17417 Historia Do Pensamento Administrativo Aula 09 Volume 02
Chapter VI Corrosion behaviour of Anodized...
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Transcript of Chapter VI Corrosion behaviour of Anodized...
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
References
[1] J. Baumeister, J. Banhart and M. Weber, Mater. Design 18 (1997) 217.
[2] S. Mezlini, K. Elleuch and Ph. Kapsa, Surf. Coat. Technol. 201 (2007) 7855.
[3] X. Li, X. Nie, L. Wang and D.O. Northwood, Surf. Coat. Technol. 200
(2005) 1994.
[4] L. Young, Anodic Oxide Films, Academic Press, London (1961).
[5] W. Bensalah, K. Elleuch, M. Feki, M. Wery and H.F. Ayedi, Surf. Coat.
Technol. 201 (2007) 7855.
[6] W. Bensalah, K. Elleuch, M. Feki, M. Wery, M.P. Gigandet and H.F. Ayedi,
Mater. Chem. Phys. 108 (2008) 296.
[7] A.W. Brace, The Technology of Anodized Aluminum, Robert Droper,
Teddington, (1968) 1.
[8] R.A. Wodehouse, Electroplating Engineering Handbook, 3rd
ed, A.K. Graham
(Ed), Nostrand Reinhold Company, New York, 456 (1971).
[9] S. Wernick, R. Pinner and P. Sheasby: The Surface Treatment of Aluminum
and Its Alloys, 5th
ed. Finishing Publication Ltd., Teddington, UK (1987).
[10] S. Mezlini, K. Elleuch, S. Fouvry and Ph. Kapsa, Surf. Coat. Technol.
200 (2006) 2852.
[11] V. Lopez, E. Otero, A. Bautista and J.A. Gonzalez, Surf. Coat. Technol.
124 (2000) 76.
[12] A. Jagminas, D. Bigeliene, I. Mikulskas and R. Tomasiunas, J. Cryst. Growth
233 (2001) 591.
[13] O. Lunder, J.C. Walmsley, P. Mack and K. Nisancioglu, Corros. Sci.
124 (2005) 1604.
[14] V. Moutarlier, M.P. Gigandet, B. Normand and J. Pagetti, Corros. Sci.
47 (2005) 937.
[15] V. Moutarlier, M.P. Gigandet, J. Pagetti and L. Ricq, Surf. Coat. Technol.
173 (2003) 87.
[16] J.M. Montero-Moreno, M. Sarret and C. Muller, Surf. Coat. Technol.
201 (2007) 6352.
[17] J.M. Montero-Moreno, M. Sarret and C. Muller, J. Electrochem. Soc.
154 (2007) C169.
[18] T. Aerts, Th. Dimogerontakis, I. De Graeve, J. Fransaer and H. Tercyn, Surf.
Coat. Technol. 201 (2007) 7310.
[19] Ch.U. Yu, Ch-Ch. Hu, A. Bai and Y.F. Yang, Surf. Coat. Technol. 201
(2007) 7259.
[20] A. Bai, Ch-Ch. Hu, Y.F. Yang and Ch-Ch. Lin, Electrochim. Acta 53
(2008) 2258.
[21] R.B. Mason and P.F. Fowle, J. Electrochem. Soc. 101 (1954) 53.
[22] J.W. Diggle, T.G. Downie and C.W. Goulding, Electrochim. Acta, 15
(1970) 1079.
[23] G. Lv, W. Gu, H. Chen, W. Feng, M. Latif Khosa, L. Li, E. Niu, G. Zhang and
S. Z. Yang, Appl. Surf. Sci. 253 (2006) 2947.
[24] H.H. Shih and S.L. Tzou, Surf. Coat. Technol. 124 (2000) 278.
[25] F. Keller, M.S. Hunter and D.L. Robinson, J. Electrochem. Soc. 100
(1953) 411.
[26] M.S. Hunter and P.E. Fowle, J. Electrochem. Soc. 101 (1954) 481.
[27] W. Bensalah, M. Fek, M. Wery and H.F. Ayedi, J. Mater. Sci. Technol.
26(2) (2010) 113.
[28] G.D. Sulka, Highly ordered anodic porous alumina formation by
self-organised anodizing: A. Eftekhari (Ed), Nanostructured Materials in
Electrochemistry, Wiley-VCH, Weinheim, Chapter 1 (2008).
[29] A.C. Harkness and L. Young, Can. J. Chem. 44 (1966) 2409.
[30] K. Shimizu, G.E. Thompson and G.C.Wood, Thin Solid Films 88 (1982) 255.
[31] G.E. Thompson, Thin Solid Films 297 (1996) 192.
[32] Y. Yamamoto and N. Baba, Thin Solid Films 101 (1983) 329.
[33] D.R. Lide (Ed), CRC Handbook of Chemistry and Physics, CRC Press,
London, New York (2000) 12.
[34] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson and G.C. Wood,
Electrochim. Acta 46 (2001) 4379.
[35] Y. Fang, Proc. Int. Conf. Surf. Sci. Eng. (1995) 333 (Eng).
[36] X.W. Yu, C.W. Yan and C.N. Cao, Mater. Chem. Phy. 76 (2002) 228.
[37] Y. Yin, T. Liu, S. Chen, T. Liu and S. Cheng, Appl. Surf. Sci. 255
(2008) 2978.
[38] C. T. Chen and G. A. Hutchins. J. Electrochem. Soc. 132 (1985) 1567.
[39] R. S. Alwitt and C. K. Dyer, Electrochim. Acta 23 (1978) 355.
[40] R. S. Alwitt, C. K. Dyer and B. Noble. J. Electrochem. Soc. 129 (1982) 711.
[41] W. J. Bernard and P. G. Russell, J. Electrochem. Soc. 127 (1980) 1256.
[42] J.W. Diggle, Electrochim. Acta 18 (1973) 283.
[43] D. Grzegorz, Sulkaa, J. Wojciech and St.epniowski, Electrochim. Acta,
54 (2009) 3683.
[44] T. B. Wei, F. Y. Yan and J. Tian, J. Alloy. Compd. 389 (2005) 169.
[45] W.B. Xue, X.L.Wu, X.J. Li and H. Tian, J. Alloy. Compd. 425 (2006) 302.
[46] X.J. Li, G.A. Cheng, W.B. Xue, R.T. Zheng and Y.J. Cheng, Mater. Chem.
Phys. 107 (2008) 148.
[47] C. Blawert, V. Heitmann,W. Dietzel, H.M. Nykyforchyn and M.D. Klapkiv,
Surf. Coat. Technol. 201 (2007) 8709.
[48] L.E. Fratila-Apachitei, J. Duszczyk and L. Katgerman, Surf. Coat. Technol.
157 (2002) 80.
[49] C.P. Lee, Y.Y. Chen, C.Y. Hsu, J.W. Yeh and H.C. Shih, Thin Solid Films
517 (2008) 1301.
[50] Y.T. Tao, J. Am. Chem. Soc. 115 (1993) 4350.
[51] W. Wilhelmsen and A. P. Grande. Electrochim. Acta, 33 (1988) 927.
[52] S. Guang-ling, A. L. Bowles and D. H. St John, Mater. Sci. Engg. A
366(1) (2004) 74.
[53] J.S. Lee, G.H. Gu, H. Kim, K.S. Jeong, J. Bae and J.S. Suh, Chem. Mater.
13 (2001) 2387.
[54] S.E. Frers, M.M. Stefenel, C. Mayer and T. Chierchie, J. Appl. Electrochem.
20 (1990) 996.
[55] G. Goeminne, H. Terryn and J. Vereecken, Electrochim. Acta 40 (1995) 479.
[56] F. Li, L. Zhang and R. Metzger, Chem. Mater. 10 (1998) 2470.
[57] D. Mcdonald and P. Bulter, Corros. Sci., 13 (1973) 259.
[58] M. Nagayama, H. Takahashi and M. Koda, J. Met. Finish. Soc. Jpn. 30
(1979) 438.
[59] R. Kent Van Horn, “Aluminum”, Robert Draper Ltd., Teddington, 3
(1971) 647.
[60] S. Tajima, Advances in Corrosion Science and Technology, M.G. Fontana and
R. W. Staehle (Eds), Plenum Press, 1 (1970) 229.
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.