Ni–P and Ni–Co–P coated aluminum alloy 5251 substrates as metallic bipolar plates for PEM fuel cell applications

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NieP and NieCoeP coated aluminum alloy 5251 substrates as metallic bipolar plates for PEM fuel cell applications Amani E. Fetohi a, R.M. Abdel Hameed b,*, K.M. El-Khatib a, Eglal R. Souaya c a

Chemical Engineering & Pilot Plant Department, National Research Center, Dokki, Giza, Egypt Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt c Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt b

article info

abstract

Article history:

This study is aimed to replace graphite bipolar plates in PEM fuel cells with surface

Received 7 December 2011

modified aluminum alloy. To improve the surface characteristics of aluminum alloy 5251

Received in revised form

(AA5251) substrate, NieP and NieCoeP coatings were deposited using electroless and

24 January 2012

electroplating deposition techniques [power supply and chronoamperometry]. Surface

Accepted 29 January 2012

morphology and chemical composition of prepared coatings have been investigated using

Available online 3 March 2012

scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques. The

Keywords:

H2SO4 þ 2 ppm HF) solution by potentiodynamic polarization technique. Lower corrosion

Bipolar plate

current densities and more positive corrosion potentials were gained after coating AA5251

corrosion behaviour of NieP and NieCoeP coated AA5251 was studied in (0.5 M

Polymer electrolyte membrane fuel

with NieP and NieCoeP deposits. Much better corrosion resistance was shown by coat-

cell

ings containing cobalt. Potentiostatic tests were carried out at þ160 mV (MMS) in air-

NieP

saturated solution to simulate cathode environment in PEM fuel cells. The current

NieCoeP

density of NieCoeP (1:1)/AA5251 was stabilized at a value lowered by 4 times relative to

Interfacial Contact resistance

that at bare AA5251 substrate. Interfacial contact resistance values between coated substrates and carbon paper were measured. NieP and NieCoeP coatings prepared by electroless method showed ICR values, twice that at ones prepared by electroplating power supply technique. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, renewable energy has become an increasingly important issue in terms of mitigating global warming and preventing the depletion of fossil fuels. There is a great potential to apply fuel cells for the generation of electricity. As an electrochemical device that converts chemical energy into electric power by means of catalytic electrochemical reaction, the main advantages of fuel cells are relatively simple mechanisms, silent operation, high efficiency and flexible scaling

between power and capacity [1]. Out of various fuel cells currently under development, direct methanol fuel cell (DMFC) is one of the most promising candidates for use as a substitute power source in portable applications, such as cellular phones and laptops [2]. In a typical PEMFC, the key components include a proton exchange membrane, catalyst, gas diffusion layer and bipolar plate. Bipolar plates constitute the backbone of a hydrogen fuel cell power stack. They conduct current between cells, facilitate water and thermal management,

* Corresponding author. Tel.: þ20 1145565646; fax: þ20 235727556. E-mail address: [email protected] (R.M. Abdel Hameed). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.145

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distribute the fuel and oxidant within the cell and provide conduits for reactant gases; namely: hydrogen and oxygen. In polymer electrolyte membrane (PEM) hydrogen fuel cell design, bipolar plates are fabricated in mass production and they must be made of materials with excellent manufacturability and suitable for cost-effective high volume automated production systems [3]. To maintain the performance and stability of PEMFC, it is important that bipolar plates possess high electrical and thermal conductivity, sufficient compressive strength, low density and excellent corrosion resistance [4e6]. The present bipolar plates are mainly composed of very thick and heavy machined graphite blocks [7]. They weigh 70e80% and require 40e45% of manufacturing cost of the whole PEMFC system [8,9]. Accordingly, to get this technology commercially viable, carbon composites and metal plates have been studied as alternative bipolar plate materials [10,11]. Metals have high thermal conductivity, are recyclable and can be easily and consistently shaped to accommodate the flow channels [12]. The thin nature of metal substrates allows for a smaller stack design with reduced weight. Aluminum, stainless steel, titanium and NieCr alloy have been examined as possible metallic bipolar plate materials [13e20]. Various schemes have been investigated for protecting metallic bipolar plates, most of which rely on a thin and inert yet electrically conductive coating [9,18,21e25]. Non-precious coatings for metals are effective for minimal permeability to gases combined with safer, more economical hydrogen consumption and easier to manufacture by the stamping process than graphite [3,26,27]. Some commercial electroless Niebased deposits, such as NieP and NieB alloys, are extensively employed as coatings for applications in electronic or chemical production due to their superior corrosion resistance and electric conductivity [28,29]. DMFC performance test demonstrated that bipolar plates coated with NieP deposits got a lower bulk resistance and an enhanced cell performance related to commercially available plates [30]. The laser treatment of NieP coatings on mild steel [31] and Al-356 aluminum alloy [32] showed a lower corrosion and pitting potential in 3.5% NaCl solution at 23  C. The electroless deposition of NieCueP layer on mild steel decreases its corrosion current and shifts its potential in the positive direction by 553 mV [33]. XPS analysis proved the presence of Cu-enriched areas as a result of NieCueP corrosion tests in 5% NaCl. They hinder the coating decomposition compared to NieP one. The addition of cobalt to NieP coating on Al6061 alloy improves its corrosion characteristics [34]. Higher corrosion resistance values are achieved at NieCoeB coatings over carbon steel, Al6061 alloy and 304 stainless steel compared to uncoated alloys; about two orders of magnitude with respect to carbon steel and an order of magnitude compared to 304 stainless steel [35]. In this study, AA5251 was coated with NieP and NieCoeP using electroless and electroplating techniques. The surface morphology and chemical composition of these formed coatings were investigated by SEM and EDX analyses, respectively. Their corrosion behaviour was examined using Tafel plots and potentiostatic polarization tests. The interfacial contact resistance of these coatings on AA5251 was also measured at different compaction force values.

Table 1 e Chemical composition of AA5251. Element Wt. %

Al

Zn

Mg

Mn

Cu

Fe

Si

Ti

Cr

95.6 0.15 1.7e2.4 0.1e0.5 0.15 0.50 0.40 0.15 0.15

2.

Experimental

2.1.

Electrode preparation

AA5251 with the chemical composition presented in Table 1 was cut in the form of rod and then mounted into glass tube of appropriate diameter with epoxy resin. The exposed surface of AA5251 was a disk shaped with apparent area of 0.1 cm2. It was pretreated by mechanical polishing with successive grades of metallurgical papers down to 1200 grit, then rubbing with a soft cloth, washing with triply distilled water and finally rinsing with acetone.

2.2.

Zincating process

AA5251 was initially cleaned in acetone for 30 s, followed by acid stripping in 1 M H2SO4 solution for 10 s. The zincating process was repeated twice, each one for 20 s. It was conducted in a NaOH and ZnO mixed solution. Its composition was fixed as (200 g NaOH þ 10 g ZnO) in 1 L H2O. Between the two zincating processes, AA5251 was cleaned in deionized water and 1 M H2SO4 solution, with each one for 10 s.

2.3.

Coating process

The bath composition of plating solutions is summarized in Table 2. Sodium citrate was used as a complexing ligand to control the rate of release of free metal ions for the reduction reaction; sodium hypophosphite was used as a reducing agent and also as a source of phosphorous. Nickel sulfate and cobalt sulfate were used as a source of nickel and cobalt, respectively. Cobalt sulfate is added to the plating bath in the desired ratio, while keeping all other components in constant amount. NieP and NieCoeP coatings were deposited on the substrate through electroless and electroplating techniques. Electroless plating was performed by immersing zincated

Table 2 e Composition of plating baths of NieP and NieCoeP coatings on AA5251. Composition

NieP

1

NiSO4$6H2O/g l CoSO4$6H2O/g l1 NaH2PO2$H2O/g l1 C6H5Na3O7$2H2O/g l1 Temperature/ C pH

30 e 20 20 70e80 5.7

NieCoeP Ni:Co

Ni:Co

1:0.2

1:1

30 6 20 20 85e90 5.6

30 30 20 20 85e90 5.4

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electrode, while the counter electrode is Pt wire. On the other hand, a third electrode as the reference one [Hg/Hg2SO4/1 M H2SO4 (MMS) electrode] was involved in a three-electrodes cell to form coated AA5251 substrates by chronoamperometry technique using a potentiostat. During the deposition of NieCoeP coatings, nickel to cobalt molar ratio in the deposition solution was varied as 1:0.2 and 1:1, respectively. The corresponding coated substrates are designated as NieCoeP (1:0.2)/AA5251 and NieCoeP (1:1)/AA5251, respectively. A scheme of the different steps of AA5251 modification with NieP and NieCoeP coatings is represented in Fig. 1.

Aluminum substrate

Mechanical polishing

Rinsing with acetone

Cleaning in 1 M H2SO4 solution

2.4.

1st zincating process

Stripping in 1 M H2SO4 solution

2nd zincating process

2.5. Electroless plating

Physical characterization of the prepared coatings

The surface morphology of bare AA5251 and coated ones with NieP and NieCoeP deposits was examined using the scanning electron microscopy. The surface changes after potentiostatic polarization experiments were also scanned. For this purpose, the scanning electron microscope “JXA-840A, Electron Prob Microanalyzer, JEOL, Japan” was used. It is equipped with EDX analysis “INCA X-sight, OXFORD instruments, England” unit, which is applied for determination of chemical composition of the coatings.

Cleaning in deionized water

Electroplating by power supply

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Electroplating by chronoamperometry

Fig. 1 e Scheme of the different steps of AA5251 modification with NieP and NieCoeP coatings.

AA5251 substrate in the deposition bath for 20 min at 70e90  C. Electroplating has been carried out at room temperature at 6 V for 7 min by two instruments; power supply and chronoamperometry using potentiostat. For electroplating using power supply, the substrate is coated through two-electrodes system; namely: working and counter electrodes. Zincated AA5251 substrate represents the working

Corrosion measurements

Potentiodynamic polarization technique was applied to study the general corrosion resistance parameters of bare AA5251 and as-deposited NieP and NieCoeP coatings. It was performed using Voltalab6 Potentiostat driven by a PC for data processing. The conventional three-electrodes cell [mentioned above] is used. Corrosion measurements were tested in (0.5 M H2SO4 þ 2 ppm HF) solution at room temperature without any gas purging. The working electrode was immersed in this solution and left until the steady-state open circuit potential was attained. It was taken as the free corrosion potential (Ei¼0). Thereafter, Tafel measurements were recorded at a scan rate of 1 mV s1, starting from ‒250 mV with respect to Ei¼0 up to 1000 mV versus mercury sulfate reference electrode.

Fig. 2 e Schematic illustration of test assembly for interfacial contact resistance measurements with one GDL (a) and with combined sample and GDLs (b).

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The stability of the formed coatings under operation conditions of PEM fuel cells was examined using potentiostatic polarization measurements. The working electrode was set at open circuit potential value for 5 min, followed by applying a constant potential value as þ600 mV (SCE) [16] [which is equivalent to þ160 mV (MMS)] in air-saturated solution to simulate the cathode environment of PEMFCs. The corresponding currentetime curves were recorded for 5 h. A freshly electrode surface and a fresh batch of solutions were used for each experiment.

2.6.

Interfacial contact resistance measurements

In order to measure the interfacial contact resistance values between coated aluminum alloy samples and carbon paper, the method developed by Wang et al. [36] had been followed. The used setup is shown in Fig. 2. It consists of two pieces of conductive carbon papers sandwiched between the coated aluminum alloy sample and two gold-plated copper plates with a stem having a surface area of 1.0 cm2. A constant electric current value of 1.0 A was passed through the whole assembly. The corresponding potential difference was registered across the cell as the compaction force was gradually increased. Therefore, the surface resistivity of measured samples can be calculated based on Ohm’s law [R ¼ V/I]. Afterwards, ICR values can be determined using the following relation: ICR ¼ A  R  Rcp




2

(1)

where Rcp is the resistivity contribution by the carbon paper/ gold-plated copper plate interface, R is the resistivity contribution by the carbon paper/gold-plated copper plate interface/ coated aluminum alloy sample and A is the sample surface area.

3.

Results and discussion

3.1.

SEM and EDX analyses of coated AA5251 samples

Fig. 3 shows the scanning electron micrographs of NieCoeP (1:1)/AA5251 coating formed by different deposition techniques. Different surface morphologies were observed. Cauliflower-like nodules are homogeneously distributed at a net structure with large vacancies for the coating formed by electroless plating method [see Fig. 3a]. On the other hand, NieCoeP deposit, formed by electroplating power supply technique in Fig. 3b, showed small-sized spherical crystals that highly accumulated in form of little groups in a perpendicular direction to the substrate surface. A compact coating layer is formed by electroplating chronoamperometry method. Highly fine grains are randomly distributed in Fig. 3c. Surface morphologies of NieP and NieCoeP (1:0.2) coatings, formed on AA5251 by electroplating chronoamperometry method, were also scanned as shown in Fig. 4. A laminar structure with sheets in different sizes is the characteristic feature of NieP deposit [see Fig. 4a], while NieCoeP (1:0.2) coating in Fig. 4b has a rigid surface with linear groves containing fine pores.

Fig. 3 e Scanning electron micrographs of AA5251 coated with NieCoeP (1:1) deposit formed by (a) electroless and electroplating [(b) power supply and (c) chronoamperometry] techniques.

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Fig. 4 e Scanning electron micrographs of AA5251 coated with (a) NieP and (b) NieCoeP (1:0.2) deposits formed by electroplating chronoamperometry technique.

Energy dispersive X-ray spectra of NieCoeP (1:1)/AA5251, formed by different deposition techniques, are shown in Fig. 5-1. The corresponding weight and atomic percentages of the chemical components of each coating are listed in Table 3. Three major elements are deposited; namely: phosphorus, nickel and cobalt. Aluminum as the substrate was also detected, besides sulfur and sodium were measured in small amounts as a result of aluminum alloy pretreatment. The phosphorus amount in NieCoeP (1:1) coating is varied based on the formation method. It increases in the order: electroplating power supply (5.22%) < electroplating chronoamperometry (12.19%) < electroless (14.48%) deposition techniques. This low phosphorus percentage in NieCoeP coating, formed by electroplating power supply method, may account for its crystalline morphology [37] as noticed from its SEM image in Fig. 3b. On the other hand, the amorphous structure with fine crystals that was observed at the surface of NieCoeP deposit, formed by electroplating chronoamperometry technique [see Fig. 3c], is a result of its moderate P content [38]. High phosphorus coating, formed by electroless method, has a typical amorphous surface as shown in Fig. 3a. Although Ni:Co molar ratio is 1:1 in the deposition solution, EDX results proved that NieCoeP deposit, formed by electroplating chronoamperometry technique, is the only one with Ni:Co atomic ratio of nearly 1:1. On the other side, applying electroplating power supply technique results in the formation of NieCoeP deposit with higher amount of Co relative to Ni (Ni:Co ¼ 1:2.86). On contrary, a trace amount of cobalt was found in NieCoeP coating formed by electroless deposition method. The chemical composition of NieP and NieCoeP (1:0.2) deposits, formed by electroplating chronoamperometry technique, was also stated in Table 4. Their corresponding EDX spectra were represented in Fig. 5-2. In absence of Co in the deposition bath, NieP coating was formed with less amount of P (6.77%). This phosphorus percentage was altered by varying cobalt content while depositing NieCoeP coating on AA5251. This is clear in Fig. 6, where maximum P% (20.35%) is attained when Ni:Co molar ratio is 1:0.2 in the deposition solution. On increasing cobalt ratio in NieCoeP (1:1) coating, the phosphorus content decreases to 12.19%. This is in a good agreement with earlier work of Huang et al. [39]. He observed that increasing cobalt sulfate concentration in the plating bath of NieCoeP ternary alloy on glass

Table 3 e Chemical composition of AA5251 coated with NieCoeP (1:1) deposit (according to EDX analysis). The coating was formed by different techniques. Element

Deposition technique Electroplating By chronoamperometry

Na K Al K PK SK Co K Ni K

By power supply

Electroless plating

Weight%

Atomic%

Weight%

Atomic%

Weight%

Atomic%

19.39 3.31 12.19 10.35 26.95 27.81

32.28 4.69 15.06 12.35 17.50 18.12

11.00 0.75 5.22 2.49 59.74 20.80

22.56 1.31 7.95 3.66 47.81 16.71

8.96 1.82 14.48 1.94 1.69 71.11

17.52 3.04 21.02 2.70 1.29 54.43

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Table 4 e Chemical composition of AA5251 coated with NieP and NieCoeP (1:0.2) deposits (according to EDX analysis). The coatings were formed by electroplating chronoamperometry technique. Element

Na K Al K PK SK Co K Ni K Zn K

NieP

NieCoeP (1:0.2)

Weight%

Atomic%

Weight%

Atomic%

22.48 39.33 6.77 7.42 _ 21.62 2.38

30.02 44.76 6.71 7.10 _ 10.33 1.08

26.92 11.26 20.35 8.04 6.68 26.75 _

38.20 13.62 21.43 8.18 3.70 14.87 _

Fig. 6 e Variation of the weight percentages of Co and P in NieP and NieCoeP coatings formed on AA5251 by electroplating chronoamperometry technique.

fibers would decrease the deposited phosphorus amount. Moreover, the addition of copper during NieP [40] and NieWeP [41] coatings formation results in a marginal decrease of P percentage. Although Al is the substrate, its weight percentage is varied according to the coating type. It decreases in the order: NieP [39.33%] > NieCoeP (1:0.2) [11.26%] > NieCoeP (1:1) [3.31%]. This observation could be supported by SEM images of NieP and NieCoeP coatings in Figs. 3 and 4. The linear groves and fine pores that distribute along NieCoeP (1:0.2) coating surface would result in a thinner layer in relation to the compact one formed at NieCoeP (1:1) deposit surface. To actually measure NieP and NieCoeP (1:1) coatings thicknesses, cross-sectional views of their samples were presented in Fig. 7. Two-layered structure was formed at AA5251 substrate. It consists of the zincating layer that was adjacent to the aluminum alloy substrate, above which NieP or NieCoeP coatings lie. The thickness of the zincating layer varies as 43.4e59.9 mm, while that of NieCoeP (1:1) coating is about 5.75 times thicker than that of NieP [it has an average of 15.9 mm for NieP deposit, compared to 91.5 mm for NieCoeP (1:1) one].

3.2.

Fig. 5 e (1) EDX spectra of AA5251 coated with NieCoeP (1:1) formed by (a) electroless and electroplating [(b) power supply and (c) chronoamperometry] techniques. (2) EDX spectra of AA5251 coated with electroplating chronoamperometry technique with (a) NieP and (b) NieCoeP (1:0.2) coatings.

Corrosion behaviour of coated AA5251 samples

Fig. 8a shows the potentiodynamic polarization curves of AA5251 coated with NieP and NieCoeP deposits [Ni:Co molar ratios are of 1:0.2 and 1:1] in 0.5 M H2SO4 solution containing 2 ppm HF. These coatings were formed by electroless deposition technique for 20 min. They were compared with the polarization curve of bare AA5251. An extended passivation range [from 50 to þ1000 mV (MMS)] is observed at uncoated AA5251. After coating with NieP, an anodic current peak appeared at a potential value of þ140 mV. The addition of cobalt to NieP [in Ni:Co molar ratio of 1:0.2] on AA5251 shifts this peak potential in the positive direction by 8 mV. It is preceded by a short passive-like zone. The coating technique was found to affect the shape of potentiodynamic polarization curve of the same deposit at AA5251 surface as shown in Fig. 8b. It represents those of aluminum alloy coated with NieP and NieCoeP deposits using the electroplating power

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Fig. 7 e Cross-sectional scanning electron micrographs of (a) NieP and (b) NieCoeP (1:1) coatings formed on AA5251 substrate by electroplating chronoamperometry technique.

supply technique for 7 min. Straightforward anodic branches were observed. On the other hand, the corresponding polarization curves of deposits formed by electroplating chronoamperometry technique for 7 min are in Fig. 8c. The anodic peak was absent at NieCoeP (1:0.2)/AA5251 in spite of the predominance of the passive-like behaviour. The electrochemical corrosion parameters derived from the polarization curves for AA5251 before and after coating with NieP and NieCoeP deposits are listed in Table 5. The values of the corrosion potential (Ecorr) and corrosion current density (Icorr) were estimated from the intersection of linear portions of anodic and cathodic curves of Tafel plot. They reflect how the substrate resists corrosion in the studied solution. As its corrosion potential gets more positive value, it is less corroded. The reported Ecorr values of different coated

Fig. 8 e Potentiodynamic polarization curves of bare and coated AA5251 with NieP and NieCoeP deposits in (0.5 M H2 SO4 D 2 ppm HF) solution. Coatings were formed by electroless deposition technique for 20 min (a) and by electroplating method for 7 min using power supply (b) and chronoamperometry (c).

AA5251 substrates as shown in Table 5 are shifted in the positive direction with respect to that of bare AA5251 [Ecorr ¼ 1112.6 mV at bare AA5251], regardless of the coating type and how it was formed. Moreover, the addition of cobalt to NieP coating on AA5251 [in Ni:Co molar ratio of 1:0.2] shifts

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Table 5 e Corrosion parameters of AA5251 coated with NieP and NieCoeP [in Ni:Co molar ratios of 1:0.2 and 1:1] deposits. These coatings were formed by electroless (a) and electroplating [power supply (b) and chronoamperometry (c)] techniques. The corresponding values of bare AA5251 substrate are inserted in section (a). The related current density values of all formed coatings at the potential of anode condition [L540 mV (MMS)] and that of cathode condition [D160 mV (MMS)] as derived from potentiodynamic polarization curves are listed in section (d). Icorr/mA cm2

Ei¼0/mV

ba/mV

bc/mV

Rp/U cm2

(a) Bare AA5251 NieP NieCoeP(Ni:Co 1:0.2) NieCoeP(Ni:Co 1:1)

78.0 18.5 14.5 8.96

1112.6 652.7 649.1 692.6

174.5 110.3 89.5 211.4

91.7 65.7 58.9 73.3

306.24 1090 1130 2540

759.0 172.2 127.8 110.6

(b) NieP NieCoeP(Ni:Co 1:0.2) NieCoeP(Ni:Co 1:1)

63 35.7 22

785 468.4 637.8

86.0 208.9 120.6

73.2 53.9 200.3

146.8 398.28 4420

208.9 481.2 61.84

597.1 619.3 619.1

224.3 247.7 252.1

150.5 139.3 140.2

Coating

(c) NieP NieCoeP(Ni:Co 1:0.2) NieCoeP(Ni:Co 1:1)

1.2 0.732 0.644

CR/mpy

38190 39260 40160

9.284 9.311 9.323

I/A cm2 Anode condition [at 540 mV (MMS)]

(d) NieP NieCoeP(Ni:Co 1:0.2) NieCoeP(Ni:Co 1:1)

Electroless

Power supply

Chronoamperometry

Electroless

Power supply

Chronoamperometry

9.34  105 1.21  104 3.21  105

1.13  104 3.36  104 4.06  106

1.13  107 1.32  106 1.20  106

1.55  102 7.11  103 6.40  104

4.01  104 3.54  104 4.92  104

4.18  106 3.10  105 2.09  105

its potential in the positive direction by 316.6 mV for the deposit formed by electroplating power supply technique. The deposition technique affects the corrosion behaviour of the coated substrate. This is clear from Ecorr values of NieCoeP (1:1)/AA5251 coating formed by different methods. It is arranged in the order: electroless (692.6 mV) < electroplating power supply (637.8 mV) < electroplating chronoamperometry (619.1 mV) techniques, implying that NieCoeP (1:1) coating, formed by the latter method, is the least corroded one. The corrosion current density (Icorr) expresses how fast the studied substrate degrades. Better corrosion resistance is observed for materials with lower corrosion current density values [42]. According to the results in Table 5, reduced current density values are shown at AA5251 after its coating with NieP and NieCoeP deposits [Icorr ¼ 78 mA cm2 at bare AA5251]. The presence of cobalt in NieCoeP coatings lowers its corrosion current density compared to those at NieP ones, irrespective of the coating technique. A further decrease in Icorr value was observed by 1.62 times when cobalt content in NieCoeP coatings is raised from Ni:Co molar ratio as 1:0.2 to 1:1 for those formed by electroless and electroplating power supply techniques, suggesting a better corrosion resistance behaviour. On the other hand, the highest corrosion resistance was achieved at AA5251 coated by electroplating chronoamperometry technique as reflected from its lowest Icorr values. The polarization resistance (Rp) can be calculated using the ¨ zyilmaz et al. [43] as follows: StearneGeary equation with O Rp ¼ b=Icorr

Cathode condition [at þ160 mV (MMS)]

(2)

where b is a constant value that can be estimated from the following equation: b ¼ ba bc =2:3ðba þ bc Þ

(3)

ba and bc as the anodic and cathodic Tafel slopes, respectively can be determined by fitting the theoretical polarization curve to the experimental one. When ba is higher than bc, the studied substrate tends to passivate, while it corrodes if its bc value is larger [44]. The higher ba values compared to those of bc for AA5251 coated with NieP and NieCoeP deposits [as evident from Table 5] support their better corrosion resistance behaviour. The polarization resistance of NieCoeP (1:1)/AA5251 coating increases by 2.33 and 30 times relative to those at NieP/ AA5251 prepared by electroless and electroplating power supply techniques, respectively. The increased cobalt content in the deposition bath, using the electroplating power supply technique, sharply increases Rp value at the formed NieCoeP/ AA5251 [Rp ¼ 398.28 and 4420 U cm2 at coated substrates with Ni:Co molar ratios of 1:0.2 and 1:1, respectively]. The highest Rp values are gained at coated samples prepared by the electroplating chronoamperometry technique. The corrosion rate can be calculated using the following equation: Corrosion rate ðmpyÞ ¼ 0:13 Icorr ðeq:wt:Þ=d

(4)

where eq.wt. is the equivalent weight in grams and d is the density in g/cm3 of AA5251 (for uncoated samples) and of Ni (for coated samples). The corrosion rate of NieCoeP (1:1)/ AA5251 formed by electroless and electroplating power supply techniques decreases by 1.56 and 3.38 times, respectively

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compared to those at NieP/AA5251. Moreover, NieP and NieCoeP coated AA5251 using the electroplating chronoamperometry technique offer the lowest corrosion rate (w9.3 mpy). The corrosion behaviour of NieP and NieCoeP coatings on AA5251 substrate is generally controlled by their surface morphology and deposited amount of phosphorous. The compact structure of NieCoeP (1:1)/AA5251, formed by electroplating chronoamperometry technique, is the reason for its best corrosion resistance among the studied coated substrates. A compact passive layer was formed by Abdel Aal et al. [34] after adding cobalt to NieP coating resulting in better corrosion behaviour as evident from Tafel plots. The lower corrosion resistance observed at NieCoeP (1:0.2)/AA5251 may be attributed to the groves and pores formed on its surface as shown in its SEM image in Fig. 4b. Regarding the effect of phosphorous content in the coating on its corrosion resistance, some reports have pointed out that lower Icorr values are shown at coatings with higher P content [45e47]. This may rationalize the improved corrosion behaviour of NieCoeP coatings [P% ¼ 20.35 and 12.19 for those formed in Ni:Co molar ratios of 1:0.2 and 1:1, respectively], compared to that of NieP one [P% ¼ 6.77]. Electronic effects in formed coatings may also play an important role [45]. When P content in the coating is increased, P can accept 0.4e0.8 electrons from Ni [48]. Accordingly, the equilibrium between NiO formation and Ni dissolution is altered by the partially covalent nature of NieP bond. In other opinion, a model has been assigned to explain the improvement in the corrosion resistance of NieP and NieCoeP coatings. It is based on the formation of an adsorbed layer of H2PO 2 on the coating surface [49]. It is formed due to the preferential dissolution of nickel to get a surface layer with enriched P that reacts with H2O to form adsorbed H2PO 2 anions [50]. As a result, the supply of water is blocked to the electrode surface. Therefore, nickel hydration is inhibited to form either soluble Ni2þ species or a passive nickel film. Bipolar plate materials should also show high corrosion resistance when operated at an anode potential of 100 mV (SCE) [it is equivalent to 540 mV (MMS)] and at a cathode potential of þ600 mV (SCE) [it equals þ160 mV (MMS)] [3]. The corresponding current density values at these potentials are marked by dashed lines in Fig. 8 and listed in Table 5d. A corrosion current density of the U.S. DOE (Department of Energy) requirements for bipolar plate materials of PEMFCs is less than 1.6  106 A cm2 at 2010 and 2015 [51]. The recorded current density value at the anode condition of NieP coating, formed by electroplating chronoamperometry technique, is 1.13  107 A cm2. It is compared to 1.32  106 and 1.20  106 A cm2 at the corresponding NieCoeP coatings, formed in Ni:Co molar ratios of 1:0.2 and 1:1, respectively. These lower values than the suggested DOE requirement confirm the best corrosion resistance of coatings formed by electroplating chronoamperometry technique and their suitability to be applied at aluminum plates used for transportation purposes.

potentiostatic polarization technique. Since corrosion is mainly detected in the cathode side [52], potentiostatic polarization test is carried out at its simulated condition. The corresponding current densityetime curves are plotted in Fig. 9 for uncoated and coated AA5251 with NieP and NieCoeP (1:1) deposits, formed by electroplating chronoamperometry technique, at þ160 mV (MMS) for 5 h in (0.5 M H2SO4 þ 2 ppm HF) solution. The current density of bare AA5251 sharply decreases at the start of polarization, then it stabilizes at about 0.64 mA cm2. This fast decay of the current density accompanies the formation of a passive film at the surface of AA5251 substrate [36,52]. The potentiostatic polarization curve of NieP/AA5251 coating also showed a rapid decay of the current density in the first 250 s, followed by a gradual current density increase to reach a value of 0.49 mA cm2 after 1.1 h, indicating that some corrosion occurs. Finally, the current density of NieP/AA5251 coating stabilizes at 0.57 mA cm2, showing a slight better corrosion resistance compared to bare AA5251 substrate. On the other hand, the current density stabilizes at NieCoeP (1:1)/AA5251 coating for 1.1 h at a value lowered by four times relative to that at bare AA5251 substrate. Afterwards, gradual current density decay was detected during the studied period, corresponding to the formation of a passive film at the coating surface. According to the result of this corrosion test under the simulated condition, the corrosion stability of the studied coatings can be arranged in the order: NieCoeP (1:1)/AA5251 > NieP/AA5251 > bare AA5251. Fig. 10 represents the surface morphologies of uncoated and coated AA5251 with NieP and NieCoeP (1:1) deposits before and after potentiostatic polarization test for cathode condition in (0.5 M H2SO4 þ 2 ppm HF) solution at þ160 mV (MMS) for 5 h. This potentiostatic polarization experiment results in a highly corroded bare AA5251 surface [see Fig. 10a0 ]. After coating of aluminum alloy surface, corrosion was observed at the grain boundaries. Fig. 10b0 showed many small-sized grain boundaries covering the whole NieP coating surface. This would increase the surface area subjected to corrosion. The situation may be different at the surface of

3.3. Potentiostatic tests of coated AA5251 samples in simulated cathode condition

Fig. 9 e Potentiostatic polarization curves of uncoated and coated AA5251 with NieP and NieCoeP (1:1) deposits at D160 mV (MMS) for 5 h in (0.5 M H2SO4 D 2 ppm HF) solution. The coatings were formed by electroplating chronoamperometry technique.

The corrosion behaviour of bipolar plate materials in real working conditions of PEMFC can be examined by

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NieCoeP (1:1) coating in Fig. 10c0 , where localized corrosion was noticed at large-sized grain boundaries. This may explain the improved corrosion resistance at NieCoeP coating surface.

3.4. Interfacial contact resistance measurements of coated AA5251 samples The interfacial contact resistance values between the bipolar plate and carbon paper represent a major factor that affects the internal impedance of the whole assembly. Accordingly,

they were measured at variable compaction forces for uncoated and coated AA5251 substrates with NieP and NieCoeP (1:1) deposits, using different deposition methods, in Fig. 11a. It was observed that the interfacial contact resistance values generally decrease with increasing the applied force [53,54]. This may be attributed to the fact that with increasing pressure, contact area would increase up to a definite value where further pressure increase does not affect this area. This may be achieved at a compaction force of 140 N cm2. The corresponding values of the interfacial contact resistance for

Fig. 10 e Scanning electron micrographs of uncoated (a) and coated AA5251 with NieP (b) and NieCoeP (1:1) (c), formed by electroplating chronoamperometry technique. [(a0 ), (b0 ) and (c0 )] are those after potentiostatic polarization test for cathode condition in (0.5 M H2SO4 D 2 ppm HF) solution at D160 mV (MMS) for 5 h.

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4.

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Conclusion

Aluminum alloy 5251 was modified by coating with NieP and NieCoeP deposits to improve its corrosion resistance. The presence of cobalt in the deposition bath increases the percentage of P in the coatings as in NieCoeP (1:0.2)/AA5251 substrate. Spherical crystalline particles, in small sizes, are arranged at the surface of NieCoeP coating formed by electroplating power supply technique, while the compact amorphous structure is a predominant feature for the coating formed by electroplating chronoamperometry method. This could explain the improved corrosion resistance of the latter one. Potentiostatic corrosion test under simulated cathode condition for NieCoeP (1:1)/AA5251 coating showed a stabilized current density that is lowered by four times relative to that at bare AA5251 substrate. Interfacial contact resistance values were affected by the deposition technique. Higher values were measured at coatings prepared by electroless deposition method.

references

Fig. 11 e (a) Variation of interfacial contact resistance of uncoated and coated AA5251 with NieP and NieCoeP (1:1) deposits with compaction force. (b) Their corresponding interfacial contact resistance values at compaction force of 140 N cmL2. These coatings were formed by electroless and electroplating power supply techniques.

uncoated and coated substrates are simply compared as bars in Fig. 11b. Bare AA5251 showed the highest interfacial contact resistance value [211.25 mU cm2] as a result of formed oxide layer that decreases its electrical conductivity. Reduced ICR values were shown after coating aluminum alloy with NieP and NieCoeP deposits. The deposition technique used to form different coatings was found to alter the resultant interfacial contact resistance values. This is apparent from the decreased ICR to about its half value when electroplating power supply technique is applied compared to that obtained by electroless method. The surface of NieCoeP (1:1), formed by electroplating power supply technique, contains small-sized crystals. This would increase the contact area of this bipolar plate with carbon paper, resulting in lowered ICR values. ICR values of samples, prepared by electroplating chronoamperometry technique, could not be measured because they require preparing coatings with surface area of 1 cm2, where the maximum current of the potentiostat is limited for small electrode area. The addition of cobalt to the deposition bath also reduces ICR values of the prepared coatings as a result of their increased electrical conductivity [45,48]. Based on the above results, interfacial contact resistance values at different coatings are decreasing in the order: AA5251 > NieP/AA5251 [electroless] > NieCoeP/AA5251 [electroless] > NieP/AA5251 [power supply] > NieCoeP/AA5251 [power supply].

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