Indian Journal of Chemical Technology
Vol. 17, May 2010, pp. 167-175
Influence of galvanic coupling on the formation of zinc phosphate coating
M Arthanareeswari1*, T S N Sankara Narayanan2, P Kamaraj3 & M Tamilselvi4
1,3
Department of Chemistry, Faculty of Engineering & Technology, SRM University, Chennai 603 203, India
2
National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India
4
Department of Chemistry, Arignar Anna Government Arts College,
Villupuram 605 602, India
Email: [email protected]
Received 17 August 2009; revised 31 March 2010
The influence of galvanic coupling of mild steel (MS) with titanium, copper, brass, nickel and stainless steel (SS) on the
phosphatability is elucidated. The galvanic couple accelerates metal dissolution, enables quicker consumption of free
phosphoric acid and facilitates an earlier attainment of point of incipient precipitation, resulting in higher amount of coating
formation. The surface morphology of the coatings exhibit more uniform coating for the mild steel substrates phosphated
under coupled conditions. XRD pattern of the zinc phosphate coating formed under coupled condition confirms the presence
of phosphophyllite rich coating. The potential-time measurements are also carried out. The study reveals that galvanic
coupling of mild steel with metals that are nobler than steel during phosphating proved to be beneficial in accelerating the
coating formation.
Keywords: Zinc phosphate, Corrosion resistance, Galvanic couple, Mild steel
Phosphating is the most widely used metal
pretreatment process for the surface treatment and
finishing of ferrous and non-ferrous metals. Due to
its economy, speed of operation and ability to afford
excellent corrosion resistance, wear resistance,
adhesion and lubricative properties, it plays a
significant role in the automobile, process and
appliance industries1–4. Majority of the phosphating
baths reported in literature require very high
operating temperatures ranging from 90 to 98°C. The
main drawback associated with high temperature
operation is the energy demand, which is a major
crisis in the present day scenario. Besides, the use
and maintenance of heating coils is difficult due to
scale formation, which leads to improper heating of
the bath solution and require frequent replacement.
Another problem is overheating of the bath solution,
which causes an early conversion of the primary
phosphate to tertiary phosphate before the metal has
been treated that results in increase in the free acidity
of the bath and consequently delays the precipitation
of the phosphate coating5. One possible way of
meeting the energy demand and eliminating the
difficulties encountered due to scaling of heating
coils and, over heating of the bath, is through the
use of low temperature phosphating baths. Though
known to be in use since the 1940s6, the low
temperature phosphating processes have become
more significant today due to the escalating energy
costs. However, low temperature phosphating
processes are very slow and need to be accelerated
by some means. Acceleration of the phosphating
process could be achieved by chemical, mechanical
and electrochemical methods. However, each of
them has some limitations and/or detrimental effects.
Chemical accelerators are the preferred choice
in many instances. The use of nitrites as the
accelerator is most common in low temperature
operated phosphating baths. However, a higher
concentration of nitrite is required to increase the
rate of deposition of phosphate coatings at low
temperatures. The environmental protection agency
(EPA) has classified nitrite as toxic in nature and
hence use of nitrite as accelerator could cause
disposal problems7.
The utility of the galvanic coupling for
accelerating low temperature zinc phosphating
processes was established recently8-10. The present
work aims at to study the utility of galvanic
coupling for accelerating the low temperature
zinc phosphating and to elucidate the effect of
cathode materials such as titanium, copper, brass,
nickel and stainless steel on the phosphatability
of mild steel.
INDIAN J. CHEM. TECHNOL., MAY 2010
168
Experimental Procedure
Mild steel specimens (hot rolled; composition
conforming to IS 1079 specifications) of dimensions
8.0 × 6.0 × 0.2 cm were used as the substrate
materials for the deposition of zinc phosphate
coating. Titanium, copper, brass, nickel and
stainless steel (AISI 304 grade) substrates were used
to create the galvanic couple with mild steel
substrate with varying anodic to cathodic area
ratio. The structural characteristic of the zinc
phosphate coating was evaluated by X-ray
diffraction measurement using Cu Kα radiation.
The surface morphology of phosphated steel
samples using galvanic coupling was assessed by
scanning electron microscope (SEM), Cambridge
Instruments (Model: Stereoscan 360).
The chemical composition of the zinc phosphating
bath and its operating conditions are given in Table 1.
Same operating conditions and phosphating bath were
used for phosphating the uncoupled mild steel for
comparison. The chemical compositions of the mild
steel and of the cathode materials used are given in
Table 2. Phosphating was done by immersion process.
The amount of iron dissolved during phosphating and
coating weight were determined in accordance with
the standard procedures11. The schematic diagram of
the experimental setup used for the phosphating
process is given in Fig. 1. The potential time
measurements during phosphating were carried out
using a multimeter (model 435 Systronics Digital
Multimeter) against the saturated calomel electrode
(SCE) using a luggin capillary. The oxygen reduction
Table 1—Chemical composition, control parameters and
operating conditions of the bath used for zinc phosphating
by galvanic coupling
Chemical composition
ZnO
H3PO4
NaNO2
5 g/L
11.3 mL/L
2 g/L
Control parameters
pH
2.7
Free acid value (FA)
3 pointage
Total acid value (TA)
25 pointage
FA:TA
18:33
Operating conditions
Temperature
27ºC
Time
30 Min
Element
Wt%
Element
Wt. %
Element
Wt. %
Element
Wt. %
Element
Wt. %
Element
Wt%
Fig. 1— Schematic diagram of the experimental setup used for
the phosphating process
Table 2—Chemical composition of (a) Mild steel (b) Stainless steel (c) Nickel (d) Brass (e) Copper and (f) Titanium
(a)
C
Si
Mn
P
S
Cr
Ni
Mo
Fe
0.16
0.17
0.68
0.027
0.026
0.01
0.01
0.02
Balance
(b)
C
Si
Mn
P
Ni
Cr
S
Fe
<0.08
<01
02
0.045
8 – 10.5
18 – 20
<0.030
Balance
(c)
Ni
99.99
(d)
Pb
Zn
Fe
Cu
0.05
34.75
0.03
65.10
(e)
Cu
99.99
(f)
N
C
H
P
Fe
O
Ti
0.03
0.10
0.01
0.027
0.20
0.18
Balance
ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE
current density was measured using a potentiostat /
galvanostat frequency response analyzer of ACM
instruments (model: grill AC).
Results and Discussion
Effect of cathode materials
The effect of galvanic coupling of mild steel
substrate with titanium, copper, brass, nickel and
stainless steel substrates on the amount of iron
dissolved during phosphating and coating weight is
given in Table 3. The corresponding values obtained
for uncoupled mild steel substrate are also included in
the same table for an effective comparison.
It is evident from the values given in Table 3 that
the extent of metal dissolution and of coating
formation are higher for mild steel substrates
phosphated under galvanically coupled condition than
the one coated without coupling. It is understandable
that galvanic coupling accelerates the initial metal
dissolution reaction and enables an earlier attainment
of the point of incipient precipitation (PIP) i.e., the
point at which saturation of metal dissolution
occurs and higher coating weight results12. Among the
169
different couples studied, namely mild steel-titanium,
mild steel-copper, mild steel-brass, mild steel- nickel
and mild steel-stainless steel, the mild steel - titanium
couple exerts a greater influence on metal dissolution
and coating weight. This is due to higher potential
difference between the anode and cathode materials of
this couple.
The anodic to cathodic area ratio is also a major
influencing factor in deciding the extent of metal
dissolution and of coating formation. Increase in
cathodic area exerts a strong influence on the mild
steel anode and increases the extent of metal
dissolution, which in turn influences the amount of
coating formation.
Effect of unaccelerated bath
Effect of galvanic coupling of mild steel with
stainless steel or titanium on the amount of iron
dissolution and phosphate coating formation from
unaccelerated bath (without sodium nitrite) is shown
in Table 4.
Compared to mild steel substrate phosphated under
uncoupled condition, the extent of metal dissolution
and coating weight are higher for substrates
Table 3—Effect of galvanic coupling of mild steel with different cathode materials of varying area ratios (1:1, 1:2, 1:3) on the amount of
iron dissolved during phosphating and phosphate coating formation
System studied
Iron dissolved during
phosphating* (g/m2)
Uncoupled mild steel
4.61
Mild steel coupled with stainless steel (area raio-MS:SS-1:1)
5.05
Mild steel coupled with stainless steel (area ratio-MS:SS-1:2)
5.66
Mild steel coupled with stainless steel (area ratio-MS:SS-1:3)
5.84
Mild steel coupled with nickel (area ratio-MS:Ni-1:1)
5.29
Mild steel coupled with nickel (area ratio-MS:Ni-1:2)
5.79
Mild steel coupled with nickel (area ratio-MS:Ni-1:3)
6.05
Mild steel coupled with brass (area ratio-MS:brass-1:1)
8.64
Mild steel coupled with brass (area ratio-MS:brass-1:2)
9.39
Mild steel coupled with brass (area ratio-MS:brass-1:3)
9.65
Mild steel coupled with copper (area ratio-MS:Cu-1:1)
8.70
Mild steel coupled with copper (area ratio-MS:Cu-1:2)
8.94
Mild steel coupled with copper (area ratio-MS:Cu-1:3)
9.30
Mild steel coupled with titanium (area ratio-MS:Ti -1:1)
9.50
Mild steel coupled with titanium (area ratio-MS:Ti-1:2)
10.00
Mild steel coupled with titanium (area ratio-MS:Ti-1:3)
10.80
*Average of five determinations (the standard deviation of the above data is within 0.16 g/m2)
Coating weight*
(g/m2)
8.04
8.75
9.21
9.98
9.72
10.05
10.70
11.50
12.86
13.85
12.83
14.15
16.73
17.50
18.80
20.00
Table 4—Effect of unaccelerated bath during phosphating using galvanic coupling of mild steel with stainless steel or titanium
Iron dissolved during
phosphating* (g/m2)
Uncoupled mild steel
0.42
Mild steel coupled with stainless steel (area ratio of MS to SS-1:3)
1.98
Mild steel coupled with titanium (area ratio of MS to Ti -1:3)
3.0
*Average of five determinations (the standard deviation of the above data is within 0.023 g/m2)
System studied
Coating weight*
(g/m2)
0.66
1.46
2.90
170
INDIAN J. CHEM. TECHNOL., MAY 2010
phosphated under galvanically coupled condition. It is
well established that phosphating reaction from
unaccelerated baths tends to be slow owing to the
polarization caused by hydrogen evolution at the
cathode13. The very slow rate of recombination of
hydrogen atoms to form hydrogen gas causes the
formation of a very low coating weight10. This effect
is evident for substrates phosphated both under
galvanically coupled and uncoupled conditions.
The presence of cathode materials in the
phosphating bath initially enhances the iron
dissolution, which enables quicker consumption of
free phosphoric acid and increases the pH at the mild
steel-phosphating solution interface. The increase in
pH causes the conversion of soluble primary
phosphate to insoluble tertiary phosphate with
subsequent deposition of the phosphate coating on
mild steel substrate1-4. Since the surface sites for
hydrogen evolution are now shifted from mild steel
substrate to cathodic substrates, it is presumed that
more surface sites are available on mild steel substrate
for coating formation which results in an increased
coating weight.
Initial potential (A)
The initial galvanic potential varies with the nature
of cathode material coupled with mild steel substrate.
Potential measured at the first minute during coating
formation in a phosphating bath having 30 min
processing time is indicative of the nature of the
metal surface undergoing corrosive attack by the
free phosphoric acid present in the bath14. Galvanic
coupling of cathode materials with the mild steel
substrate is found to shift the measured potential at
the first minute to a less negative value as compared
to the initial potential of uncoupled mild steel.
Potential - time measurements
During phosphating, the potential of the galvanic
couple is monitored continuously as a function of
time for the entire duration of coating formation. A
typical potential-time curve depicting the following
classification is shown in Fig. 2. The potential-time
curves obtained for mild steel-stainless steel and mild
steel-titanium [Fig. 2 (a and b)] could be analysed by
the following significant points.
Fig. 2— A typical potential-time curve depicting the classification
of different points of the curve to analyze the changes that occur
during phosphating using galvanic coupling.
A - Initial potential; B -Maximum potential; C - Final potential
and ti -Induction time
Fig. 2a— Variation of potential with time during phosphating of
uncoupled mild steel and mild steel-stainless steel couple (area
ratio of MS to SS 1:1, 1:2 and 1:3)
Fig. 2b- Variation of potential with time during phosphating of
uncoupled mild steel and mild steel – titanium couple (area ratio
of MS to Ti 1:1; 1:2 and 1:3).
ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE
Maximum potential (B)
The maximum potential represents the onset of
conversion of soluble primary phosphate to insoluble
tertiary phosphate (point of incipient precipitation),
following the rise in interfacial pH. At this point,
the potential of the galvanic couple is shifted towards
more cathodic direction. This is also observed in
conventional phosphating process. It is due to the
corrosive attack by the free phosphoric acid present
in the bath15. The extent of shift in potential
in conventional phosphating process is moderate
(50-100 mV)15. In zinc phosphating, utilizing galvanic
coupling the extent of shift in potential from initial to
maximum potential (point of incipient precipitation),
following the rise in interfacial pH is similar to the
conventional phosphating process. However, the
maximum potential obtained at this point is found to
shift towards anodic values from mild steel-stainless
steel couple to mild steel-titanium couple. Increase in
the potential difference between the galvanic couple
results in a shift in maximum potential towards anodic
direction.
Final potential (C)
The potential near the coating completion time
(30 min) can qualitatively suggest the extent to which
coating formation has occurred16. The potential
measured at this stage is more anodic for coupled
mild steel substrates than the uncoupled mild steel
substrate. Among the couples studied, the final
potential is more noble for mild steel – titanium
couple which implies better coating.
From the maximum potential there is a shift in the
anodic direction. The anodic shift in potential
represents the progressive build up of the phosphate
coating formation. Even though metal dissolution and
coating formation occur throughout the process, the
predominant reaction at this stage is the deposition of
zinc phosphate coating. The stabilization in potential
value noted at the end of phosphating is due to the
decrease in the rate of conversion of primary
phosphate to tertiary phosphate and hydrogen
evolution. The extent of shift in potential from
maximum to final potential observed at this stage is
due to the competition between hydrogen evolution
and deposition of zinc phosphate.
Increase in the potential difference between the
galvanically coupled mild steel and the cathode
materials results in an increased shift in final potential
towards anodic direction. Increase in the area ratio
171
between the mild steel and the cathode materials also
results in an increased shift in potential towards
anodic direction.
Induction time (ti)
The time taken for saturation of metal dissolution
i.e., the induction time (point at which ennobling
of potential occurs) is an important parameter in
indicating the rate and the extent of coating formation
in a phosphating bath10.
Induction time decreases from mild steel-stainless
steel couple to mild steel-titanium couple. The
decrease in induction period is one of the significant
effects of galvanic coupling. This is because the
pronounced metal dissolution due to galvanic
coupling enhances the consumption of free
phosphoric acid at the metal-solution interface
and enables an earlier attainment of the point of
incipient precipitation. The time taken for attainment
of point of incipient precipitation for mild
steel-titanium couple is the lowest out of all the
five galvanic couples utilized for coating formation.
This is due to the higher potential difference between
the anode and cathode of this couple which in turn
increases the metal dissolution and accelerates the
attainment of PIP which results in an increased
coating weight.
Mechanism of coating formation
Conventional phosphating baths consist of dilute
phosphoric acid based solutions of one or more alkali
metal/heavy metal ions1-4. These baths essentially
contain free phosphoric acid and primary phosphates
of the metal ions. When a mild steel substrate
is introduced into the phosphating solution, a
topochemical reaction takes place, during which the
metal dissolution is initiated at the micro-anodic sites
on the substrate by the free phosphoric acid present
in the bath. Hydrogen evolution occurs at the microcathodic sites.
Fe + 2H3PO4 → Fe(H2PO4)2 + H2 ↑
The formation of soluble primary phosphate leads
to the subsequent depletion of free phosphoric acid
concentration in the bath which results in the rise of
pH at the metal-solution interface. This change in pH
alters the hydrolytic equilibrium that exists between
the soluble primary phosphates and the insoluble
tertiary phosphates of the heavy metal ions present in
172
INDIAN J. CHEM. TECHNOL., MAY 2010
the phosphating bath resulting in a rapid conversion
and deposition of insoluble heavy metal tertiary
phosphate1-4. In a zinc phosphating bath, these
equilibria may be represented as follows:
Zn(H2PO4)2 ↔ ZnHPO4 + H3PO4
3ZnHPO4 ↔ Zn3(PO4)2 + H3PO4
In galvanically coupled condition both metal
dissolution and coating formation occur at the mild
steel substrate whereas hydrogen evolution occurs
at the cathode. While in uncoupled condition all
these reactions occur on the mild steel substrate
itself.
The decrease in the induction period is one of the
significant effects of galvanic coupling. This is
because of the pronounced metal dissolution resulting
from galvanic coupling which forces quicker
consumption of free phosphoric acid at the metalsolution interface and enables an earlier attainment of
the point of incipient precipitation. Potential-time
measurements suggest the occurrence of iron
dissolution as the predominant reaction during the
initial period, followed by the deposition of zinc
phosphate with a simultaneous metal dissolution
through the pores of the coating.
The continuous evolution of hydrogen at the
cathode enables deposition of zinc phosphate on
the entire surface of the anode. The continuous
evolution of hydrogen visually observed at the
cathode material throughout the entire duration
of deposition suggests the availability of metallic
sites at the mild steel substrate at any given time.
In conventional phosphating, the hydrogen
evolution also occurs at the mild steel substrate,
whereas in using galvanic coupling for zinc
phosphating, the surface sites of hydrogen
evolution are shifted from mild steel to stainless
steel or titanium substrates. It is presumed
that more surface sites are available for phosphate
coating formation which results in the increased
coating weight. Moreover, another advantage
resulting from galvanic coupling of mild steel
with more noble metals is the formation of
phosphate coatings richer in phosphophyllite
[Zn2Fe(PO4)2.4H2O] phase. With the advent
of cathodic electrophoretic painting, the need
for phosphate coatings that are richer in
phosphophyllite phase is greatly felt as they
offer better chemical stability than phosphate
coatings richer in hopeite phase, towards the
alkaline conditions created during electrophoretic
painting.
The formation of a phosphophyllite rich coating
is expected when the mild steel substrate is
galvanically coupled with metals more nobler than
it, as the metal-solution interface is most likely to
be populated with relatively more amount of ferrous
ions than the one phosphated under uncoupled
condition. However, the deleterious effect of
accumulation of ferrous ions at the metal solution
interface is not reflected on the corrosion
performance of phosphate coating. The presence
of sufficient concentration of nitrite ions in the bath
enables the oxidation of ferrous ions to ferric ions,
which are subsequently precipitated as ferric
phosphate sludge.
Surface morphology & XRD
SEM images [Fig. 3(a-f)] reveal that galvanic
coupling increases the coating formation and
improves the fineness of the coating. Coating on
mild steel specimens phosphated under uncoupled
condition (Fig. 3a) is found to be little less compact.
Introducing galvanic coupling [Figs 3(b-f)] gives
smooth and compact deposits with reduced porosity.
This is confirmed by the electro chemical method
of porosity testing. The formation of needle like
crystals confirmed the presence of phosphophyllite
phase15. X-ray diffraction pattern (Fig. 4) of
zinc phosphate coating formed under coupled
condition has shown the presence of both
hopeite and phosphophyllite phases. It is proved
from the figure that the coating is richer in
phosphophyllite phase.
Porosity of the phosphate coating
The electrochemical method, which measures
the oxygen reduction current density, clearly
indicates the amount of porosity involved. This
method involves the measurement of the oxygen
reduction current density when immersed in airsaturated sodium hydroxide solution (pH 12)17-19.
The current density values measured at -550 mV
versus SCE (Table 5) reveal that the panels
coated using galvanic coupling have a low porosity
value as compared to the uncoupled specimen.
The mild steel panel coated using titanium as the
ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE
173
Fig. 3— Surface morphology of the zinc phosphate coated mild steel specimens: (a) mild steel(MS) under uncoupled condition
(b) MS coupled with SS(1:3) (c) MS coupled with Ni (1:3) (d) MS coupled with brass(1:3) (e) MS coupled with Cu (1:3)
(f) MS coupled with Ti (1:3)
coupling material (area ratio 1:3) has the lowest
porosity value when compared to the other mild
steel substrates coated using different cathode
materials. Thus, it can be concluded that the
galvanic coupling of mild steel substrates with the
cathode materials during phosphating results in the
formation of uniform, fine grained coatings of
reduced porosity.
INDIAN J. CHEM. TECHNOL., MAY 2010
174
Table 5-Current densities of phosphated mild steel substrates (under galvanically coupled and uncoupled conditions)
System studied
Current density at – 550 mV
versus SCE (µA/cm2)
14.01
11.90
11.12
10.25
10.11
8.97
8.05
9.23
8.12
7.03
8.00
7.31
5.99
6.05
5.10
4.12
Uncoupled mild steel
Mild steel coupled with stainless steel (area ratio of MS to SS - 1:1)
Mild steel coupled with stainless steel (area ratio of MS to SS - 1:2)
Mild steel coupled with stainless steel (area ratio of MS to SS - 1:3)
Mild steel coupled with nickel (area ratio of MS to Ni - 1:1)
Mild steel coupled with nickel (area ratio of MS to Ni - 1:2)
Mild steel coupled with nickel (area ratio of MS to Ni - 1:3)
Mild steel coupled with brass (area ratio of MS to brass - 1:1)
Mild steel coupled with brass (area ratio of MS to brass - 1:2)
Mild steel coupled with brass (area ratio of MS to brass - 1:3)
Mild steel coupled with copper (area ratio of MS to Cu - 1:1)
Mild steel coupled with copper (area ratio of MS to Cu - 1:2)
Mild steel coupled with copper (area ratio of MS to Cu - 1:3)
Mild steel coupled with titanium (area ratio of MS to Ti - 1:1)
Mild steel coupled with titanium (area ratio of MS to Ti - 1:2)
Mild steel coupled with titanium (area ratio of MS to Ti - 1:3)
Fig. 4— Xray diffraction pattern of zinc phosphate coating
developed under coupled condition (mild steel coupled with
titanium, area ratio of mild steel to titanium is 1:3)
Conclusion
The extents of metal dissolution and of coating
formation are higher for mild steel substrates
phosphated under galvanically coupled condition
than for the one coated without coupling. The coating
weight is a function of galvanic potential exerted
by the couple. The increase in the area ratio of
anode to cathode increases the coating weight
formation. Among the different couples studied,
mild steel – titanium couple of area ratio 1:3 exerts
a greater influence on metal dissolution and
coating weight. The experiments performed using
phosphating bath without sodium nitrite (accelerator)
showed that galvanic coupling not only promotes
the iron dissolution but also favours the phosphate
coating formation by shifting the hydrogen evolution
reaction to cathode. Effective coating formation
by galvanic coupling technique is influenced by
the nature of the cathode material, anode to cathode
area ratio and processing time. Potential time
measurements strongly support the mechanisms
proposed to explain the role of cathode materials
and their area ratios with respect to mild steel
anode. These results are in excellent agreement
with the conclusions drawn from coating weight
measurements. Thus, the galvanic coupling of
mild steel with metals that are nobler than steel
during low temperature phosphating proved to be
beneficial in accelerating the rate of coating formation
and producing uniform, less porous and higher
weight coatings. Hence, this methodology proved to
be cost effective in accelerating low temperature
phosphating.
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