1. INTRODUCTION:
Aluminum and its alloys are increasingly used in many fields due to their physical properties such as low density, high specific strength, thermal and electrical conductivities that make them suitable for various applications[1,2]. It has been widely used in automotive industry, aircrafts, medical appliances, and electronic equipments[3-6]. In general, aluminum alloys present complex microstructure, with the presence of intermetallics of different composition, rendering that the alloys are very prone to be corroded locally [7,8]. These problems lead to the development of surface treatments for aluminum alloys which would be protect by the other metal alloys.
Ni-P alloys are very useful in automobiles, aviation, printing, chemical industrial machine, and other fields. They are used especially to plate cylinders, pistons, revolving shafts, compressors, rollers machine, and shaping molds [9-12]. There are various types of Electroless Nickel (EN) coatings, including binary alloy, ternary alloy and composite coatings. Parker et al reported [13] that EN widely used either as protective or decorative coatings in many industries, including petroleum, chemical, plastic, optics, printing, mining, aerospace, nuclear, automotive, electronics, computer, textile, paper, and food.
The EN method was developed by Mandich and Krulik[14]. EN plating is a chemical reduction process in a solution to reduce metallic ion to the metal state by using a reducing agent. The metals capable of being deposited by electroless plating such as Ni,Co, Pd, Cu, Au and Ag with co-metal P, B and W. The EN process, there was no electric current used but it has charge transfer.
Electroless nickel process produces a hard and uniform coating of nickel - phosphorous alloy. Electroless nickel is used to protect the surface of metals like aluminum, magnesium, titanium, copper and steel from corrosion and wear and its plating is an effective method due to their good corrosion resistance and high wear resistance[7]. Some ternary based alloys are developed to further increase the properties of binary metals by adding another metal. Codeposition of W element in binary Ni-P plating has been considerable interest because of its corrosion-resistant, wear-resistant, electrical-resistant and high temperatures resistant. The corrosion resistances of electroless ternary Ni-W-P and Ni-W-Co alloys with various structures are higher than that of Ni-P deposits[15,16].
In this present study, the electroless plating of Ni-P/Ni-W-P were deposited on Al 6061 alloy in order to improve the corrosion-resistant and microhardness of the coating. The corrosion protection action was compared using different techniques like potentiodynamic polarization, electrochemical impedance spectroscopy (EIS). The surfaces have been analyzed by using SEM, EDAX and XRD measurements.
2. EXPERIMENTAL METHODS:
Samples made of Al 6061 with size of 20x20x1 mm were mechanically polished to get smooth surfaces. The chemical composition of AA6061 is Al-97.9%, Mg-1.0%, Si-0.6%, Cu-0.25% and Cr-0.25%. The aluminium samples were then pre-cleaned by degreasing in trichloroethylene for removel of oils and greases. And then etched in sodium carbonate (25g/L), trisodium orthophosphate (30 g/L), sodium metasilicate ( 20g/L) and sodium dodecyl sulphate (1g/L) aqueous mixed solution to remove oxide layers on Al substrate at a temperature of about 60 °C. In order to neutralize the surface, the samples are immersed in a acid solution containing sulphuric acid (10 ml/L), hydrofluoric acid (11 ml/L) and nitric acid (20 ml/L) at a temperature of about 25 °C. The double layer zincated coating formed on aluminum samples to get good adhesion between aluminium substrate and Ni-P coating and then the aluminium substrate were immersed into an electroless nickel bath. The formulations of the electroless nickel phosphorus and electroless nickel tungsten phosphorus plating are given in table 1. The pH of the solution was adjusted by adding 15% ammonia and 15% H2SO4 to increase or decrease the pH, respectively. All the chemicals used were of analytical grade and de-mineralized water was used throughout the experiment.
The corrosion resistance of the coating was evaluated by EIS and potentiodynamic polarization method and it was performed in a conventional three electrode cell using a computer-controlled potentiostat /galvanostat (Autolab PGSTAT 302N potentiosat from Eco-chemie, Netherlands). Platinum electrode was used as the counter electrode, Ag/AgCl, 3M KCl as the reference electrode and the Ni coated sample with an exposed area of 1 cm2 as working electrode (WE). All experiments were performed in 3.5 wt.% NaCl solution at room temperature.
Table 1: Bath compositions and conditions of electroless Ni-P/Ni-W-P coating on aluminium 6061 alloys.
|
Baths |
Bath Composition and conditions |
||
|
Electroless Ni-P |
Nickel Sulphate |
: |
30 g / L |
|
Sodium Hypoposphite |
: |
10 g / L |
|
|
Sodium Citrate |
: |
10 g / L |
|
|
Sodium Acetate |
: |
5 g / L |
|
|
Thiourea |
: |
1 mg / L |
|
|
Temperature |
: |
82 ± 2 °C |
|
|
Time |
: |
45 min |
|
|
pH |
: |
4.8 ± 0.2 |
|
|
Electroless Ni-W-P |
Agitation |
: |
moderate |
|
Nickel sulphate |
: |
15 g / L |
|
|
Sodium Hypoposphite |
: |
20 g / L |
|
|
Sodium tungstate |
: |
10 g/L |
|
|
Sodium Citrate |
: |
40 g / L |
|
|
Temperature |
: |
85 ± 2 °C |
|
|
Time |
: |
240 min |
|
|
pH |
: |
5 ± 0.2 |
|
|
Agitation |
: |
moderate |
|
Before each EIS and potentiodynamic polarization (Tafel) studies, the electrode was allowed to corrode freely and its open circuit potential (OCP) was recorded as a function of time up to 30 min to attain a stable state. EIS measurements were carried out using AC signals at the amplitude of 10 mV and measurement frequency range from 30 kHz-10 mHz at the stable OCP. The potentiodynamic polarization measurements were started from cathodic to the anodic direction (E = Ecorr ± 250 mV) at a scan rate of 10 mVs-1. The potentiodynamic polarization and EIS data were analyzed and fitted using impedance software, Nova 1.4. Fresh solution and fresh samples were used after each sweep.
The surface and cross section morphology and chemical composition of the coatings were investigated using ESEM Quanta 200, FEI a Scanning Electron Microscope (SEM) which equipped with Energy dispersive X-Ray spectroscopy (EDAX). Phase structure was characterized by X-ray diffraction (XRD) spectroscopy using a JDX-8030, JEOL. Microhardness of the electroess Ni-P/Ni-W-P was evaluated using a Vickers microhardness tester, Wolpert Wilson Instruments, Germany. It was measured using a diamond indenter at a 50 gf load for 10 s. Five measurements were taken on each plated coatings and the values were then averaged.
The thickness of the EN plating layer was determined from the amount of deposited material and material density by using the equation given below. W1 was the before EN plating and W2 was the after EN plating. W1 and W2 were measured using an analytical balance with the precision of 0.0001g. The density (ρ) of EN plating was 8.9 g/cm3. A was the total surface of the samples.
x 10-4 (1)
3. RESULTS AND DISCUSSIONS:
3.1. Electroless deposition of Ni-P/Ni-W-P coatings
In electroless plating, phosphorus could not be deposited without presence of a metal of the iron group such as nickel [17]. This process is called as induced co-deposition [18]. Nickel sulphate used as nickel ion source and sodium hypoposphite used as reducing metal ion. The approximate mechanism was proposed by many researchers [19-22] for the formation of Ni-P and W alloy deposit. Here, we suggest some chemical reaction that occurs when deposition of Ni, P and W on aluminium.
(a) H2PO2 -
+ H2O H2PO3-+ H2 (2)
Ni2+ + + H2PO2o
+ H2O Ni° + H2PO3 + 2H (3)
H2PO2 + H+
P + OH- + H2O (4)
(b) Ni2+ + 2e Ni
(5)
WO42- + 6e + 4H2O
W + 8OH- (6)
H2PO2- +
3OH- HPO32- +2H2O
+ 2e (7)
H2PO2- +
2H+ + e P + 2H2O (8)
3.2. Surface morphology and structure of the as plated coatings
The X- ray diffraction patterns of the 6061 aluminium alloy substrate, the electroless Ni-P coating and the Ni-W-P coating are shown in Fig. 1. From the XRD pattern of aluminium 6061 alloy indicate that the substrate consisted of well crystalline α Al phase. It can be observed that as-deposited electroless Ni-P (b) shows that the diffraction patterns of Ni-P have a single peak at 2θ value of 450 corresponding to the plane of a face centered cubic (fcc) phase of nickel. The as-deposited Ni-P and Ni-P/Ni-W-P coatings (C) showed broad diffraction at about 2θ value of 45° that can be attributed to (111) plane o Ni, indicating that the deposits have amorphous structure [23] with increase in the grain size. Fig. 1.C shows that the Ni-W-P XRD pattern has an only single very broad peak of an amorphous structure.
Fig.1: XRD pattern of as-deposited Ni-P and Ni-P/Ni-W-P coatings
The compositions of the as-plated Ni-P and Ni-W-P alloys were determined by energy-dispersive analysis of X ray EDX are given in figures 2 and 3. The Ni-P coating thickness was around 12 -15 µm. From the figure 2 it can be seen that the Ni-P deposit contains 9.93wt.% P and 90.07 wt.% Ni and the codeposition of tungsten in ternary Ni-W-P alloy (Fig.3.) the tungsten content was found to be 4.28 wt.% and phosphorus content was 10.72 wt.% and rest was found to be nickel 85 wt.%. According to the study when the phosphorous content is 7-10%, it is considered as medium phosphorous [24] and in the present study the phosphorous content is found to be 10.72 wt.%. The Ni-P and Ni-W-P gives a mixture of amorphous plus nanocrystalline phase. The W was codeposited due to the addition of sodium tungstate in the EN bath and the reduction of the tungsten was determined by its electrochemical potential as well as its catalytic activity for the reduction process [25].
Fig2: EDAX pattern of as-deposited Ni-P coatings on Al 6061
Fig3: EDAX pattern of as-deposited Ni-W-P coatings on Al 6061
The surface morphology of as-deposited electroless Ni-P and Ni-W-P alloys deposition on 6061 aluminum has been studied using SEM and the results are given in Fig. 4. The surface morphology of the coating is influenced by the chemical constituents present in the electroless nickel bath [26]. Fig.4a shows the surface morphology of the Ni-P coating on the 6061 aluminum substrate. It can be seen that the surface morphology of the Ni-P deposit exhibits a smooth morphology with fine nodular structure. It is clearly shown from the fig.4b that the ternary Ni-W-P deposit contains more nodular cluster with fine grains. This may be due to the incorporation of W element in Ni-P. The surface morphology of the cross-section of as deposited Ni-P/Ni-W-P coatings shown in Fig.4c reveals 27.5 µm of Ni, P and W are coated on the surface of Al 6061 alloy.
Fig. 4: SEM morphology of as-deposited Ni-P and Ni-P/Ni-W-P coatings
3.3. Study of Microhardness
Microhardness of the electroess Ni-P and Ni-W-P was evaluated using a Vickers microhardness tester in as plated conditions. The as plated Ni-P coating has the microhardness of 400 ± 18 HV which is lower than that of as plated Ni-W-P coating 485 ± 20. The increase of microhardness in Ni-W-P is due to the incorporation of W matrix in Ni-P coatings. In the present study, W and P contents influence the as-deposited hardness on Al 6061 alloy. In general, the codeposited W is in solid solution form and P is in supersaturated form in EN matrix [27].
3.4. Study of corrosion resistance
3.4.1. Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) method is used to investigate the corrosion resistance of electroless Ni-P and Ni-W-P double layer coating in as-plated on aluminium 6061 alloy. The Figs. 5 and 6 shows the Nyquist and Bode plots obtained for bare aluminium alloy substrate, as-plated Ni-P and Ni-W-P coatings in 3.5% sodium chloride solution at their respective open circuit potentials. The diameter of the capacitive loop stands for the resistance of the corrosion and it can be seen that the resistance decreases significantly with the decrease in diameter. In general, it consists of a capacitive loop in the high frequency region and a small inductive loop in the low frequency region of the impedance spectra. The high frequency capacitance loop was indicated to the charge transfer resistance and the low frequency inductive loop was attributed to the corrosion resistance at aluminium 6061 alloy interface. The impedance values are calculated using by the diameters of capacitance.
Fig.5: Nyquist plots as-deposited Ni-P and Ni-P/Ni-W-P coatings immersed in 3.5 wt.% NaCl solutions.
Fig.6: Bode plots of as-deposited Ni-P and Ni-P/Ni-W-P coatings immersed in 3.5 wt.% NaCl solutions
Fig.6. shows the Bode plots of Al 6061, Ni-P and Ni-W-P coatings on Al 6061 alloy and the results indicates the higher value and broader phase angle Ni-P and Ni-W-P compare to aluminium substrate and this could be attributed to the corrosion resistance of the Ni-P/Ni-W-P coating deposited on the aluminium substrates. Fig.7. shows the equivalent circuit model of the EIS spectra in which Rs is the solution resistance, Rct is the charge transfer resistance and CPE is a constant phase element, which is parallel to the charge transfer resistance of the coatings and its impedance is given by Z=A-1(iω)-n.
Fig.7: Equivalent circuit fit for EIS
Where, A is proportionality coefficient, ω is the angular frequency and i is the imaginary number, n is an exponent related to the phase shift and can be used as a measure of surface non-homogeneity. For an ideal electrodes, when n=1, CPE can be considered as a real capacitor. The charge transfer resistance (Rct) values are calculated based on the difference in impedance at lower and higher frequencies. From the Fig.5, it can be seen that the constant phase element decreased and the charge transfer resistance value increased for the Ni-W-P coating compared to Ni-P coating deposited on the aluminium substrates. From these results, the decrease in this capacitor values can be attributed to the formation of dense structure of electroless Ni-W-P coating than that of Ni-P coating. The quantitative results of impedance measurements are given in Table 2.
3.4.2. Potentiodynamic polarization studies
Potentiodynamic polarization curves for electroless Ni-P and Ni-W-P double layer coating in as-plated on 6061 aluminium alloy immersed in 3.5% sodium chloride solution are given in Fig. 7. From the figure it is evident that the corrosion current density (Icorr) and corrosion potential (Ecorr) were obtained from the intercept of the Tafel slopes extrapolated from the cathodic and anodic potentiodynamic polarization curves. The polarization resistance Rp was calculated from the linear polarization curves and the corrosion potential and corrosion current density of the coatings obtained from the electrochemical polarization curves are listed in Table 2. Potentiodynamic polarization curves for electroless Ni-P/Ni-W-P coatings show a similar corrosion trend as studied in EIS measurments.
The percentage of corrosion inhibition efficiency (% IE) was calculated from potentiodynamic polarisation curves based on the following equation
% IE =(1 − (jcorr(i)/jcorr(o))) × 100 (9)
Fig.7: Tafel plots of potentiodynamic polarization studies in as-deposited Ni-P and Ni-P/Ni-W-P coatings immersed in 3.5 wt.% NaCl solutions
Where, jcorr(i) and jcorr(o) are the corrosion current densities for the Ni-P/Ni-W-P coatings and the substrate, respectively obtained from Tafel plots. The corrosion potential Ecorr of the Ni-P/Ni-W-P coatings on the substrate is shown more positive values than that of the substrate.