· Web viewThe role of cathodic current in plasma electrolytic oxidation of aluminium: “Scanning waves” of the current density on complex-shape substrates Aleksey B. Rogova,b,c

The role of cathodic current in plasma electrolytic oxidation of aluminium ldquoScanning wavesrdquo of the current density on complex-shape substrates

Aleksey B Rogovabc Aleksey Yerokhina Allan Matthewsa

a) School of Materials the University of Manchester Manchester M13 9PL United Kingdomb) Nikolaev Institute of Inorganic Chemistry Novosibirsk Russia 630090

c) Scientific and Technical Centre ldquoPokrytie-Ardquo Novosibirsk Russia 630015

Abstract This paper is focused on the factors influencing the surface distribution of the

current density during plasma electrolytic oxidation of 2024 Al alloy under alternating

polarisation It was found that employment of combined current mode including relatively long

(100-2000 ms) pulse trains improved coating uniformity even when electrolyser provided severe

non-uniform primary electric field Experimental investigation employing a sectioned sample

showed that the non-uniform distribution of the primary electric field could be compensated by

the changes in the coating properties induced by previous cathodic polarisation Temporary

changes in the secondary distribution of current density across the sample surface (attributed to

the coating properties) caused dynamic redistribution of the anodic current density during

following AC pulse train resulting in so-called ldquoscanning waverdquo effect ie migration of the

maximum current density along sample surface Factorial experimental design finite element

modelling and analysis of the transient current-voltage curves were applied Mechanistic

explanation underlying considered phenomenon has been suggested In addition the discussion

on the increase in PEO efficiency under soft sparking conditions were provided as

ldquoelectrocatalysisrdquo of anodic oxidation by previous cathodic treatment

Keywords Plasma electrolytic oxidation coating uniformity aluminium alloy scanning wave

effect

- corresponding author

1 Introduction

Plasma electrolytic oxidation (PEO) is an electrochemical surface treatment under high anodic

polarisation conditions (up to 1000V) [12] A specific feature of the PEO is the presence of

localised current channels caused by dielectric breakdown of the forming oxide layers The

breakdown appears as localised microdischarges where energy is dissipated triggering plasma-

assisted chemical reactions and heat effects that influence the charge and mass transfer through

the metal-oxide-electrolyte system and may cause phase transformations sintering calcination

and dehydration of the formed oxide layers As a result new composition and microstructure

may develop in the coatings leading to new properties For instance hard and dense sintered

oxide layers enriched with α-Al2O3 can be obtained on the surface of aluminium alloys at near to

ambient bulk substrate temperatures Due to ecologically friendly nature of the process as well as

high wear- and corrosion resistance of the coatings they become suitable for a wide range of

applications

Despite some positive results a broad industrial application of PEO is limited by a number of

technological issues caused by complexity of the process and a lack of basic information about

the coating formation mechanisms One of the most promising current mode in PEO is so-called

ldquosoft sparkingrdquo regime wherein the development of large destructive discharges at the later

stages of the process is suppressed providing a deeper substrate oxidation leading to more

uniform coatings with higher hardness and better adhesion (see overview in [34]) PEO under

alternating polarisation conditions including soft sparking mode is described by a number of

process parameters in terms of initial (electrolyte composition substrate materials) and boundary

(polarisation mode) conditions As a result it is difficult to find an appropriate set of the process

parameters satisfying requirements of the final coating quality One of the most important

generic properties of coatings is thickness uniformity on the substrates with complex shape

geometry such as blind holes cavities sharp edges and large protrusions which represents the

main challenges in PEO technology

It is clear that any non-uniformity of the PEO coating is caused by non-uniform distribution

of the current density which in turn depends on both the primary electric field distribution in the

electrochemical cell and the local properties of the resulting coating Therefore there are two

main approaches to achieve the uniform PEO coating The first one consists in deliberate

arrangement of the uniform primary electric field distribution by using conformal auxiliary

counter electrodes which shape fits the substrate geometry thereby creating a uniform primary

field distribution [56] Wei et al [7] mentioned that the smaller interelectrode distance the better

corrosion resistance of the coating moreover they have found that current pulses earlier reached

maximum amplitude on the nearer parts of substrate then on the farther ones Importance of the

interelectrode distance in PEO under galvanostatic conditions was reported as well in [8][9]

The extension of this technique is so called ldquoscanning micro arc oxidationrdquo wherein a small-area

counter electrode is moved through the surface of larger substrate allowing uniform coatings to

be produced on the large and complex substrates [1011] Numerical calculation of the primary

electric field distribution through the surface of complex shape substrates with deep internal dead

holes under PEO conditions has been performed in [12]

The second approach utilises specific coating properties to influence the secondary

distribution of current density at the sample surface providing a self-aligning coating growth

behaviour Those approaches are based on the local difference in transient current-voltage curves

(CVC) of the particular coating regions It is known that typical anodic (characterising substrate

oxidation) CVC for PEO of aluminium possess severe non-linear character that may include both

the sharp increase in current once threshold voltage is achieved and the regions with negative

differential resistance [313ndash16] The CVC at any given local point within PEO coating depends

on the set of available electrochemical reactions under given particular conditions Those

reactions are defined by properties of the coating (composition microstructure) as well as by the

electrolyte composition It was suggested that transition to the soft sparking PEO which occurs

at certain excess of cathodic current density over anodic one is accompanied by associated

processes under anodic and cathodic polarisations which in turns significantly change current

response on applied voltage (CVC) Only a few papers concerning coating uniformity in view of

electrolyte composition or polarisation conditions can be found Yerokhin et al [17] noted that

coating uniformity can be improved with increase in alkali and decrease in silicate

concentrations in electrolyte solution Terleeva et al [18] has found that application of combined

current mode including alternating current and additional cathodic current pulse trains (ACC)

allows the coating uniformity on the internal surface of a conical shape substrate to be improved

A few works were devoted to the influence of electrolyte ageing on the appearance of non-

uniform features in the coating regions far away from the substrate edges [1920]

Disadvantages of the primary field control lay in the necessity of manufacturing a counter

electrode which shape may be even more complex than the targeted substrate and any changes

in substrate shape would require the counter electrode to be redesigned and remanufactured At

the same time a control over the secondary field distribution by means of adjusting polarisation

conditions and tailoring electrolyte composition to the specific alloy provides considerable

advantages especially in situations where substrates can vary to some degree Such variation can

be caused for example by a bespoke shape of medical implants or in other applications (eg

marine or aerospace) by the fact that batches could be formed by parts of similar but not exactly

the same geometry due to the small-scale nature of manufacturing (eg by 3D printing)

In this paper experimental investigation is performed in regards of the PEO coating

uniformity and secondary current density distribution under intentionally non-uniform

distribution of the primary electric field and how this distribution is affected by timings and

current densities of the pulse trains in combined current modes Some general considerations

regarding the mechanisms underlying PEO treatments and efficiency of the coating formation

are also provided

2 Experimental

All substrates were made from A2024T3 aluminium alloy (AlCu4Mg) Particular sample

geometry is described in following subsections An aqueous electrolyte solution comprising 15

gdm3 of technical water glass (specific gravity 141 gcm3 SiO2Na2O = 302) and 2 g dm3 of

potassium hydroxide was contained in a cubic stainless steel tank (300 dm3) served as a counter

electrode During the PEO process electrolyte solution was pumped throughout the internal part

of the screen along the coated surface in order to prevent electrolyte overheating above 40oC

The power supply provided a combined current waveform composed by pulse trains of

symmetrical alternating current (AC) characterised by equal positive and negative average

currents J+ = J- = JAC with addition of either cathodic (C) or anodic (A) half-wave current pulses

with frequency 50 Hz (period = 20 ms) for all pulse trains (see Fig1) An obstruction to the

primary electric field was formed by a special insulating screen made from PTFE which

surrounded the sample as shown in Fig2 In order to exclude bottleneck effect but provide

suitable obstruction the cross-sectional area of the open end of the screen was equal to the total

surface area of the sample exposed to the treatment

Fig 1 Schematic representation of the complex alternating current modes with additional

cathodic (AC-C) and anodic (AC-A) pulse trains

21 Factorial experiments Application of the combined current mode (AC-C or AC-A) is

complicated by a number of characteristic parameters that hindering optimisation or general

relations to be found Moreover some parameters are interconnected for instance cathodic

charge may be increased either by increasing in τC or Jc Therefore the factorial experimental

design may help us find out the most valuable effect of individual parameters as well as effects

of their combinations

The samples were fabricated in the form of tubes (Oslash14x12x30mm) which cylindrical sides

subjected to PEO treatments Variable parameters in experiments were as follows current

densities in AC and C pulse trains (JAC and JC) and their durations (τAC and τC respectively)

However some particular effects in factorial experiments were represented by combination of

two variable parameters since we already know the importance of those combinations in PEO

Detailed explanation of factorial experiments and real experimental conditions are given below

and listed in Table 1

Fig2 Drawings of the insulating screen and sample assembly used for a) factorial experiment b) for studying the current density distribution It is sectioned for demonstration the internal construction

As the central point (designated as 0) of the factorial design the following set of

experimental parameters was chosen JAC0 = 90mAcm2 Jc0 = 18 JAC = 1125 mAcm2 τAC =

1000 ms τC = 280 ms t = 251 hours because this was one of the main current modes for a

semi-industrial application that provided hard and well adhered coatings on the outer surfaces of

many industrial components made of aluminium alloys with low (lt12) content of silicone

However such current mode could not often provide satisfactory quality of the coatings for

substrates with complex geometry especially on the inner and concave surfaces The main

effects (Xn) and their levels (plusmn1) in factorial design were chosen as follows (Xn = Xn0 plusmn ΔXn)

1) current density within AC pulse train X1 JAC = JAC0 (1 plusmn 13)

2) ratio of current densities within C and AC pulse trains X2 JCJAC = 18(1 plusmn 13)

3) ratio between durations of AC and C pulse trains X3 τACτC = 357 plusmn 114

a) b)

PTFE

Al

1234

Al ringsPTFE

PTFEcap

PTFEcap

wire

Spacers

inner end

outer end

4) duration of C pulse train X4 τC = 280 plusmn 140 ms

Total process duration for every experimental condition were chosen to achieve equivalent

total anodic charge in the PEO processes This allows us to estimate the process efficiency by

comparison of the average coating thicknesses

The triple interaction (eg X1X2X3) was assumed negligible therefore fractional replication

could be applied and the forth factor X4 was introduced using defining contrast ndashX1X2X3 which

appears from the defining relation 1=X1X2X3X4 and conditions of orthogonality XimiddotXj = 0 (for inej)

and Xi2 = 1 Validity of this assumption was estimated as discussed in the Results section The

sequence of treatments among total 24 experiments was defined by randomization Runs at each

experimental condition were repeated three times As responses the following characteristics

were taken

1) The average increase in diameter Y1

2) The mean average coating thickness Y2

3) Dispersion of the diameter increase through the sample length Y3

4) Dispersion of the coating thickness increase through the sample length Y4

5) Visual appearance of coating uniformity which was evaluated subjectively within 0 (the

worst) to 1 (the best) scale with 01 step at equal intervals Y5

The first two responses (Y1 Y2) represented general process efficiency (since anodic charge

was kept constant for all experiments) the other three (Y3 Y4 Y5) ndash coating uniformity The

coating thickness was measured by a Quanix 1500 eddy-current gauge equipped with a stand

which provided reproducible measurements in respect to the sample axis with an error 4microm

Differences in sample diameters ΔDi = Diafter ndash Dibefore where i represents displacement 1 to 5

along the main sample axis were measured using a LIN 0-25 digital micrometre with accuracy

of plusmn4microm The schemes of measurements are depicted on Fig 3

Fig 3 Scheme of thickness and diameter measurements from the outer (1) to the inner (5) end of

the sample

Usually current mode is characterised by a value ofR=J C J A where JC and J A are the

average negative and positive current densities In this experimental design because of small

variation in the R-value the usage of the cathodic current excess percentage ΔR () = (R - 1)

100 may be more convenient

Taking into account that the AC pulse train contains both positive and negative polarisation

an estimation of ΔR values for different experimental conditions was carried out in accordance

with expression

∆ R ( )=( J C

J A

minus1)∙ 100 =([ J AC ∙ τ AC+J C ∙ τC

J AC ∙ τ AC+J A ∙ τ A ]minus1)∙ 100 (1)

Table 1 Factorial design parameters and corresponding experimental conditions for fractional

replication 2-1

X0JAC JCJAC τACτC τC JAC

mAcm2JC

mAcm2τCms

τACms t h ΔR X1 X2 X3 X4

1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343

22 Finite element method (FEM) FEM calculations were performed using ldquoComsol

Multiphisicsrdquo software for a 2D cross-section including the main symmetry axis of the sample

and screening holder Evaluation of the current density distribution along the sample length has

been carried out for two cases of ldquothinrdquo and ldquothickrdquo coating The thin coating was modelled as a

layer with uniform thickness such situation is typical for PEO right after the voltage had reached

breakdown level The thick coating was modelled as a layer with non-uniform thickness which

is three times thicker at the outer end compared to the inner The difference between anodic and

cathodic polarisations was modelled by different conductivity the average values of which were

taken as 01 and 10 Sm respectively The electrolyte solution was modelled as an aqueous

medium with typical for the alkali-silicate electrolyte conductivity of 10 Sm The counter

electrode (not shown on figures) was represented by a grounded at zero potential metal circle 04

m in diameter Net anodic and cathodic currents in the system were set at 15 A corresponding

to the current density of about 100 mAcm2 which is similar to those maintained in the

experiments The results of calculations are presented as 2D map of current line in electrolyser

as well as normal component of the local current density in respect to the metal-coating

interface

23 Effects of R on coating structure In this case we were interested only in the effect of

ΔR-value on the coating microstructure with no regards to the non-uniform current density

distributions Therefore substrates were fabricated as disks (Oslash25x5mm) Influence of ΔR varied

between -462 and 135 on the coating microstructure was studied using combined AC-A and

AC-C current modes with constant timings and variable current densities within AC A and C

pulse trains (see Table 2) SEM investigation were performed by table top device Hitachi T3000

with EDS facility

Table 2 Polarisation conditions for PEO of aluminium A2024 alloy in silicate-alkaline electrolyte in AC-C (1-3) and AC-A (4-6) modes f = 50Hz

JAC

mAcm2

JC mAc

m2

JA mAc

m2 τAC ms τC ms τA ms ΔR 123456

117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462

24 Redistribution of current densities Substrates were fabricated in the form of rings

(Oslash14x12x75 mm) and installed by four into special holder with insulating spacers and

individual electrical connections (See Fig 2b) This provided the cell layout similar to that

applied in the factorial experiment (Fig2a) but allows differentiating substrate currents

depending on the distance from the open end All connections were made in accordance with the

wiring diagram shown in Fig4 In this part of the study the current mode was set corresponding

to run 5 in the factorial experiment which was found to provide the best results

Fig4 Wiring diagram for experiments with sectioned samples

3 Results

31 Influence of combined current mode on the coating thickness distribution Appearances of the coatings obtained under different current modes are shown in Fig5 High

quality PEO coatings obtained in dilute silicate-alkaline electrolytes on A2024 alloy are

typically of uniform light-grey colour whereas appearance of brownish regions indicates

deterioration in coating quality due to destructive action of powerful ldquoarcrdquo microdischarges or

insufficient coating thickness The brownish colour might possibly be attributed to the formation

of copper enriched compounds in the vicinity of the powerful discharges by direct oxidation of

substrate containing about 4 of Cu

From Fig5 it is clear that spatial distribution of defects associated with the brownish regions

is strongly influenced by polarisation conditions The samples can be divided to three groups

with relatively uniform spatial distribution of defects (runs 1 5 6) and with higher defect

densities in the inner (runs 2 4 7 8) and outer (run 3) parts of the sample

Fig 5 Appearance of the samples PEO treated at different combinations of factors (see Table

1) Top of the pictures corresponds to the outer end of the specimen bottom to the inner one

Moreover comparison of the pairs of samples produced at the same R value but with

different other conditions (1 vs 2 3 vs 4 5 vs 6 and 7 vs 8 see Table 1) shows that the

coating quality and distribution of defects depend on the parameters of the combined current

mode rather than solely on the value of R

Fig 6 shows relative distributions of coating thickness (normalised in respect to the values at

the outer end of the sample) and the increment in the sample diameter (averaged within three

repetitions) along the sample axis

Fig6 Relative coating thicknesses hih0 (a) and relative increases in diameter ΔDiΔD0 (b) at given distances x from the outer end of the sample Numbers 0 and 1-8 correspond to the central point and factorial experiment points (see Table 1) respectively

Responses Y1-Y5 for each experimental conditions (averaged within three repetitions) are

presented in Table 3 The regression coefficients for linear model (2) are shown in Table 4

Y i=X0 i+b1 i X1+b2i X 2+b3 i X3+b4 i X 4+b5i X1 X2+b6 i X1 X3+b7 i X2 X3 (2)

Table 3 Averaged responses of factorial experiment design for runs 0 to 8 increase in the

sample diameter (ΔD) coating thickness (h) corresponding variations σ(ΔD) and σ(h) visual

estimation Error level for Y1-Y4 was 4 μm

ΔD μm h μm σ(ΔD) μm σ(h) μm

Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286

Table 4 Regressions coefficients (bij) for linear model of the factorial design for the averages

(X0) individual effects (X1-X4) and their interactions (X1X2 X1X3 X2X3) ΔXimin ndash confidence

interval - effects without influence (|bijmiddotXi| lt ΔXimin) - effects close to an error level (|bijmiddotXi|

asymp ΔXimin)

Effects bi1 microm bi2 microm bi3 microm bi4 microm bi5

X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100

From Table 4 it follows that the process efficiency estimated by the coating thickness Y2 and

increase in sample size Y1 (bi2 bi1 are the respective regression coefficients) increases when the

AC current density (X1) decreases The inverse dependence of the coating growth rate on the

current density (b11 b12 lt 0) indicates that possible optimisation is restricted due to the current

density cannot be reduced indefinitely Increases in both absolute (τC) and relative (τACτC) values

of duration of the cathodic pulse train improve the process efficiency as well However the

effect of current density during C-pulse train is ambiguous namely its increase causes a

decrease in the coating thickness (b22 lt 0) together with no effect in the sample diameter (b21 lt

ΔX1min) This means the substrate oxidation is suppressed the process efficiency decreases and

the coating becomes enriched with electrolyte components (eg silica)

The coating uniformity was evaluated by variations of both increment in diameter σ 2(ΔD) or

Y3 and thickness σ2(h) or Y4 along the main axis of the specimen (coefficients bi3 bi4 respectively)

from the outer to inner end The variation of ΔD can be reduced with the decrease in τACτC and

increase in JCJAC however the significance in the latter effect is lower as its value is close to

the corresponding error level (marked as in Table 4) Other individual effects are

insignificant (marked as in Table 4) The variation in coating thickness is not influenced by

any individual effects (all coefficients bi4 are less than their error levels) however from Fig6a it

is clear that different conditions produce different coating thicknesses This however can be

accounted for by interactions of individual factors

It is important that for coefficients bi1 bi3 bi4 and bi5 at least one of the interactions from X1X2

X1X3 X2X3 is below the error level thereby supporting the initial assumption about insignificance

of the triple interaction for Y1 Y3 Y4 Y5 responses However for the coating thickness

coefficients bi2 shows considerable values for every effect and their interactions Therefore there

appears to be a confounding between estimates for X4 and the triple effect combination -X1X2X3

The other important feature of interacting effects is that the above independence of σ(h) or Y4 on

any individual effects may be attributed to interacting pairs of X1X2 and X1X3 effects

It is obvious that both the final local coating thickness and changes in diameter indicate local

process efficiency at given local polarisation conditions averaged within the treatment duration

Therefore the variation in the process parameters in complex combined polarisation conditions

allows local current densities to be redistributed on the surfaces with concave geometry

32 Finite element modelling (FEM) of the current density distribution

Before experiments illustrated above it was expected that coating non-uniformity in

particular distribution of the defects would have similar trend for every set of conditions

However the difference in the defects distribution was qualitatively in some cases they were

concentrated at the outer end in other cases they were concentrated at the inner end or they were

uniformly distributed It was clear that such behaviour could not be explained only by non-

uniform distribution of the primary electric field therefore considered phenomenon appeared to

be more complex

Following calculations were performed to clarify the influence of the coating properties on the

current density distribution It is known that metal-oxide-electrolyte system under PEO

conditions possess severe non-linear properties We will use simplified approach taking into

account only valve effect (difference in effective conductivity under positive and negative

polarisation) and thickening of the coating at the outer end which were found in above

experiments Accordingly we used two values for coating conductivity and two types of coating

geometry (see Sec22) The modelling results as 2D map of the current paths are illustrated on

Fig7 Moreover distributions of the normal component of current density for thin and thick

coatings under anodic and cathodic polarisation are shown in Fig8

Fig7 Current densities distributions in cases of thin coating (ab) and thick coating (cd) under

anodic (ac) and cathodic (bd) polarisation

Fig8 Dependencies of the normal component of current densities for (a) thin and (b) thick

coatings under anodic and cathodic polarisations

It can be seen that thicker coatings tend to provide more uniform current density distribution

however a relative difference is still quite considerable A specific feature of the current density

distribution consists in the fact that not only a decrease in current along the sample axis can be

seen but also a variation in local values of R = JcJa As a result formation of the coating

fragments located at the inner end of the sample occurs not only at a lower growth rate but also

under substantially different value of R lt 1 As can be seen from Fig8b at the outer end R is

about 11 and soft sparking PEO conditions could arise then R decreases to 10 at the distance of

about 5mm (characteristic point) after that the main part of the internal coating is formed under

conditions corresponding to R lt 10 which could promote the arcing PEO mode It is known

that PEO coating formed in soft sparking PEO (R gt 1) are quite different from sparking PEO

coating (R lt 1) in structure composition and properties

33 Influence of R-factor in combined current mode

In order to clarify how the coating microstructure is influenced by the R-value in combined

current modes (AC-C and AC-A) a series of treatments has been carried out under experimental

conditions provided in Table 2 Values of ΔR were varied from -462 to 135

The question about application of optical spectroscopy instead of electron microscopy sounds

quite often For copper containing aluminium alloys this is reasonable because of lack in

contrast in SEM images which can be easily seen in optical microscopy as dark and white

sublyers Fig9 illustrates the dark field light microscopy (a) and SEM BSE (b) images We

assume that copper from 2024 alloy (~1 at Cu) changes colour of the PEO coating Point EDX

analysis (Table 5) revealed no considerable difference in the coating sub regions This could

happen due to different oxidation state of copper (2+ and 1+) in interfacial and internal layers

however we had no successful evidence of the assumption due to low copper concentration

which also unable to provide enough contrast in BSE images Therefore optical microscopy

appears to be the best way to see internal microstructural features of the PEO coatings on Cu-

containing alloys

Fig 9 Microscopy images in dark field visible light (a) and backscattering electrons (b) Inset of BSE image in equivalent scale is provided in (a)

Table 5 EDX analysis in points 1 and 2 on Fig9 AtPointelement O Al C Cu Si Na Mg12

57896104

32733143

785579

034017

034043

037028

048085

Cross-sectional microstructure of the resulting coatings is shown in Fig10 It can be seen that

the coating produced at ΔR = -462 has non-uniform morphology where white and dark

regions are mixed together PEO processes with ΔR = -364 and higher produced layered

coatings composed from well distinguishable white interfacial layer and dark dense inner layer

(loose outer layer was partly removed) Moreover the total coating thickness increases from 70

to 125 microm whereas the interfacial layer occupies between 34 to 8 of the total coating

thickness Since the charge passed in the anodic direction was the same for all current modes

listed in Table 2 the increase in the coating thickness indicated improvement of the process

efficiency As a result non-uniform distributions of both local current densities and R-values

cause considerable variation in the coating microstructure (and therefore properties)

Fig10 Dark-field optical micrographs of cross-sections of PEO coatings on formed on A2024

alloy using current modes with different R values (see Table 2) h ndash total thickness (by eddy-

current gauge) bottom percentages indicate fractions of the white interfacial layer

34 Redistribution of the current density

From the results of the factorial experiment discussed in Section 31 it could be seen that the

coating uniformity may vary depending on the current mode Simplified calculations of the

current density distribution (Sec 32) could not help explaining the difference in the coating

uniformity Therefore it became necessary to study in a direct experiment what happens with the

current density distribution during the PEO process under combined polarisation conditions

Experimental investigations of redistribution of current densities caused by the non-uniform

distribution of the primary electric field have been carried out using substrate composed from

rings which were located at specific distances from the open end of the insulating screen (see

Fig1b) One experiment included four individual specimens insulated from each other by PTFE

spacers providing independent electrical connections of the rings (1-4) to the individual current

shunts as shown in Fig4 Current signals from rings 1 2 and 4 were recorded in channels 1 2

and 3 of the four-channel oscilloscope and the voltage signal ndash in channel 4 Due to this current

signal from section 3 was left unrecorded however general considerations could still be

provided

Fig11 shows behaviour of the measured electrical parameters during AC pulse train

following the C pulse train (not shown) It can be seen that a well-known gradual increase in

anodic voltage at the very beginning of the AC pulse train is accompanied by reduction in

current through ring 1 and by increase in current through ring 2 besides a slight increase in the

current through the ring 4 could be noticed during a whole AC-pulse train Such behaviour

indicates redistribution of the electrical current within the sample surface caused by relaxation of

the coating after the polarisation conditions were switched from C- to AC-pulse train Such

redistribution can also be considered as a wave of the maximum current density as if it

ldquoscannedrdquo the surface thereby resulting in improvement of the coating uniformity at a given set

of electrical parameters

Moreover comparison of the transient current-voltage curves (CVCs) recorded at 15 and 55

min (Fig12) captured within steady part (the last period) of the AC pulse train shows that

maximum of hysteresis between upward and downward anodic branches (one of the

characteristic feature of soft sparking PEO) shifts from the outer ring 1 (at 15 min) to the inner

ring 4 (at 55 min) Therefore ldquowave scanningrdquo phenomenon provides redistribution of the soft

sparking conditions in addition to the current density redistribution This is not obvious because

very different CVC for anodic and cathodic polarisations generally speaking may provide

waves with different characteristic length magnitude and propagation rate In particular such

local conditions may be illustrated by Fig12a where maximum of cathodic current density

corresponds to the second ring whereas maximum of the anodic current density corresponds to

the first ring Finally from Fig12b it can be seen that in course of such treatment local CVCs

becomes more similar to each other illustrating uniform formation of the PEO coating

Fig 11 Oscillograms of voltage and current density waveforms for individual subsections (1 2

and 4) during AC pulse train following the C pulse train (not shown) in a combined current

mode at 45 min of the process

Fig 12 Current-voltage curves for the last period of AC pulse train within AC-C current mode

(5 Table 1) at 15 min (a) and 55min (b) of the process for the sectioned specimen Arrows

show time sweep

4 Discussion

41 Redistribution of anodic current density

Improvements in the coating uniformity observed in the above experiment are driven by

current density redistribution under certain polarisation conditions which may be explained

based on the qualitative analysis discussed below This analysis is performed on the basis of

recently suggested concept of the ldquoactive zonerdquo [4] and the discussion within this section should

therefore be considered in close connection with that work The main idea of that concept

consists in the existence of a dielectrically volatile region at the metal-oxide interface where the

main voltage drops It was also assumed that the effective resistance of the active zone could

change due to incorporation of protons under cathodic polarisation providing increase in the

local electric field at the metal-oxide interface hence increasing the rate of electrochemical

reactions

Coating formation under PEO conditions includes electrochemical oxidation of Al substrate

and precipitation of electrolyte components which are followed by plasma assisted reactions It

is obvious that local charge and mass transfers are the more intense the higher the current density

within a particular local region of the specimen Initial coating thickness distribution is caused by

non-uniform primary distribution of the electric field in the electrolyte (defined by the geometry

of conductive media and electrodes) influenced by the secondary current density distribution

defined by local properties of the forming layer

Let us consider local changes within the active zone which may occur during the switch

between cathodic and anodic polarisation (Fig13) If the total coating thickness is sufficient to

enable commencement of soft sparking the thickness of the active zone over the sample surface

is relatively uniform and the main difference in the coating thickness should be attributed to the

other region defined as the ldquoproduct zonerdquo in ref [4] The zone structure in the coating straight

after anodic polarisation (without incorporated hydrogen species) is taken as initial point

(Fig13a) When subsequent cathodic polarisation is applied the distribution of incorporated

hydrogen species indicated by bold black dots within the active zone (Fig13b) becomes non-

uniform with higher hydrogen concentration at the outer end of the sample The reason for such

distribution is clear considering current density distribution along the sample inside the

insulating screen depicted in Fig8 The same reason lies behind uneven distribution of local

extraction rates of hydrogen species under subsequent anodic polarisation with the higher

current density indicating the higher extraction rate As a result the coating at the outer end of

the sample becomes depleted in hydrogen species earlier than in the inner region (Fig13cd)

After total extraction the coating goes to the initial state (Fig13a) It should be noted here that

exact shape of the line between depleted and enriched regions depends on the mechanism of

hydrogen species extraction however further discussion is not affected by this difference

Fig 13 Local changes within the active zone under cathodic and anodic polarisation

Taking into account that hydrogen enriched regions in the active zone possess significantly

higher conductivity (and therefore lower effective resistance) the hydrogen extraction can be

expected to be accompanied by gradual evolution in distribution of effective resistance along the

sample as depicted in Fig14b with curves 0 to 3 corresponding to stages (a) to (d) in Fig13

respectively Taking into account Ohmrsquos law the primary electric field distribution (Fig 14a)

and changes in resistance (Fig14b) the local current density distributions along the sample can

be evaluated (Fig14c) It can be clearly seen that during the AC-pulse train the maximum

current density gradually shifts from the outer end towards the inner end of the sample

From the results of factorial experiments (Sec 31) it can be seen that the coating quality

depends not only on the ratio between cathodic and anodic currents (R) but also on the temporal

parameters of polarisation conditions Fig15 provides an example of the evolution of voltage

waveform in the AC-C current mode It clearly demonstrates that switching from both AC to C

and C to AC is accompanied with voltage relaxation during considerable period of time For this

condition the relaxation lasts for about 3 to 6 periods (or 30 to 60 ms excluding pauses) after

which the system achieves a steady state

Fig 14 Schematic qualitative representation of the primary electric field effective resistance

and current densities during AC-pulse train

The life-time of hydrogen species in the coating without external polarisation (within pause

between cathodic and anodic pulses) has been found to be at least 10 ms [3] Hence chemical

reactions that accompany switching between positive and negative polarisation are responsible

for associated processes under cathodic and anodic polarisation finally resulting in the soft

sparking PEO mode can be assumed to have characteristic times of about few tens of

milliseconds In other words in order to achieve noticeable interaction between the effects of

anodic and cathodic polarisations it is necessary to use appropriate time scale

Fig14 Overview of the voltage behaviour during AC-C current mode

There are two possible options in the analysis of this situation On one hand too short

duration of the pulse train might not be sufficient for complete relaxation of the metal-oxide-

electrolyte system hence a decrease in the process efficiency would be expected On the other

hand slow rates of chemical reactions may serve as a low-pass filter which cut off high-speed

switching effects providing an averaging effect on the overall coating formation process Of

course the latter case is inappropriate for processes that include considerable changes in the

coating properties eg accumulation and total extraction of the hydrogen species as discussed

above in this Section

Besides it is worth noting that scanning phenomena considered above should be clearly

distinguished from known feature of some PEO processes where discharges are not occupying

whole sample surface but form some sort of aggregated group acting only within the surface

part Usually the collective behaviour of discharges is accompanied by gradual migration within

sample [21ndash25] Such behaviour is thought to be attributed with thermal effects of the

discharges promoting predictable ignition of the following discharges rather than with

switching between positive and negative polarity

42 Influence of cathodic current on the PEO process efficiency

Finally some simplified considerations can be provided regarding the mechanisms underlying

the increase in efficiency of PEO treatments that are carried out at increased R values in general

and in the soft sparking mode in particular (Sec 33) Troughton et al [26] have estimated that

almost all energy injected into the system via microdischarge events is irreversibly transformed

into heat absorbed by electrolyte solution (by mechanisms different from Joule heating) As the

dissipated energy is difficult to recover the efficiency of the coating formation process appears

to be reduced At near to ambient bulk temperatures (typical for PEO) formation of alumina due

to the chemical reaction between aluminium and oxygen is strongly favourable

thermodynamically

2Al + 32O2 = Al2O3 ΔGf0 = -15823 kJmolAl2O3 (1)

Often this leads to a temptation to assess the PEO of metals based solely on the formation

energy of the corresponding oxides [27] However such oversimplified approach is not always

credible due to more complex route of transformations In particular such estimations neglect

the fact that despite highly negative Gibbs energy in the case of oxide formation on the surface

of a bulk Al substrate (regardless of PEO anodising or just chemical oxidation) reaction (1) runs

with strong kinetic limitations mainly caused by the barrier properties of the formed oxide layer

which restricts transfer of reacting species Hence additional energy is required to overcome this

barrier and enable charge and mass transfer through the oxide-electrolyte interface the bulk

oxide and the metal-oxide interface

Fig16 shows a simplified energy diagram for typical reactions of alumina formation under

PEO conditions As a starting point aluminium metal (Al) and reduced form of oxygen (O2-) is

considered Depending on electronic properties of the electrode surface two main

electrochemical reactions can take place under anodic bias If electrode provides noticeable

electronic conductivity the oxygen evolution may occur Otherwise the main reaction should be

oxidation of aluminium metal which in turn is limited by reacting species transport either to or

from metal-oxide interface We are interested in the latter case ie in the coating formation

process From this point of view it is clear why most of electrical energy is transformed into

heat which is dissipated in the electrolyte under the conditions of PEO The system consumes

electrostatic energy to excite species increase their kinetic energy (velocity) provide avalanche

breakdown migration etc However there are no ways to transform this excess energy back to

electrical form The only way is to absorb this energy via thermalisation of excited species with

solvent molecules typically H2O (as in the case of PEO) which leads to the increase in the bulk

electrolyte temperature

Fig16 Potential energy diagram for alumina formation process during PEO where IAl ndash

ionisation energy for aluminium AO ndash electron affinity for oxygen molecule with following

dissociation G0f(α) ndash free Gibbs energy for α-alumina formation under standard conditions Ea

ndash an activation energy EDL ndash electric double layer at oxide-electrolyte interface

The activation energy (Ea) of the entire process is defined by the step with the highest energy

For conventional PEO this is likely to be breakdown of electrical double layer (EDL) at the

oxide-electrolyte interface After that high-field migration of reacting species may occur

yielding in particular oxide layer as depicted by the ldquosparking PEOrdquo route in Fig16 The high-

field migration depends mainly on the local strength of electric field (providing hopping

mechanism) which in turn depends on the ratio between the applied potential difference and

the distance at which it is applied At the same time as previously suggested [4] the role of

cathodic current consists in particular in increasing the effective electric field at the metal-oxide

interface due to narrowing the non-conductive region in the active zone as well as in eliminating

the oxide-electrolyte EDL due to the local acidification of the electrode region in the vicinity of

it As a result the total activation energy appears to be reduced due to exclusion of the EDL

barrier and facilitation of migration under increased electric field at the metal-oxide interface

(ldquosoft PEOrdquo route in Fig16)

In general chemistry the phenomenon of energy reduction for an intermediate state (with the

same energies of the reagent and the product) is known as ldquocatalysisrdquo Thus temporal injection

of protons under cathodic polarisation may be considered as an ldquoelectrocatalyticrdquo effect Besides

cathodic current the chemical composition of the coating can potentially affect the activation

energy for PEO

5 ConclusionsIn this paper the improvement in uniformity for PEO coatings on Al 2024 alloy with complex

shape subjected to treatments in combined current modes (AC-C and AC-A) has been

considered The following conclusions can be drawn

1 It was found that the application of combined current modes (AC-C) allowed the coating

uniformity to be improved even if the primary current density distribution is substantially

non-uniform The key role in this phenomenon belongs to associated processes under

cathodic and anodic polarisations The former decreases the local effective resistance of

the coating thereby affecting the secondary distribution of the current density under

subsequent anodic polarisation Local switching between low and high resistance of the

coating occurs in such way that the maximum current density is gradually shifted along

the surface during the AC pulse train thereby facilitating formation of more uniform

coatings Since the shift of the maximum current density takes some time temporal

parameters of polarisation become critical Experiments have shown that even at equal

average R-value the coatings formed at different durations of pulse trains possess

substantially different quality

2 From the finite element modelling it was found that the complex surface geometry might

influence not only the local current density but also the ratio between cathodic and anodic

currents (R) Therefore at any particular set of processing parameters local PEO

processes may differ depending on the location of the cite not only in the coating

formation rate but also in its structure and quality as indicated by the local R value

3 Increase in PEO process efficiency with application of excessive cathodic current

particularly in the soft sparking mode can be explained by electro-catalytic action of the

cathodic current which consists both in the elimination of the oxide-electrolyte EDL due

to the local acidification and in the injection of protons into the coating active zone

thereby reducing potential barrier at the metal-oxide interface These effects decrease the

activation energy of alumina formation process which causes lowering the anodic

threshold voltage of the PEO process and reduction of total power consumption

References

[1] VI Belevantsev OP Terleeva GA Markov EK Shulepko AI Slonova V V Utkin Microplasma electrochemical processes Prot Met 34 (1998) 416ndash430

[2] AL Yerokhin X Nie A Leyland A Matthews SJ Dowey Plasma electrolysis for surface engineering Surf Coatings Technol 122 (1999) 73ndash93 doi101016S0257-8972(99)00441-7

[3] AB Rogov VR Shayapov The role of cathodic current in PEO of aluminum Influence of cationic electrolyte composition on the transient current-voltage curves and the discharges optical emission spectra Appl Surf Sci 394 (2017) 323ndash332 doi101016japsusc201610115

[4] AB Rogov A Yerokhin A Matthews The Role of Cathodic Current in Plasma Electrolytic Oxidation of Aluminum Phenomenological Concepts of the ldquoSoft Sparkingrdquo Mode Langmuir 33 (2017) 11059ndash11069 doi101021acslangmuir7b02284

[5] AG Rakoch V V Khokhlov VA Bautin NA Lebedeva Y V Magurova I V Bardin Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process Prot Met 42 (2006) 158ndash169 doi101134S003317320602010X

[6] A Melhem G Henrion T Czerwiec JL Brianccedilon T Duchanoy F Brochard T Belmonte Changes induced by process parameters in oxide layers grown by the PEO process on Al alloys Surf Coatings Technol 205 (2011) S133ndashS136 doi101016jsurfcoat201101046

[7] CB Wei XB Tian SQ Yang XB Wang RKY Fu PK Chu Anode current effects in plasma electrolytic oxidation Surf Coatings Technol 201 (2007) 5021ndash5024 doi101016jsurfcoat200607103

[8] X Ma C Blawert D Houmlche ML Zheludkevich KU Kainer Investigation of electrode distance impact on PEO coating formation assisted by simulation Appl Surf Sci 388 (2016) 304ndash312 doi101016japsusc201601030

[9] ES Karakozov AV Chavdarov NV Barykin Microarc oxidation - a promising method of producing ceramic coatings Weld Int 8 (1994) 218ndash222

[10] L Xia J Han JP Domblesky Z Yang W Li Investigation of the Scanning Microarc Oxidation Process Adv Mater Sci Eng (2017) 12 doi10115520172416821

[11] L Xia J Han JP Domblesky Z Yang W Li Study of Scanning Micro-arc Oxidation and Coating Development J Mater Eng Perform (2017) doi101007s11665-017-2861-x

[12] E V Parfenov A Yerokhin RR Nevyantseva M V Gorbatkov CJ Liang A Matthews Towards smart electrolytic plasma technologies An overview of methodological approaches to process modelling Surf Coatings Technol 269 (2015) 2ndash22 doi101016jsurfcoat201502019

[13] PS Gordienko ES Panin VA Dostovalov VK Usoltsev Current-Voltage Characteristics of the Metal-Oxide-Electrolyte System when Polarizing Electrodes with Pulse Voltage Pacific Sci Rev 10 (2008) 300ndash306

[14] H Duan Y Li Y Xia S Chen Transient Voltage-Current Characteristics New Insights into Plasma Electrolytic Oxidation Process of Aluminium Alloy Int J Electrochem Sci 7 (2012) 7619ndash7630

[15] AV Timoshenko YV Magurova Application of oxide coatings to metals in electrolyte solutions by microplasma methods Rev Met Madrid 36 (2000) 323ndash330

[16] AG Rakoch AA Gladkova Z Linn DM Strekalina The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-

discharge intensity depending on pH of the electrolyte Surf Coatings Technol 269 (2015) 138ndash144 doi101016jsurfcoat201502026

[17] AL Yerokhin AA Voevodin V V Lyubimov J Zabinski M Donley Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys Surf Coatings Technol 110 (1998) 140ndash146 doi101016S0257-8972(98)00694-X

[18] OP Terleeva V V Utkin AI Slonova Current density distribution through the growing oxide on duralumin surface during microplasma discharges Fiz i Him Obrab Mater (1999) 60ndash64

[19] OP Terleeva AI Slonova VI Belevantsev IB Kireenko AP Ryzhikh Correlations of electrolyte state and characteristics of microplasma coatings with quantity of transmitted electricity Prot Met Phys Chem Surfaces 47 (2011) 80ndash85 doi101134S2070205111010199

[20] J Martin P Leone A Nomineacute D Veys-Renaux G Henrion T Belmonte Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium Surf Coatings Technol 269 (2015) 36ndash46 doi101016jsurfcoat201411001

[21] T Mi B Jiang Z Liu L Fan J Kan X Zhang C Wang Self-Organization Kinetics of Microarc Oxidation Nonequilibrium-State Electrode Reaction Kinetics J Electrochem Soc 163 (2016) C184ndashC197 doi10114920631605jes

[22] VS Rudnev Growth of anodic oxide layers under electric discharge conditions Prot Met 43 (2007) 275ndash280 doi101134S0033173207030125

[23] AB Rogov AI Slonova VR Shayapov Peculiarities of iron-containing microplasma coating deposition on aluminum in homogeneous electrolyte Appl Surf Sci 261 (2012) 647ndash652

[24] Y Cheng Z Peng X Wu J Cao P Skeldon GE Thompson A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte Electrochim Acta 165 (2015) 301ndash313 doi101016jelectacta201503020

[25] S Moon Y Kim Lateral Growth of PEO Films on Al1050 Alloy in an Alkaline Electrolyte JKoreanInstSurfEng 50 (2017) 10ndash16 doi105695JKISE201750110

[26] SC Troughton A Nomineacute A V Nomineacute G Henrion TW Clyne Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation Appl Surf Sci 359 (2015) 405ndash411 doi101016japsusc201510124

[27] TW Clyne SC Troughton A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals Int Mater Rev 0 (2018) 1ndash36 doi1010800950660820181466492

1 Introduction

Plasma electrolytic oxidation (PEO) is an electrochemical surface treatment under high anodic

polarisation conditions (up to 1000V) [12] A specific feature of the PEO is the presence of

localised current channels caused by dielectric breakdown of the forming oxide layers The

breakdown appears as localised microdischarges where energy is dissipated triggering plasma-

assisted chemical reactions and heat effects that influence the charge and mass transfer through

the metal-oxide-electrolyte system and may cause phase transformations sintering calcination

and dehydration of the formed oxide layers As a result new composition and microstructure

may develop in the coatings leading to new properties For instance hard and dense sintered

oxide layers enriched with α-Al2O3 can be obtained on the surface of aluminium alloys at near to

ambient bulk substrate temperatures Due to ecologically friendly nature of the process as well as

high wear- and corrosion resistance of the coatings they become suitable for a wide range of

applications

Despite some positive results a broad industrial application of PEO is limited by a number of

technological issues caused by complexity of the process and a lack of basic information about

the coating formation mechanisms One of the most promising current mode in PEO is so-called

ldquosoft sparkingrdquo regime wherein the development of large destructive discharges at the later

stages of the process is suppressed providing a deeper substrate oxidation leading to more

uniform coatings with higher hardness and better adhesion (see overview in [34]) PEO under

alternating polarisation conditions including soft sparking mode is described by a number of

process parameters in terms of initial (electrolyte composition substrate materials) and boundary

(polarisation mode) conditions As a result it is difficult to find an appropriate set of the process

parameters satisfying requirements of the final coating quality One of the most important

generic properties of coatings is thickness uniformity on the substrates with complex shape

geometry such as blind holes cavities sharp edges and large protrusions which represents the

main challenges in PEO technology

It is clear that any non-uniformity of the PEO coating is caused by non-uniform distribution

of the current density which in turn depends on both the primary electric field distribution in the

electrochemical cell and the local properties of the resulting coating Therefore there are two

main approaches to achieve the uniform PEO coating The first one consists in deliberate

arrangement of the uniform primary electric field distribution by using conformal auxiliary

counter electrodes which shape fits the substrate geometry thereby creating a uniform primary

field distribution [56] Wei et al [7] mentioned that the smaller interelectrode distance the better

corrosion resistance of the coating moreover they have found that current pulses earlier reached

maximum amplitude on the nearer parts of substrate then on the farther ones Importance of the









































are also provided

2 Experimental










































a) b)

PTFE

Al

1234

Al ringsPTFE

PTFEcap

PTFEcap

wire

Spacers

inner end

outer end











were taken















the sample







with expression

∆ R ( )=( J C

J A




replication 2-1


mAcm2JC

mAcm2τCms


1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
















interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References





































































are also provided

2 Experimental










































a) b)

PTFE

Al

1234

Al ringsPTFE

PTFEcap

PTFEcap

wire

Spacers

inner end

outer end











were taken















the sample







with expression

∆ R ( )=( J C

J A




replication 2-1


mAcm2JC

mAcm2τCms


1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
















interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References


































are also provided

2 Experimental










































a) b)

PTFE

Al

1234

Al ringsPTFE

PTFEcap

PTFEcap

wire

Spacers

inner end

outer end











were taken















the sample







with expression

∆ R ( )=( J C

J A




replication 2-1


mAcm2JC

mAcm2τCms


1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
















interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References



















































a) b)

PTFE

Al

1234

Al ringsPTFE

PTFEcap

PTFEcap

wire

Spacers

inner end

outer end











were taken















the sample







with expression

∆ R ( )=( J C

J A




replication 2-1


mAcm2JC

mAcm2τCms


1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
















interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References







































were taken















the sample







with expression

∆ R ( )=( J C

J A




replication 2-1


mAcm2JC

mAcm2τCms


1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
















interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References






























the sample







with expression

∆ R ( )=( J C

J A




replication 2-1


mAcm2JC

mAcm2τCms


1 1 1 1 1 -1 120 20 140 660 185 3542 1 -1 1 1 1 60 10 420 1980 347 3543 1 1 -1 1 1 120 10 420 1980 173 1774 1 -1 -1 1 -1 60 5 140 660 369 1775 1 1 1 -1 1 120 20 420 1020 206 6866 1 -1 1 -1 -1 60 10 140 340 456 6867 1 1 -1 -1 -1 120 10 140 340 228 3438 1 -1 -1 -1 1 60 5 420 1020 412 343
















interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References









































interface







with EDS facility


JAC

mAcm2

JC mAc

m2

JA mAc


117117117927461

37120000

0005898122

280280280280280280

120120120000

000

120120120

1354500

-213-364-462









3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References






























3 Results





























Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References










































Visual estimation

Y1 Y2 Y3 Y4 Y5

0 1879 1252 269 393 -1 1770 1339 247 238 07152 2079 1653 238 268 01433 1985 1473 502 385 04294 2039 1573 198 300 00005 2169 1670 126 164 10006 2223 1869 227 367 08587 1791 1395 236 242 05728 2187 1692 225 269 0286




asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References
































asymp ΔXimin)


X0 203 158 25 28 0470X1 (JAC) -10 -32 3 -2 0102

X2 (JCJAC) 3 -13 -4 -2 0102

X3 (τACτC) -6 -28 5 2 -031X4 (τC) 7 24 2 -1 0061

X1X2 1 -24 -5 -4 -0102

X1X3 1 -18 5 4 -0020

X2X3 -7 -26 -1 -2 -0184ΔXimin 4 4 4 4 0100






































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References











































be more complex







































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References


























































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References




































containing alloys



57896104

32733143

785579

034017

034043

037028

048085




























provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References














































provided




























show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References



















































show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References


































show time sweep

4 Discussion











reactions











































































thermodynamically














































energy for PEO




























References



































































































thermodynamically














































energy for PEO




























References








































































thermodynamically














































energy for PEO




























References






















































thermodynamically














































energy for PEO




























References










































































energy for PEO




























References


















































energy for PEO




























References













































References





























References









































Documents

· Web viewThe role of cathodic current in plasma electrolytic oxidation of aluminium: “Scanning waves” of the current density on complex-shape substrates Aleksey B. Rogova,b,c