influence of galvanic coupling on the formation of zinc phosphate … · 2018-12-11 · influence...

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 Narayanan

2, P Kamaraj

3 & M Tamilselvi

4

1,3Department of Chemistry, Faculty of Engineering & Technology, SRM University, Chennai 603 203, India 2National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India

4Department 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 5 g/L

H3PO4 11.3 mL/L

NaNO2 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

Table 2—Chemical composition of (a) Mild steel (b) Stainless steel (c) Nickel (d) Brass (e) Copper and (f) Titanium

(a)

Element C Si Mn P S Cr Ni Mo Fe

Wt% 0.16 0.17 0.68 0.027 0.026 0.01 0.01 0.02 Balance

(b)

Element C Si Mn P Ni Cr S Fe

Wt. % <0.08 <01 02 0.045 8 – 10.5 18 – 20 <0.030 Balance

(c)

Element Ni

Wt. % 99.99

(d)

Element Pb Zn Fe Cu

Wt. % 0.05 34.75 0.03 65.10

(e)

Element Cu

Wt. % 99.99

(f)

Element N C H P Fe O Ti

Wt% 0.03 0.10 0.01 0.027 0.20 0.18 Balance

Fig. 1— Schematic diagram of the experimental setup used for

the phosphating process

ARTHANAREESWARI et al.: INFLUENCE OF GALVANIC COUPLING ON FORMATION OF ZINC PHOSPHATE

169

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

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)

Coating weight*

(g/m2)

Uncoupled mild steel 4.61 8.04

Mild steel coupled with stainless steel (area raio-MS:SS-1:1) 5.05 8.75

Mild steel coupled with stainless steel (area ratio-MS:SS-1:2) 5.66 9.21

Mild steel coupled with stainless steel (area ratio-MS:SS-1:3) 5.84 9.98

Mild steel coupled with nickel (area ratio-MS:Ni-1:1) 5.29 9.72



Mild steel coupled with brass (area ratio-MS:brass-1:1) 8.64 11.50



Mild steel coupled with copper (area ratio-MS:Cu-1:1) 8.70 12.83



Mild steel coupled with titanium (area ratio-MS:Ti -1:1) 9.50 17.50

Mild steel coupled with titanium (area ratio-MS:Ti-1:2) 10.00 18.80

Mild steel coupled with titanium (area ratio-MS:Ti-1:3) 10.80 20.00

*Average of five determinations (the standard deviation of the above data is within 0.16 g/m2)

Table 4—Effect of unaccelerated bath during phosphating using galvanic coupling of mild steel with stainless steel or titanium

System studied Iron dissolved during

phosphating* (g/m2)

Coating weight*

(g/m2)

Uncoupled mild steel 0.42 0.66

Mild steel coupled with stainless steel (area ratio of MS to SS-1:3) 1.98 1.46

Mild steel coupled with titanium (area ratio of MS to Ti -1:3) 3.0 2.90

*Average of five determinations (the standard deviation of the above data is within 0.023 g/m2)


170

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.

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.

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.

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).


171

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

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 micro-

cathodic 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

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 metal-

solution 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 air-

saturated 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


173

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.

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)


174

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.

References 1 Freeman D B, Phosphating and Metal Pretreatment – A

Guide to Modern Processes and Practice (Industrial Press

Inc., New York), 1986.

2 Rausch W, The Phosphating of Metals (Finishing

Publications Ltd., London), 1990.

3 Guy Lorin, Phosphating of Metals (Finishing Publications

Ltd., London), 1974.

4 Rajagopal & Vasu K I, Conversion Coatings: A Reference

for Phosphating, Chromating and Anodizing (Tata McGraw-

Hill Publishing Company Ltd., New Delhi), 2000.

5 Sankara Narayanan T S N, Met Finish, 94(9) (1996) 40.

6 Streicher M A, Met Finish, 46(8) (1948) 61.

7 U.S. Environmental protection agency, EPA Enforcement

Alert, 3(3) (2000) 1.

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S N, Kamaraj P & Rajeswari S, J Curr Sci, 2(2) (2002) 153.

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)

Uncoupled mild steel 14.01

Mild steel coupled with stainless steel (area ratio of MS to SS - 1:1) 11.90



Mild steel coupled with nickel (area ratio of MS to Ni - 1:1) 10.11



Mild steel coupled with brass (area ratio of MS to brass - 1:1) 9.23



Mild steel coupled with copper (area ratio of MS to Cu - 1:1) 8.00



Mild steel coupled with titanium (area ratio of MS to Ti - 1:1) 6.05



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)


175

10 Arthanareeswari M, Ravichandran K, Sankara Narayanan T

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influence of galvanic coupling on the formation of zinc phosphate … · 2018-12-11 · influence...

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