2.1.2. Preparation of specimens
Pure aluminium (Al) specimens (99.2%, 0.25 mm thick) in flag shape pieces
of size 7 x 2 cm were cut from the sheet. Appropriate sized threaded holes were made
in samples, to ensure good electrical connection with jigging arrangements. All
samples were prepared from the same aluminium sheet with same area in order to
minimize the differences of samples in composition and microstructure.
2.1.3. Surface Treatments
2.1.3.1. Mechanical Polishing.
The aluminium specimens were mechanically polished by simple mechanical
buffing to remove so many surface defects such as scratches, deep corrosion pits, die
lines, scales etc.
2.1.3.2. Degreasing
The mechanical polishing will leave many organic contaminations such as oil,
grease etc., on the surface of aluminium. These organic impurities were removed by
degreasing.
It was done by soaking cotton in acetone and rubbing the aluminium
surface with the soaked cotton in a single direction.
2.1.3.3. Alkaline Cleaning
After degreasing, the aluminium specimens were immersed in 5% NaOH
solution at room temperature for 3 min to remove the naturally formed oxide film and
to get chemically and metallurgically cleaned surface.
2.1.3.4. Washing and Rinsing
After the alkali cleaning, the excess amount of alkali solution adhered on the
surface of the aluminium was washed out in running tap water thoroughly and rinsed
in demineralised water.
2.1.3.5. Desmutting
Every alkali treatment will leave smut or sludge on the surface. Likewise,
alkali cleaning also leaves some smut on the surface of aluminium. If this smut is not
properly removed, it will affect the further treatment such as anodization and
deposition. The removal of smut or sludge called desmutting, was done by immersing
the aluminium specimens in 25% HNO3 solution at room temperature for 2 min.
2.1.3.6. Washing and Rinsing
After desmutting, the excess amount of nitric acid adhered on the surface of
the aluminium was washed out in running tap water thoroughly and rinsed in
demineralised water.
2.1.4. Corrosion behaviour of microwave assisted immersion tin deposited
aluminium
Tin immersion bath was prepared using stannous chloride, hydrochloric acid
and phosphoric acid. Immersion plating was done by immersing the pretreated and
preweighed specimens for various durations from 0.5 to 2.5 min at various
temperature from 30 to 50ºC. The concentration range of immersion plating bath were
stannous chloride (1-5 g/l), hydrochloric acid (100-500 ml/lit) and phosphoric acid
(200 ml/lit) (Table 2.1). The concentration of stannous chloride and hydrochloric acid
were optimized to get thick and uniform coating. The immersion plating was done in
a conventional microwave oven (LG, Ws700 watts and frequency 2.45 GHz), by
heating at a temperature from 30-50ºC at atmospheric pressure for 0.5–2.5 min. After
treatment, the specimens were washed with tap water and rinsed with deionised water,
dried and weighed (w2).
2.1.5. Corrosion behaviour of microwave assisted chromate conversion coated
aluminium
Chromate conversion bath was prepared using chromium trioxide and sodium
fluoride. Conversion coating was done by immersing the pretreated and preweighed
specimens in the chromating bath placed in a a conventional microwave oven (LG,
Ws700 watts and frequency 2.45 GHz), by heating at various temperature from 3070ºC at atmospheric pressure for 0.5–2.5 min. The concentration range of conversion
coating bath were chromium trioxide (3-7 g/l) and sodium fluoride (0.2-0.4 g/lit)
(Table 2.1). The concentration of chromium trioxide and sodium fluoride were
optimized to get thick and uniform coating. Chromating was carried out in After
chromating, the specimens were washed with tap water and rinsed with deionised
water, dried and weighed (w2).
2.1.6. Corrosion behaviour of microwave assisted chromate conversion coated
aluminium with surfactants
The corrosion resistance behaviour of chromate conversion bath can be
improved by adding surfactants such as sodium lauryl sulphate and cetyl pyridinium
chloride respectively. The concentration range of sodium lauryl sulphate was 0.0010.005 g/l and cetyl pyridinium chloride was 0.001-0.005 g/l (Table 2.1). The
concentrations of sodium lauryl sulphate and cetyl pyridinium chloride were
optimized. Chromating was carried out by immersing the samples in surfactant
containing chromate bath in a conventional microwave oven (LG, Ws700 watts and
frequency
2.45
GHz)
by
heating
at
a
temperature
from
30-70ºC at atmospheric pressure for 0.5–2.5 min. After chromating, the specimens
were washed with tap water and rinsed with deionised water, dried and weighed (w2).
2.1.7. Corrosion behaviour of molybdate conversion coated aluminium
The molybdate conversion coating bath was prepared using sodium
molybdate, nitric acid and additives such as manganese acetate, calcium nitrate and
sodium nitrate. Molybdate conversion coating was done by immersing the pretreated
and preweighed specimens into the molybdate bath for various durations from 5 to 25
min at various temperature from 30 to 60ºC. The concentration range of molybdate
conversion coating bath were sodium molybdate (6-10 g/l), manganese acetate (12
g/l), calcium nitrate (8 g/l), sodium nitrate (2 g/l) and nitric acid (80 ml/l) (Table 2.1).
The concentration of sodium molybdate was optimized to get thick and uniform
coating. After treatment, the specimens were washed with tap water and rinsed with
deionised water, dried and weighed (w2).
2.1.8. Corrosion behaviour of anodized aluminium
Anodization of aluminium was carried out in a simple electrolytic cell
equipped with a magnetic stirrer, thermostat (Thermo Haake, DC10-K15) and water
circulating system. Aplab DC regulated power supply (Model: 0 – 5 A/30 V & 01A/120 V) was used for anodization. The working surface of the sample was 1 cm2
and
the
rest
of
aluminum
plate
was
insulated
with
an
epoxy
resin.
A lead sheet of size 10 cm x 3 cm x 0.3 cm was used as a cathode, and the distance
between both electrodes was kept at about 3 cm. Pretreated and preweighed (w1)
aluminium specimens were taken as anode and connected to positive terminal and
lead cathode to the negative terminal of the DC power supply. The coatings were
formed by galvanostatic anodization.
The anodization bath consisted of potassium tetra oxalate (5 – 25 g/l).
Anodization of aluminium was carried out at various temperatures from 25 to 45ºC
for 45 min. The concentration of potassium tetra oxalate was optimized to get thick
and uniform coating. After anodization, the specimens were washed with tap water
and rinsed with deionised water, dried and weighed (w2).
2.2.
Surface examination studies
The structure of immersion, conversion and anodic oxide coatings have been
examined and characterized using optical and scanning electron microscopy, energy
dispersive X-ray spectroscopy, X-ray diffraction and Atomic force microscopy.
2.2.1. Scanning electron microscopic analysis
The surface morphology and nanostructure of the coatings were analyzed by
Scanning Electron Microscope (Philips XL 30 ESEM using beam voltage of 20 kV
and JEOL JSE 5600)/ Field Emission Scanning Electron Microscope (model: FEI
Sirion200), Because of the low electrical conductivity of the coatings, it was
necessary to either use a low accelerating voltage and low vaccum mode, or to sputter
coat the specimens with a thin conductive film of gold or platinum. Platinum was
used when preparing specimens for examination in FESEM, since it deposits more
uniformly on fine scale features.
2.2.2. Energy dispersive X-Ray spectroscopic analysis (EDS)
The elemental analysis of all the coatings were carried out using Energy
Dispersive x-ray Spectroscope (Model: INCA Oxford). X-ray counting was done by
measuring the X-ray photon energies with a Si(Ni) solid state detector. Different
characteristic X-ray lines of elements represent the types and relative amounts of
elements in the sample. The number of counts of each peak can be converted to
weight concentration using standard (more accurate) or standardless calculations.
Samples were flat-polished (scratches < 0.1 μm), grounded and made
conductive to prevent charging. A qualitative analysis was performed to identify the
elements present. Standard materials of known compositions and homogeneous at the
microscopic level were used. The electron beam energy was selected 2 or 3 times that
of the measured X-ray energy. Once the spectrum was collected, the background was
removed, usually by a digital filtering algorithm.
2.2.3. X-Ray Diffraction Method
The elemental composition of the coatings as determined by SEM-EDS (see
section 2.3.1 and 2.3.3) is not sufficient to identify the materials present. The
crystallographic characteristics of the all types of coatings were investigated with
Powder X-Ray Diffractometer (XPERT-PRO, The Netherlands/Philips Model PW
1830, Cu kα radiation). A standard θ-2θ trace was recorded in the range of
10º-120º for complete phase identification. A Cu Kα radiation source was used, with a
40 kV accelerating voltage and a 30 mA filament current. The signal was recorded at
0.02º steps, with a dwell between 4 s and 20 s at each step, according to the desired
data quality.
The X-ray diffraction peak position and full width of peak at half of the
maximum intensity (FWHM) were obtained by fitting the measured peaks in order to
find the true peak position and width corresponding to monochromatic
Kα1/ Kα2
radiation.
Peak fitting was then applied to the diffraction traces and for the angular
positions of peaks. The crystalline phases were identified by comparison with
reference patterns from the Powder Diffraction File (PDF) from the International
Centre for Diffraction Data.
2.3.
Corrosion studies
2.3.1. Tafel Polarization
The Tafel polarization studies were carried out in a glass cell using a typical
three-electrode system consists of coated Al as working electrode, platinum as counter
electrode and a saturated calomel electrode (SCE) filled with saturated KCl as a
reference electrode along with Agar-Agar-KCl salt bridge in all experiments
(Electrolyte: 3.5 % NaCl solution). The tip of the reference electrode was positioned
very close to the surface of working electrodes by the use of a fine luggin capillary in
order to minimize ohmic potential drop. The working electrode was embedded with
epoxy resin to leave 1 cm2 surface contacting with the electrolyte. For all corrosion
studies, the OCP (Open circuit potential) was established first and then the
polarization measurements were carried out after 800s. When the OCP remained
stable, the potential was scanned from initial to final potential at a rate of 10 mV/s
using an electrochemical Workstation (Model No CHI 760, CH Instruments, USA)
and all experiments were carried out at a constant temperature of 28±2°C. Tafel
experiments were carried out at the scan rate of 10 mV/s and quit time was set as 2 s.
This scan rate was selected based on the differential trial experiments to get
reproducible results representing some steady state conditions for the electrode
reaction. The Tafel slopes ba and bc were determined from the slopes of the anodic
and cathodic slopes of the Tafel plots. In addition to this, corrosion current (Icorr),
Corrosion rate (Rcorr) and corrosion potential (Ecorr) were obtained from the
workstation software.
2.3.2. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance measurements were performed in 3.5 % NaCl
solution (Artificial sea water). The applied AC perturbation signal was about
10 mV within the frequency range of 100 MHz to 1Hz. The cell setup used was
exactly same as that used for Tafel Polarization studies. Nyquist impedance plots
were recorded for coated as well as non coated samples. The range of current
densities was dependent upon the choice of the electrolyte and additive. The solution
resistance (Rs), capacitance (C) and polarization resistance (Rp) values were obtained
from CHI workstation after fitting with electrochemical equivalent circuit.
Table 2.1 The bath composition, operating and optimized parameters for various
surface treatment of aluminium.
Constituents
Treatment
Microwave
assisted tin
Immersion
platings
Electrolyte
Additives
Treatment
Temperature
time (min)
(ºC)
HCl (10–
Stannous
50 ml/l)
chloride
H3PO4
(SnCl2)
(200 ml/l)
30 ml/l HCl,
200 ml/l
0.5-2.5
30-50
2.5 min
Sodium
5 g/l CrO3,
fluoride
trioxide
(NaF)
(CrO3)
(0.2–0.4
(3 – 7 g/l)
H3PO4 , 40ºC
and
(1 – 5 g/l)
Chromium
Optimum
bath
composition,
Current
density,
Treatment
time and
Temperature
5 g/l SnCl2,
0.3 g/l NaF,
0.5-2.5
30-70
50ºC
and
1.5 min
g/l)
(i) Sodium
Microwave
fluoride
irradiated
(NaF)
Chromate
(0.2 – 0.4
5 g/l CrO3,
conversion
Chromium
g/l)
0.3 g/l NaF,
coatings
trioxide
(ii)
0.005 g/l
(CrO3)
Sodium
(3 – 7 g/l)
lauryl
sulphate
(SLS)
(0.001–
0.005 g/l)
0.5-2.5
30-70
SLS,
50ºC and 1.5
min
(i) Sodium
fluoride
(NaF)
Chromium
trioxide
(CrO3)
(3 – 7 g/l)
(0.2–0.4
5 g/l CrO3,
g/l)
0.3 g/l NaF,
(ii) Cetyl
pyridinium
0.005 g/l
0.5-2.5
30-70
Chloride
CPC,
50ºC and 1.5
min
(CPC)
(0.001–
0.005 g/l)
(i)
Manganse
80 g/l
Na2MoO 4, 80
ml/l HNO 3,
acetate
Molybdate
Sodium
conversion
molybdate
coatings
(Na2MoO 4)
(60 – 100
g/l)
(12 g/l)
(ii)
Calcium
nitrate (8
5-25
30-60
12 g/l
MnCOOCH3 ,
8 g/l
Ca(NO3)2, 2
g/l NaNO3,
g/l)
(iii)
50ºC and 20
min.
Sodium
nitrate (2
g/l)
25
g/l
potassium
Potassium
Anodization
tetra oxlate,
tetra
oxalate
(5 – 25 g/l)
-
15-75
30-50
0.05
A/cm2
current
density,
45 min and
30ºC
Chapter III
Corrosion behaviour of microwave assisted immersion
tin deposited aluminium
3.1. Introduction
The applications of immersion deposition allow considerable improvement on
the adhesion of the metal coating to the substrate, as well as in promoting the
corrosion resistance of the substrates. The basis of this method is an exchange
reaction leading to the dissolution of the electronegative metal phase (Al) and
deposition of a more electropositive metal (e.g., Sn) during the so called „immersion
treatment‟ of aluminium.
Tin immersion platings are especially noted for their low cost, bright
appearance, good frictional properties, and ease of application to many common
metals such as copper, brass, bronze, aluminum and steel. Tin is a useful metal for the
food processing industry since it is non-toxic, ductile and corrosion resistant.
Tin immersion platings are popular for decorative finishing of small parts such
as safety pins, thimbles and buckles. They are also applied to copper tubing to prevent
discoloration from water, and to aluminum engine pistons to provide lubrication
during break-in periods.
There have been some studies on the formation and characterization of tin
platings deposited on Al and its alloys by the simple immersion method [1-3].
To meet the challenging in-service requirements, surfaces of many mechanical
components need to be modified. This can be achieved by altering the surface
properties through deposition technique. Apart from the conventional methods, laser
treatment is the most popular post processing technique. Of late, microwave heating is
emerging as one of the potential post processing sources. Microwave processing of
materials is fundamentally different from traditional techniques. In microwave
processing, energy is directly transferred to the material through interaction of
electromagnetic waves with molecules leading to volumetric heating.
Number of organic and inorganic compounds were synthesized by using
microwave heating [4, 5]. Many reports are available on MW assisted deposition of
thin films and coatings [6-14].
It was reported by many researchers that the rate and efficiency of the reaction
can be increased by using MW heating [15, 16]. Based on thorough literature survey,
we came to know that there is no report on microwave assisted immersion deposition.
This tempted us to use microwave for the immersion deposition of tin on aluminium.
In the present study, tin immersion platings were applied to aluminium at
various temperatures, time intervals from various bath compositions by using MW
assisted immersion deposition method. The influence of the process parameters on
surface characteristics and the corrosion behaviour of tin coated samples were studied.
The details concerning the operation of the bath can be found in the paper of
Huttunen-Saarivirta [17]. The bath contained hydrochloric acid for pH adjustment,
phosphoric acid for bath stabilisation and stannous chloride for delivering Sn2+ ions.
3.2. Effect of experimental conditions
3.2.1. Effect of temperature
Fig. 3.1 (a-c) reports the dependence of the amount of tin deposited, growth
rate and thickness of the tin immersion platings formed on aluminium in 2.5 min on
temperature. From the Figs, it can be seen that, as temperature of the bath increases,
the weight of the tin deposit, thickness and rate of deposition are found to increase up
to 40 C. During these stages, the tin coatings grew gradually and quickly with
increase of temperature up to 40 C. Maximum values were obtained at 40 C and the
values begin to decrease when the temperature crosses 40 C, because high
temperature will gradually damage the deposition. The results indicate that increase in
plating temperature above 40 C deteriorates the deposit. Moreover, surface corrosion
occurs above 40 C and is enhanced at higher temperature. Enhanced surface corrosion
is also marked by sudden fall in the plating rate. Similar observations were also
reported elsewhere [18, 19]. According to them, such type of corrosion is due to the
resultant effect of Cl- ion and plating temperature. The present results are in good
agreement with their findings.
The nucleation and growth rate of the particle increased with increasing bath
temperature. This observation is in good consistence with the previous report [3] that
the bath temperature had little effect on the maximum weight gain of the specimen.
From the results described above, it can be concluded that, the best plating in terms of
uniformity and compactness was obtained from a bath containing 5g/l SnCl2 + 30 ml/l
HCl + 200 ml/l H 3PO 4 at 40 C. Platings under MW irradiation has a higher growth rate
and higher conversion than traditional heating [20].
Faster heating is achieved under microwave heating than conventional heating.
Approximately 20–30% of the electric energy consumed by the microwave oven is
converted to thermal energy in the vessel during (non-reactive) heating of the
individual components [21].
3.2.2. Effect of treatment time
In order to study the influence of treatment time, tin immersion plating was
carried out on aluminium in various microwave irradiation time from 0.5 to 2.5 min
and its influence on weight of tin deposit, growth rate and thickness of the coating is
given in Fig. 3.2 (a-c).
Usually, for immersion platings, weight of tin deposit and thickness increase
with plating time obeying a linear trend [22] until a certain thickness level. In this
study also, linear dependence of tin coating thickness and weight of deposit on
immersion time is observed. However, at short plating times, the coating nucleation
and growth concentrate on the sites of the nearest plating solution accessibility
yielding surface irregularities. At longer plating times, coating growth takes place
more evenly throughout the surface. The observed levelling is in agreement with the
observations of Iacovangelo [23].
It is commonly understood that immersion (or displacement) deposition
continues as long as the substrate material is removed and its by-products can diffuse
throughout the porous regions in the layer of deposits. But eventually, the process is
self-limiting [24]. Similarly, Sn deposition in the initial state occurred by preferential
nucleation on Al surface rather than building up of the as formed Sn particles. It was
suggested that, once a layer of deposits was formed, deposition would thereafter
proceed to the second layer over the first one resulting in less uniform particle size
and contribute to an enhanced surface roughness [25]. So far, little experiment has
been reported upon a long-term situation of such deposition.
Nevertheless, it is deposited as small islands on the substrate metal. These
islands will be cathodic surfaces whereas the rest of the substrate metal will be anodic
surfaces. That is, initially the anodic surfaces will be much larger than the cathodic
surfaces. As more and more of the surface is coated with the plating metal, the
cathodic areas increase in size and at the end of the process, they are much larger than
the anodic areas. As a consequence, the plating rate on the cathodic surfaces will
decrease with time since the amount of metal reduced at the cathodic areas must
balance the amount of metal oxidised at the anodic areas. This also means that, the
oxidation rate of the anodic areas will increase as more of the total area is coated with
the plated metal and may finally become very high.
3.2.3. Effect of Sn2+ion concentration
Effect of concentration of Sn2+ ion on the formation and properties of
immersion plating was studied by varying the concentration between 1 - 5 g/l at
constant concentration of 30 ml/l HCl and 200 ml/l H 3 PO4 and its influence on weight
of tin deposit, thickness and growth rate of the tin coating is given in Fig. 3.3 (a-c).
The concentration of Sn2+ ion significantly affected the rate of deposition and
thickness. The rate of deposition, amount of tin and thickness are found to increase
with increasing concentration of Sn2+. The difference in the weight gain between
various platings correlates well to their surface morphology. As the concentration of
tin increases in the bath, the formation of Sn2+ ions increases which increases the
deposition rate and thickness of the tin coatings. So, the rate increases at higher
concentration of stannous chloride (5g/l).
Among the different type of combinations of SnCl2 with HCl and
H3 PO4
concentration, the specimen treated in 5g/l SnCl2 + 30 ml/l HCl + 200 ml/l H3PO4 has
the maximum tin deposit (2.279×10-5gm/min), thickness (0.42 m) and growth rate
(0.28 m/min). So, that was chosen as optimum composition for the formation of
better quality immersion platings.
Bailey [26] has also reported that, dilute zincate solutions give coarse, thicker
deposits resulting in poor adhesion, whereas more concentrated solutions produce
finer grained and more compact deposits giving much better adhesion.
It was reported by Karunakaran and Nayak [27] that at higher tin concentration,
the rate of immersion plating is faster but the quality of the deposit deteriorates. In the
present study also, the reaction rate is faster at higher tin concentration but, the quality
of deposit deteriorates.
3.2.4. Influence of chloride ions
The dependence of Sn deposition on Cl - ion concentration at constant Sn2+
concentration was also investigated. Fig. 3.4 (a-c) reports the dependence of the
amount of tin deposited, growth rate and thickness of the tin immersion platings
formed on aluminium in 2.5 min from various Cl - concentrations. From the Fig. 3.4
(a-c), it can be seen that, as concentration of chloride ion increases, the weight of the
tin deposit, thickness and rate of deposition are found to increase up to 30 ml/l.
Beyond that, the properties are found to decrease because pitting corrosion occurred
at very high concentration of Cl- ion. Hence, compact and strong deposit is obtained at
optimum chloride ion concentration (30 ml/l).
The amount of deposited Sn decreases at higher Cl- concentration (30 ml/l).
Least amount of Sn is detected at a Cl- ion concentration higher than 30 ml/l.
According to the above results, we can conclude that, Sn deposition is progressively
inhibited as the concentration of Cl- increased in the solution. In addition, the
effective Cl- concentration, at which the deposition vanishes, is comparable to the
case of aqueous solutions [28].
Halide ions have been used [18, 27] to remove the oxide layer present on
aluminium surface. In the present study, chloride ion was used to evaluate its effect on
the plating of tin onto aluminium. In the absence of chloride ion, the rate of tin plating
is extremely slow indicating the presence of oxide coverage on the aluminium
surface. Similar results were reported by Xue et al [29].
The dissolution of the oxide layer in presence of hydrochloric acid may
proceed according to the following reaction.
Al2O3 + 6HCl
2AlCl3 + 3H2O
(Reaction 3.1)
The higher hydrochloric acid concentration decreases the deposition rate and
affects the plating quality. But at the same time, pit formation also occurred at very
higher hydrochloric acid concentration. The decrease in plating rate at higher chloride
ion concentration may be attributed to relatively more roughening of the aluminium
surface. It favours the formation of thin deposition.
3.3. Corrosion behaviour of the immersion tin platings
The corrosion behaviour of the tin immersion platings formed in various
immersion conditions were evaluated through potentiodynamic polarization technique
and electrochemical impedance spectroscopy. The corrosion potential (Ecorr),
corrosion current density (Icorr), and rate of corrosion (Rcorr) values were determined.
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 tin plated samples have semicircle with a larger diameter indicating the higher
corrosion resistance compared to that of the uncoated sample.
As seen from the equivalent circuit (Fig. 3.5), the impedance of the measured
system between reference electrode (SCE) and working electrode (tin coated) consists
of two R components in series with the solution resistance (R s), and the polarization
resistance Rp parellel with capacitance C. The corrosion resistance behaviour of tin
plated aluminium formed at various deposition conditions are discussed below.
3.3.1. Effect of treatment temperature on the corrosion parameters
Fig. 3.6 shows the Tafel polarization curves of the tin immersion platings
formed at various treatment temperatures. The potential increases from -1.2 V (SCE)
for bare aluminium to -0.6 V (SCE) for the tin coated aluminium samples at various
temperatures. 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 tin coated aluminium
samples. This indicated that the addition of tin significantly increased the potential
and decreased the corrosive current, suggesting that the corrosion resistance of
aluminium could be obviously improved by addition of the tin.
The corrosion parameters of the immersion plating as a function of bath
temperature are presented in table 3.1. From the table, it can be observed that, the
corrosion current density (Icorr) and corrosion rate (Rcorr) decrease on increasing the
bath temperature from 30 to 40 C and the corrosion potential (Ecorr) is shifted to more
noble potential. This indicates that on increasing the temperature from 30 to 40 C, the
coating formation is rapid and further increase of temperature to 50 C, the values are
reversed. Hence corrosion rate decreases. Surface corrosion is initiated at 40 C and is
enhanced at higher temperature. Enhanced surface corrosion is also marked by sudden
fall in the plating rate. The current density for the tin coated samples is three orders of
magnitude lower than that for bare aluminium. Hence the optimum temperature for
formation of more corrosion resistant immersion plating is 40 C. The least corrosion
rate obtained at 40ºC is 9.169×10-2 mpy.
The Nyquist impedance plots of the uncoated and tin coated samples at various
temperatures are shown in Fig. 3.7 and the impedance parameters are given in the
table 3.2. From the table, it can be seen that, on increasing the bath temperature from
30 to 40 C, the corrosion resistance (Rp) increases rapidly ( 8000
) and the
capacitance (C) decreases. But for the coating formed at 50 C, the corrosion
resistance (Rp) is decreased ( 5000
) and capacitance (C)
increases. This is
because, the enhanced surface corrosion arises above 40 C. So, there is fall in the
plating rate and subsequently corrosion resistance is decreased.
3.3.2. Effect of treatment time on the corrosion parameters
The electrochemical behaviour of the uncoated and tin coated samples
obtained in various irradiation time (0.5, 1.0, 1.5, 2.0 and 2.5 min) were studied by
recording the anodic and cathodic potentiodynamic polarization curves and reported
in Fig. 3.8. For bare aluminium, the potential increases in the anodic region, which is
characteristic of an active state and dissolution of the aluminium alloy. This correlates
with observed fluctuations in the corrosion potential curve of the untreated sample.
The curves of the tin coated aluminium rise to a higher potential of
-0.5 V (SCE), whereas the uncoated aluminium remained at the low potential of
-1.23 V (SCE), as illustrated in Fig. 3.8. A shift in the Tafel line towards the upper
right of the diagram indicates an increase in corrosion current density and a decrease
in corrosion resistance of bare aluminium. The current density for the tin coated
samples at various irradiation times is three orders of magnitude lower than that of
bare aluminium indicating the higher corrosion resistance.
The corrosion resistance of the samples with and without tin immersion
platings is evidently different. In contrast to the bare aluminium, the samples with tin
immersion platings all have the positive corrosion potential and lesser corrosion
current density which is shown in Fig. 3.8.
The corrosion parameters of the immersion plating as a function of immersion
time are presented in table 3.3. From the table, it can be observed that, corrosion
current density (Icorr) and corrosion rate (Rcorr) decrease on increasing the treatment
time from 0.5 to 2.5 min. On increasing the irradiation time initially, that is from
0.5 to 1.5 min, there is gradual decrease of corrosion rate (Rcorr). This indicates that,
initial dissolution of the substrate which in turn promotes the initiation of the coating
process. On further increasing the irradiation time to 2.0 min as well as 2.5 min, there
is rapid decrease in corrosion rate (Rcorr) value and the corrosion potential (Ecorr) is
shifted to more noble potential. This indicates the proceeding of the coating over the
surface rather than dissolution. Lowest corrosion rate (Rcorr) is obtained at 2.5 min
time of immersion.
It has been shown that, when the Al alloy is immersed in a plating solution, it
dissolves and a corrosion film is formed at the initial stage of coating process [1, 2, 3].
Subsequent metal dissolution then proceeds through this initially formed porous
corrosion film. Thinning of the corrosion film also occurs simultaneously during
metal dissolution [2]. Continuous metal dissolution provides a sufficient concentration
of Al3+ ions for the coating film deposition.
The Nyquist impedance plots of the tin coated and uncoated samples
determined from the magnitude of the impedance data at 40ºC is shown in Fig. 3.9 as
a function of immersion time and the impedance parameters are given in the table 3.4.
From the table, it can be seen that, the polarization resistance (Rp ) is very much higher
for the tin coated samples than for the bare sample as expected, i.e.,
times (less than 2.0 min) to
8000
2000
at short
at very long times (more than 2.0 min).
The corrosion resistance increases with increase of plating time from 0.5 to 2.5 min.
3.3.3. Effect of Sn2+ ion concentration on the corrosion parameters
Fig. 3.10 shows potentiodynamic polarization curves of the tin immersion
depositions formed from the bath containing various concentration of tin chloride.
From these curves, it can be obviously seen that, the potential of the tin coated
aluminium substrate and the bare aluminium are -0.5 V (SCE) and -1.2 V (SCE)
respectively. The potential of the tin coated sample shifts positively about
0.7 V (SCE) compared with that of bare aluminium. The coating showed passivation
state in a wide potential region. These results demonstrated that, the corrosion
resistance of aluminium has been improved through the tin immersion plating. The
Sn containing layer is very stable and protective over a long range of anodic potential.
The corrosion parameters of the tin immersion platings as a function of various
concentrations of Sn2+ (1-5 g/l) are presented in table 3.5. The corrosion current
density (Icorr) and corrosion rate (Rcorr) decreases and corrosion potential (Ecorr)
increases on increasing the concentration of Sn2+ from (1-5 g/l). The corrosion current
density (Icorr) decreased about three orders of magnitude compared with that of
substrate. This indicates that, increase in concentration of Sn 2+ increases the corrosion
resistance. At higher concentration, more amount of Sn is deposited. This enhances
the plating rate as well as corrosion resistance.
The Nyquist impedance plots of the uncoated and tin coated samples obtained
from the bath containing various concentration of Sn2+ at 40ºC in 2.5 min are shown
in Fig. 3.11 and the impedance parameters are given in the table 3.6. At lower
concentration of stannous chloride, (1 g/l), the corrosion resistance (Rp) is less ( 900
), but on raising the concentration to 3 g/l, there is a rapid increase in corrosion
resistance (Rp) ( 4000
). Further increase of concentration to 5 g/l, yields a thick
and strong deposit and the corrosion resistance becomes doubled ( 8000
) and the
capacitance (C) value decreases. The increase in corrosion resistance implies the
presence of a conversion layer on the aluminium. Best corrosion resistance is
achieved for the tin immersion plating formed from the bath containing 5g/l SnCl2 +
30 ml/l HCl + 200 ml/l H 3PO 4 at 40ºC for the deposition time of 2.5 min.
In several papers, it was claimed that, the growth rate increases with increasing
phosphorus content [30-33].
3.3.4. Effect of chloride ion concentration
Tafel polarization curves of tin platings formed from different hydrochloric
acid concentrations (10-50 ml/l) are shown in Fig. 3.12. At a lower chloride ion
concentration (10 ml/l), the potential move towards cathodic direction. On increasing
the chloride ion concentration to 30 ml/l, the potential increases and moved towards
anodic direction. Further increasing the concentration to 50 ml/l, the potential
decreases and moved towards cathodic direction. This indicates that at very high
chloride ion concentration, the pitting corrosion occurred. So the potential decreases.
At lower HCl concentration, there is insufficient amount of chloride ions to break the
oxide layer on aluminium surface as a result of which, relatively fewer active sites are
available for nucleation of tin. The rate of tin dissolution is also minimum at lower
chloride ion concentration. The amount of tin deposit is higher at higher chloride ion
concentration. But at very high chloride ion concentration, the pitting corrosion
occurred. Hence compact and strong tin deposition was obtained at optimum chloride
ion concentration (30 ml/l). Similar results were also reported [34].
The corrosion parameters of the immersion plating as a function of various
HCl concentrations are presented in table 3.7. The corrosion current density (Icorr) and
corrosion rate (Rcorr) decrease on increasing the concentration of Cl- from 10-30 ml/l.
This indicates that on increasing the concentration, the corrosion resistance increases.
But on further increasing the concentration to 50 ml/l, the corrosion rate (Rcorr)
increases.
The Nyquist impedance plots of the uncoated samples and tin coated samples
obtained from various chloride ion concentrations are shown in Fig. 3.13 and the
impedance parameters are given in the table (3.8). At a lower concentration (10 ml/l),
the corrosion resistance (Rp ) is less (500
), but on raising the concentration to
30 ml/l, there is rapid increase in corrosion resistance (Rp) (
8000
). Further
increase of concentration to 50 ml/l, the polarization resistance (Rp) becomes
decreased (2500
) and yields a porous deposit which has less corrosion resistance.
3.4. Surface examinations
3.4.1. Surface Morphological studies: Scanning electron microscopy
SEM examinations were used to get a deeper insight into the surface
morphology and its dependence on the processing parameters. These studies revealed
the dependency of coating nucleation process on the processing temperature.
Figs. 3.14 – 3.16 show the scanning electron microscope images of tin
immersion platings obtained from 5g/l SnCl2, 30 ml/l HCl and 200 ml/l H3PO4 at
different temperatures.
The variation in the plating kinetics due to change in the bath temperature is
well reflected in the deposit morphology. At room temperature (30 C), (Fig. 3.14) the
deposit is thick and compact and the crystallites are in flake shape and distributed
uniformly over the aluminium surface. When the temperature is raised to 40 C,
(Fig. 3.15) the deposit appears to be more compact. In addition, secondary growth is
found to appear. A further increase in plating temperature to 50 C (Fig. 3.16) results
in the formation of thick but relatively coarse deposit. Increase in plating temperature
to 50 C further deteriorates the surface morphology of tin.
The SEM characterization of immersion platings disclosed the sensitivity of
immersion plating morphology to temperature. Temperature regulates both the
coating nucleation and growth behaviour of immersion tin platings deposited from
hydrochloric acid based bath. At 30°C, (Fig. 3.14) the nucleation sites for coating
build-up are less in number, leading to a heterogeneous coating formation mainly into
surface irregularities. At the temperature of 40°C, (Fig. 3.15) optimal conditions for
coating nucleation and growth phenomena were received. At 50°C (Fig. 3.16) the
coating nucleation is successful but growth is retarded, yielding a porous and thin
coating.
Accordingly, for immersion tin platings, the appropriate coating structure was
achieved at 40 °C, (Fig. 3.15) where the nucleation sites are regularly distributed and
accompanied by a multiple grain growth as compared with the platings formed at
lower temperatures. This suggests that, the coating build-up is controlled by
nucleation process up to the temperature of 40°C. At higher temperatures, the factor
regulating the coating build-up is of grain growth character. The grain size relatively
decreases and the deposit seems to be more compact at 40 C.
Immersion tin plating produced in hydrochloric acid based bath turned out to
be fairly homogeneous in terms of the grain size. The size of equiaxed tin grains near
the aluminium surface is 110–460 nm, which equals the size of dislocation cells
observed in aluminium substrate. This further confirms the close relation of formed
tin coating structure and the structure of substrate material. The size of tin grains in
coating is found to increase with increasing coating thickness. This resembles more
for the coating formation of amorphous than crystalline surfaces [35], where deposit
initiates as a formation of fine-grained crystals and the size of crystals increases with
film growth. It is therefore suggested here that, not alone the microstructure of
substrate but more efficiently, the plating bath chemistry and the mechanism of
deposition process influence the development of microstructure in chemical coatings.
There are no voids or inclusions of organic molecules during defocusing the image
[36]. This indicates the purity of tin coatings deposited from hydrochloric acid based
bath.
In the studied temperature range from 30 to 50°C, the deposition was uniform.
However, the coating growth process is found to be very sensitive to the plating
temperature. Figs. 3.14 – 3.16 also address the value of plating bath temperature in the
coating growth process. High temperature favours effective grain growth leading to
surface roughness and, thus, changing the overall coating morphology. An increase in
the coating thickness is also associated with higher deposition temperatures. These
results show that, the temperature sensitivity of the grain growth process is evident.
3.4.1.1. The growth of tin whiskers and Inter Metallic Compounds
Tin and aluminium are necessary materials for inter metallic compounds
(IMC) layer formation; they must originate from the tin coating and aluminium
substrate, respectively. Therefore, the formation of the IMC layer decreases the
volume of the tin layer. It has been reported that, by using the density of the material,
the specific volume can be calculated. The tin-rich area containing oxygen near the
surface is due to the formation of oxide during storage. There is no whisker growth,
if the surface is oxide-free because the oxide effectively removes all the vacant
sources and sinks on the surface [37]. Therefore, only metals that grow protective
oxide layers, such as aluminum and tin, are known to have whisker growth. In other
words, metals with non protective oxides, such as iron or copper, do not grow
whiskers.
It has been reported that, there are three necessary and sufficient conditions for
the spontaneous formation of tin whiskers. The first is atomic diffusion at room
temperature; the second is formation of a protective surface oxide, which is required
for developing a stress gradient; and the third is the driving force to develop a
compressive stress [37, 38]. For immersion tin platings, the first is available since
aluminium is found to diffuse into the coating layer. The second is also available
since oxide is observed to form on the surface of the coating. The third is due to the
formation of IMCs: as long as the formation of the IMC proceeds at room
temperature, the driving force is maintained. When the stress is sufficiently high, tin
whiskers form to release the stress. But the XRD data reveals that, no such
intermetallic compounds are formed and
from the morphology of tin coating it is
observed that, some tin whiskers are formed but not in much amount. So, it is
concluded that, the formed tin coating is thick and free from tin whiskers.
3.4.2. Elemental analysis: Energy Dispersive X-ray Spectroscopy
The characteristic EDX (Energy Dispersive X-ray Spectroscopy) spectrum of
the tin plating formed in 5g/l SnCl2 , 30ml/l HCl and 200ml/l H3 PO4 at 40ºC in 2.5 min
is shown in Fig.3.17 and the elemental composition of the tin plating obtained on
aluminium is given in table 3.9. The EDX spectrum shows peaks corresponding to the
element Sn. It indicates that qualitatively, the chemical composition of topcoat layer is
mainly composed of Sn, P and O. The element P and O may come from the
phosphoric acid, and there is no whiskers or intermetallic compounds. This confirms
that the formed immersion platings are composed of tin only.
3.4.3. Phase compositional analysis: X-Ray diffraction method
Figs. 3.18 – 3.20 show the XRD patterns of tin platings prepared from 5g/l
SnCl2, 30 ml/l HCl and 200 ml/l H3 PO4 at various temperatures in 2.5 min. The XRD
pattern indicates that, these immersion platings are mainly composed of tin only. The
average size of the crystallites in the coating ranges from 110-460 nm. The
crystallographic data discussed below are in perfect agreement with what reported by
JCPDS International Centre for Diffraction Data (2003).
The XRD pattern of tin deposition formed at 30ºC is shown in Fig. 3.18. When
the treatment temperature was maintained at 30ºC, tin phase appears at 44.906°
(d spacing=2.0168) [JCPDS card= 65-2631, tetragonal/Body-centered, a5.831 c3.181]
with preferred orientation of (2 1 1), 65.80° (d spacing=1.4181) [JCPDS
card=65-0297, tetragonal/Body-centered, a5.831 c3.181] with preferred orientation of
(2 1 1), 79.47° (d spacing=1.205) [JCPDS card=04-0673, tetragonal/Body-centered,
a5.831 c3.182] with preferred orientation of (3 1 2) and 99.577° (d=1.0086) JCPDS
card=65-0296, tetragonal/Body-centered, a5.833 b c3.182] with preferred orientation
of (4 2 2).
The XRD pattern of tin deposition formed at 40ºC is shown in Fig. 3.19. If we
increase the temperature to 40ºC, tin phase appears at 45.045° (d spacing=2.0109)
[JCPDS card= 86-2265, tetragonal/Body-centered, a5.831 c3.181] with preferred
orientation of (2 1 1), 65.410° (d spacing=1.4256) [JCPDS card=65-0297,
tetragonal/Body-centered, a 5.831 b c 3.181] with preferred orientation of (2 1 1),
78.533° (d spacing=1.2170) [JCPDS card= 04-0673, tetragonal/Body-centered,
a5.831 c3.182] with preferred orientation of (3 1 2), 99.371° (d =1.0108) [JCPDS
card= 65-0296, tetragonal/Body-centered, a5.833 c3.182] with preferred orientation of
(4 2 2) and 112.09° (d spacing=0.9286) [JCPDS card= 65-7657, tetragonal/Bodycentered, a 5.831 b c 3.181] with preferred orientation of (5 1 2).
The XRD pattern of tin deposition formed at 50ºC is shown in Fig. 3.20. On
further increasing the temperature to 50ºC, tin phase is observed at 38.71°
(d spacing=2.3242) [JCPDS card= 65-0298, cubic/Body-centered, a 3.287] with
preferred orientation of (1 1 0), 44.902° (d spacing=2.017) [JCPDS card= 65-7657,
Tetragonal/Body-centered, a5.831 c3.181] with preferred orientation of (2 1 1),
65.316° (d spacing=1.4274) [JCPDS card= 65-0297, tetragonal/Body-centered,
a5.831 c3.181] with preferred orientation of (2 1 1), 78.421° (d spacing=1.2185),
[JCPDS card= 04-0673, tetragonal/Body-centered, a5.831 c3.182] with preferred
orientation of (3 1 2), 97.427° (d =1.0251) JCPDS card= 65-2631, tetragonal/Bodycentered, a5.831 b c3.181] with preferred orientation of (4 2 2) and 112.17°
(d spacing=0.9282) [JCPDS card= 65-7657, tetragonal/Body-centered] with preferred
orientation of (5 1 2).
The tin phases appearing at 30ºC, with orientations (2 1 1), (2 1 1), (3 1 2) and
(4 2 2) are also appearing again in the case of 40ºC and 50ºC. In addition to this, a
new tin phase appears at 112.09° (d spacing=0.9286) with preferred orientation of
(5 1 2) at 40ºC and another new tin phase appears at 38.71° (d spacing=2.3242) with
preferred orientation of (1 1 0) at 50ºC. On increasing the temperature, new tin phases
are appearing more and more. This indicates that on compared to 30ºC, more tin
phases are observed at 50ºC. Hence the coating growth is maximum with the
formation tin at higher temperature (50ºC). From the Figs. 3.18 – 3.20, it is clear that
no separate peak of aluminium is observed and only sharp peaks of tin are observed
indicating that the aluminium is covered completely by tin during deposition.
From the results of X-ray diffraction (XRD), we confirmed that the device
improvement originates from better crystallization at low temperature made possible
by the use of microwave irradiation [39].
3.5. Mechanism of immersion platings
The following reactions are assumed to takes place during immersion plating
[40, 41]. When no dissolved oxygen is present, immersion plating proceeds as
coupled reactions of Al oxidation and Sn deposition. These reactions are described as
follows.
Al PO 4 + AlCl3 +6H+ + 6e
2Al + 3HCl + H3 PO4
(Reaction 3.2)
E0 = 1.70 V vs. SHE
Sn2+ + 2e-
Sn
(Reaction 3.3)
E0 = 0.14 V vs. SHE
After the entire Al surface is oxidized or covered with Sn, reaction (3.2) does
not proceed, leading to no change in the amount of Sn.
In the presence of dissolved oxygen, the reduction of oxygen is possible in
addition to reactions (3.2) and (3.3).
O2 + 4H+ + 4e
2H2O
(Reaction 3.4)
E0 = 1.23 V vs. SHE
The amount of the deposit increases as long as reaction (3.2) is available, but it
starts to decrease due to the lack of available sites for reaction (3.1). The surface is
covered with Sn at this stage. Since the standard reduction potential of reaction (3.4)
is much higher than that of reaction (3.3), reaction (3.3) proceeds in the reverse
direction i.e, Sn dissolution. The re-dissolution rate of Sn depends only on the activity
of dissolved oxygen. These processes cause the deposition of Sn and its re-dissolution
on the Al surface.
In chemical displacement or immersion plating, the spontaneous exchange of
electrons between the atoms of the solid base metal and the ions of the nobler metal in
solution takes place in a common potential. However, in the presence of complexing
agents, the electrode potentials of metals will differ remarkably from their standard
electrode potentials, and even the order of metals in the electrochemical series may be
changed.
Thus, the chemical coating of metal with aluminium can be conducted
without applying an external current. Chemical metallic coatings are deposited on
aluminium by a replacement reaction.
Al + Sn2+ → Al3+ + Sn
(Reaction 3.5)
First, an increase in the aluminium ion concentration and a decrease in the
metal ion concentration of plating solutions were observed as plating processes
proceeded. The change in the amount of these ions exactly follows the stoichiometry
of the presented replacement reaction.
The immersion metal deposition process can be characterized by coupled
redox reactions, one corresponding to the metal ion reduction and the other to the
aluminium oxidation, as shown in the following equations:
Oxidation
Al+3H2O → Al2O3+6H++6e−
(Reaction 3.6)
Reduction
Sn2++2e− → Sn
(Reaction 3.3)
It is worth noting that, the oxidation of aluminium to Al2O3 is indispensable to
the metal deposition. Consequently, as the result of formation of oxide layer on the
surface saturation for metal deposition is reached, the aluminium surface is no longer
in contact with the solution. Under this condition, the displacement reaction between
metal and aluminium is stopped because aluminium is completely oxidized and is not
able any more to supply electrons for the reduction of metal ions.
Figures and Tables
Fig 3.1 Influence of temperature on (a) deposited tin (b) thickness and (c) growth rate
of the tin immersion coatings on aluminium.
Fig 3.2 Influence of irradiation time on (a) deposited tin (b) thickness and (c) growth
rate of the tin immersion coatings on aluminium.
Fig 3.3 Influence of concentration of stannous chloride on (a) deposited tin (b)
thickness and (c) growth rate of the tin immersion coatings on aluminium.
Fig 3.4 Influence of concentration of HCl on (a) deposited tin (b) thickness and (c)
growth rate of the tin immersion coatings on aluminium.
Fig. 3.5
Equivalent circuit used for fitting the impedance data for immersion
coatings on aluminium.
Fig. 3.6 Comparative Tafel polarization curves of the immersion tin platings formed
at various temperatures (30-50°C).
Fig. 3.7 Comparative Nyquist impedance plots of the immersion tin platings formed at
various temperatures (30-50°C).
Table 3.1 Influence of temperature on calculated Tafel parameters of the immersion
tin platings.
Temperature (ºC)
Icorr (A)
Ecorr (V vs. SCE)
Rcorr (mpy)
Bare
7.3070×10-4
-1.2312
3.131×10+02
30
4.701×10-6
-0.6256
2.017×100
40
2.138×10-7
-0.5472
9.169×10-2
50
9.553×10-7
-0.5846
4.098×10-1
Table 3.2 Influence of temperature on impedance parameters of the immersion tin
platings.
Temperature (ºC)
Rs (Ω)
C (F)
Rp (Ω)
Bare
1.92
2.182 x10-5
185
30
3.58
1.097 x10-5
2026
40
5.597
8.286 x10-6
8677
50
4.968
1.169 x10-5
4615
Fig. 3.8 Comparative Tafel polarization curves of the immersion tin deposition
formed in various irradiation time (0.5-2.5 min).
Table 3.9 Comparative Nyquist impedance plots of the immersion tin deposition
formed in various irradiation times (0.5-2.5 min).
Table 3.3 Influence of irradiation time (0.5-2.5 min) on calculated Tafel parameters of
the immersion tin platings.
Time (min)
Icorr (A)
Ecorr (V vs. SCE)
Rcorr (mpy)
Bare
7.3070×10-4
-1.2312
3.131×10+02
0.5
5.885×10-5
-0.7408
2.524×101
1.0
3.738×10-5
-0.7082
1.604×101
1.5
6.717×10-6
-0.6410
2.881×100
2.0
2.251×10-6
-0.5490
9.654×10-1
2.5
2.138×10-7
-0.5472
9.169×10-2
Table 3.4 Influence of irradiation time (0.5-2.5 min) on impedance parameters of the
immersion tin platings.
Time (min)
Rs (Ω)
C (F)
Rp (Ω)
Bare
1.92
2.182 x10-5
185
5
2.42
8.344x10-6
483
1
0.9278
2.995x10-5
2500
1.5
5.36
8.435x10-6
3424
2
2.857
1.039 x10-5
5317
2.5
5.597
8.286 x10-6
8677
Fig. 3.10 Comparative Tafel polarization curves of the immersion tin coatings formed
from various stannous chloride concentrations (1-5 g/l).
Fig. 3.11 Comparative Nyquist impedance plots of the immersion tin coatings formed
from various concentration of stannous chloride (1-5 g/l).
Table 3.5 Influence of concentration of stannous chloride (1-5 g/l) on calculated Tafel
parameters of the immersion tin platings.
Conc. of Sn2+ (g/l) Icorr (A)
Ecorr (V vs. SCE)
Rcorr (mpy)
Bare
7.3070×10-4
-1.2312
3.131×10+02
1.0
4.841×10-6
-0.5600
2.077×100
3.0
2.2451×10-6
-0.5420
9.664×10-1
5.0
2.138×10-7
-0.5472
9.169×10-2
Table 3.6 Influence of concentration of stannous chloride on impedance parameters of
the immersion tin platings.
Conc. of Sn2+ (g/l)
Rs (Ω)
C (F)
Rp (Ω)
Bare
1.92
2.182 x10-5
185
1
3.04
1.052x10-5
2609
3
4.979
1.012x10-5
5870
5
5.597
8.286 x10-6
8677
Fig. 3.12 Comparative Tafel polarization curves of the immersion tin platings formed
from various concentrations of HCl (10-50 ml/l.
Fig. 3.13 Comparative Nyquist impedance plots of the immersion tin platings formed
from various concentrations of chloride ions (10-50 g/l).
Table 3.7 Influence of Cl- concentration on calculated Tafel parameters of the
immersion tin platings.
Conc. of HCl (ml/l)
Icorr (A)
Ecorr (V vs. SCE)
Rcorr (mpy)
Bare
7.3070×10-4
-1.2312
3.131×10+02
10
1.084×10-5
-0.6563
4.648×100
30
2.138×10-7
-0.5472
9.169×10-2
50
2.577×10-5
-0.6845
1.080×101
Table 3.8 Influence of Cl- concentration on impedance parameters of the immersion
tin platings.
Conc. (ml/l)
Rs (Ω)
C (F)
Rp (Ω)
Bare
1.92
2.182 x10-5
185
1
5.36
8.435 x10-6
3424
3
5.597
8.286 x10-6
8677
5
4.58
1.097 x10-5
2026
Fig. 3.14 SEM image of the immersion tin plating formed on aluminium at 30ºC.
Fig. 3.15 SEM image of the immersion tin plating formed on aluminium at 40ºC.
Fig. 3.16 SEM image of the immersion tin plating formed on aluminium at 50ºC.
Fig. 3.17 EDX spectra of the immersion tin plating obtained on aluminium at 40°C.
Table 3.9 Elemental composition of the tin plating obtained on aluminium at 40°C.
S.NO.
Element
kev
Atomic %
1
OK
0.525
3.37
2.
Al K
1.486
95.73
3.
PK
2.013
0.16
4.
Cl K
2.621
0.21
5.
Sn K
3.446
0.53
Fig. 3.18 X-Ray diffraction pattern of the immersion tin plating obtained on
aluminium at 30°C.
Fig. 3.19 X-Ray diffraction pattern of the immersion tin plating on aluminium at
40°C.
Fig. 3.20 X-Ray diffraction pattern of the immersion tin plating on aluminium at
50°C.
References
[1]
M.A. Gonzalez-Nunez, C.A. Nunez-Lopez, P. Skeldon, G.E. Thompson,
H. Karimzadeh, P. Lyon and T.E. Wilks, Corros. Sci. 37 (1995) 1763.
[2]
M.A. Gonzalez-Nunez, P. Skeldon, G.E. Thompson and H. Karimzadeh,
Corros. 55 (1999) 1136.
[3]
C.S. Lin, H.C. Lin, K.M. Lin and W.C. Lai, Corros. Sci. 48 (2006) 93.
[4]
R. David Baghurst, D. Michael and P. Mingos, J. Chem. Soc. Chem.
Commun. 12 (1988) 829.
[5]
Y.K. Srivastava, Rasayan J. Chem. 1 (2008) 884.
[6]
L. Rassaei, E. Vigil, R. W. French, M. F. Mahon, R. G. Compton and
F. Marken, Electrochim. Acta, 54 (2009) 6680.
[7]
H. Huang, S. Zhang, L. Q. X. Yu and Y. Chen, Surf.
Coat. Technol.
200 (2006) 4389.
[8]
N.S.L.S. Vasconcelos, J.S. Vasconcelos, V. Bouquet, S.M. Zanetti, E.R. Leite,
E. Longo, L.E.B. Soledade, F.M. Pontes, M. Guilloux-Viry, A. Perrin,
M.I. Bernardi and J.A. Varela, Thin Solid Films 436 (2003) 213.
[9]
Y.C. Chen, R. L.H. Liu, X. L. Chen, H. J. Shu and M. D. Ger, Appl. Surf. Sci.
257 (2011) 6734.
[10]
M. Xin, K.W. Li and H. Wang, Appl. Surf. Sci. 256 (2009) 1436.
[11]
R. Wang, C. Liu, J. Zeng, K.W. Li and H. Wang, J. Solid State Chem.
182 (2009) 677.
[12]
Y. Liu, T. Tan, B. Wang, R. Zhai, X. Song, E. Li, H. Wang and H. Yan,
J. Colloid Interface Sci. 320 (2008) 540.
[13]
Y. P. Guo, Y.B. Yao, C.Q. Ning, L.F. Chu and Y. J. Guo, Mater. Lett.
65 (6) (2011) 1007.
[14]
N.M. Bahadur, T. Furusawa, M. Sato, F.Kurayama, and N. Suzuki, Mater.
Res. Bull. 45 (10) (2010) 1383.
[15]
A.R. Phani and S. Santucci, J. Noncryst. Solids 352 (2006) 4093.
[16]
B. Sanjaya and S.A. Shivashankar, Thin Solid Films 518 (2010) 5905.
[17]
E.H. Saarivirta, Surf. Coat. Technol. 160 (2002) 288.
[18]
V. Annamalai and J.B. Hiskey, Soc. Min. Eng. (1978) 650.
[19]
V. Annamalai and L.E. Murr, Hydrometall. 3 (1978) 249.
[20]
S. Xiong, X.Guo, L. Li, S.Wu, P. K. Chu and Z. Xu, J. Fluor. Chem.
131(3) (2010) 417.
[21]
M. Komorowska, G.D. Stefanidis, T. Van Gerven and A.I. Stankiewicz,
Chem. Eng. J. 155 (3) (2009) 859.
[22]
B.D. Barker, Surf. Technol. 12 (1981) 77.
[23]
C.D. Iacovangelo, J. Electrochem. Soc. 138 (1991) 976.
[24]
Y. Okinaka, R. Sard, C. Wolowodiuk, W.H.
Craft and T.F. Retajczyk,
J. Electrochem. Soc. 121 (1974) 56.
[25]
L.A. Nagahara, T. Ohmori, K. Hashimoto and A.J. Fujishima, Vac. Sci.
Technol. A11 (1993) 763.
[26]
C.L.J. Bailey, J. Electrodep. Tech. Soc. 27 (1951) 233.
[27]
K.K. Karunakaran and B.J. Nayak. Electrochem. Soc. 30 (1981) 15.
[28]
C.D. Gu, J.S. Lian, J.G. He, Z.H. Jiang and Q. Jiang, Surf. Coat. Technol.
200 (2006) 5413.
[29]
T. Xue, W. C. Cooper, R. Pascual and S.J. Saimoto, J. Appl. Electrochem.
21 (1991) 238.
[30]
Y.C. Sohn, J. Yu, S.K. Kang, D.Y. Shih and T.Y. Lee, Proc. 54 th Electronic
Components Technology Conference (2004) 75.
[31]
Y.C. Sohn, Y. Yu, S.K. Kang, D.Y. Shih, and T.Y. Lee, IBM Research Report
RC23513, (Presented at the 55th Electronic Components and Technology
Conference) (2005).
[32]
K.W. Paik, Y.D. Jeon and M.G. Cho, Proc. of the 54th Electronic Components
and Technology Conference (2004) 675.
[33]
M.O. Alam, Y.C. Chan and K.N. Tu, J. Appl. Phys. 94(6) (2003) 4108.
[34]
M. Kanungo, V. Chakravarty, K.G. Mishra and S.C. Das, Hydrometall.
61 (2011) 11.
[35]
S. Nakahara and Y. Okinaka, Acta Metall. 31 (1983) 713.
[36]
R. Weil, Plat. Surf. Finish. 84 (1997) 28.
[37]
W.J. Choi, T.Y. Lee, K.N. Tu, N. Tamura, R.S. Celestre, A.A. MacDowell,
Y.Y. Bong and L. Nguyen, Acta Materialia, 51 (2003) 6253.
[38]
B.Z. Lee and D.N. Lee, Acta Materialia, 46 (1998) 3701,
[39]
K. Song, C. Y. Koo, T. Jun, D.Lee, Y. Jeong and J. Moon, J. Cryst. Growth
326 (1) (2011) 23.
[40]
T.K. Sham, S.J. Naftel and I. Coulthard, J. Appl. Phys. 79 (1996) 7134.
[41]
I. Coulthard and T.K. Sham, Solid State Commun. 105 (1998) 751.