titanium duty cycle plasma electrolytic oxidation, influence on...

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Titanium duty cycle plasma electrolytic oxidation: influence on film porosity and corrosion resistance

Davide PRANDO1, Luca CASANOVA2,

MariaPia PEDEFERRI3, Marco ORMELLESE4

1 Politecnico di Milano, Milan, Italy, [email protected] 2 Politecnico di Milano, Milan, Italy, [email protected]

3 Politecnico di Milano, Milan, Italy, [email protected] 4 Politecnico di Milano, Milan, Italy, [email protected]

Abstract: among different form of anodic oxidation on titanium, plasma electrolytic oxidation (PEO) is recognized to produce oxide layers up to two orders of magnitude thicker that the ones produced by standard anodization. This treatment increases corrosion resistance in a variety of environments. However, the micro-arcs produced during film growth induce nano-porosity in the oxide film. This porosity limits oxide barrier effect and allows aggressive species to approach metallic titanium. In this work, pulsed signal at different duty cycle values (25% and 75%) and frequencies (ranging from 20 Hz to 1000 Hz) is applied to reduce the energy of each spark and create a smoother surface. The obtained samples are characterized for corrosion resistance in bromides rich environment, for oxide morphology using image recognition for pores count statistical analyses and for crystallinity. Process parameters are optimized in order to achieve the best corrosion resistance for each given anodizing potential while maintaining an energy efficiency higher than the one obtained with direct current anodizing. The best anodizing condition to achieve the lower porosity and the higher or lower amount of crystalline phase is highlighted to allow fine oxide tuning for different applications.

Keywords: plasma electrolytic oxidation (PEO), duty cycle, porosity, localized corrosion

mailto:[email protected]:[email protected]:[email protected]:[email protected]

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Introduction Titanium is worldwide used and appreciated for its excellent corrosion resistance. This resistance is due to a thin (around 2 to 10 nm) [1] chemically stable oxide layer that is formed when titanium is exposed to atmosphere moisture. This important property, together with high specific strength, low density and high fracture toughness [2,3], makes titanium ideal for applications in critical environments, ranging from offshore plants, acid environment, aerospace [4,5], automotive, high temperature, chemical and food industry [6–8], marine hydrometallurgical application and even nuclear fuel wastes containment [1,9–12]. Despite his outstanding resistance, even titanium may undergo several forms of corrosion if the environment is aggressive enough. The most critical form is localized corrosion due to the breaking of passive layer, this phenomenon is highly favored by halides presence [13,14]. The higher price of titanium compared to competitive alloys justify its usage on application where no failure is allowed. In these conditions, a further improvement in corrosion resistance may be required. Corrosion resistance enhancing treatments can be divided in two families: chemical composition changing treatments and oxide thickening treatments. The first family includes alloying with other elements (mainly Pd, Mo and Ni) [15], nitration [16,17], vacuum plasma spray coating, plasma spray coating and chemical vapor deposition [18]. Treatments of the second family rely on the possibility to thicken the naturally formed TiO2 film with an external electrical, chemical or thermal driving force, using respectively anodic, chemical or thermal oxidation, respectively [19]. Among them, anodization is the easiest, more reliable and precisely controllable [20]. However, the highest corrosion resistance achievable with this technique is obtained at the price of high-energy consumption, and leads to undesired porous and crystalline oxide. To improve results achievable with regular direct current (DC) anodic oxidation, pulsed anodizing was studied. The application of pulsed signal allows the interruption of power supply during anodic sparking in anodic spark deposition anodizing regime. On light metals other than titanium, such as magnesium and aluminum, this is reported to lead to oxides with lower porosity [21–23]. Pulsed anodizing was recently transferred on titanium by Torres-Cerón et al [24] with promising results. The aim of this study is to investigate the effect of anodic oxidation treatment performed with different waveform applied at different frequencies on the so obtained titanium dioxide corrosion resistance, porosity and crystallinity. Materials and methods

Square samples 20×20×1.6 mm were cold cut from a titanium UNS R50400 (ASTM grade 2) sheet, and polished with 100, 320 and 800 grit SiC paper. To remove possible surface contaminations, the specimens were sonicated for 4 minutes in ethanol and 4 minutes in distilled water. Anodizing During this study, a monopolar pulsed anodizing treatment was applied. An example of waveform is schematically depicted in Fig. 1 where the prescription of a voltage waveform (blue line) requires the use of a current waveform (red line) with a shape necessary to allow the voltage step. The pulsed signal is applied in voltage, thus, no residual polarization is present in between consequent pulses. In order to understand the trend in power consumption among different waveforms, the current signal was analyzed by considering a peak value, i.e. the value of current required to reach the voltage step prescribed by the waveform, and the plateau current i.e. the value of current required to sustain the voltage plateau (Fig. 1).

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Fig. 1 Example of monopolar pulsed waveform of voltage and current

In order to study the effect of pulsed anodizing on titanium corrosion resistance, oxide crystallinity and porosity, a set of relevant parameters were identified:

• Anodizing voltage • Anodizing frequency • Anodizing duty cycle (% ON-time vs total time)

Anodic oxidations were carried out with a California Instruments® Asterion AST751 power supply, able to supply 750 W. This power supply was chosen for its fully programmable waveform capability, that allows to set an arbitrary waveform and to vary its frequency up to 1000 Hz. This wide range of variable parameters allowed the construction of a comprehensive experimental matrix. The three most important parameters, according to literature, were divided in ranges with two or three values each; the result is a set of five different conditions:

• Duty cycle 100% - 1 Hz (corresponding to a standard DC anodizing) • Duty cycle 25% - 20 Hz • Duty cycle 25% - 1000 Hz • Duty cycle 75% - 20 Hz • Duty cycle 75% - 1000 Hz

Each condition was repeated for three different voltages selected into the anodic spark deposition (ASD) range: 120 V, 160 V and 220 V. Each anodizing was repeated twice to ensure repeatability. Thus, this study considers thirty different samples. All the anodizing treatments were performed in H2SO4 0.5 M to allow direct comparison with the results obtained in previous studies [25]. Surface morphology After anodizing, XRD analyses were performed using a Panalytical Empyrean XRD with Cu Kα1 radiation with 1.54058 Å wavelength. Scanning angle was maintained between 20° and 30° to find the characteristic peaks of anatase and rutile TiO2 crystalline form. SEM analyses were conducted with a Cambridge Stereoscan 360 SEM, to investigate morphology and porosity. To measure quantitatively oxide surface porosity, an image recognition procedure was developed using ImageJ software. In Fig. 2, an example of SEM image used for image recognition is shown.

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Fig. 2 Example of a SEM image used for pores analysis

For this analysis, a minimum of five images were taken from each sample. Considering fifteen different anodizing condition, and a repeatability of two, more than 150 images were acquired. After acquisition, each image has to be cropped from its canvas (containing scale and SEM parameters) and uniformed with the others in term of contrast and brightness. At this stage, pictures were converted in 8 bits images, in this way, each pixel can assume a value between 0 and 255 (28 possible values), that represents its color, where 0 is total black, and 255 total white. Implemented pores recognition working principle is that pixel color inside pores is darker that outside pores. The darkness of the black representing a pore is not constant for all the pixels in the pores, but the transition is smooth, while as pore boundaries are reached, color difference between close pixels become higher. Thus, it is possible to proceed dividing the image in color thresholds, according to the following steps:

1. The maximum pore dimension is measured manually 2. A first threshold is applied from 0 to 10 while transforming the image in binary one.

This means that the pixel that has a color value between 0 and 10 becomes total black, and the other total white, in this way the first mask is created and stored

3. A second mask is then generated considering gray scale from 0 to 20. Now the previously recognized regions become wider because a broader color range is considered, expanding their area. However, after each successive step, a filter is applied to remove all the spots with area bigger than the biggest pore measured manually before the procedure. In this way, if a spot become bigger than the biggest pore with an abrupt increase of area, it is discarded because it passed pore boundaries, and only the previous area value is maintained, stored in the image recorded from the previously applied threshold. In Fig. 3 an example of four successive threshold applied to image reported in Fig. 2 is shown; notice that initially only the center of few pores is recognized, while increasing threshold value, more pores are detected and with more accuracy

4. By continuing this procedure for an adequate number of thresholds, all the pores are detected. An image is now generated by stacking all the ones previously generated and a filter is applied to remove spare pixels detected as noise. The result of this stack is visible in Fig. 4

5. The image is then superimposed with the original SEM picture to visually verify the pores recognition accuracy. An example is shown in Fig. 5.

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Fig. 3 Four successive threshold applied to Fig. 2 image to make it binary

Fig. 4 Image obtained by stacking all the mask saved after having applied different threshold

to the original image

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Fig. 5 Image obtained by stacking all the mask saved after having applied different threshold

to the original image At the end of this analysis, the area of each pore is obtained, allowing the analysis of pores size and pores’ number; the final data is a statistical distribution in form of average pores number per image and average percentage of sample surface covered by pores at each anodizing condition. Corrosion resistance Samples corrosion resistance was characterized with potentiodynamic tests performed with MetroOhm Autolab potentiostat M204. A standard 1 liter, 3-electrode cell was used, including an activated titanium counter electrode, a silver/silver chloride (SSC) reference electrode and the anodised titanium working electrode with 0.4 cm2 of exposed area. Open circuit potential (OCP) was recorded after 1 h of sample exposure to the testing solution. Then potential was scanned from 0.1 V below the OCP up to 8 V SSC with a scan rate of 20 mV/min. Anodic current and potential difference between metal and reference electrode were registered with Nova® 2.1.1 software. To assess titanium resistance to corrosion it is necessary to distinguish different forms of corrosion. The effectiveness of different electrolytes in promoting localized oxide breakdown was studied [14]: NaF, K3Fe6CN, FeCl3 and NaBr were tested at different pH and temperature to identify the best localized corrosion promoting solution. As a result, corrosion tests were performed in 0.5 M NaBr solution at 50°C. A current density of 10 A/m2 was chosen as threshold to define localized oxide breakdown events. Results Sample crystalline phase analysis Sample crystallinity was studied through XRD measurements: peaks at angle 2𝜃 = 25.5° and 2𝜃 = 27.5° are associated with anatase and rutile crystalline phase, respectively. Each plot shown in Fig. 6 reports the trend of anatase and rutile growth with increasing anodizing voltage at each value of duty cycle and frequency used. As could be seen, in DC modality the use of a voltage equal to 160 V allows the formation of a large anatase quantity compared to voltages equal to 120 and 220 V, while using higher biases determines a dramatic increase of the rutile phase at the expenses of the anatase, due to a larger heating effect. For what concern the investigation of the trend in crystalline phase for the pulsed waveform, it was considered

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the quantification of the area covered by anatase and rutile peaks, thus allowing an easier classification: results will be discussed later in the paper.

Fig. 6 XRD results of sample anodized in different regimes

Surface morphology Each sample was observed with SEM and a minimum of five pictures were taken for each sample, thus, a minimum of ten pictures were analyzed for each anodizing condition. The morphologies of samples anodized at each duty cycle value and each frequency considered are shown in Fig. 7, Fig. 8 and Fig. 9 for samples anodized up to 120 V, 160 V and 220 V respectively. Pores size increases increasing anodizing voltage, with a final transition from porous to grooved structure at 220 V. In between the same anodizing voltage, pore size and distribution vary with anodizing parameters, this will be discussed in detail in later paragraphs. Corrosion resistance

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Results of corrosion resistance tests in each anodizing condition of duty cycle and frequency are reported in Figs. 10-14. In direct current regime (Fig. 10) all the samples anodized at different voltages have a similar OCP value, with the one treated at 220 V showing the lowest passive current density for potential values lower than 2 V SSC. For larger polarization voltage the trend is reversed, with the 220 V sample showing the lowest onset of pitting corrosion for potential slightly greater than 3 V SSC. The use of a pulsed signal is particularly effective in increasing the corrosion resistance of samples treated at 220 V, slightly increasing the pitting potential near the highest possible value (8 V SSC) applied by the potentiometer used to perform the experiment (Fig. 12). The application of a pulsed signal is particularly detrimental for all the different waveforms tested at voltage equal to 160 V, in fact both the OCP and the pitting potential were considerably decreased with respect the direct current condition.

Fig. 7 SEM pictures of the samples anodized at a final voltage of 120 V:

a) Duty cycle 25% 20 Hz, b) Duty cycle 25% 1000 Hz, c) Duty cycle 75% 20 Hz, d) Duty cycle 75% 1000 Hz and e) DC

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Fig. 10 Results of corrosion resistance tests performed in NaBr 0.5 M solution at 50°C on

samples anodized in DC


samples anodized with duty cycle 25% at 20 Hz



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samples anodized with duty cycle 75% at 1000 Hz Discussion Sample crystallinity To analyze oxide phases and quantify its oxide crystalline forms, the area of XRD peaks corresponding to anatase and rutile phase were measured, and the signal background was removed. This data is considered more accurate than the peak intensity only and, upon a proper calibration of the x-ray diffractometer, can be used for direct comparison between samples. Regardless the anodizing duty cycle and the frequency used the rutile crystalline phase shows the same trend on all the samples: it is absent or low at 120 V, and then it grows abundantly before 160 V and maintain a similar value with a modest growth up to 220 V. If rutile phase is required at the lower voltage, a duty cycle of 25% at 1000 Hz stimulates its development. On the contrary, anatase phase showed a marked dependency on anodizing condition used, without any univocal trend passing from 120 V to 160 V and to 220 V. For this reason, a comparison at the same voltage with different anodizing duty cycle and frequency is shown in Fig. 15, Fig. 16 and Fig. 17. At 120 V the application of a duty cycle 25% at low frequency (20 Hz) lower the anatase presence, this is attributed to the lower quantity of current necessary to reach that voltage,

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with consequent lower heating of the sample. At 1000 Hz the trend is not clear but the ability of anodizing performed with duty 75% at 1000 Hz to reduce total crystallinity is promising for all the applications for which amorphous oxide is beneficial. At 160 V both anatase and rutile crystalline phases are abundant. Once again, the treatments at 1000 Hz are interesting to reduce the amount of crystalline phase. In particular performed with duty cycle 25% to minimize anatase and at duty cycle 75% to minimize rutile. The transition above 200 V is known to transform most of the anatase crystalline phase into rutile. This is verified for DC and low frequency anodized samples. However, the usage of high frequency during pulsed anodizing prevent this effect, leading to a residual quantity in case of treatments carried out with duty cycle 25% and to a preponderant quantity of anatase respect to rutile in case of treatments carried out with duty cycle 75%. In conclusion, duty cycle and frequency add new possibilities to fine tuning crystalline phases without acting on final anodizing voltage and must be taken into account for all the application that requires not only oxide crystallization, but a specific crystalline phase (i.e. photocatalysis).

Fig. 15 Anatase and rutile XRD peaks area measured on samples anodized at 120 V

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Surface Morphology In all the anodizing condition tested, the same trend in pore distribution evolution with anodizing voltage can be noticed. Anodic oxidation carried out up to 120 V produces the narrowest pore distribution and the lower area covered by pores, regardless the duty cycle and frequency used. As 120 V is close to the ASD regime establishing potential, a smaller area covered by pores was expected compared to higher anodizing voltage. Increasing anodizing voltage leads to a broadening of pore size distribution, without a clear difference between 160 V and 220 V. A slightly narrower pore size distribution is observed on sample anodized at

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220 V. As general trend, anodizing with duty cycle 25% at 1000 Hz produces the highest area covered by pores, while changing duty cycle value to 75% and keeping the same frequency (1000 Hz), the lowest porosity is achieved, as shown in Fig. 18. Pores numerousness, on the contrary, does not show any clear trend with increasing anodizing voltage, duty cycle or frequency, as shown in Fig. 19. By comparing Fig. 15 and Fig. 18 it is possible to notice that porosity of samples anodized at 120 V follows the same trend of oxide total crystalline phase amount. This phenomenon is limited to the lowest voltage considered and is not present at higher anodizing voltage. As 120 V is very close to ASD regime, a more vigorous sparking lead to both an increase in porosity and sample heating, with consequent crystallization. At higher voltages, this correlation is lost because pores are not limited to oxide surface anymore and other mechanisms starts to play a role in oxide crystallization.

Fig. 18 Area covered by pores on samples anodized up to 120 V, 160 V and 220 V with

different duty cycles and frequencies

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Fig. 19 Pores numerousness of samples anodized up to 120 V, 160 V and 220 V with different

duty cycles and frequencies Corrosion resistance To directly compare corrosion resistance, oxide breakdown potentials are plotted as a function of anodizing voltage and regime in Fig. 20. Where possible, potentials were averaged between two measures, if one or more experiments did not lead to oxide breakdown, a value of 8 V was reported as oxide breakdown as this is the higher cap of instrument measure range. Thus, for some anodizing condition, the plotted oxide breakdown potential value represents a conservative approximation.

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Fig. 20 Summary of oxide breakdown potentials of samples anodized up to 120 V, 160 V and

220 V with different duty cycles and frequencies Anodic oxidation carried out up to 120 V are typically less corrosion resistance than the one carried out at higher voltages [26]. However, passing from DC anodizing to high frequency anodizing regime the corrosion resistance increases of about 3 V in bromides 0.5 M solution. The effect of duty cycle value is less pronounced, with a higher resistance associated with higher duty cycle at both 20 Hz and 1000 Hz. Passing to 160 V the corrosion resistance trend is inverted, with the maximum resistance obtained with DC anodizing. Comparing the discontinuous anodizing treatments, a higher duty cycle value produces a slightly higher corrosion resistance, coherently with what observed at 120 V. Discontinuous anodizing was found to be particularly beneficial for anodizing treatments at 220 V. The introduction of discontinuous process leads to an oxide breakdown potential increase of at least 3 V, regardless the pulse regime. As both duty cycle 25% at 1000 Hz and duty cycle 75% at 20 Hz produce samples able to resist up to 8 V of anodic polarization in bromides 0.5 M at 50°C, the best treatment has to be chosen looking at different criteria, for example, energy saving that will be discussed in the following paragraph. However, it is interesting to notice that these two anodizing conditions produces a higher porosity compared to the others (Fig. 18), and this suggest that the surface porosity is not directly linked to corrosion resistance. Energy saving One of the main advantages of discontinuous anodizing is the possibility to reach the same voltage obtain with DC anodizing in a shorter period of current ON, then using less energy. In Table 1 the current recorded at the end of anodizing process, when the desired voltage is reached, is shown. However, when a duty cycle waveform is applied, the current is supplied for a fraction of time depending on duty cycle percentage. An approximation of the current used at the end of the anodizing treatment can be calculated neglecting the current spikes in both, anodic and cathodic direction, and multiplying the current plateau for the duty cycle value. In Table 1 this average current is reported for each anodizing condition. The almost perfect reproducibility of current measurements between each sample and its repeatability increases its reliability.

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The use of a duty cycle 25% waveform at 20 Hz to reach 120 V allows a total energy saving of more than 70% with respect to a standard DC anodizing. For higher voltages, the most energy saving condition passes from duty cycle 25% at 20 Hz to duty cycle 75% at 20 Hz. The lower energy consumption of 20 Hz respect to 1000 Hz was expected because the latter requires a higher number of polarization and depolarization per second. However, the mechanism behind the higher consumption of one duty cycle respect to the other is more complex. At lower voltages, close to ASD regime establishment, the oxide film growth is facilitated, and it is sufficient to stay at anodizing voltage for the 25% of the total time, during each cycle, to grow the film. At higher voltages, on the contrary, the thicker dielectric layer and the higher local temperature make hinder film growth, and the 25% is not sufficient anymore to fulfill oxide growth kinetic requirements. For this reason, increasing duty cycle value allows the usage of lower current to reach the same film growth rate, resulting in an increased energy saving. Although less performant, even at 220 V pulsed anodizing results in more than 55% of energy saved respect to DC anodizing.

Table 1 Average current consumed at each potential for each anodizing condition

Duty Cycle Voltage [V] Freq. [Hz] IPLATEAU IAVG

100% (DC)

120 - 0.36 0.36

160 - 0.8 0.8

220 - 2.4 2.4

25%

120 20 1.7 0.1

1000 3 0.4

160 20 6 0.6

1000 0.6 0.75

220 20 2.4 1.75

1000 7 1.5

75%

120 20 0.4 0.45

1000 1.3 0.3

160 20 4.6 0.5

1000 0.6 0.97

220 20 0.7 1.05

1000 1.4 3.45

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Conclusion Discontinuous anodizing proved to be a powerful technique to perform modification on titanium anodically grown oxide. The wide range of parameters allows the fine tuning of each desired properties provided a full comprehension of their effect is achieved. The selection of proper process parameters in terms of voltage, duty cycle and frequency allows the formation of the required amount and type of crystalline phase, taking advantage of the lower energy consumption with respect a direct current process. Acknowledgments This work has been financially supported by the PRIN project “Monitoraggio, consolidamento, conservazione e protezione dei beni culturali” n. 2015WBEP3H. References 1. D. Prando, A. Brenna, M.V. Diamanti, S. Beretta, F. Bolzoni, M. Ormellese, M.

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IntroductionMaterials and methodsResultsDiscussionConclusionAcknowledgmentsReferences

titanium duty cycle plasma electrolytic oxidation, influence on...

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