Chapter VI

Corrosion behaviour of Anodized aluminium

6.1. Introduction

The use of aluminum alloys in building and construction industry has

increased at the last few decades [1] due to their specific properties such as

appearance, low density and high corrosion resistance in combination with relatively

good mechanical properties. However, in many instances, inadequate corrosion

properties and low surface hardness have greatly restricted their application.

Mechanical and corrosion resistance can be further improved when the aluminum

substrate is anodized [2–9].

Anodizing [7–9], which is an electrochemical process, consists of converting

aluminium into its oxide by appropriate selection of the electrolyte and the anodizing

conditions such as current density, voltage and temperature.

Numerous works dealing with anodization were focused on the anodizing

treatment conditions and the composition of single acid electrolyte, i.e. solution of

sulphuric acid, chromic acid, phosphoric acid or oxalic acid, in order to optimize the

properties of the anodic layer such as corrosion resistance, microhardness and

abrasion resistance [10–13]. Over the past decades, modified electrolytes were

implemented by addition of oxianions having two oxidation states such as the

chromates (molybdates, permanganates etc.) [14,15] to improve the properties of the

anodic layer and/ or to find an alternative of chromic acid anodizing process which

will be forbidden by the year 2007.

Montero-Moreno et al. have investigated the effect of the aluminum surface

pretreatment [16] and anodizing voltage applied in the first and second anodizing

steps [17] on porous AAO membranes produced by two-step anodization of AA1050

alloy in oxalic acid at 20ºC.

A potential-controlled one-step anodization of AA1050 alloy in a H2SO4

(145 g dm−3

) and Al2(SO4)3·18H2O (5gdm−3

) mixture at 17 V has been reported by

Aerts et al. [19]. The effect of anodizing temperature ranging from 5 to 55ºC on the

porosity and mechanical properties of fabricated AAO films was investigated [18]. On

the other hand, Bai et al. [19,20] proposed, for the AA1050 alloy, a new anodizing

procedure for obtaining highly uniform AAO layers with variying pore diameters. The

anodization was carried out in a mixture of sulfuric acid and oxalic acid with an

addition of the commercially available Al protection agent.

In the present work, anodization of aluminium was carried out in potassium

tetra oxalate (PTO) bath and the effect of several processing factors, such as

temperature, treatment time, current density and concentration of potassium tetra

oxalate on the corrosion resistance of anodized coatings in sodium chloride media

were systematically investigated. The interpretation of the results obtained from the

fitting procedure of the impedance spectra using electrochemical equivalent circuit

was supported by means of surface analysis technique like SEM. Oxalate ions are

bidentate ligands capable of forming strong surface complexes and at the same time

they are of very low toxicity.

Anodization of aluminium was carried out by immersing preweighed

aluminium specimens (w1) in potassium tetra oxalate (5–25 g/l) bath at various bath

temperatures (30-50°C) and current densities (0.02-0.05 A/cm2). After the treatment,

the specimens were washed with tap water, rinsed with deionized water, dried and

weighed (w2). The bath solutions were prepared using triply distilled water.

6.2. Effect of process parameters

The influence of process parameters on thickness and growth rate of oxide

film has been discussed in detail.

6.2.1. Effect of temperature

Fig. 6.1 reports the dependence of the thickness and growth rate of the

alumina coatings formed by anodization using PTO bath (25 g/l potassium tetra

oxalate) with temperature (30 – 50°C). From the Fig, it can be seen that, as

temperature of the bath increases, the thickness and growth rate of the coatings are

found to decrease. During these stages, the coatings grew gradually and quickly with

increase of temperature. High temperature can cause the dissolution rate of Al2O3 to

increase rapidly and hence the film become thinner. If the temperature is too high, the

rate of dissolution is faster than that of oxide formation, the film even vanishes,

resulting in electropolishing of aluminum. So the growth rate and thickness of the

coating decrease at higher temperature. Maximum thickness (30.74 µm) and growth

rate (0.6831 µm/min) were obtained at this temperature. This can be explained as

follows.

A large amount of heat is released during the formation of oxide coating

because of the exothermic reaction of aluminium oxide formation and from the

electric current. Due to this, the electrolyte near the specimen is heated to a

maximum. At this maximum temperature, the electrolyte becomes more aggressive

and hence its dissolution ability gets higher. At this condition, the formed anodic

coating undergoes dissolution at a faster rate than that of its formation. At higher

temperature, the dissolution of aluminium predominates, but at lower temperature, the

formation of oxide coating predominates. As a result of the overheating, tribological

properties of the alumina layer are degraded. During the regular oxide growth, the

anodization temperature along the edges is slightly lower than that at the centre and is

distributed accordingly in the remaining regions.

Together with the mechanical properties, the process of oxide formation and

the microstructure of the anodic alumina films are also influenced by the variation of

the electrolyte temperature. By increasing electrolyte temperature, the aggressiveness

of the electrolyte towards the oxide also increases, thereby, enhancing the chemical

dissolution of the coating by the electrolyte [21, 22].

At very low temperature, the liberated heat during coating growth is

completely and effectively dissipated from the specimen to the bulk of the solution.

Hence, maximum thickness, growth rate and coating ratio were obtained at low

temperature (30°C). At this lower temperature, the formation of Al3+

and O2-

ions

from the solution is maximum and they combined together to form various kinds of

alumina compounds.

An increase in electrolyte temperature will increase proportionally the rate of

dissolution of the anodic film resulting a thinner, more porous and softer film. Low

temperatures are used to produce hard coatings normally in combination with high

current densities and vigorous agitation.

In decorative and protective coatings, anodizing temperatures in the range of

15-25°C are normally used. If temperature is increased further, the maximum

thickness is reduced to lower values due to the higher dissolving power of the

electrolyte. Hence, 30°C was considered as the optimum temperature for fabricating

better quality coatings (i.e., showing maximum thickness and growth rate).

6.2.2. Effect of treatment time

Fig. 6.2 reports the dependence of the thickness and growth rate of the

alumina coatings formed by anodization using PTO bath (25 g/l potassium tetra

oxalate) with treatment time (15 – 75 min). From Fig. 6.2, it can be observed that, as

the treatment time increases, the thickness is found to increase only up to 45 min.

After reaching a maximum, i.e., after 45 min, it decreases. At the initial stage, the

formation of oxide film predominantly takes place than the chemical dissolution of

film by the electrolyte. But as the time increases, the specimen is in contact with

electrolyte for longer time and the liberated heat is more concentrated on the

specimen. Hence the chemical dissolution of oxide coating gradually increases with

time even though formation of coating also increases. As a result, the thickness and

growth rate are found to decrease with time for longer treatment time.

The growth rate of coating formed in 15 min is 1.3 μm/min whereas that

formed in 75 min is 0.3 μm/min. i.e., the growth rate is gradually decreasing with

treatment time. This may be explained as follows: As the anodizing time increases,

chemical dissolution of the formed oxide film by the electrolyte increases and hence

the rate of growth of oxide film decreased with time. When the treatment time is

increased, oxide film formation is taking place gradually and hence thickness and the

growth rate of oxide film increase with time [23].

6.2.3. Effect of current density

Effect of current density on the formation and properties of anodized alumina

coating was studied by varying the current density between 0.02 – 0.05 A/cm2

for 45

min at 30ºC and the results are presented in Fig. 6.3. The current density of the

process significantly affected the rate of deposition and thickness. The rate of

deposition and thickness are found to increase with increasing current density. The

maximum thickness and growth rate are gained at 0.05 A/cm2. At very high current

density (0.05 A/cm2), the applied current is maximum and hence the rate of formation

of the coating is more. The difference in the weight gain between various coatings

correlates well to their surface morphology. The thickness of the thin barrier layer at

the bottom of the porous structure is only dependent on the anodizing CD/voltage,

regardless of anodizing time. It is generally accepted that, the thickness of barrier-

type alumina is mainly determined by the applied voltage, even though there is a

small deviation depending on the electrolytes and temperature.

The aluminium dissolution process has complicated character, representing a

combination of chemical dissolution of aluminium and dissolution assisted by electric

field. In fact, the increase in current density provokes high dissolution of the oxide in

the bottom of pores and favour, thus, the layer growth [24].

The thickness of the barrier layer is extremely important from point of view of

applications of AAO films. The thickness of the barrier layer depends directly on the

anodizing CD. The anodizing ratio being the proportionality constant correlating the

barrier layer thickness is about 1.3–1.4 nm [25,26]. In fact, high values of the growth

rate are obtained for high current densities [27].

The range of CD used in standard anodizing varies from 1-2 A/dm2, for certain

application up to 3 A/dm

2. Current densities below this range produce soft, porous

and thin films. As the current density is increased, the film forms more quickly with

relatively less dissolution by the electrolyte, consequently the film is harder and less

porous. At very high current densities, there is a tendency for the oxide film to "burn",

which occurs due to the development of excessively high current flow at local areas

with overheating at such areas [27].

The minimum lower current density generates growth of the compact and

high-resistant oxide layer on aluminum at the beginning of the process. A local

increase in current density and appearance of the maximum in the current-time curve

is a consequence of rapid transformation of the compact layer to porous oxide

followed by the rearrangement of pores occurring on the surface [28]. It is generally

accepted that, the presence of the maximum in the current–time curve is ascribed to

the pore rearrangement process occurring on the surface. The rearrangement process

is a result of interaction of neighboring pores and leads to a network close-packed

pore on the surface. Due to the lower current densities recorded during the

anodization in oxalic acid (lower growth rates of the oxide layer), the rearrangement

of pores on anodized surface occurs at the later time than typically in sulfuric acid.

6.2.4. Effect concentration of potassium tetra oxalate

Effect of concentration of potassium tetra oxalate on the formation and

properties of anodized Al was studied by varying the concentration between 5 - 25 g/l

at constant current density of 0.05 A/cm2

for 45 min and the results are presented in

Fig. 6.4. The concentration of oxalate ion significantly affects the rate of deposition

and thickness. The rate of deposition and thickness are found to increase with

increasing concentration of oxalate ion. The maximum thickness and growth rate are

obtained from bath containing 25 g/l potassium tetra oxalate. The difference in the

weight gain among various coatings correlates well to their surface morphology. As

the concentration of PTO increases in the bath, the formation of oxlate ions increases

which increases the formation and thickness of the coatings. So the rate increases at

higher concentration of potassium tetra oxalate (25g/l). The increase in concentration

imitates the maximum film thickness due to the higher dissolving power of the

concentrate solutions.

It is well known [29,30], that the oxide films are formed by anodic

polarization of aluminum in aqueous solutions of di(tri)basic acids according to the

reaction:

2Al3+

+ 3H2O →Al2O3 + 6H+

+ 6e-

(Reaction 6.1)

As claimed by Thomson [31], in oxalic acid solutions, this reaction takes place

only at the metal/barrier oxide layer interface causing a low oxalate (2–3 wt.% [32])

entrapment. In addition, the Faradaic efficiency for pure aluminum is close to

100% [33].

Some oxalate ions are also migrated into the porous coating of aluminum, thus

causing an increase of oxide film thickness [34]. It was found that this method

produced better oxide coating with good corrosion resistance as compared to the

method developed by Fang using sulphuric/oxalic acid system [35].

6.3. Corrosion behaviour of the anodized coatings

The corrosion behaviour of anodized formed in various anodizing conditions

were evaluated through Tafel polarization method and Electrochemical impedance

spectroscopy and the respective curves are shown in Figs. 6.6 to 6.13. Corrosion

potential (Ecorr), corrosion current density (Icorr) and corrosion rate (Rcorr) values were

determined and the results are represented in tables (6.1-6.8).

The simulation fitting procedure was performed using the equivalent circuit of

Fig.6.5 and the parameters are Rs, Rct and Cdl, where Rs is the solution resistance,

Rct is the charge transfer resistance and Cdl is the double layer capacitance [36].

The shape of the Nyquist diagram is similar for all samples and the shape is

like a semicircle. The impedance data are mainly capacitive. The Nyquist diagram of

the anodized aluminium have semicircle with a larger diameter and higher corrosion

resistance compared to that of the bare aluminium.

6.3.1. Effect of treatment temperature on the corrosion parameters

Fig. 6.6 shows the Tafel polarization curves of the anodized Al formed at

various treatment temperatures. The curve of the anodized aluminium rises to a higher

potential of -0.6 V (SCE), whereas the bare aluminium remained at the low potential

of -1.2 V (SCE). The dense layer of alumina on the surface of the anodized

aluminium resulted in an insulated barrier at a higher potential.

The corrosion parameters of the anodized aluminium samples as a function of

bath temperatures are presented in table 6.1. From the table, it can be observed that

the corrosion current density (Icorr) and corrosion rate (Rcorr) increase on increasing the

temperature from 30 to 50 C. This indicates that on increasing the temperature from

30 to 50°C, the oxide formation is slow and so corrosion rate decreases. The lowest

corrosion rate is observed at 30ºC for 45 min is 5.099×10-04

mpy. Anodized

aluminium revealed the highest nobility with a lowest corrosion current density.

It is observed that, there is a large decrease in the anodic current of the

anodized samples compared to the uncoated aluminum sample. The corrosion current

density of the anodized samples is four orders of magnitude lower than that of

uncoated aluminum. The corrosion protection efficiency of the anodic coatings can be

explained and interpreted by both the increase in corrosion potential as well as the

decrease in the corrosion current density [3, 37].

The Nyquist impedance curves for the bare and anodized aluminium formed at

various temperatures are shown in Fig. 6.7 as a function of treatment temperatures

and the impedance parameters are given in the table 6.2. The semicircle of bare

aluminium is much smaller than that of the anodized aluminium. The reactions across

interfaces might thus be explained with reference to the equivalent circuit shown in

Fig. 6.5. On increasing the temperature from 30 to 40 C, the corrosion resistance (Rp)

decreases rapidly (9998 ). Further increasing the temperature from 40 to 50 C, the

corrosion resistance (Rp) still decreases (4683 ). The solution resistance of the

coatings at this particular current density is also high compared to others (7.458 Ω).

Crystalline films have higher capacitance because of larger dielectric constant [38],

but exhibit an electrical instability [39] which is related to the presence of voids [40]

and/or to the trapped oxygen [41]. As temperature increases, the dissolution of Al is

increases. Since there is fall in the oxide formation and subsequently corrosion

resistance is decreased. The highest corrosion resistance (Rp) obtained at 30ºC is

47640 Ω.

6.3.2. Effect of treatment time on the corrosion parameters

Fig. 6.8 shows the Tafel polarization curves of the anodized Al formed in

various treatment times. The potential increased from -1.2 V (SCE) for bare

aluminium to -0.55 V (SCE) for the anodized aluminium samples at various treatment

times. The current density for the anodized samples is four orders of magnitude lower

than that for bare aluminium. Moreover, the current increasing speed of anodic branch

of bare aluminium is slower than that of its cathodic branch, which is just reverse and

passivation behaviour is observed in the cathodic branch for anodized aluminium

samples. This indicated that anodization significantly increased the potential and

decreased the corrosive current, suggesting that the corrosion resistance of aluminium

could be obviously improved after anodization.

The corrosion parameters of the anodized aluminium samples as a function of

treatment times are presented in table 6.3. From the table, it can be observed that the

corrosion potential (Ecorr) of anodized aluminium is more positive than the bare

aluminium. The corrosion current density (Icorr) and corrosion rate (Rcorr) decrease on

increasing the treatment time from 15 to 45 min. This is due to the fact that, at the

initial stage, the formation of oxide film predominantly takes place than the chemical

dissolution of film by the electrolyte. The corrosion current density (Icorr) and

corrosion rate (Rcorr) increase on further increasing the treatment time from 45-75

min, This is because, the chemical dissolution predominates over the formation of the

coatings at longer duration. The least corrosion rate is observed for Al anodized in 45

min at 30ºC is 5.099×10-04

mpy.

The Nyquist impedance curves of the bare and anodized Al formed in various

treatment times at 30ºC are shown in Fig. 6.9 and the impedance parameters are given

in the table 6.4. On increasing the treatment time from 15 to 45 min, the corrosion

resistance (Rp) increases rapidly (47640 ). When the treatment time is increased,

oxide coating formation is taking place gradually and hence the thickness of oxide

coating increases with time [42]. So, the corrosion resistance increases with anodizing

time. Further increasing the anodizing time from 45 to 75 min, the corrosion

resistance (Rp) decreases rapidly (5097 ). This is due to the chemical dissolution of

oxide film by the electrolyte during longer duration.

6.3.3. Effect of current density on the corrosion parameters

Fig. 6.10 shows the Tafel polarization curves of the anodized Al formed at

various current densities. The potential value increased from -1.2 V (SCE) for bare

aluminium to -0.5 V (SCE) for the anodized aluminium samples at various current

densities. The corrosion current density was found by extrapolation of the Tafel

portions of the anodic and cathodic polarization curves. Passivity of oxide coatings

are commonly accompanied by a positive shift of the electrode potential of the metal.

Therefore, the corrosion potential Ecorr of the anodized aluminium can serve as

indicator of its state. The corrosion current density for the anodized samples is four

orders of magnitude lower than that for bare aluminium. Moreover, the current

increasing speed of anodic branch of bare aluminium is slower than that of its

cathodic branch, which is just reverse and a passivation behaviour is observed in the

cathodic branch for anodized aluminium samples. This indicated that anodization

significantly increased the potential and decreased the corrosive current, suggesting

that the corrosion resistance of aluminium could be obviously improved after

anodization.

The corrosion parameters of the anodized aluminium samples as a function of

current density are presented in table 6.5. From the table, it can be observed that, the

corrosion current density (Icorr) and corrosion rate (Rcorr) decrease on increasing the

current density from 0.02 to 0.05 A/cm2. This indicates that on increasing the current

density from 0.02 to 0.05 A/cm

2, the coating formation increases and the corrosion

rate decreases. The least corrosion rate is observed at 0.05 A/cm2

for 45 min at 30ºC

is 5.099×10-04

mpy.

It is worth noting that, the anodizing current is related with the movement of

oxygen containing ions (O2−

or OH−) from the electrolyte through the barrier layer at

the pore bottom to the metal/oxide interface and with simultaneous outward drift of

Al3+

ions across the oxide layer. When the rate of oxide layer formation is relatively

high, especially at the high anodizing CD or a high-field anodization regime

(hard anodizing), a diffusion limited anodization processes is observed. A rapid

increase in oxide thickness results in a significant extension of the diffusion path

along the channels of porous layer and gradual decrease of the ionic current over time.

According to the Faraday‟ s law, the thickness of oxide layer formed

during anodization is directly proportional to current density and anodizing time

when the Faradaic current efficiency equals 100%. For a given duration of the

process (30 min or 60 min), the thickness of oxide layer increases with increasing

current density and consequently with increasing anodizing potential.

Therefore, with increasing anodizing potential, an exponential increase of the

oxide layer thickness is observed for all studied temperatures [43].

The Nyquist impedance curves of the bare and anodized Al formed at various

current densities at 30ºC for 45 min are shown in Fig. 6.11 and the impedance

parameters are given in the table 6.6. On increasing the current density from 0.02 to

0.03 A/cm2, the corrosion resistance (Rp) increases gradually. Further increasing the

current density from 0.03 to 0.05 A/cm2, the corrosion resistance (Rp) increases

rapidly (47640 ). As the current density is increased, the film forms more quickly

with relatively less dissolution by the electrolyte, consequently the film is harder and

less porous. As thickness of the oxide coating increases with CD, the corrosion

resistance increases. The corrosion resistance of the coating is mainly dependent on

its thickness, microstructure and phase composition [44-47].

6.2.3. Effect of concentration of potassium tetra oxalate on the corrosion

parameters

Fig. 6.12 shows the Tafel polarization curves of the anodized Al formed from

various PTO concentrations. The curve of the anodized aluminium rises to a higher

potential of -0.6 V (SCE), whereas the bare aluminium remained at the low potential

of -1.2 V (SCE). The dense layer of alumina on the surface of the anodized

aluminium resulted in an insulated barrier and a higher potential. The current density

for the anodized samples is four orders of magnitude lower than that for bare

aluminium. Passivity of oxide coatings are commonly accompanied by a positive shift

of the electrode potential of the metal. Therefore, the corrosion potential Ecorr of the

anodized aluminium is shifted to more positive than the uncoated aluminium.

The corrosion parameters of the anodized aluminium samples as a function of

oxalate ion concentration are presented in table 6.7. From the table, it can be observed

that the corrosion current density (Icorr) and corrosion rate (Rcorr) decrease and

corrosion potential (Ecorr) increases on increasing the concentration from 5 to 25 g/l.

This indicates that, on increasing the concentration from 5 to 25 g/l, the rate of oxide

formation is more than the dissolution, and so corrosion rate decreases. The least

corrosion rate is observed at 25 g/l potassium tetra oxalate for 45 min at 30ºC is

5.099×10-04

.

The corrosion rate is less, which is probably due to the involvement of

oxalate/sulphate and borate ions in the oxide film making it more corrosion resistant

[3,48-50]. The corrosion rate of aluminum sample is also quite less for this coating.

The oxalate concentration influenced markedly both the general shape and the

partial characteristics of the polarization curves. The most significant difference may

be seen in the anodic branch which upon increasing oxalate concentration showed an

extended passive region. Moreover the current densities corresponding to the anodic

branch increased when the specimens were anodized in solutions of higher oxalate

concentrations. In the literature, this type of behaviour is reported [51].

The Nyquist impedance curves of the bare and anodized Al formed from

various PTO concentration (5 - 25 g/l) at 30ºC in 45 min is shown in Fig. 6.13 and the

impedance parameters are given in the table 6.8. On increasing the concentration from

5 to 10 g/l, the corrosion resistance (Rp) increases gradually (5384 ). Further

increase in concentration from 10 to 20 g/l, the corrosion resistance (Rp) still increases

(26750 ) and maximium corrosion resistance (Rp) is observed (47640 Ω) for the Al

anodized in 25 g/l PTO concentration. At higher concentration of PTO, more oxalate

anions are produced which also retard the oxide dissolution. Corrosion resistance at

higher concentration of PTO increases at a CD of 0.05 A/cm2. The diameter of the

capacitive semicircle of a measured Nyquist impedance spectrum is closely related to

the corrosion rate [52]. The larger the semicircles, the better will be the corrosion

resistance.

6.3. Surface Examinations

6.3.1. Surface Morphological Studies: Scanning Electron Microscopy

Anodized specimens formed at 30ºC in 45 min at various current densities

(0.02-0.05 A/cm2) were analyzed by SEM in order to study the growth of anodized

coatings. Figs. 6.14 – 6.17 show the scanning electron microscope image of anodized

aluminium obtained from 25g/l potassium tetra oxalate bath at various current

densities.

Fig. 6.14 shows the SEM image of anodized Al obtained at 30ºC in 45 min at

CD of 0.02 A/cm2. SEM micrograph reveals cracks in the oxide film and is most

likely caused by the internal stress generated by the growth of the oxide at the

substrate/oxide interface [48]. The cracks may be as a result of non-uniform oxidation

cracks and flaws can be formed on the surface.

SEM of anodized Al obtained at 30ºC in 45 min at CD of 0.03 A/cm2

is shown

in Fig. 6.15. It is known that, a carefully controlled anodization of aluminum in an

acidic electrolyte produces a thin layer of dense aluminum oxide, followed by an

ordered array of nanopores [53]. The advantages of porous anodic alumina include

self-assembly, high aspect ratio, uniform pore size, uniform channel length and easy

fabrication.

Fig. 6.16 shows the SEM image of anodized Al obtained at 30ºC in 45 min at

CD of 0.04 A/cm2. The top surfaces of the oxide layers exhibit nano-pores uniformly

distributed, together with some spherical shaped dots heterogeneously distributed in

the intervening areas. In addition, the pores are roughly circular in section and

gradually merge along domain boundaries. The pore spacing is relatively high

compared to the pore diameter [27].

Scanning electron microscope image of anodized coating obtained at 30ºC in

45 min at CD of 0.05 A/cm2

is shown in Fig. 6.17. A self-organized anodization of

aluminum results in a porous structure of oxide with a dense and compact dielectric

layer at the pore bottoms known as the barrier layer. It is clearly visible that,

anodization of pure aluminum results in formation of nanoporous alumina layers with

the near ideal, hexagonal arrangement of pores, independently of the used anodizing

electrolyte. Moreover, for the given anodizing potential and type of anodized

substrate, porous anodic nanostructures exhibit uniform pore diameters.

This complex influence of the solution specific conductivity on the

electrochemical measurements was considered very probable to result from a porous

structure of the surface oxide film formed in the presence of the oxalate ions. This

consideration is in agreement with the results obtained by Wilhelmsen et al [54] who

showed that anodization of aluminum in neutral oxalate solutions resulted in the

formation of a porous oxide layer.

It should be also noted that the porous structure of the covering layer obtained

in the presence of oxalate seemed to be comparable to that reported by Goeminne et al

[55] for the case of conversion coatings formed on aluminum treated in chromate-

phosphate containing solutions.

6.3.2. Elemental Analysis: Energy Dispersive X-Ray Spectroscopy

Fig. 6.18 shows the EDX spectrum of anodized Al obtained from 25g/l

potassium tetra oxalate bath at 30 C for 45 min at CD of 0.05 A/cm2. The element

contents (%) of different anodic alumina coatings were determined from EDX results

and the results are given in table 6.9. The anodized coatings formed from the bath

containing potassium tetra oxalate are all contain C, O, S and K (came from

solutions) and Al (came from substrate). From the elemental analysis, it can be seen

that qualitatively, the chemical composition of topcoat layer is mainly composed of

various phases of aluminium oxides. The elements K, C and S may come from the

anodizing bath indicating the presence of some other oxalate compounds.

6.3.3. Phase Compositional Analysis: X-Ray Diffraction Method

Figs. 6.19 – 6.21 show the XRD patterns of anodized Al from 25g/l potassium

tetra oxalate in 45 min at various current densities. In the XRD pattern of anodized Al

at CD of 0.03 A/cm2

(Fig. 6.19), aluminium oxalate (C6Al2O12) phase appears at

38.99° (d spacing=2.31) [JCPDS card= 37-0488]. Potassium aluminium oxalate

(C6AlK3O12.3H2O) phase appears at 44.848° (d spacing=2.0193) [JCPDS card=

51-0619, monoclinic/primitive, a10.28 b19.54 c7.704; β108.33] with preferred

orientation of (4 3 1). θ-alumina (θ-Al2O3) phase is also observed at 65.813°

(d spacing=1.419) [JCPDS card= 47-1771. Aluminium hydroxide (Al(OH)3) phase

appears at 78.304° (d =1.22) [JCPDS card= 26-0025, cubic/primitive a7.20] with

preferred orientation of (5 3 1).

In the XRD pattern of anodized Al at CD of 0.04 A/cm2

(Fig. 6.20),

aluminium oxalate (C6Al2O12) phase appears at 38.99° (d spacing=2.31) [JCPDS

card= 37-0488]. δ-alumina (δ-Al2O3) phase appears at 45.105° (d spacing=2.01)

[JCPDS card= 47-1770, tetragonal/primitive, a7.943 c23.90] with preferred

orientation of (3 1 7). θ-alumina (θ-Al2O3) phase is also observed at 65.813° (d

spacing=1.419) [JCPDS card= 47-1771]. Aluminium hydroxide (Al(OH)3) phase is

observed at 78.304° (d =1.22) [JCPDS card= 26-0025, cubic/primitive a7.20] with

preferred orientation of (5 3 1). Hence on increasing the current density from 0.03-

0.04 A/cm2, potassium aluminium oxalate phase is disappeared and new alumina

phase is appeared.

(oxide)

Fig. 6.21 shows the XRD pattern of anodized Al at the CD of 0.05 A/cm2

δ-alumina (δ-Al2O3) phase appears at 45.105° (d spacing=2.01) [JCPDS card= 47-

1770, tetragonal/primitive, a7.943 c23.90] with preferred orientation of (3 1 7). θ-

alumina (θ-Al2O3) phase appears at 65.813° (d spacing=1.419) [JCPDS card= 47-

1771]. Aluminium hydroxide (Al(OH)3) phase appears at 78.304° (d =1.22) [JCPDS

card= 26-0025, cubic/primitive a7.20] with preferred orientation of (5 3 1). Hence on

increasing the current density from 0.04-0.05 A/cm2, aluminium oxalate phase also

disappeared and only alumina phases are predominates.

6.5. Mechanism of Anodization

F. Li et al. described the formation of oxide coating during anodization via a

chemical model. The reactions are explained with the following mechanism [56]:

Al3+

ions form at the metal/oxide interface:

Al(s) → Al3+

+ 3e-

(Reaction 6.2)

At the oxide/electrolyte interface, the water-splitting reaction takes place,

which is considered as the rate determining step:

3/2 H2O(1) → 3H+

(aq) + 3/2 O2-

(oxide) (Reaction 6.3)

The O2-

(oxide) ions migrate due to the electric field and form Al2O3 at the

metal/oxide interface.

2Al3+

+ 3O2-

→ Al2O3 (Reaction 6.4)

The protons can locally dissolve more oxide:

l/2Al2O3(s) + 3H+(aq) → Al

3+ (aq) + 3/2H2O(l) (Reaction 6.5)

The hydronium ions can also migrate toward the cathode, where they leave the

system in the form of hydrogen and complete the circuit:

3H+

(aq) + 3e- → 3/2H2(g) (Reaction 6.6)

The mechanism of anodic oxidation is complex and not completely

understood. In 1973, a possible mechanism had been deduced by McDonald and

Bulter [57]. The following reactions are assumed to involve in the anodization

process.

(1) Ionization reaction:

Al →Al3+

+ 3e-

(Reaction 6.2)

(2) Chemical reaction:

Al3+

+ 3OH- → Al(OH)3 (Reaction 6.7)

(3) Aging process:

2Al(OH)3 →Al2O3.H2O + 2H+

+2OH-

(Reaction 6.8)

(4) Dissolution reaction:

Al2O3 + 6H+

→ 2 Al3+

+3H2O (Reaction 6.9)

In anodizing, the barrier layer is formed first and its thickness varies directly

with the forming voltage and current density. Porous oxide coating soon develops

above the barrier zone due to the dissolving action of acidic electrolyte [58]. The

porous structure permits continued growth in thickness of the coating until

equilibrium is established between formation and dissolution of coating [59].

The observation of the anodic film on aluminum by the electron microscopy, it

has been documented that Al3+

migrates through the metal/oxide interface and O2-

migrates through the oxide/electrolyte interface so that Al3+

can react with O2-

to

produce Al2O3. The migration rates of the ions depend on their size, the temperature

of the surrounding and the electric field strength.

The principal reactions occurring in sulphuric acid anodizing have been

summarized by Tajima [60] as follows

Al →Al3+

+ 3e-

(Reaction 6.2)

2Al3+

+ 3H2O → Al2O3 + 6H+

(Reaction

6.10)

SO42-

→ SO3 + O2-

(Reaction

6.11)

2Al3+

+ 3O2-

→ Al2O3

(Reaction

6.4)

The growth of the oxide layer in sulphuric acid involves the simultaneous

formation of a new barrier layer as pores are being formed in the previous barrier

layer.

Based on the above mechanisms, the following reactions are assumed to takes

place during anodization of aluminium from potassium tetra oxalate (PTO) in addition

to the formation of various phases of alumina, aluminium oxalate and potassium

aluminium oxalate which is confirmed by EDX and XRD studies.

Al → Al3+

+ 3e-

(Reaction 6.2)

KH3(C2O4)2 . 2H2O → 3K+

+ 6CO2 + 6O2-

+ 6OH- +3H

+ + 2H2O

(Reaction 6.12)

2Al3+

+ 3O2-

→ Al2O3

(Reaction

6.4)

Al2O3 + 3 H2O → 2Al(OH)3

(Reaction

6.13)

3K+

+ Al3+

+ 6CO2 → K3AlC6O12

(Reaction

6.14)

2Al3+

+ 6CO2 → Al2C6O12

(Reaction

6.15)

Al3+

+ 3(OH)- → Al(OH)3

(Reaction

6.16)

Figures and Tables

Fig. 6.1 Influence of temperature on thickness and growth rate of the oxide

coatings obtained by anodization of aluminium at CD of 0.05 A/cm2

for 45

min.

Fig. 6.2 Influence of time on thickness and growth rate of the oxide coatings

obtained by anodization of aluminium at CD of 0.05 A/cm2.

Fig. 6.3 Influence of current density on thickness and growth rate of the oxide

coatings obtained by anodization of aluminium at 30ºC for 45 min.

Fig. 6.4 Influence of concentration of potassium tetra oxalate on thickness and

growth rate of the oxide coatings obtained by anodization of aluminium at CD

of 0.05 A/cm2

for 45 min.

Fig. 6.5 Equivalent circuit used for fitting the impedance data for oxide coatings by

anodization of aluminium.

Fig. 6.6 Comparative Tafel polarization curves of the oxide coatings formed by

anodization by aluminium at various temperatures (30-50ºC) in 3.5% NaCl solution

at 0.05 A/cm2

for 45 min.

Fig. 6.7 Comparative Nyquist plots of the oxide coatings formed by anodization of

aluminium at various temperatures (30-50°C) at 0.05 A/cm2

for 45 min.

Table 6.1 Influence of temperature on calculated Tafel parameters of the coatings

formed by anodization at 0.05 A/cm2

for 45 min.

Temperature (ºC) Icorr (A) Ecorr (V vs. SCE) Rcorr (mpy)

Bare 7.3070×10-4

-1.2312 3.131×10+02

30 1.189×10-9

-1.2912 5.099×10-04

40 2.079×10-6

-0.7285 8.916×10-01

50 5.574×10-5

-0.6566 2.391×10+01

Table 6.2 Influence of temperature on impedance parameters of the oxide coatings

formed by anodization at 0.05 A/cm2

for 45 min.

Temperature (ºC) Rs (Ω) Cdl (F) Rct (Ω)

Bare 1.92 2.182 x10-5

185

30 7.458 8.852x10-9

47640

40 2.9985 5.402x10-9

9998

50 2.5258 2.768x10-6

4683

Fig. 6.8 Comparative Tafel polarization curves of the oxide coatings formed by

anodization of aluminium in various treatment times (15-75 min) in 3.5% NaCl

solution at 30ºC.

Fig. 6.9 Comparative Nyquist plots of the oxide coatings formed by anodization of

aluminium in various treatment times (15-75 min) at 30°C.

Table 6.3 Influence of treatment times on calculated Tafel parameters of the oxide

coatings formed by anodization of aluminium at 30ºC.

Time (min) Icorr (A) Ecorr (V vs. SCE) Rcorr (mpy)

Bare 7.3070×10-4

-1.2312 3.131×10+02

15 2.885×10-6

-0.7411 1.238×1000

30 5.434×10-8

-0.6761 2.331×10-02

45 1.189×10-9

-1.2912 5.099×10-04

60 4.008×10-7

-0.7450 1.719×10-01

75 1.460×10-4

-0.5956 6.263×10+01

Table 6.4 Influence of current density on impedance parameters of the oxide coatings

formed by anodization of aluminium at 30°C.

Time (min) Rs (Ω) Cdl (F) Rct (Ω)

Bare 1.92 2.182 x10-5

185

15 7.350 1.122x10-6

7635

30 5.754 x10-8

2.088x10-6

19680

45 7.458 8.852x10-9

47640

60 0.009926 6.001x10-8

30710

75 0.4706 4.554 x10-6

5097

Fig. 6.10 Comparative Tafel polarization curves of the oxide coatings formed by

anodization of aluminium at various current densities (0.02-0.05 A/cm2) in 3.5% NaCl

solution at 30ºC for 45 min.

Fig. 6.11 Comparative Nyquist plots of the oxide coatings formed by anodization of

aluminium at various current densities (0.02-0.05 A/cm2) at 30°C for 45 min.

Table 6.5 Influence of current density on calculated Tafel parameters of the oxide

coatings formed by anodization of aluminium at 30ºC for 45 min.

Current density (A/cm

2) Icorr (A) Ecorr (V vs. SCE) Rcorr (mpy)

Bare 7.3070×10-4

-1.2312 3.131×10+02

0.02 7.418×10-6

-0.7317 3.152×1000

0.03 9.066×10-7

-0.7539 3.889×10-01

0.04 1.854×10-7

-0.4433 7.952×10-02

0.05 1.189×10-9

-1.2912 5.099×10-04

Table 6.6 Influence of current density on impedance parameters of the oxide coatings

formed by anodization of aluminium at 30°C for 45 min.

Current density (A/cm2) Rs (Ω) Cdl (F) Rct (Ω)

Bare 1.92 2.182 x10-5

185

0.02 2.75 2.245x10-6

1148

0.03 3.45 3.695x10-9

3748

0.04 0.006393 4.705x10-9

21180

0.05 7.458 8.852x10-9

47640

Fig. 6.12 Comparative Tafel polarization curves of the oxide coatings formed by

anodization of aluminium from various concentrations of PTO (5-25 g/l) in 3.5%

NaCl solution at 0.05 A/cm2

for 45 min at 30ºC.

Fig. 6.13 Comparative Nyquist plots of the oxide coatings formed by anodization of

aluminium from various concentration of PTO (5-25 g/l) at 0.05 A/cm2

for 45 min at

30ºC.

Table 6.7 Influence of concentration of PTO on calculated Tafel parameters of the

oxide coatings formed by anodization of aluminium bath at 0.05 A/cm2

for 45 min at

30ºC.

Conc (g/l) Icorr (A) Ecorr (V vs. SCE) Rcorr (mpy)

Bare 7.3070×10-4

-1.2312 3.131×10+02

5 1.544×10-4

-0.7262 6.624×10+01

10 3.347×10-5

-0.6733 1.436×10+01

15 2.177×10-7

-0.6472 9.340×10-02

20 1.988x10-8

-0.7010 8.527x10-3

25 1.189×10-9

-1.2912 5.099×10-04

Table 6.8 Influence of concentration of PTO on impedance parameters of the oxide

coatings formed by anodization of aluminium at 0.05 A/cm2

for 45 min at 30ºC.

Conc. (g/l) Rs (Ω) Cdl (F) Rct (Ω)

Bare 1.92 2.182 x10-5

185

5 0.78 5.53x10-8

1604

10 1.99 5.596x10-9

5384

15 1.362x10-6

1.59x10-9

21180

20 7.606x10-5

8.316x10-7

26750

25 7.458 8.852x10-9

47640

Fig. 6.14 SEM image of the oxide coating formed by anodization of Al at CD

of 0.02 A/cm2

at 30ºC for 45 min.

Fig. 6.15 SEM image of the oxide coating formed by anodization of Al at CD

of 0.03 A/cm2

at 30ºC for 45 min.

.

Fig. 6.16 SEM image of the oxide coating formed by anodization of Al at CD

of 0.04 A/cm2

at 30ºC for 45 min.

.

Fig. 6.17 SEM image of the oxide coating formed by anodization of Al at CD

of 0.05 A/cm2

at 30ºC for 45 min.

.

Fig. 6.18 EDX spectrum of the oxide coating formed by anodization of aluminium at

CD of 0.05 A/cm2

for 45 min at 30ºC.

Table 6.9 Elemental composition of the oxide coating formed by anodization of

aluminium at CD of 0.05 A/cm2

for 45 min at 30ºC.

S.NO. Element kev Atomic %

1. C K 0.277 6.18

2. O K 0.525 9.65

3. Al K 1.486 81.5

4. K K 3.312 2.62

5. S K 2.307 0.05

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Chapter VII

Summary and conclusions

Various surface coatings were prepared on aluminium from different baths

with additives. The corrosion behaviour of various coatings on aluminium was

analyzed. Influence of temperature and current density on surface morphology (SEM)

and structural aspects (XRD) have been studied. In order to optimize other

experimental conditions, such as electrolyte/additive concentration, temperature and

treatment time, surface treatments were carried out in the possible range of the above

parameters. This thesis is divided into seven chapters. Chapter I deals with

introduction of theory and concepts of corrosion, immersion platings, conversion

coatings, anodization and surfactants along with review of literature and aim and

scope. In chapter II experimental details were given. The obtained results were

discussed in chapter III to chapter VI. In the chapter III, the corrosion behaviour of

microwave assisted immersion tin deposited aluminium was studied. The influence of

temperature, irradiation time and concentration of bath constituents on the properties

of the coatings were discussed. Corrosion behaviour of microwave assisted chromate

conversion coated aluminium and the effect of surfactants on the properties of

chromate conversion coating are discussed in chapter IV. In the chapter V, corrosion

behaviour of molybdate conversion coated aluminium and the effect of various

parameters are discussed. Anodization of aluminium using potassium tetra oxalate

was carried out and its influence on various current densities, temperature and

concentration on properties of oxide coatings are discussed in chapter VI.