Electrochemical impedance spectroscopy studyon multi layered coated steel sheets

Corrosion Science 49 (2007) 794–817

www.elsevier.com/locate/corsci

Electrochemical impedance spectroscopy studyon multi-layered coated steel sheets

Tapan K. Rout *

Research and Development Department, Tata Steel, Jamshedpur 831 007, India

Received 18 October 2005; accepted 7 June 2006Available online 10 August 2006

Abstract

Electroplating technique is used to coat nickel over the coated substrates like galvannealed andgalvanized, and uncoated substrate of cold rolled closed annealed steel sheets as well. These coatedsubstrates are exposed to 3.5% aqueous NaCl solution in a flat cell to carry out the corrosion tests.The effect of nickel coating on galvannealed, galvanized and cold rolled closed annealing steel sheetsis studied for their anti-corrosive performance by electrochemical impedance spectroscopy. Electro-chemical impedance data were analysed by using Bode and Nyquist plot by considering themetal-coating-electrolyte interface as an electrical equivalent circuit model. The circuit elementsi.e. polarisation resistance and coating capacitance are determined to evaluate the corrosion behav-iour of these multilayered coatings. The changes in the impedance characteristics of the systems arefound to occur as a function of exposure time in all the three cases. These results have showed dif-ferences in the protective characteristics of the three systems, where the nickel coated galvannealedexhibited superior corrosion resistance compared to nickel coated cold rolled closed annealed andgalvanised.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Nickel; B. EIS; B. SEM; C. Passive film; C. Potential parameters

0010-938X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2006.06.008

* Tel.: +91 0657 2147445; fax: +91 0657 2271510.E-mail address: [email protected]

mailto:[email protected]

T.K. Rout / Corrosion Science 49 (2007) 794–817 795

1. Introduction

The coated steels with added advantages of light weight, high strength and excellentcorrosion resistance with paint have been emerging as a prerequisite raw material forthe automotive industries. However, in comparison to mild steel sheets, the high strengthsteel sheets are difficult to be formed due to limited elongation properties [1]. Interstitialfree high strength galvannealed (IFHS-GA) coated steel sheets are widely used in autobody application because of its superior formability, weldability and corrosion resistancewith paint [2,3]. IFHS-GA steel sheets are produced by annealing hot dipped galvanizedIFHS steel sheets for about 20–30 s at temperatures ranging from 450 to 550 �C [4]. Dur-ing annealing the substrate gets softened by releasing locking stresses and the pure zinccoating on the surface is converted to iron-zinc intermetallic phases due to diffusion of ironto zinc and vice versa. These phases then comprise the galvannealed coating on the steelsubstrate. The intermetallic phases present are f (6% Fe), d (7–12% Fe), C (16–28% Fe)and C1 (16.5–22% Fe) [5]. Improvement in corrosion resistance of GA in 3.5% NaCl solu-tion can be attained by maximizing the presence of delta phase. The potential of deltaphase is �950 mV which is equivalent to the protective potential of zinc to iron [6]. Inorder to get longer protection against corrosion, still there is tremendous scope for furtherdevelopment of coatings for IFHS-GA steels. Current developments in this field includemodification of zinc bath composition, substrate chemistry, pre-treatment and post-treat-ment processes. Many steel industries are focusing on post treatment processes because ofadditional surface quality with less investment in their continuous production line. As faras post treatment is concerned, many modern steel producers in Europe and Japan providesurface treatment such as rust preventive oils (RPO), thin organic coatings (TOC) andphosphate conversion coatings etc. for the improvement of formability and corrosionresistance. Surface treatment reduces coefficient of friction between die and sheet whichenhances the formability as well as the corrosion resistance by suppressing surface oxida-tion in high humidity atmosphere [7,8]. Recently nickel based lubricants are being used forimproving formability as well as corrosion resistance of GA, as nickel improves corrosionresistance by suppressing the attacks of chloride and chlorate ions from the corrosive envi-ronment [9,10].

In this paper, a thin layer nickel coating has been developed over GA by electroplatingtechnique and the corrosion performance of this multi-layered coating (Steel–GA–Ni) areevaluated by Electrochemical Impedance Spectroscopy (EIS). This is also compared withtwo different multi-layered coatings such as nickel coated GI (Steel–GI–Ni) and nickelcoated cold rolled closed annealed (CRCA-Ni). The schematic diagram of the multilay-ered coating systems are shown in Fig. 1.

2. Experimentals

2.1. Substrate details

CRCA, GA and GI substrates are used for this investigation. Where CRCA stands forcold rolled closed annealing steel (bare steel), GA stands for galvannealed coated steel andGI stands for galvanized coated steel. The chemical composition of the CRCA steel sheetis C 0.03%, Mn 0.4%, Si 0.04%, P 0.02%, S 0.02%, Al 0.06%, N 70 ppm. Similarly, GA

Fig. 1. Schematic diagram of multilayered coating systems.

796 T.K. Rout / Corrosion Science 49 (2007) 794–817

coating composition is 10% Fe, 90% Zn and GI coating composition is 100% Zn, bothover CRCA steel sheet.

2.2. Substrate surface preparation

These substrates were cut into dimensions of 150 mm · 100 mm. The surface of the sub-strates was cleaned by using proprietary degreasing chemicals followed by distilled waterrinsing and drying.

2.3. Bath composition for electro-deposition

The electro-deposition bath solution was prepared by mixing boric acid, nickel chloride,nickel sulfate and citric acid into 1 l of water. The detail bath chemistry is given in theTable 1 below.

2.4. Calculation of cathode efficiency for metal deposition

In a plating operation, it is expected that the entire current passed to be utilized for elec-troplating [11]. If a fraction of the current is used for any other reaction (including heatingof bath) it is a waste. In the present case the cathode efficiency was calculated by usingfollowing formula, Cathode efficiency is defined as the percentage of the total current use-fully employed for the cathodic deposition of the metal.

Cathode current efficiency ¼ W 1

W 2

� 100

Table 1Bath chemistry

Nickel sulphate (NiSO4 Æ 6H2O) 75 g/lNickel chloride (NiCl2 Æ 6H2O) 75 g/lCitric acid 30 g/lBoric acid 30 g/l


whereW1 weight of metal actually deposited.W2 weight of metal theoretically calculated from the quantity of electricity passed.

Weights of metal actually deposited (W1) were calculated by using following formula;

W 1 ¼ S1 � S2

whereS1 weight of the sample before deposition.S2 weight of the sample after deposition.

Weight of metal was calculated theoretically by using following formula,

W 2 ¼I � t � A96500� z

whereI current passed in ampere.t time for plating in seconds.A atomic weight of coated metals.z valence of the metal to be coated.

2.5. Cross-sectional analysis of coated substrate by SEM and EDS

The interfaces of the multi-layer coated steel are observed by Scanning ElectronMicroscopy (SEM) and concentration of various elements at different depth of the coatingare calculated through Energy Dispersive X-ray Spectroscopy (EDS). This instrument issupplied by JEOL, Japan.

2.6. Electrochemical Impedance Spectroscopy (EIS)

The Electrochemical Impedance Spectroscopy (EIS) measurement is carried out in a3.5% NaCl solution by using EIS-300, Gamry Instrument, USA. The samples are maskedwith portholes (3 cm2 area) and are exposed to the solution. The current is recorded as thepotential increased at a scanning rate of 5 mV/s, where applied potential is very small inthe range of 10 mV over a nominal frequency range of 10 kHz–10 mHz. The currentresponses thus obtained by input voltages over a wide frequency range are analysed byNyquist and Bode plots [12–14]. In the plots, the three different parts indicated with highfrequency, medium frequency and low frequency reflect the interfacial behaviour of thecoated sheets. The interface behaviour in solution environment is modeled with electricalequivalent circuit. The values of the circuit element such as coating capacitance (Cc),polarization resistance (Rp) and impedance (Z) are used to characterize the performanceof the coating in the corrosive environment. At high frequency, the changes of Cc withexposure time can be used to determine water/ions uptake of the coating. The decreaseof Rp enhances the coating degradation [15–19] while the interfacial impedance (Z) whichoccurs only at low frequency provides the protective properties of the coatings.

Power supply

cathode

Voltmeter V

A

anode Solution

Water Ni+2, Cl- , SO4

-2

Amperometer

+ ve

Fig. 2. Schematics of nickel electrolytic cell for plating ‘‘Ni’’ from a solution of its metal salt.


3. Results and discussion

3.1. Coating deposition

Nickel is deposited cathodically from the solution onto the different substrates [20–22].The nickel coating on the different substrates were optimised by measuring coating thick-ness, cathode efficiency and surface quality of the deposits. The applied voltage, pH of thesolution, exposure time for deposition and temperature of the solution control the abovementioned coating properties. Nickel salts in the nickel plating bath as shown in Fig. 2 existas ions in the aqueous solution. The Ni+2 in the solution move towards cathode and getsdeposited (Ni+2 + 2e! Ni: �0.257 V) as metal. The anode (nickel plate) reacts with elec-trons from solution ions and pass into solution as Ni+2. These processes of dissolution atanode and deposition at cathode would continue as long as there is a potential differencebetween anode and cathode. The extent of deposition or dissolution is determined by cath-ode efficiency which is a ratio of weight of metal actually deposited to the weight of metalcalculated from the quantity of electricity passed. Anode replenishes the metal deposited atcathode so that the bath composition remains stable. In the plating bath, nickel sulphateand nickel chloride provides most Ni+2 ions and chloride from nickel chloride helps inenhancing solution conductivity and throwing power. Boric acid in the bath stabilizesthe pH of the cathode film due to its buffer action and citric acid in the bath controls thedeposition rate by acting as a leveling agent. The superior surface quality of the depositshas been discussed with varying pH, temperature, voltage and deposition time.

3.2. Effect of variation of voltage, time, pH and temperature for obtaining nickel coating

on CRCA, GA and GI steel sheets

A silicon-controlled rectifier is used for carrying out the experiments. The effect ofabove mentioned parameters on coating thickness and cathode efficiency are optimizedwhich decide the nickel coating with respect to surface quality of deposits.

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Fig. 3a. Effect of variation of voltage at constant temp: 55 �C, time: 90 s, pH: 2.8, conductivity: 57.1 for CRCAsheets.


The variation of voltage at constant solution pH: 2.8–2.9, temperature: 55 �C and depo-sition time: 90 s, is studied to optimise effective voltage for obtaining suitable coatingthickness at maximum cathode efficiency on CRCA steel sheets. It is observed inFig. 3a that the coating thickness is 1.6 lm obtained at optimum cathode efficiency of61% with the required out put voltage of 6.9 V. It is also noticed that the coating quality,coating thickness and cathode efficiency get deteriorated below this voltage level. The cur-rent is wasted below this voltage level, it may be due to consumption of current by solutionfor the heating of plating bath.

When the bath is heated by raising the temperature, the coating thickness and cathodeefficiency are found to be simultaneously increased with temperature which is shown inFig. 3b. This may be attributed to increase of hydrogen overvoltage at the cathode surfaceand also adequate supply of ions to the cathode. Therefore, the quality of deposits wasfound to be good [23].

Coating deposition time is varied at a constant temperature: 55 �C, optimized voltage:6.9 V and pH: 2.8–2.9 for obtaining best possible time for the coating deposition with

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Fig. 3b. Effect of variation of temperature on coating deposition at constant time: 360 s, voltage: 6.9 V, pH: 2.9and conductivity: 57.1 for CRCA sheets.

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Fig. 3c. Effect of variation of time on coating deposition at constant voltage: 6.9 V, pH: 2.9, temp: 55 �C andconductivity: 57.1 mS/cm for CRCA sheets.


maximum cathode efficiency. It is found that the cathode efficiency is constant at differentdeposition time and the coating thicknesses increased with increase in time. The results areshown in Fig. 3c. The coating thickness is found to be 7 lm at 360 s with superior surfacequality.

The control of pH of the plating bath is necessary in order to operate the bath withoptimum cathode efficiency and to maintain the desired physical properties of the deposits.If nickel plating is carried out outside the recommended pH range, the deposit maybecome pitted or cracked. Low pH may lead to hydrogen evolution and a consequentdecrease in metal deposition efficiency. It is necessary to increase and decrease the pH val-ues by using nickel carbonate or ammonia sodium hydroxide and sulphuric acid, respec-tively. With the constant voltage, time and temperature, the pHs are varied to exploremaximum cathode efficiency. The results are shown in Fig. 3d. It is found that the cathodeefficiency is above 87%, at pH ranges of 3.2–3.5 which is efficient for plating [24], andunder these conditions the coating quality is also found to be superior. The optical

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Fig. 3d. Effect of variation of pH on coating depostion at constant temp: 70 �C, voltage: 6.9 V, time: 360 s andconductivity: 57.1 mS/cm for CRCA sheets.

Fig. 4. Optical photograph of good quality nickel coating surface.


photograph of the coating top surface is shown in Fig. 4. Above and below this pH level,coating quality is found to be uneven with lots of scratch marks. This is shown in Fig. 5.

In case of GA, the nickel coating thickness and plating efficiency are evaluated withrespect to voltages. The results are shown in Fig. 6a. It is observed from Fig. 6a thatthe coating thickness is linearly varying with the voltages. The cathode efficiency is foundto be 27% at 7.5 V, which is considered to be very low. The coating thickness is found to be1 lm. The coating surface quality is visually observed and found to be uneven in this case.This may be due to the consumption of total current for the reduction reaction at cathodesurface. During reduction reaction, the hydrogen gas thus generate may trap in the depos-its and causes damage to quality of deposits. The coating deposition time is enhanced toincrease coating thickness with aim to superior surface quality. The coating deposition

Fig. 5. Optical photograph of bad quality nickel coating surface.

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Fig. 6a. Effect of variation of voltage on coating thickness and cathode efficiency at constant pH: 2.88, time: 60 s,temp: RT for GA sheets.


time is varied at constant voltage of 7.5 V, at pH of 2.88 and temperature of 30 �C. Theresult is presented in Fig. 6b. It is observed from Fig. 6b that the coating thicknessincreases with increased time and the cathode efficiency also increased initially up to120 s to about 50% and plateaus are found thereafter. This indicates that 50% of the cur-rent is effectively utilised for getting permissible coating thickness of 3.4–3.6 lm.

In order to enhance the cathode efficiency further, it is intended to control the reductionreaction at cathode surface by varying pHs of the solution. The effect of pH on coatingthickness and cathode efficiency is shown in Fig. 6c. It is observed that cathode efficiencysignificantly increases with increased pHs. Cathode efficiency of more than 70% at pHranges between 3.8 and 5.0 is obtained with coating thickness of 5.1–5.4 lm.

The effect of temperature is studied at constant pH: 3.8, voltage: 7.5 V and time: 240 s inorder to optimise temperature for operating bath solution. It is found from Fig. 6d thatthe cathode efficiency decreases with increased temperature (from room temperature to60 �C) where as coating thickness is found to be varying between 4 lm and 6 lm. There-

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Fig. 6b. Effect of variation of time on coating depostion thickness and cathode efficient at constant voltage:7.5 V, pH: 2.88, temp: RT for GA sheets.

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Fig. 6d. Effect of variation of temperature on coating deposition thickness and cathode efficiency at constantvoltage: 7.5, pH: 2.88, time: 240 s for GA sheets.

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Fig. 6c. Effect of variation of pH on coating depostion thickness and cathode efficiency at constant voltage: 7.5,temp: RT, time: 240 s for GA sheets.

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Fig. 7a. Effect of variation of voltage on coating deposition thickness and cathode efficiency at constant pH: 2.86,temp: RT, time: 60 s for GI sheets.



fore, room temperature is found suitable for operating bath solution for efficient platingon GA substrate.

In case of GI, the variations of different parameters are studied similar to nickel coatedGA steel sheet. The results of variation of voltage on cathode efficiency and coatingthickness is shown in Fig. 7a. It is observed that cathode efficiency is almost 90% whereascoating thickness is about 1 lm at the operating out put voltage of 7.0 V. The coatingthickness is intended to increase further by increasing deposition time. It is found fromFig. 7b that coating thickness is linearly varying with deposition time. A maximum coatingthickness of 4 lm is obtained at about 240 s of deposition time, whereas cathode efficiencyis found to be 30%. The higher coating thickness could be due to the availability of ions atthe cathode surface for participating in coating deposition. On the other hand, the reduc-tion of cathode efficiency with time can be attributed to consumption of current by hydro-gen evolution at the cathode surface which is evident from rough coating surface. In orderto get higher coating thickness with higher cathode efficiency and good surface quality, the

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Fig. 7b. Effect of variation of voltage on coating deposition thickness and cathode efficiency at constant voltage:10, temp: RT, pH: 2.86, for GI sheets.

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Fig. 7c. Effect of variation of temperature on coating deposition and cathode efficiency at constant pH: 2.88,voltage: 10, time: 240 s for GI sheets.

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Fig. 7d. Effect of variation of pH on coating deposition thickness and cathode efficiency at constant voltage: 10,temp: 45 �C, time: 240 s.


temperature and pH are varied separately. The effect of temperature on coating thicknessand cathode efficiency is presented in Fig. 7c. It is observed that at pH: 2.88, voltage:7.0and time: 240 s, the cathode efficiency decreases from 32% to 2% with increased tempera-ture from room temperature to 60 �C, and the coating thickness is found in between 4 lmand 6 lm. Such a low efficient plating bath is not suitable for coating industries. Therefore,pHs are varied to obtain desirable cathode efficiency, coating thickness and surface qual-ity. It was found from Fig. 7d that the cathode efficiency is more than 70% at the operatingpH of 4–5 with the desired coating thickness of 5–4 lm at room temperature. The coatingsurface topography is found to be same as Fig. 4.

3.3. Cross-sectional analysis of the Ni-coated CRCA, GI and GA substrates

The cross-sectional microstructure of the coating is studied by Scanning ElectronMicroscopy (SEM) and the elemental analysis is done by Energy Dispersive X-raySpectroscopy (EDS) in order to confirm the coating deposition. The presence of nickel

Fig. 8. Cross-sectional analysis of nickel coated CRCA.

Fig. 9. Cross-sectional analysis of nickel coated GI.

Fig. 10. Cross-sectional analysis of nickel coated GA.


at different points in CRCA, GI and GA confirms the nickel deposition. A thick coating ofNi on CRCA as evident from Fig. 8 and point 1, 2 and 3 in Fig. 9 reveals that a sharpnickel coating has been obtained over GI coating. However in the case of GA, the samecan be seen in point 1, 2 and 3 of Fig. 10, indicating the formation of nickel coating ofuniform thickness over GA sheets.

3.4. Corrosion behaviour of nickel coated CRCA, GI and GA

The impedance spectra for Ni-coated GA, GI and CRCA are analysed using the elec-trical equivalent circuit model is shown in Fig. 11. In this equivalent circuit model, Rs rep-resents solution resistance between Ni-coating and the reference electrode, Rcp is thecoating pore resistance representing the resistance of areas in the coating with more rapidsolution uptake, Rct = Rp is the charge transfer resistance representing the corrosion resis-tance of the metal, Cc is the coating capacitance represents the area where the coatingremains intact during immersion, Cdl is the double layer capacitance represents metal-coating interface and Zw is called Warburg impedance and represents the diffusion processof corrosive ions. When coated steel is immersed into the solution, the high mobile

Fig. 11. Equivalent circuits for EIS data generation.


chloride ion specifically diffuses through the coating pores because it is less hydrated [25],therefore the impedance is deviated from its ideal capacitive behaviour. So the impedanceof the non-ideal behaviour of capacitance is given by [26],

Z ¼ ½QðjxÞn��1;

where j! the imaginary number, Q is the frequency-independent real constant,x = 2pf! angular frequency (radians), f! the frequency of the applied signal and n! isthe Randles exponent.

For diffusion controlled process the Warburg impedance is given by [27],

Zw ¼ ½tan BðjxÞ1=2�=Y 0ðjxÞ1=2

where B = l/(D)1/2, D is the diffusion coefficient, l is the diffusion layer thickness,=Yo = [,(2)1/2]1/2, , – Warburg coefficient.

When a constant phase element [28,29] is introduced into the system then the imped-

ance of the double layer capacitance is expressed by 1Z ¼

ðjxRctCdlÞbRct

where Z = impedanceof double layer capacitance, b = 0.5.

The CRCA sheets (with and without nickel coating) are studied by EIS in 3.5% NaClsolution and the impedance diagrams are generally found by modeling metal/coatinginterface or metal/electrolyte interface with an equivalent circuit as shown in Fig. 11.The generated data are analysed by the help of Nyquist and Bode plots. The experimentaldata are fitted with the proposed model when Warburg impedance is absent in the circuit.It is like a randles model. The Nyquist plot for nickel coated CRCA and Bode plot forbare and nickel coated CRCA is shown in Figs. 12 and 13a and 13b, respectively. The

Fig. 12. Nyquist plot for nickel coated CRCA in 3.5% NaCl solution.


diameter of the Nyquist circles of Ni coated CRCA increase with increased immersiontime indicating the protective properties of the nickel coatings. From Table 2 andFig. 13b, it is found that the increase in polarization resistance from 30 X cm2 to89 X cm2 with decreasing capacity from 6.4 · 10�4 F/cm2 to 5.2 · 10�4 F/cm2correspondsto the formation of passivation film. It is evident from the reproducible impedance valuesthat the impedance increases from 1130 X cm2 to 1220 X cm2 over the exposure time. It issupported by potentiodynamic measurement (Fig. 14) that Ni coating provides a passiv-ation band in 3.5% NaCl solution between �750 mV and 450 mV, which may be due tothe formation of a thin nickel passivation film. Whereas in case of bare CRCA, it wasobserved from Table 3 and Fig. 13a that the impedance values decrease from 875 X cm2

to 693 X cm2 over the exposure time indicating that this is corroding over the time andthe thin oxide/hydroxide film thus formed provides poor corrosion resistant against per-meation of water/ions [30–32]. This is evident from decrease in polarization resistance(Rp / 1/corrosion rate) from 56 X cm2 to 34 X cm2 and increase in coating capacitancefrom 1.1 · 10�4 F/cm2 to 4.1 · 10�4 F/cm2 (Table 3). This is also evident from reproduc-ible experimental results that the wide shifting of phase angle in Bode plot (Fig. 13a) from�900 at lower frequency range towards less negative values compared to nickel coatedCRCA, indicating uptake of water/ion for the corrosion process.

In case of nickel coated GI, the behaviour of the Nyquist plots as shown in Fig. 15 isdifferent for different exposure time. After 1 h of exposure in 3.5% NaCl solution, there isno time constant either low, medium or high frequency range. The occurrence of only onesemicircle may be due to a smaller difference between the time constants of the coating andthe corrosion reaction at the metal surface [33]. After 24 h of exposure, a time constant is

Fig. 13a. Bode plot for bare CRCA in 3.5% NaCl solution.


observed at medium frequency range. The criteria for a coating system to take part in bothcharge transfer as well as diffusion control process is that the ratio of Rct/Rcp rangesbetween 0.2 6 Rct/Rcp 6 5 [34]. The circuit parameters are calculated from the experimen-tal data (Fig. 15) by fitting the proposed equivalent circuit model in Fig. 11 incorporatingWarburg impedance. It is found that the ratio of charge transfer resistance to polarizationresistance is one which fall in that range. The ratio is calculated by knowingRs + Rct + Rcp = 5.5 X at higher frequency and Rs + Rcp = 3 X at medium frequencyand Rs = 0.5 X at lower frequency in the Nyquist plot. So, the coating is taking part inboth charge transfer and diffusion control processes. Therefore, two semicircles withouttail are being observed in continuation indicating the diffuse ions may oxidize the metallicgrains of zinc coatings through charge transfer reaction and fill the nickel coating poresmaking the film more resistance against further diffusion of ion/water. The mechanismof corrosion process is shown in schematic Fig. 16. The following reactions are taking

Fig. 13b. Bode plot for nickel coated CRCA in 3.5% NaCl solution.

Table 2Nickel coated CRCA

Time (Z), X cm2 Rp, X cm2 Cc, F/cm2

1 h 1130 30 6.4 · 10�4

24 h 1190 74 4.9 · 10�4

72 h 1220 89 5.2 · 10�4


place while immersing the coated sheet into the solution is given below. Zinc dissolves aszinc ions and participate in cathodic reaction to form zinc hydroxide rust. This is then con-

Fig. 14. Potentiodynamic measurements of nickel coated CRCA, GI and GA steel sheets in 3.5% NaCl solution.

Table 3Bare CRCA


1 h 875 56 1.1 · 10�4

24 h 832 44 2.5 · 10�4

72 h 693 34 4.1 · 10�4

Fig. 15. Nyquist plot for nickel coated GI in 3.5% NaCl solution.


Fig. 16. Mechanism of simultaneous charge transfer and diffusion process in corrosion of nickel coatedgalvanized steel sheets.


verted into a complex compound of zinc hydroxychloride product by charge transfer anddiffusion of chloride ion. This product blocks the nickel coating pores and act as a corro-sion resistant layer.

Zn! Znþ2 þ 2e ðanodic reactionÞH2Oþ 1=2O2 þ 2e! 2OH� ðcathodic reactionÞZnþ2 þ 2OH� ! ZnðOHÞ2 ðzinchydroxide rustÞZnþ2 þ 4H2Oþ 2Cl� þ 4ZnO! Zn5 ðOHÞ8Cl2 �H2O ðzinc hydroxychloride rustÞ

After 72 h of exposure, the time constant at high frequency range can be attributed to thecapacitive behaviour of the coatings. The simultaneous increase of Rp from 559 X cm2 to598 X cm2 and decrease of Cc from 6.8 · 10�4 F/cm2 to 1.6 · 10�4 F/cm2 over the expo-sure time is observed (Table 4). This may be due to participation of coating in chargetransfer and diffusion control process with different diffusion coefficient of Na+ andCl�through the passive film. This is also evident from the ratio of Rct to Rcp is 0.3, whichis laying between 0.2 and 5 as a criteria for protective coating system. Therefore, this coat-ing lasts for longer period in 3.5% NaCl solution.

In case of bare GI, the occurrence of first semicircle after 1 h of exposure may be due toinsufficient time difference between the time constant of the coating (Rcp · Cc) and thecorrosion reaction at metal surface. With the passage of exposure time, the circle of highfrequency loop is found to be decreased with a simultaneous emergence of a linear regionor tail at the lower frequency side of Nyquist plots as shown in Fig. 17. The distortion is

Table 4Nickel coated GI


1 h 559 26.0 6.8 · 10�4

24 h 559 24.0 3.4 · 10�4

72 h 598 36.0 1.6 · 10�4

Fig. 17. Nyquist plot for bare GI in 3.5% NaCl solution.


observed due to Warburg impedance in the form of a straight line 45� angle to the hori-zontal axis. This showed that Warburg impedance diverged from real axis. This indicatesan appreciable corrosion of the coating due to diffusion effect. This behaviour is occurredwhen a steel sheet treated with a conversion coating [35]. The curve is found 50%–60%fitted with equivalent circuit model including Warburg impedance. The derived circuit ele-ments are presented in Table 5. The increase in both polarization resistance from18.7 X cm2 to 49 X cm2 and coating capacitance from 1.0 · 10�4 F/cm2 to 7.9 · 10�4 F/cm2 over the exposure time indicates the gradual permeation of water/ions through thezinc coating. The schematic diagram (Fig. 18) representing the mechanism of corrosionof bare galvanized steel sheets in 3.5% NaCl solution, where diffusion and charge transferreaction occurs simultaneously.

The corrosion product thus formed is come out from the substrate and dissolve in thesolution. Therefore, zinc coating is gradually degraded with exposure time. This is evidentfrom significant decrease of coating capacitance over the exposure time (Table 4).

In case of nickel coated GA, the radius of the circles are gradually increasing over theexposure time indicating coating is protective in nature. This is shown in Fig. 19. The for-mation of circles without any time constant indicates, the coating is taking part in chargetransfer process only. The equivalent circuit parameters are derived by fitting curves with

Table 5Bare GI


1 h 262 18.7 1.0 · 10�4

24 h 587 38.5 6.9 · 10�4

72 h 693 49.0 7.9 · 10�4

Fig. 18. Mechanism of simultaneous charge transfer and diffusion process in corrosion of galvanized steel sheets.

Fig. 19. Nyquist plot for nickel coated GA in 3.5% NaCl solution.


proposed model at Fig. 11 excluding Warburg impedance. The circuit elements are pre-sented in Table 6. It is evident from Table 6 that the polarization resistance increases from78.4 X cm2 to 91 X cm2 and coating capacitance decreases from 1.5 · 10�3 F/cm2 to1.2 · 10�4 F/cm2 over the time which suggests the thickening of the film and improvement

Table 6Nickel coated GA


1 h 788 78.4 1.5 · 10�3

24 h 875 74.4 1.0 · 10�3

72 h 916 91.0 1.2 · 10�4


in the film passivity. The film passivity in GA coating occurs in between �700 mV and�600 mV in 3.5% NaCl solution as describe by Rout et al. elsewhere [6]. The GA coatingitself provides protection against corrosion in 3.5% NaCl solution due to self-galvanicaction of delta phase (d), which comprises 9–13% Fe in Zn.

In case of bare GA, it is found from Fig. 20 and Table 7 that the radius of the circle isquite high after 72 h of exposure in 3.5% NaCl solution with high polarization resistanceof 133 X cm2 indicating protective nature of the film. This is also due to occurrence of pas-sivation film as well as self-galvanic action of delta phase (d) in GA coating, which hasbeen described earlier. The coating capacitance value of bare GA is found to be higher(3.7 · 10�4 F/cm2) as compared to nickel coated GA (Cc: 1.2 · 10�4 F/cm2) after 72 h of

Fig. 20. Nyquist plot for bare GA in 3.5% NaCl solution.

Table 7Bare GA


1 h 788 124 7.4 · 10�4

24 h 802 117 8.0 · 10�4

72 h 916 133 3.7 · 10�4


exposure indicating water/ion uptake by bare GA coating is comparatively at higher ratefor the corrosion process. Therefore, nickel coated GA is showing superior corrosion resis-tance in 3.5% NaCl solution.

4. Conclusions

1. Nickel coating thickness of 7 lms with superior coating surface quality is obtained onCRCA steel sheets at optimum voltage of 6.9 V, pH of 3–2 to 3.5, temperature of 55 �Cwith coating deposition time of 360 s.

2. Nickel coating thickness of 4–5 lm with superior surface quality is obtained on GA andGI steel sheets at optimum voltage of 7.5 V, pH ranges from 3.8 to 5.0 with coatingdeposition time of 240 s at room temperature.

3. Nickel coated CRCA sheets provide excellent corrosion resistance compared to bareCRCA sheets in 3.5% NaCl solution as obtained from EIS measurements. It is evidentthat Rp increases with decrease in Cc.

4. Nickel coated GI provides superior corrosion resistance over bare GI due to simulta-neous increase and decrease of Rp and Cc, respectively.

5. Coating capacitance value is found 2.8 times less in case of nickel coated GA comparedto bare GA coating after 72 h of exposure in 3.5% NaCl solution indicating nickelcoated GA is less permeable to water/ions.

6. Nickel coated GA provides superior corrosion resistance as compared to nickel coatedCRCA and nickel coated GI due to higher polarization resistance (Rp) and lower coat-ing capacitance (Cc) in 3.5% NaCl solution.

Acknowledgements

The authors would like to thank the Tata Steel management for getting permission forthe publication and also gratefully acknowledge Dr. P. Srinivasan, Unit Leader of Scien-tific Services for the technical guidance and Miss Nitu Rani and Mr Vikram Sharma fortheir assistance during carrying out experiments and SEM studies, respectively.

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Electrochemical impedance spectroscopy studyon multi layered coated steel sheets