*

Ni-W Alloys Coatings as Ecological Alternative for Chromium Plating – Evaluation of

Corrosion Behaviour

L.Anicai**

ICPE-Advanced Research, ICPE-CA, Laboratory of Non-conventional Engineering, Splaiul

Unirii 313, sector 3, Bucharest, Romania

Among electrochemical processes having a considerable impact on technical

development in a large range of industrial areas and including implications from

environmental viewpoints, electrodeposition and electroforming play an important role.

Metallic layers as structures/microstructures formed through electrodeposition may be

further used as final products or accurate moulds whose shapes are then copied and replicated

involving sometimes again electrochemical techniques. In all cases the deposits must posses

very good mechanical, thermal stability and corrosion protection characteristics. These

complex performances may be especially assured when alloys of tungsten/molybdenum with

iron group metals are applied that can be also electrochemically obtained as coating layers or

thin metallic foils.

Moreover, under the frame of the efforts to find ecological alternatives to chromium

plating, alloyed Ni with tungsten or molybdenum as well as boron or SiC based composites

are showing promise for specific applications [1-4].

Generally alloys electrodeposition processes are more complex and more difficult to

investigate as compared with those involving pure metals, due to at least two metals

codeposition, of hydrogen evolution reaction that affects both current efficiency and deposit

* Present address: PETROMSERVICE SA, Implementation of Ecological Technologies Division, Calea Grivitei 8-10, secdtor 1, Bucharest, Romania

*

characteristics, as well as to the multiple crystallographic structures that may form according

to phase diagrams [5]. Metallic alloys electrodepositions properties are in close correlation

with their chemical composition as well as with the applied operating parameters, including

electrolyte type and composition that influence deposit appearance, uniformity and their

physical chemical characteristics (e.g. mechanical, electrical, anticorrosive properties).

Ni-W coatings belong to the so-called induced co-deposition systems, when nickel's

reaction rate enhances the codeposition of tungsten. These alloys show good mechanical

properties, good protection against certain aggressive environments and specific thermal,

electrical and magnetic properties [6-9].

Many studies have been dedicated on induced codeposition mechanism investigation

and a specific dependence of Ni-W alloys against operation parameters was evidenced [6-11].

Thus, based on catalytic reduction it was considered that a tungsten oxide is initially formed

at cathode, that furtherly is reduced by atomic hydrogen In this case, metallic species

belonging to iron group should catalyze tungsten oxide reduction to metallic W. Non-

homogenous coatings might form, that contain a certain arrangement of very thin layers of

the two metals alternatively deposited [7,12]. Obradović et al. [11] took into account Ni-W

alloy formation when Ni is mainly deposited from its citrate-ammonium complex. At low

overpotentials this reduction is activation controlled and for more electronegative values

diffusion control is evidenced due to the low electroactive species concentration and to the

low other complexes formation rate. Metal deposition takes place just in the presence of Ni-

citrate-ammonium complexes that affects W oxide layer structure. According to the reported

results in [11] Ni species reduction is not an independent process, being affected by the W

oxide presence on electrode surface at a certain potential modified by Ni itself deposition. Ni

reduction within the alloys is proposed to occur through 2 steps, involving Ni(I) intermediary

species formation whose concentration may be virtually in equilibrium with that of freshly

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formed Ni(0) ones. These Ni(0) metallic particles in contact with electrode surface may

facilitate electrons transport towards W complex to activate it; afterwards the complete

reduction and alloy formation occurs.

Composition, structure and morphology of Ni-W electrodeposited alloys are strongly

influenced by applied operation parameters, mainly of electrolyte concentration in metallic

species and complexing agent, as well as of current density, hydrodynamic regime and

temperature [6,8,9]. Thus, it may consider that electrochemical obtaining of Ni-W alloys

represents a complex process that strongly influence their further application domain.

For practical use it is essential to get more information on their corrosion behaviour.

Hence, some investigations have been devoted to this subject, as well. It was reported [4,13

and ref.included] that corrosion is affected by the phase composition, chemical composition,

morphology and grain size of the deposit. However, few details are available on the subject.

Based on previous considerations, some preliminary investigations regarding direct

current electrodeposition and corrosion behaviour of Ni-W alloys (5-25% W) as coating

layers and thin metallic foils are presented.

EXPERIMENTAL

Deposition of Ni-W alloys has been performed in solutions containing NiSO4 and

Na2WO4 as electroactive species and Na3Cit as complexing agent. NH3 has been used to

adjust pH. To minimize the internal stress of the deposit, additions of saccharine and boric

acid have been applied. Current-potential curves for single metal and alloy deposition in

citrate-ammonium electrolytes were recorded, for a glassy carbon working electrode of 3 mm

diameter and a geometrical surface of 0.07 cm2 at a sweep rate of 10 mV/s, against a

Ag/AgCl reference electrode; the counter electrode was a Pt mesh. Electrolytes compositions

and experimental conditions are shown in Table 1.

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Table 1- Electrolytes composition and operating parameters for Ni-W alloys

electrodeposition

Electrolyte

[NiSO4.7H2O], mole/L

[Na2WO4.2HO] mole/L

[Na3Citrate] mole/L

H3BO3 mole/L

pH T, oC

NiW-1 0.4 0.1 0.5 - 7.5 - 9 (NH3 addition)

60-80

NiW-2 0.4 0.2 0.5 - 8 (NH3 addition)

60-80

NiW-3 0.2 0.1 0.5 8.5-9 (NH3 addition)

60-80

NiW-4 0.4 0.1 0.8 - 9-10 (NH3 addition)

60-80

NiW-5 0.4 0.05 0.9 0.5 8.5-9 (NH3 addition)

60-80

Alloys structure for various formation conditions have been evaluated through XRD

involving a DRON UM-1 equipment (CuKα radiation, 35kV, 30mA, graphite plane

monochromator on detector, 2θ∈ [20,100]0, ∆θ=0.050, τ=2s) coupled with a computer for data

acquisition, that have processed with specialized software. Deposit morphology has been

evidenced involving SEM associated with EDX analysis. To evaluate the alloys compositions

as well as cathodic efficiency, the electrodeposition has been performed involving a stainless

steel or Ti working electrodes that allow further the detaching of the deposit as a thin metallic

foil. When the electrodeposition has been done as coating layer, it was applied on brass

metallic support and copper foils, of 70x35 mm sizes.

To evaluate corrosion behaviour, the following accelerated laboratory tests have been

performed, respectively: (i) potentiodynamic polarization curves in aerated 0.5M NaCl

against a saturated calomel electrode (SCE) reference electrode and Pt mesh

counterelectrode, with a sweeping rate of 1 mV/s and 5 mV/s, for Ni-W alloys

electrodeposited on brass as working electrode; (ii) electrochemical impedance spectra in

0.5M NaCl at open circuit potential in the same three electrodes cell. For both

electrochemical investigations the geometrical surface of working electrode was 0.785 cm2.

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A Zahner IM6e potentiostatic equipment have been used. EIS spectra have been processed

using Zview 2.4 produced by Scribner Association Inc., Derek Johnson; (iii) continuous

immersion in 0.5M NaCl at 20oC for 240 hours with intermediary visual examinations and

recording of corrosion potential for at least 3 specimens (70x35 mm) of Ni-W coatings on

brass having a thickness of 10-15µm. For all experiments, electrolytes and investigation

solutions have been prepared involving p..a. grade chemical reagents and distilled water.

RESULTS AND DISSCUSSIONS

Ni-W alloy electroformation – the influence of operating parameters

The cathodic polarization curves both for single metal solutions (0.4 M NiSO4; 0.1M

Na2WO4) and complete electrolyte containing both metallic cations in stationary conditions

are presented in Figure 1. In the case of tungstate solution, the curve represents in fact only

hydrogen evolution because in aqueous medium tungsten cannot be deposited [14]. The

current-potential curves for Ni and Ni-W deposition are quite close one by another, especially

for low current densities. This is in agreement with other literature data that evidenced a

small difference between deposition potentials of alloy and iron group metals of about 15-20

mV [15]. Ni-W alloy cathodic curve shows a small limiting current plateau, due to the

deposition limitation through diffusion process, more evidenced in stationary conditions.

Ni-W co-deposition is complicated by simultaneous discharge that occurs both during

Ni electrodeposition from individual solution and during both metallic species reduction to

form the alloy.

Partial deposition currents for Ni and W can be calculated using Equation 1:

ipartial = i alloy X (metal percentage in the alloy) (1)

and the consumed current for hydrogen discharge can be evaluated according to Equation 2:

iH2 = itotal - (iNi +iW) (2)

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Figure 1 - Cathodic polarization curves for Ni, H2 from single metal solutions and for

Ni-W alloy from NiW-1 electrolyte (Table 1) in stationary conditions, 25oC, 10 mV/s.

Based on the above mentioned relationships and on the chemical analysis of the

deposited alloy for various current values, it was noticed that during potential displacement

towards electronegative values, Ni reduction partial current shows a maximum at a value of

around –1.4 V/Ag/AgCl; after that the current diminishes due to H2 discharge reaction

contribution increase.

Electrolyte pH value is an important parameter that considerably affects alloy

composition. Figure 2 shows an example of pH influence on cathodic polarization limiting

current for Ni-W deposition from electrolyte NiW-1, in semilogarithmic coordinates, at 25oC.

As it can be seen in Fig.2, pH increase from 7.5 to 9 leads to a considerable rising of cathodic

limiting current of about two magnitude orders. However, from alloy composition view point

it determines a slightly diminishing of W content, from about 20% to 16% and also a

decrease of cathodic efficiency from around 68% for pH =7.5-8 to 51-52% for pH=9-9.5.

-2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

-0.0025

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

0.4M NiSO4 0.1M Na2WO4

0.4M NiSO4+0.1M Na2WO4+0.5M Na3Citr

I, A

E, V/Ag/AgCl

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-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

1E-7

1E-6

1E-5

1E-4

1E-3 pH = 7.5 pH = 9

I, A

E, V/Ag/AgCl

Figure 2 – Cathodic polarization curves for Ni-W electrodeposition from NiW-1 electrolyte

at different pH values (100 rpm, 25oC, 10 mV/s)

Usually pH value is corrected involving NH3 additions. Also, it is known that NH3 represents

a strong ligand for Ni2+ cations, being possible some complex compounds formation of

[Ni(NH3)n]2+ form with n=2-6. Ammonium role as complexing agent together with citrate

anion in Ni-W alloys electrodeposition has been recently taken into account [1]. In this way,

through formation of Ni2+ complexes with NH3 and citrate, the concentration of ternary

complex Ni/W/Citrate-considered as W deposition precursor is modified and consequently

the final alloy composition.

An important effect on Ni-W alloy electrodeposition from internal stress view point

decrease is assigned to alkaline citrate within the electrolyte. This aspect is correlated with

the possible deposition mechanism [1,11] that suggests formation of some precursors as

ternary complexes of [(Ni)(HWO4)(Cit)]2- type according to reactions:

[(Ni)(Cit)]- + [(HWO4)(Cit)]4- → [(Ni)(HWO4)(Cit)]2- + Cit3- (a)

*

[(Ni)(HWO4)(Cit)]2- + 8e- + 3H2O → NiW + 7(OH)- +Cit3- (b)

On one side these complexes have a high stability constant [1, 11], that leads to alloy

composition limiting through deposition partial currents limitation for a maximum citrate

concentration and on the other side cationic species reduction through these precursors

influence nucleation and growth phenomena and thus facilitate formation of microcrystalline

deposits that qualitatively are materialized by a better ductility [9]. The experimental results

confirmed these presumtions. Thus, an increase of citrate concentration from 0.5M to 0.9M

determined an improvement of deposit quality from appearance point of view, especially as

metallic foil, but W content decreased from 21% to 19%. Correspondingly, cathodic

efficiency significantly decreased from 85% to 55%.

From morphological view point, SEM microscopy revealed a coherent , crack free deposit,

with a granular structure, as is exemplified in Figure 3 for an alloy containing 20% W.

By means of XRD analysis, some information of the electroformed alloys structure

were obtained. Generally, besides the Ni characteristic peaks (at 2θ = 44o, 51o and 76o)

phases of Ni169W31 and Ni4W type have been also identified. W characteristic peaks have

not been evidenced so that it is suggested that the deposit keeps Ni structure. Taking into

account that W atomic radius is 0.1370 nm, larger than that of Ni (of 0.1245 nm) it should be

considered that Ni-W alloy is a solid solution in which Ni is the solvent and W is the solute

[8,9]. Figure 4 shows an example of X-ray diffractogram for Ni-W alloy (20% W)

electrodeposited from NiW-1 electrolyte (37 mA/cm2, 70o).

Usually, electrolytes containing more than 0.2M alkaline tungstate lead to deposits

having strong internal stress due to intense H2 discharge reaction, when alkaline citrate is in

the range 0.4-0.6M. These electrolytes having Ni:W ratio of 4:1, 2:1, 1:1 produce rich W

alloys (20-40% W) and these deposits may be used as coating layers with a good adherence

on non-ferrous metallic supports such as brass and copper.

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Figure 3 – SEM micrograpy of Ni-W alloy (20% W) electrodeposited from

NiW-1 electrolyte (37 mA/cm2, 70o)

Figure 4 - X-ray diffractogram for Ni-W alloy (20% W) electrodeposited from

NiW-1 electrolyte (37 mA/cm2, 70o)

Beneficial effect of citrate complexing agent in high concentrations of minimum 0.8M

associated with NH3 addition has been evidenced in all cases, materialized in a significant

decrease of internal stress.

Additionally, metallic foils with a suitable flexibility and minimum internal stresses

have been obtained from low W content electrolytes as NiW-5 type from Table 1.

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Corrosion behaviour of Ni-W alloys

To characterize Ni-W alloys from corrosion protection view point, potentiodynamic

polarization curves in 0.5M NaCl and 0.1M H2SO4 have been recorded as well as

electrochemical impedance spectra in 0.5M NaCl at open circuit potential, for immersion

periods between 0-240 hours. Figure 5 shows several polarization curves for various

immersion periods in 0.5M NaCl in the case of a Ni-W alloy (10-12% W) electrodeposited on

brass metallic support from electrolyte NiW-5 (acc.to Table 1) at a thickness layer of 10 µm.

1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

E, V

/SC

E

i, A

initial 48 h 144 h

Figure 5 - Polarization curves in semilogarithmic coordinates for Ni-W alloy (10-12%W)

deposit in 0.5 M NaCl for various immersion periods (25oC, 5 mV/s)

At the beginning of conditioning, a corrosion potential around -0.86V/SCE was evaluated,

that moved towards more electropositive values for longer immersion times. Also, pitting

potential has the same evolution, from about -0.2 V/SCE towards about 0.010 V/SCE for

conditioning durations longer than 96 hours. Corrosion currents of 4 -7 µA/cm2 have been

determined, slightly higher for longer immersion periods. Table 2 summarizes characteristic

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values resulted from polarization curves in 0.5M NaCl processing for different conditioning

periods, in the case of Ni-W alloy deposit containing 10-12% W.

Table 2 – Characteristic values resulted from polarization curves in 0.5M NaCl for

Ni-W alloy (10-12% W) during continuous immersion

Immersion period, hours Ecor, mV/ECS Epit, mV/ECS I cor, µA/cm2

0 -860 -200 4.09 48 -340 0 5.35 96 -772 10 7.5 144 -450 10 5.5 192 -475 10 6

For the same Ni-W alloy, electrochemical impedance spectra have been recorded in

0.5M NaCl at the same immersion periods, at open circuit potential in a frequency range

between 3.105 – 10-1 Hz. Figure 6 presents the recorded impedance spectra for alloy deposit

in 0.5M NaCl during immersion in Nyquist plots.

Figure 6 – Nyquist plots for Ni-W alloy deposit (10-12%W) in 0.5M NaCl at open circuit

potential, after various continuous immersion periods

0 10000 20000 30000 40000 50000 60000 700000

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

110000

- Zim

, Ω

Zre, Ω

initial 48 h 96 h 144 h

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The shape of impedance spectra suggests that the alloy has protective properties.

Nyquist plots evidence the presence of semicircle, characteristic to layers that exhibit

relatively high polarization resistance values. To fit experimental data, an equivalent circuit

model has been proposed (as shown in Figure 7), comprising a combination of a constant

phase element (CPE) in parallel with coating resistance (Rc), in series with solution resistance

(Rsol). This CPE behaviour may suggest a variation of composition of the layer along its

thickness [13], phenomenon that can be taken into account, due to the presence of various

phases formation as also XRD revealed during electroformation process. Also, the use of

CPE takes into account the coating possible inhomogeneities and better simulates corrosion

processes [16,17]. The fitting parameters are presented in Table 3.

Figure 7 – Nyquist plot for Ni-W alloy deposition in 0.5 M NaCl at open circuit

potential (Uoc =-0.287 V/SCE) after 48 hours of immersion, representing both experimental

and fited points. Inset: the proposed equivalent circuit

-10000 0 10000 20000 30000 40000 50000 60000 70000-10000

0

10000

20000

30000

40000

50000

60000

70000

80000

Z im, Ω

Zre, Ω

NiW/Am in 0.5M NaCl after 48 h of immersion

experimental fit results

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Table 3 - Fitting parameters of Ni-W alloy deposition in 0.5 M NaCl for various

immersion periods using the proposed equivalent circuit from the inset of Figure 7

CPE Immersion period, hours

Rş Ω.cm2

RP Ωcm2 C , F/cm2 n

Initial 36 2,4*104 1,67*10-5 0,859 48 54 2,2*105 1,18*10-5 0,827 96 59 1,45*105 1,85*10-5 0,768

144 43 7,7*105 1,27*10-5 0,855

Based on fitting results, polarization resistance values of about 24 kΩ have been

determined at the beginning of immersion, that increased with about one magnitude order

after 48 hours. This behaviour suggests formation of a passive film on metallic alloy surface

that enhance protective characteristics, materialized also by no major aspect modifications

even after 240 conditioning hours.

Ni-W alloys specimens (10-12% W) have been also evaluated by corrosion potential

evolution against immersion time in aerated 0.5M NaCl at 25oC, as is shown in Figure 8.

-24 0 24 48 72 96 120 144 168 192 216 240 264-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

Ope

n ci

rcui

t pot

entia

l, U

oc, V

/EC

S

immersion time, hours

Figure 8 – Open circuit potential against continuous immersion time in 0.5M NaCl for Ni-W

alloy deposits (10-12%W)

According to Figure 8, corrosion potential slightly moves towards electronegative values

in the first 48 hours, followed by a displacement in positive direction, as an indicative of the

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protective effect of the coating. This behaviour is in a good agreement with results presented

above, that assign the protective capacity to formation of an additional passive film that

enhance corrosion protection.

CONCLUSIONS

According to the experiments that have been performed it should be conclude that:

• Ni-W alloys with low internal stress and W contents between 5-25% W may be deposited

from citrate-ammonium type electrolytes with relatively low tungstate concetrations (0.05M-

0.2M);

• From XRD analysis phases of Ni169W31 and Ni4W type have been identified. W

characteristic peaks have not been evidenced so that it is suggested that the deposit keeps Ni

structure;

• Based on the experimental results, it should be mentioned that Ni-W alloys coatings offer

good corrosion protective characteristics materialized in corrosion currents of around 4-6

µA/cm2 and polarization resistances of minimum 25 kΩ. After 240 hours of continuous

immersion in chloride containing aggresive medium the exposed specimens didn’t exhibited

any major surface modification and no pits have been evidenced.

• Future experiments will be performed for a deeper investigation of corrosion protection in

close correlation with alloy composition and structure, as well as the application field.

Acknowledgements

This work was supported by the Romanian Ministry of Education and Research,

MATNANTECH Program, Contract 169(305)/2003. The authors wish to thank prof. T.Visan

from POLITEHNICA University of Bucharest, Department of Applied Physical Chemistry

and Electrochemistry for useful disscussions.

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