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Z. Phys. Chem. 2015; aop Waheed A. Badawy*, Sahar A. Fadl-Allah, and Ahlam M. Fathi Influence of Ni–Cu–P Deposits on the Surface Characteristics of Anodized Al, Al2014 and Al7075 Abstract: Nickel-copper-phosphorous layers were electro-less deposited on the surface of anodized aluminum and aluminum alloys. The electrochemical be- havior of the improved surface was investigated in 0.5 M Na 2 SO 4 . The corrosion resistance of the modified alloy’s surface is ten folds that measured on the an- odized materials. The surface morphology was examined by scanning electron microscopy, SCE, and the deposit composition was subjected to x-ray energy dis- persive analysis, EDAX. The experimental impedance data were fitted to theoret- ical data according to a proposed equivalent circuit model. The results show that the NiCuP layer was deposited homogeneously on the anodized Al surface but on the anodized-alloy surface the layer had a defective nature. Keywords: Alloys, Coatings, Electrochemical Techniques, SEM, Electrochemical Properties. DOI 10.1515/zpch-2014-0645 Received November 7, 2014; accepted January 20, 2015 1 Introduction Anodized aluminum or aluminum alloys of definite porous structure represent good substrates for the fabrication of nanostructures important for many appli- cations [14]. A simple, anodization process in aqueous electrolyte was reported; where no sophisticated and expensive techniques were needed. The process leads to the formation of hexagonal cells of nano-porous alumina layer. At the bottom of the pores, continuous and dielectric oxide layer, called “barrier layer” is built [5]. *Corresponding author: Waheed A. Badawy, Chemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt, e-mail: [email protected] Sahar A. Fadl-Allah: Chemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt Ahlam M. Fathi: Department of Physical Chemistry, National Research Centre, Giza, Egypt

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Page 1: zpch-2014-0645 (1)

Z. Phys. Chem. 2015; aop

Waheed A. Badawy*, Sahar A. Fadl-Allah, and Ahlam M. FathiInfluence of Ni–Cu–P Deposits on theSurface Characteristics of Anodized Al,Al2014 and Al7075Abstract: Nickel-copper-phosphorous layers were electro-less deposited on thesurface of anodized aluminum and aluminum alloys. The electrochemical be-havior of the improved surface was investigated in 0.5M Na

2SO4. The corrosion

resistance of the modified alloy’s surface is ten folds that measured on the an-odized materials. The surface morphology was examined by scanning electronmicroscopy, SCE, and the deposit composition was subjected to x-ray energy dis-persive analysis, EDAX. The experimental impedance data were fitted to theoret-ical data according to aproposed equivalent circuit model. The results show thattheNi–Cu–P layer was deposited homogeneously on the anodizedAl surface buton the anodized-alloy surface the layer had a defective nature.

Keywords: Alloys, Coatings, Electrochemical Techniques, SEM, ElectrochemicalProperties.

DOI 10.1515/zpch-2014-0645Received November 7, 2014; accepted January 20, 2015

1 IntroductionAnodized aluminum or aluminum alloys of definite porous structure representgood substrates for the fabrication of nanostructures important for many appli-cations [1–4]. A simple, anodization process in aqueous electrolyte was reported;where no sophisticated and expensive techniqueswere needed. The process leadsto the formation of hexagonal cells of nano-porous alumina layer. At the bottom ofthe pores, continuous and dielectric oxide layer, called “barrier layer” is built [5].

*Corresponding author: Waheed A. Badawy, Chemistry Department, Faculty of Science, CairoUniversity, 12613 Giza, Egypt, e-mail: [email protected] A. Fadl-Allah: Chemistry Department, Faculty of Science, Cairo University, 12613 Giza,EgyptAhlam M. Fathi: Department of Physical Chemistry, National Research Centre, Giza, Egypt

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2 | W.A. Badawy et al.

Recently, anodic aluminum oxide was investigated as hydrogen sensor [6].The oxide consists of highly uniform nano-pores with large aspect ratio(depth/width > 1000). Characteristic parameters of porous anodic alumina in-cluding pore diameter, inter-pore distance, porosity, pore density and thicknessof the oxide layer can be easily controlled by adjusting anodizing conditions suchas type of electrolyte, anodizing potential, current density, temperature and an-odization time. Most of the anodization processes were carried out in sulfuricacid [4, 7, 8], oxalic acid [1, 9], and phosphoric acid [11, 12] solutions.

High purity aluminum foil (min 99.99%) is used as a substrate for anodiza-tion; and only few studies on the anodization of aluminum alloys were re-ported [13–16]. From the economic point of view, aluminum alloys are interestingbecause of their reasonable price and better workability compared to high purityAl. The corrosionprotectionof aluminumalloys is an important aspect indifferentindustrial applications. The corrosion resistance depends essentially on the al-loying elements and the alloy surface modification. In the engineering processesmost of alloying elements are added to improve the mechanical and wear proper-ties by solid solution strengtheningor agehardening [17]without regard to the cor-rosion characteristics of the alloys. Alloy anodization is, generally, not adequateto protect the alloy surface from corrosion. So, electro-less plating was adoptedto produce aprotective top layer on the anodized surfaces, especially nickel [17].The electro-less deposited Ni-coatings have several advantages such as uniformdeposition, good corrosion and wear resistance, good magnetic properties, goodelectrical and thermal conductivity, and good solder ability, as well as the abil-ity of co-deposition of metallic or non-metallic materials [18–20]. The depositedlayer always contains phosphorous in addition to nickel, as a result of the pres-ence of sodium hypophosphite [21]. Electrochemical deposition ofNi–P layer onanodizedAl or its alloys is widespread due to the unique physical, chemical andmechanical properties of the surface layer. Theproperties of the alloy aremanifoldand highly dependent upon the phosphorus content and the plating conditions.Layers of various thicknesses find application as protective coatings to improvecorrosion resistance,wear resistance andhardness. The amorphous structure andhardness of the Ni–P deposit is used in making molds for optical applicationsand wheel bearings and valves in automotive industry. The presence of copper inthe deposited layer, i.e. the electro-lessNi–Cu–P coatings, exhibit better wettingproperty and higher thermal stability than the electro-less Ni–P deposit [22–25].Moreover, the addition of copper into the electro-less Ni–P matrix improves thecorrosion resistance of the coatings [26, 27]. Deposition by electro-less plating us-ing the after pretreatment anodization ofAl orAl alloys is considered as newly de-veloped process that needs further research to optimize the corrosion resistanceand to obtain brilliant finishing.

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 3

In this paper, the surface characteristics of Al, Al2014 and Al7075 were im-proved by the electro-less deposition ofNi–Cu–P layer on the anodized surface.The surface morphology of the specimens was examined by the scanning elec-tron microscope, SEM, and the composition of the surface layer was analyzed byx-ray energy dispersive technique, EDAX. The electrochemical characteristics ofthe electrode/electrolyte interface were investigated by polarization techniquesand electrochemical impedance spectroscopy, EIS. It was important to emphasizethe improvement of the alloy surface by the Ni–Cu–P deposit to benefit from itsrelatively low price and the extraordinary workability.

2 Experimental

2.1 Surface preparation and electro-less plating

Al, Al2014 andAl7075 specimens were cut as cylindrical electrodes of 0.12 cm2

surface areas to contact the test electrolyte. The mass spectrometric analysis ofthe specimens is presented in Table 1. Prior to each experiment the electrodewas abraded by successive grades emery papers down to 2000 girt, then rubbedagainst a soft cloth until it acquired amirror bright surface. The electrode wasthen washed with triple distilled water and transferred quickly to the electrolyticcell. The cell is an all glass double walled three electrode cell with Al or Al al-loy as working electrode and aPt spiral as counter electrode. The working elec-trode potential was measured against and referred to aHg/Hg

2SO4/Na2SO4ref-

erence electrode [𝐸o = 0.640 V vs the standard hydrogen electrode, SHE]. The an-odization process was carried out potentiostatically at 20 V for 30min in a stirred1.0MH

2SO4solution with a regulated DC power supply (Sci-tech RDC 3002T) at

25∘C. The anodizing conditions were chosen to produce porous oxide films [16].

The freshly anodized specimen was transferred to the electro-less plating bath,where the deposition process was carried out for 1 h at 90 ∘C. The plating bath isa hypophospite solution of the composition presented in Table 2, which includes

Table 1: The mass spectrometric analysis of the Al 2014 and Al 7075 samples.

Chemical composition (wt.%)

Sample Si Fe Cu Mn Mg Cr Zn Ti Al

Al2014 0.5 0.5 5.20 0.4 0.8 0.10 0.25 0.15 BalanceAl7075 0.4 0.5 1.20 0.3 2.1 0.18 5.10 0.20 Balance

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4 | W.A. Badawy et al.

Table 2: Plating bath composition and deposition parameters used forNi–Cu–P electro-lessdeposition.

Constituents of Concentration Deposition conditionsthe plating bath (g/L)

NiSO4.7H

2O 26.27 Parameters Value

NaH2PO

238.12 Bath temperature 90

∘C

Na3C6H

5O

7.H

2O 38.71 Deposition time 60 min

CH3COONa 27.2 Solution PH 9 ± 0.1

NH2CH

2COOH 21

CuSO4.5H

2O 0.015

also the deposition parameters. The specimens were then rinsed with triply dis-tilled water, dried, and then the different investigations were conducted.

2.2 Electrochemical measurements

2.2.1 Polarization tests

The potentiodynamic current-potential curves were recorded by changing theelectrode potential automatically from −800mV to +200mV. To achieve quasi-stationary condition a potential scan rate of 1mV s−1 was used. All polariza-tion measurements were accomplished with Autolab Galvanostat/Potentiostat,(GPES), connected to a computer. The polarization measurements were carriedout in naturally aerated stagnant 0.5M Na

2SO4solution at room temperature

(25 ± 1 ∘C). The corrosion parameters i.e. corrosion current density, 𝑖corr, corro-sion potential, 𝐸corr, and polarization resistance, 𝑅p, were evaluated from the in-tersection of the linear anodic and cathodic branches of the Tafel plots.

2.2.2 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy, EIS, is a non-destructive sensitivetechnique which enables the detection of any changes occurring at theelectrode/electrolyte interface. In our investigations, the impedance data werepresented in the Bode plot format. This format enables the whole impedancedata to be presented and the presence of the phase angle 𝜃, as a sensitive pa-rameter for any surface changes, as a function of the frequency is beneficial. TheEIS experiments were performed using the IM6d.AMOS system (Zahner Elektrik

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 5

GmbH & Co., Kronach, Germany). The input signal was usually 10mV peak topeak in the frequency domain 10−1–105 Hz. Before each experiment, the work-ing electrode was immersed in the test solution until a steady-state was reached,then the impedance data were recorded. The effect of immersion time on theimpedance characteristics of each electrode was also studied. The impedancedatawere fitted to theoretical data according to proposed equivalent circuitmodelusing a complex non-linear least squares (CNLS) circuit fitting software [28].

2.3 Surface characterization

The surfacemorphology of the different sampleswas investigated by the scanningelectron microscopy, SEM, using a JEOL-840 Electron probe micro-analyzer. Themicro-analyzer with accelerating voltage 30 kV enables energy diffraction x-rayanalysis of each specimen. Details of all experimental procedures are as describedelsewhere [29–31].

3 Results and discussionThe properties of the anodic films formed on the aluminum alloys are influencedby the alloying elements. For example, magnesium and manganese elements arebeneficial for aluminum anodization, while copper and silicon have harmful ef-fects, so the 2000 series aluminum alloys are hard to anodize [15]. The essentialalloying elements in the two alloys under investigation are copper inAl2014 andzinc in Al7075. It is well known that that the abraded non-anodized surfacesof the alloys are rough and contain cracks or flawed regions compared to pureAl [32]. Generally, the surfacemorphology plays an important role in the anodiza-tion process and also influences the electrochemical behavior of the metallic ma-terial [30, 32]. The surface morphology of the anodized Al, Al2014 and Al7075is presented in Figure 1a, b and c, respectively. For anodized Al, the anodizingprocess converts aluminum into aluminum oxide which possesses a unique or-dered geometrical structure with hexagonally arranged cells containing cylindri-cal pores in each cell-center (cf. Figure 1a) [16]. The anodized surfaces of the twoalloys i.e. Al2014 and Al7075 are more defective compared to the anodized Alsurface (cf. Figure 1). The SE micrograph of the anodized Al2014 surface (Fig-ure 1b) represents a porous surface with random distribution of pores with differ-ent size. Morphology differences between anodized aluminum and anodized alu-minum alloys can be attributed to the influence of the alloying elements. During

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6 | W.A. Badawy et al.

Figure 1: SE micrographs of anodized Al (a), anodized Al2014 (b), and anodized Al7075 (c).

anodization, the alloying element enriches the alloy surface and influences thealloy/anodic film interface [33, 34]. In most cases a disordered film with porousstructure is formed. The nature of the film disorder depends on the anodizationconditions e.g. anodizing bath composition and temperature. The presence ofpores leads to poor corrosion resistance and hence surface instability of the an-odized alloys. A smooth compact anodic film leads to high corrosion resistanceand stability of metallic material. Such surfaces are not accessible for coatings.On the other hand, rough anodized surfaces are accessible for coatings, whichimprove the surface characteristics of the anodic film, especially its stability incorrosive solutions. These findings can be confirmed by the electrochemical in-vestigations. In this respect, potentiodynamicpolarizationmeasurements andEISwere used.

In the polarization experiments, anodized Al, Al2014 and Al7075 free andafter electro-less deposition of Ni–Cu–P coatings were investigated. Typicalpotentiodynamic polarization curves of these measurements are presented in

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 7

Figure 2: Potentiodynamic polarization curves of anodized Al and Ni–Cu–Pmodified anodizedAl (a), anodized Al2014 andNi–Cu–Pmodified anodized Al2014 (b), and anodized Al7075 andNi–Cu–Pmodified Al7075 (c) measured in naturally aerated stagnant 0.5 M Na

2SO

4aqueous

solution at 25 ∘C and scan rate 1 mV s−1.

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8 | W.A. Badawy et al.

Figure 2: Continued.

Figure 2 (a, b and c) for the three electrodes, respectively. The values of the corro-sion parameters are presented in Table 3. These values show clearly that the cor-rosion potential of pure Al was shifted to more negative value and the corrosionresistance was decreased. On the other hand, the surface modified anodized al-

Table 3: Corrosion parameters obtained from the polarization curves of anodized and Ni–Cu–Pmodified anodized Al and Al-alloys in stagnant naturally aerated 0.5 M Na

2SO

4solution at

25∘C.

Type of film 𝑖corr 𝐸corr 𝑅𝑝

𝑏𝑎

𝑏c Corrosion rateμA/cm

2mV Ohm mV/dec mV/dec mm/year

Anodized Al 4.19 −508 476 124 59 4.5 × 10−3

Ni–Cu–P/ 4.58 −544 450 29 26 5.0 × 10−3

Anodized Al

Anodized Al2014 1.36 −407 548 15 18 1.5 × 10−3

Ni–Cu–P/ 7.85 −301 460 37 36 8.6 × 10−3

Anodized Al2014

Anodized Al7075 3.18 −605 302 24 15 3.5 × 10−3

Ni–Cu–P/ 0.499 −577 330 26 24 0.5 × 10−3

Anodized Al7075

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 9

loys show apositive shift in the corrosion potential. The corrosion resistance andthe corrosion rate of the surface modified alloys were found to be dependent onthe alloying element. The corrosion rate of Al7075 was decreased by about oneorder of magnitude, whereas the corrosion rate ofAl2014was increased by aboutthe same order of magnitude. This means that the morphology of the surface andthe alloying elements affect the Ni–Cu–P coatings, which depends on three fac-tors [35]:1. The amorphous nature of the surface.2. The extent of internal stresses.3. The extent of phosphorous in the deposited film.

In general, the corrosion resistance of a surface film depends on the formationspeed of the film and its protective nature. Some alloying elements can decreasethe corrosion current by promoting anodic and/or cathodic reactions during thecorrosion process [36]. Copper improves the corrosion resistance ofNi–P coatingsprovided that the coatings thickness has reached a certain range [37]

To confirm the potentiostatic polarization results, EIS investigations of allspecimens were carried out. For comparison between anodized Al and Al-alloysand the Ni–Cu–P modified anodized electrodes, the impedance spectra were

Figure 3: Bode plots of anodized Al, Al2014 and Al7075 after 3 h immersion in naturally aeratedstagnant 0.5 M Na

2SO

4aqueous solution at 25 ∘C.

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10 | W.A. Badawy et al.

Figure 4: Bode plots of Ni–Cu–Pmodified anodized Al, Al2014 and Al7075 after 3 h immersionin naturally aerated stagnant 0.5 M Na

2SO

4aqueous solution at 25 ∘C.

recorded after 3 h of electrode immersion in the electrolyte and presented as Bodeplots in Figure 3 for the anodized electrodes and in Figure 4 for the Ni–Cu–Pmodified electrodes under the same conditions. In general, anodization of Alor Al-alloys in sulfuric acid produces anodic films of duplex nature, which con-sist of a thin non-porous compact layer known as the barrier layer and an outerporous layer [16, 29]. The presence of porous structures, surface inhomogeneityand roughness factors leads to frequency dispersion and enables formation ofnano-Ni–Cu–P deposits. The advantage of EIS is that it shows relaxation phe-nomena over awide frequency range. The Bode plots of Figures 3 and 4 show oneclear phasemaximumat high frequency and an indication of another phase max-imum at lower frequency. This means that the electrode/electrolyte interface iscontrolled by two time constants. The impedance data were fitted to theoreticaldata according to the equivalent circuit model presented in Figure 5 in which twoparallel combination terms are used, the first is the charge transfer resistance,𝑅ct, and the double layer capacitance,𝐶dl, and the second is the barrier film resis-tance,𝑅b, and the barrier film capacitance,𝐶b, in series to the solution resistance,𝑅s [38]. The experimental impedance data are consistent with the data obtainedaccording to the proposed model. The impedance behavior of the systems underinvestigation can be represented by the dispersion formula, which is given by the

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 11

Figure 5: Equivalent circuit model used for fitting of impedance data of anodized and modifiedanodized Al, Al2014 and Al7075. 𝑅s = solution resistance, 𝑅ct = charge transfer resistance,𝐶dl =double layer capacitance, 𝑅b =barrier film resistance and 𝐶b =barrier film capacitance.

following equation:

𝑍 = 𝑅s + [𝑅ct/{1 + (2𝜋𝑓𝑅ct𝐶dl)𝛼}] (1)

where𝑍 is the total impedance and𝛼 is an empirical parameter (0 < 𝛼 < 1) and f isthe frequency in Hz. The above relation takes into account the deviation from theideal RC-behavior in terms of distribution of time constants due to surface inho-mogeneity, roughness effects and variations in properties or compositions of sur-face film [39, 40]. The calculated impedance parameters of the modified anodizedmaterials after 3.0 h immersion in stagnant naturally aerated 0.5M Na

2SO4at

25∘C according to the fitting procedure are presented in Table 4. These values

show clearly that the corrosion resistance of the modified anodized Al surface ismore than 10 times that of the modified anodized alloys. This can be attributedto the galvanic interactions between the micro-structural constituents of the al-loy [41, 42]. Corrosion resistance of the anodized films of duplex nature is repre-sented by the sum of the resistances of the inner and outer layers and also thesolution resistance. In most cases the barrier film resistance is much larger thanthe others and hence it controls the corrosion process.

The impedance data of the Ni–Cu–P modified anodized alloy elec-trodes show that the phase angle has two maxima, which means that the

Table 4: Equivalent circuit parameters after 3 h of immersion of the modified anodized Al andAl-alloys in 0.5 M Na

2SO

4at 25 ∘C.

Modified anodized 𝑅s/ 𝐶b/ 𝛼1

𝑅b/ 𝐶p/ 𝛼2

𝑅p/

sample Ω μF cm−2

Ωcm2μF cm

−2Ωcm

2

Al 62.6 52.5 0.85 72.0 8.8 0.99 318Al2014 75 47.5 0.61 42.2 82.5 0.71 31Al7075 101 39.7 0.79 24.2 40.8 0.58 300

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12 | W.A. Badawy et al.

Table 5: Equivalent circuit parameters at different time intervals of immersion in stagnantnaturally aerated 0.5M Na

2SO

4at 25 ∘C for:

(a) Ni–Cu–P/ anodized Al(a) 𝑅s/ 𝐶b/ 𝛼

1𝑅b/ 𝐶p/ 𝛼

2𝑅p/

Time/min Ω μF cm−2

Ω cm2μF cm

−2kΩ cm

2

5 62.0 64.2 0.81 63.8 35.8 0.87 0.9130 62.0 56.7 0.79 80 20.0 0.99 1.2660 62.5 54.2 0.84 78.1 18.9 0.99 1.90120 61.7 51.6 0.81 66.5 12.5 0.99 2.50180 62.6 52.5 0.85 72.0 8.8 0.99 3.20(b) Ni–Cu–P/ anodized Al2014(b) 𝑅s/ 𝐶b/ 𝛼

1𝑅b/ 𝐶p/ 𝛼

2𝑅p/

Time/min Ω μF cm−2

Ω cm2μF cm

−2kΩ cm

2

5 76.5 51.7 0.75 33.4 150 0.59 0.1230 77 50.8 0.59 29.4 125 0.74 0.3760 79 56.7 0.61 40.3 84.2 0.72 0.37120 75 53.3 0.61 41.9 82.5 0.71 0.32180 75 47.5 0.61 42.2 82.5 0.71 0.31(c) Ni–Cu–P/ anodized Al7075(c) 𝑅s/ 𝐶b/ 𝛼

1𝑅b/ 𝐶p/ 𝛼

2𝑅p/

Time/min Ω μF cm−2

Ω cm2μF cm

−2kΩ cm

2

5 93 50.8 0.95 14.3 91.7 0.68 0.2030 91 55.8 0.93 11.6 66.7 0.58 0.1760 95 54.1 0.81 18.2 53.3 0.53 0.18120 101 48.3 0.81 15.1 39.2 0.50 0.2180 101 39.7 0.79 24.2 40.8 0.58 0.3

electrode/electrolyte interface is controlled by two time constants. For modifiedanodized Al, the single phase maximum means that the porous layer proper-ties has high conductivity due to the inclusion of the electrolytic solution insidethe pores [43]. The capacitance of the modified anodized alloys is higher thanthose recorded for modified anodized Al. This indicates the presence of thickerand rougher porous layer on the modified anodized alloy surfaces. The calcu-lated values of the empirical parameter 𝛼, for the modified anodized Al elec-trode approach 1 indicating that the film is homogeneous and behaves like idealcapacitor. On the other hand, all 𝛼 values of the modified anodized alloys devi-ate from 1, which means that the formed film is defective and deviates from idealRC behavior.

The obtained impedance values are in good agreement with those calculatedfrompolarizationmeasurements. The values obtained for the corrosion resistancefrom both impedance and polarization measurements for Al2014 show that thedeposition ofNi–Cu–P did not improve the surface protection, whereas themod-

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 13

Figure 6: SE micrograph of Ni–Cu–Pmodified anodized Al surface (a) and EDAX analysis ofNi–Cu–Pmodified anodized Al electrode (b).

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14 | W.A. Badawy et al.

Table 6: The composition of theNi–Cu–Pmodified anodized Al after 3 h of electrode immersionin 0.5 M Na

2SO

4at 25 ∘C as obtained from EDAX analysis.

CompositionNi K P K Cu K

Atomic % 72.99 25.52 1.48Weight % 82.89 15.29 1.82

ified anodized Al7075 was improved. This difference can be attributed to the al-loying elements, such as Zn which is used for activating the coating on the sur-face [21].

To investigate the stability of the deposited film, the effect of immersion timeon the different Ni–Cu–P modified anodized samples in sulfate solution wasstudied by EIS. The values of the fitting parameters calculated at different timeintervals from electrode immersion in the electrolyte according to the equivalentcircuitmodel of Figure 5 arepresented inTable 5. It is clear that the corrosion resis-tance of the anodized surface increases with increasing the immersion time. Thiscanbe attributed to an improvement in the protective properties of the barrier filmwith ageing [4].

The difference in behavior betweenAl,Al2014 andAl7075 can be attributedto the difference in the surface morphology presented in Figure 1. While the SEmicrograph of modified anodized Al is homogeneous and smooth, the anodizedalloy surfaces are porous and contain voids and flawed regions (cf. Figure 1). Ineither case the deposition of the nano-Ni–Cu–P deposit takes place successfully.A compact defect–freeNi–Cu–P deposit on anodizedAlwithout distinct cavitiesand crevices, was recorded and presented in the SE micrograph of Figure 6a. Theappearance of cauliflower–like nodule is typical of amorphousmaterials [44]. Thecomposition of surface film was confirmed by EDAX and presented in Figure 6b.The EDAX data analysis is presented in Table 6.

Figure 6b and Table 6 indicate that the anodic film is completely covered byNi, P andCu and there is no appearance for anyAl on the surface. The presenceof copper in the plating bath enhances the full surface coverage byNi–Cu–P de-posit. The good dispersion of Ni–Cu–P deposit over the anodized surfaces mayaccount for its high corrosion resistance.

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Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 15

4 Conclusions1. The corrosion resistance of the Ni–Cu–P modified anodized Al or Al-alloy

surfaces increases with ageing in sulfate solutions.2. Modified anodizedAl surface behaves like ideal RC, whereas themodified an-

odized alloy surfaces are defective anddeviate from ideal capacitivebehavior.3. The presence of copper in the plating bath resulted in the formation of more

dispersed film.4. The improved surfaces of theNi–Cu–P alloys suggest their application as cor-

rosion resistant materials.

Acknowledgement: The authors are grateful to the AvH foundation and the CairoUniversity for providing the electrochemical work station and continuous sup-port.

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