance(～3.68E+06) was found to be: A2B3C3D1E1 (Na3PO4concentration at level 2 (15g/L), KF concentration at level 3(10g/L),frequency at level 1(500Hz),current density at level 3 (200mA/cm3) and duty cycle at level 1 (30%).Moreover,the corrosion resistance of the optimized coating is more than 1000 times better than that of the magnesium alloy (ZX504).

Appendix A.Initial microstructure of the magnesium alloy (substrate)Fig.A.1, A.2 and Table A.1.

Fig.A.1.SE-SEM of the ZX504 magnesium alloy (a) before homogenizing and (b) after homogenizing.

Table A.1The results of ICP analysis for chemical composition of the magnesium substrate.