Indian Journal of Engineering & Materials Sciences Vol. 24, August 2017, pp. 306-312

Improvement of adhesion, corrosion and wear resistance of Ni electrodeposited coating by applying Cu intermediate layer after zincate process

M Adabia* & A Amadehb

aYoung Researchers and Elite Club, Roudehen Branch, Islamic Azad University, Roudehen, Iran bSchool of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

Received 25 February 2015; accepted 6 April 2017

Zincate treatment is extensively used for preparation of aluminium surface prior to deposition process. However, the zinc layer may be dissolved in acidic solutions such as Watts bath used for electrodeposition of Ni coating. Hence, the use of an intermediate layer between Zn film and top coating has been suggested. In this study, the effect of Cu intermediate layer on properties of Ni coating electrodeposited on 6061 Al alloys substrate is investigated. The properties of the coatings are studied by means of peel test, potentiodynamic polarization techniques and wear test. The results indicated that the use of Cu intermediate layer increased the adhesion strength of the coating from 0.85×103 N/m to 4.01×103 N/m. Furthermore, the corrosion and wear resistance of samples improved when Cu intermediate layer is applied after zincate process.

Keywords: Ni coating, Zincate treatment, Peel test, Potentiodynamic polarization, Wear resistance

Al and its alloys due to the high specific strength, low density, high thermal and electrical conductivities, general corrosion resistance and abundant availability are extensively used in many applications such as aerospace and automotive industry.1-3 However, the poor resistance to wear and localized corrosion of aluminium can limit its more widespread usage.4 Different coating approaches namely physical vapor deposition (PVD)5, sputtering6, ion implantation7 and electrodeposition8-10 have been developed to enhance the wear and corrosion resistance of aluminum alloys. Among these methods used for producing wear and corrosion resistant coatings, electrodeposition is known as an economic and simple technique.11,12

Electrodeposition of metals on Al alloys due to the existence of aluminium oxide film on the surface of aluminium is difficult. This oxide layer reduces the adhesive strength of electrodeposited layer. Hence, a surface pretreatment is necessary to remove the oxide layer and suppress its further formation prior to plating process. The zincate treatment is frequently used for this purpose.13-15 In this process, the surface of the substrate is covered by a zinc film via displacement reaction of aluminium and zinc. In fact, zincate reactions occur at two distinct sites on the aluminium. These two sites are known as the anode, or aluminium dissolution site, and the cathode, or zinc reduction site. The zincate reactions are:16-20

Cathodic reactions:

Zn(OH)42- = Zn2+ + 4OH- … (1)

Zn2+ = Zn + 2e- … (2)

Sum cathodic reactions:

Zn(OH)42- + 2e- = Zn + 4OH- … (3)

Anodic reactions: Al+3OH- = Al(OH)3 + 3e- ... (4)

Al(OH)3 = AlO2- + H2O + H+ … (5)

Sum anodic reactions: Al + 2H2O = AlO2

- + 4H++ 3e- … (6)

Finally, the total reaction (anodic and cathodic) is:

2Al + 3Zn(OH)42- = 2AlO2

- + 3Zn + 4H2O + 4OH- … (7)

The cathodic reaction is the reduction of zinc from alkaline zincate solution. The anodic reaction is the dissolution of aluminium. From the overall reaction, zinc particles are deposited from the solution and aluminium is dissolved by galvanic coupling. Sometimes, the double zincate treatment which repeats a conversion treatment twice is performed to increase the adhesive strength of the coating to aluminium alloy substrates.21,22 To prevent the dissolution of Zn film in acidic Watts electrolyte, copper plating in alkaline bath prior to Ni electroplating has been suggested.23-25

____________ *Corresponding author (E-mail: adabi@riau.ac.ir)

ADABI & AMADEH: Ni ELECTRODEPOSITED COATING

307

In this work, a Ni layer was electrodeposited on the 6061 Al alloy following the Cu layer applied after the zincate treatment. The effect of Cu intermediate layer on adhesion coating to substrate and corrosion resistance was investigated using peel and corrosion test, respectively. Finally, wear resistance of the coatings with and without copper intermediate layer was compared together. Experimental Procedure

Plates of 6061 aluminium alloys were used as cathode. The chemical composition of the alloy is given in Table 1. The plates with a size of 2 cm × 3 cm × 0.2 cm and 1 cm × 5 cm × 0.2 cm mechanically ground up to 2000-grit SiC papers. The longer specimens (1 cm × 5 cm × 0.2 cm) were used for adhesive strength test of the plating layer to the substrate. The anode was a nickel plate with 99.9% purity. Before electrodeposition, some treatments were carried out to prepare a suitable surface. These treatments with their details used in this study were summarized in Table 2. As shown in this table, three different processes (sample A–C) were performed to clarify the effect of Zn and Cu as an intermediate layer. In other words, the variations of surface preparation include:

(i) The zincate treatment with single or double dip procedures in zincate solution. It is noted that the second dip in zincate solution (double zincate) was performed after removing the first zinc layer by dipping in nitric acid solution at room temperature.

(ii) Electrodeposition of copper at different current densities (0.1and 0.5 A/dm2).

In all treatments mentioned above, specimens were washed with distilled water after exiting each solution and before immersing in the following bath. After surface pretreatment, the specimens were dipped in Watts solution for nickel electrodeposition process. Table 3 lists the composition of electrodepostion electrolyte. The pH and temperature of the bath were fixed at 4C and 50C, respectively. During electrodeposition, the bath was agitated by means of magnetic stirrer (250 rpm). The Ni coatings with 22 µm thickness were prepared at current density of 5 A/dm2.

The surface morphology and elemental analysis of the samples were studied using a VegaTescan

Table 1 Composition of 6061 aluminium alloy Element Si Fe Cu Mn Mg Cr Zn Ti Alwt% 0.36 0.2 0.01 0.02 0.52 0.03 0.01 0.02 Balance

Table 2 Bath compositions and various pretreatments for preparing 6061 Al alloy surface

Process Bath composition Concentration Condition Sample A Sample B Sample C

Alkaline Clean NaOH 50 g/L Temp.= 70C, 20 s Acid Etch HNO3 65 vol% Room temp., 5 s First zincate treatment

Ni(SO4)2

ZnSO4

NaOH KCN

KHC4H4O6

CuSO4

FeCl2

30 g/L 40 g/L

106 g/L

10 g/L

40 g/L

5 g/L

2 g/L

Room temp., 20 s

Second zincate treatment

Ni(SO4)2

ZnSO4

NaOH KCN

KHC4H4O6

CuSO4

FeCl2

30 g/L 40 g/L

106 g/L

10 g/L

40 g/L

5 g/L

2 g/L

Room temp., 20 s

-

Cu Electrodeposition CuCN NaCN

Na2CO3

22 g/L 35 g/L 3 g/L

Temp.= 45C, 1000 spH= 12

current density= 0.1 and 0.5 A/dm2

- -

Table 3 Composition of Watts bath

Nickel sulfate (NiSO4_6H2O) : 300 g/L Nickel chloride (NiCl2_6H2O) : 45 g/L Boric acid (H3BO3) : 45 g/L

INDIAN J. ENG. MATER. SCI., AUGUST 2017

308

scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDS).

The 90° peel test was used to calculate the bonding strength of coatings using MTS-810 instrument at room temperature. The experimental setup used for peeling test is shown in Fig. 1. The peeling rate was 1 mm/min.

Corrosion behavior of the samples was compared using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The testing equipment was an EG&G potentiostat model 273A. A conventional three electrode cell, with the specimens as the working electrode, an Ag/AgCl as reference and a platinum sheet as counter electrode, were employed in those tests. Working electrodes were embedded into epoxy resin with an exposure area of 1 cm2. The specimens were immersed in the electrolyte for 3 h prior to each test, allowing the system to be stabilized. The test electrolyte was neutral 3.5 wt% NaCl solution and the test temperature was maintained at room temperature. The scan rate for potentiodynamic polarization curves was 1 mV/s.

Dry sliding wear tests were carried out using a pin-on-disk wear apparatus at room temperature. The normal load of 5 N was used in the wear tests and the rotation speed was 0.314 m/s (i.e. 300 rpm) with a radius of 10 mm for 300 m sliding. The pin material was AISI 52100 hardened steel with the hardness of 45-50 HRC. The pins, with 6 mm in diameter, had a tip radius of 5 mm. Before wear test, all contact surfaces were cleaned with acetone and dried. The weight loss of disk specimens and steel pins was measured by an electric balance with 0.1 mg accuracy.

Results and Discussion

Morphology study Figure 2(a,b) shows the surface morphologies of

single, double zincate deposits on the Al 6061 surfaces, respectively. Comparison of Fig. 2a with Fig. 2b indicates that double zincate provides more dense and uniform zinc layer than single one. It is also observed that the copper strike treatment covers zinc layer with continues Cu layer (Fig. 2c).

To investigate the effect of Cu film as an intermediate layer, Ni was electrodeposied on the samples with and without Cu layer. The surface and cross-section of these samples were observed in Fig. 3.

Fig. 1 Schematic diagram of generic peel test

Fig. 2 The surface SEM micrographs of (a) single zincate, (b) double zincate, and (c) copper strike

ADABI & AMADEH: Ni ELECTRODEPOSITED COATING

309

As shown in this figure, the surface of sample with Cu film as intermediate layer shows a dense and uniform microstructure in comparison with sample without Cu one. The cross-sectional SEM image seen in Fig. 3c also indicates the existence of defects between the Ni layer and substrate.

The coating process for the 6061 Al alloy prepared with and without Cu intermediate is schematically illustrated in Fig. 4. In the case of the sample without Cu intermediate layer, the zinc layer was partly dissolved in the Watts bath and, therefore, a non-uniform Ni layer was formed on it during electrodepostion of nickel. On the other hand, Cu electroplating on zinc layer suppressed the dissolution of Zn layer in Watts bath. Adhesion test

Figure 5 exhibits the force-distance curves of Ni coatings for each pretreatment achieved by the peeling test. According to this figure, the peeling force of coating increases with increasing the distance, passes through a maximum and then drops slightly and attains an approximately constant value until failure. The existence of peak at the first region in the curves is related to provide the driving force needed for crack initiation. Once the crack initiates, its propagation needs less driving force and so the peeling force reduces. The observed fluctuations can be attributed to successive stop-go crack growth

during detachment of coating from the substrate. The value of peeling force in latter region was placed in the following equation to calculate bonding toughness,26,27 and results are given in Table 4:

G = F (1-cosθ)/b … (8) Where F is the critical peeling force, b is the width of the film on the substrate, F/b is peel strength and θ is peel angle (90° in present study). From Table 4, it is seen that the nickel coating on Cu intermediate layer shows the highest bonding toughness. This is related to the fact that the Watts bath is acidic solution and

Fig. 4 Schematic of Ni coating formation on (a) Zn layer and (b) Cu layer

Fig. 5 The force-distance curves of Ni coatings for each pretreatment

Fig. 3 SEM images of (a, c) without Cu intermediate layer and(b, d) with Cu intermediate layer after Ni electrodeposition

INDIAN J. ENG. MATER. SCI., AUGUST 2017

310

this matter may cause damage of zincate layer.23 Hence, the existence of a copper intermediate layer before electroplating of nickel can protect the zincate layer. It is also found that the bonding toughness decreased with increasing current density used for electrodeposition of copper. This may be due to the effect of deposition rate on the bonding toughness, i.e, high current density results in high deposition rate which reduces the coating adherence.28 In addition, as evident from Table 4, the adhesion of coating after second zincate was more than first one. This can be attributed to more dense and uniform zinc layer in double zincate in comparison with single one.

The fracture surface morphology of sample with Cu film as intermediate layer, related to behind of coating, after performing of peel test is shown in Fig. 6. The surface shows many dimples. EDS analysis obtained these dimples indicated that Al exists in these areas. This result presents that the aluminum was detached from substrate and adhered to the coating. Corrosion properties

Potentiodynamic polarization The potentiodynamic polarization curves of nickel

coating on Zn and Cu layer, determined in the 3.5% NaCl corrosive medium, are presented in Fig. 7. The results of the potentiodynamic polarization measurements are summarized in Table 5. Corrosion current density (Icorr), calculated by using the Stern–Geary equation29, and self-corrosion potential (Ecorr) are used to evaluate the protective property of coating. Based on the extracted data from the polarization curves, the Ecorr of nickel coating on Cu intermediate layer (sample C) is approximately 25 mV positive than that of nickel coating on zincate

layer (sample B). In addition, the corrosion current density (icorr) of sample C is lower than the one of sample B which implies that the corrosion rate of sample C is lower than sample B due to the formation of more uniform and compact film of Ni on the Cu intermediate layer. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy method is powerful technique to investigate the corrosion protection of different coatings for substrate. The EIS characteristics of the Ni coating on the Cu intermediate

Table 4 Adhesion strength

Zincate procedure Peeling force (N)

Adhesion strength (N/m)

Without zincate treatment < 1 < 0.11 × 103 Single zincate 7.65 0.85 × 103 Double zincate 10.80 1.20 × 103 Double Zincate treatment + copper strike at different current densities

- -

0.5 A dm-2 18.90 2.10 × 103 0.1 A dm-2 36.09 4.01 × 103

Table 5 Corrosion potential (ECorr) and corrosion current density (ICorr) for different samples in 3.5 wt% NaCl solution at room temperature

Sample Ba (mV) Bc (mV) ICorr (mA cm-2) ECorr (mV) CR (mpy)

Ni coating without Cu intermediate layer 70 80 120×10-4 -635 5.1 Ni coating with Cu intermediate layer 90 140 85×10-4 -610 3.6

Fig. 6 The fracture surface morphology of sample with Cu film as intermediate layer

Fig. 7 Potentiodynamic polarization curves of coatings in 3.5 wt% NaCl solution (a) the Cu intermediate layer and (b) the zincate undercoat layer immersed in 3.5 wt% NaCl solution

ADABI & AMADEH: Ni ELECTRODEPOSITED COATING

311

layer (sample C) and zincate layer (sample B) in 3.5 wt% NaCl solution were respectively examined at open circuit potential to illuminate their different corrosion resistance as shown in Fig. 8. The equivalent circuit model used for analysis of EIS spectra is also presented in the insert of this figure. In this circuit Rs represents the solution resistance between the reference and working electrodes; Rct is the charge transfer resistance; and a constant phase element (CPE) which replaces the capacitance of the double layer (Cdl) due to the roughness and inhomogeneity of the electrode surface.

The values of the circuit’s particular electric elements for the samples are shown in Table 6. From this table, it is found that the Rct values of sample C is 2652 which is about 2 times that of the sample B with the Rct of 1337. The EIS analytic results also confirm that corrosion resistance of sample C is better than sample B according to the results of analysis of potentiodynamic polarization technique. Wear

The wear test on the samples A and B led to detachment of the coating from the substrate. It can be attributed to the fact that the maximum shear stress locates close to coating/substrate interface and/or the low adhesion of the coating to the substrate. However, the wear sample C due to a good adhesion coating to substrate shows appropriate anti-wear properties. Table 7 demonstrates the wear rate and friction coefficient of Ni coated on Cu intermediate layer. The friction coefficient value is in good agreement with the results presented in the literature.30

The SEM micrographs of worn surface of Ni coated sample on Cu intermediate layer are shown in Fig. 9. As seen, tribological films were formed on the worn surface of sample. The EDS analysis indicates that these films are probably the iron and nickel oxides created during the contact between the coated sample and the steel pin. Formation of mixed layer on the worn surface of coated sample was also found by Fini et al.31

Table 6 Electrochemical parameters fitted from EIS measurement impedance data of the Ni electrodeposited on

(a) Zn and (b) Cu layer

sample Rct

(Ω/cm2) Yo

(Ω/cm2 .sn) Rs

(Ω cm2) n

Ni coating without Cu intermediate layer

1337 2.207 ×10-4 6.98 0.74

Ni coating with Cu intermediate layer

2652 1.071 ×10-4 5.75 0.79

Table 7 The wear rate and coefficient friction of Ni coating on Cu intermediate layer sample wear rate (mg/m) coefficient friction Ni coating on Cu intermediate layer

0.043 0.76

Fig. 8 Nyquist plots of Ni plated layer formed on (a) the Cu intermediate layer and (b) the zincate layer immersed in 3.5 wt% NaCl solution (inset showing equivalent circuit model used to fit the experimental data of EIS plots)

Fig. 9 SEM micrograph of worn surface of the Ni coated sample on Cu intermediate layer against 52100 steel pin under a load of 5 N for sliding distance of 300 m

INDIAN J. ENG. MATER. SCI., AUGUST 2017

312

Conclusions The following conclusions can be drawn from this

study: (i) The adhesion strength of the Ni layer

electrodeposited on the Cu intermediate layer was better than the zincate layer due to a less defective interface between the elctrodeposited nickel layer and the substrate.

(ii) Corrosion tests in 3.5 wt% NaCl solution confirmed that the elctrodeposited nickel layer formed on the Cu intermediate layer presented higher corrosion resistance than the one applied on zincate layer, due to its dense and uniform structure.

(iii) The wear results indicated that Ni coating on the Cu intermediate layer possesses better wear behavior in comparison with Ni coating on the Zn layer. This effect can be attributed to better adhesion of Ni coating on the Cu intermediate layer to the substrate.

References 1 Abdel A, Mater Sci Eng A, 474 (2008) 181-187. 2 Abdel Rehim S, H. Hassan H &Amin M, Corros Sci, 46

(2004) 5-25. 3 Abdel Hamid Z & Abou Elkhair M, Mater Lett, 57 (2002)

720-726. 4 Paloumpa I, A Yfantis, P Hoffmann, Y Burkov, D Yfantis &

D Schmeißer, Surf Coat Technol, 180 (2004) 308-312. 5 Lugscheider E, Krämer G, Barimani C & Zimmermann H,

Surf Coat Technol, 74 (1995) 497-502. 6 Pauleau Y, Juliet P & Gras R, Wear, 210 (1997) 326-332. 7 Zhan Z, Ma X, Sun Y, Xia L & Liu Q, Surf Coat Technol,

128 (2000) 256-259. 8 Adabi M & Amadeh A, Surf Eng, 31 (2015) 650-658. 9 Adabi M & Amadeh A, Trans Nonferrous Met Soc China, 24

(2014) 3189-3195.

10 Adabi M & Amadeh A, Trans Nonferrous Met Soc China, 25 (2015) 3959-3966.

11 Aruna S, William Grips V & Rajam K, J Alloy Compd, 468 (2009) 546-552.

12 Rashidi A & Amadeh A, J Mater Sci Technol, 26 (2010) 82-86.

13 Monteiro F, Barbosa M, Gabe D & Ross D, Surf Coat Technol, 35 (1988) 321-331.

14 Critchlow G & Brewis D, Int J Adhes Adhes, 16 (1996) 255-275.

15 Hino M, Murakami K, Mitooka Y, Muraoka K & Kanadani T, Trans Nonferrous Met Soc China, 19 (2009) 814-818.

16 Lee S, Lee J & Kim Y, J Electron Mater, 36 (2007) 1442-1447.

17 Papis K, Hallstedt B, Löffler J & Uggowitzer P, Acta Mater, 56 (2008) 3036-3043.

18 Huang X, Li N, Li D & Jiang L, Trans Nonferrous Met Soc China, 16 (2006) 414-420.

19 Khan E, Oduoza C & Pearson T, J Appl Electrochem, 37 (2007) 1375-1381.

20 Stoyanova E & Stoychev D, J Appl Electrochem, 27 (1997) 685-690.

21 Monteiro F, Barbosa M, Ross D & Gabe D, Surf Interface Anal, 17 (1991) 519-528.

22 Jin J, Lee S & Kim Y, Thin Solid Films, 466 (2004) 272-278. 23 Totten G & MacKenzie D, Handbook of Aluminum: Vol. 2:

Physical Metallurgy and Processes, (CRC Press), 2003. 24 Tang J & Azumi K, J Appl Electrochem, 158 (2011)

D535-D540. 25 Tang J & Azumi K, Electrochim Acta, 56 (2011) 8776-8782. 26 Kinloch A, Lau C & Williams, J Int J Fract, 66 (1994)

45-70. 27 Ren F, Zhao W, Zhou G, Ju X & Zheng M, Met Mater Int, 8

(2002) 129-134. 28 Ning Z H & He Y D, Trans Nonferrous Met Soc China, 18

(2008) 1100-1106. 29 Stern M & Geary A L, J Electrochem Soc, 104 (1957) 56-63. 30 Ul-Hamid A, Quddus A , Al-Yousef F, Mohammed A,

Saricimen H & Al-Hadhrami L, Surf Coat Technol, 205 (2010) 2023-2030.

31 Fini M & Amadeh A, Trans Nonferrous Met Soc China, 23 (2013) 2914-2922.