Microstructure characteristics and corrosion resistance of Ni–Co alloy coating prepared on AZ91D magnesium alloy

Abstract

In this article, a protective Ni–Co alloy coating was prepared on AZ91D magnesium alloy which was pretreated with electroless plating. The surface morphologies, microstructure and chemical composition of the coatings were studied using environmental scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy respectively. The surface morphologies and microstructure analysis showed that the Ni–Co alloy coating possessed cone shaped grain congeries and formed face centred cubic solid solution. The energy dispersive spectroscopy analysis revealed that the content of cobalt element in the alloy coating was ∼31 wt-%. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy were employed to investigate the corrosion resistance of different corrosive systems in neutral 3·5 wt-%NaCl and 0·5 mol L^–1 Na₂SO₄ solutions, which were chosen to simulate the effects of marine and industrial environments respectively. The results showed that the corrosion resistance of the AZ91D magnesium alloy was obviously improved by the Ni–Co alloy coating.

Keywords

Magnesium alloys Electroless plating Ni–Co electroplating Potentiodynamic polarisation Electrochemical impedance spectroscopy

Introduction

Owing to its excellent properties, such as high strength/weight ratio, low density, high dimensional stability and damping characteristics, magnesium alloys have been widely applied to the fields of automobile, electronic products, aerospace, etc.1, 2 Unfortunately, the high reactive nature of magnesium makes itself highly susceptible to corrosion under most ambient conditions, which severely hindered its wide applications in many fields.3, 4 Various surface treatment techniques, such as plating, anodising, conversion coating, gas phase deposition, laser surface alloying and polymer coating, have been applied to the corrosion protection of magnesium alloys, and each method has its own advantages and disadvantages.5 ^– 10 These methods have been reviewed in detail by Gray and Luan.11 The electrodeposited Ni–Co alloy coating, which possesses high strength, good wear resistance, good adhesion, high thermal stability, high heat conductivity and high electrocatalytic activity properties, has attracted much attention.12 ^– 15 Through review of researches on the Ni–Co alloy coating, it can be found that most of these works are focusing on its preparation, mechanical properties, thermal stability, and so on, while the available information with respect to its corrosion protection properties is limited. Furthermore, so far, no work has reported on the researches of the Ni–Co alloy coating prepared on magnesium alloys.

It is well known that magnesium alloys are ‘difficult to plate metal’,16 so it is vital to choose an appropriate pretreatment method for magnesium alloy before electroplating. Generally speaking, electroless plating and conversion coating are the simplest and frequently used pretreated techniques, and it has been found that electroless plating exhibits a certain degree of corrosion resistance and excellent conductivity, but the thickness of electroless coating is limited and could not provide the effective protection for substrates. The main disadvantage of conversion coating is the toxicity of the treatment solutions, which is harmful to the human body and the environment.11 Thus, we have chosen electroless plating as the pretreatment for the following electrodeposition.

In this study, Ni–Co alloy coating was successfully applied to AZ91D magnesium alloy, which was pretreated with electroless Ni–P coating. The main aim of this study is to understand the microstructure characteristics and corrosion protection properties of Ni–Co alloy coating provided for AZ91D magnesium alloy in the simulated marine (chloride) and industrial (sulphate) environments and provide some references for the expansion of its applications.

Experimental

Materials

Die casted AZ91D magnesium alloys, whose main chemical composition is Mg–8·77Al–0·74Zn–0·18Mn (wt-%), were used for the present investigation. The AZ91D magnesium alloys with the size of 20×20×5 mm were cut and then mechanically polished by emery papers of successively finer grit down to 2000 grit before the pretreatment process. The composition and operating conditions of the pretreatment process of electroless Ni–P coating are listed in Table 1.

Table 1

Bath compositions and operating conditions for electroless plating

Bath composition and operating conditions	Quantity
NiCO₃.2Ni(OH)₂.4H₂O	10 g L⁻¹
NaH₂PO₂.H₂O	20 g L⁻¹
C₆H₈O₇.H₂O	5 g L⁻¹
NH₄HF₂	20 g L⁻¹
SC(NH₂)₂	1 mg L⁻¹
pH	6·1
Temperature	75°C
Time	30 min

Electrodeposition process

The Ni–Co alloy coating was electrodeposited under direct current conditions. The optimised bath compositions and other parameters obtained from large numbers of orthogonal experiments are given in Table 2.

Table 2

Bath compositions and operating conditions for electrodeposition of Ni–Co alloy coating

Bath composition and operating conditions	Quantity
NiSO₄.6H₂O	250 g L⁻¹
CoCl₂.6H₂O	18 g L⁻¹
H₃BO₃	30 g L⁻¹
NaCl	10 g L⁻¹
Current density	5 A dm⁻²
pH	3·7
Temperature	40°C
Time	30 min

Tests

The surface morphologies and compositions of the coatings were studied using a Quanta 200 environmental scanning electron microscope (FEI Co., Ltd, The Netherlands) coupled with an INCA energy dispersive analyser system (Oxford Co., Ltd, UK). The phase structure of the coatings was analysed by a D8 DISCOVER X-ray diffractometer (Bruker AXS Co., Ltd, Germany) operated at 40 kV and 40 mA with Cu K_α radiation.

Electrochemical corrosion tests were carried out using a classical three-electrode cell with platinum as counter electrode, saturated calomel electrode (SCE) [+0·242 V(SHE)] as reference electrode and the samples with an exposed area of 1 cm² as working electrode. The neutral 3·5 wt-%NaCl solutions and 0·5 mol L^–1 Na₂SO₄ solutions (open to air at 20±2°C) were used as corrosive media. The PAR system, which included an M273A potentiostat, an M5210 lock in amplifier and the PowerSuite software, was used for measuring the potentiodynamic polarisation curves with a constant voltage scan rate of 1 mV s^–1 and electrochemical impedance spectroscopy (EIS). The employed amplitude of the sinusoidal signal for EIS test was 5 mV, and the frequency range studied was from 105 to 10^–2 Hz. The acquired data were curve fitted and analysed using ZsimpWin 3·20 software. All the corrosion tests were normally repeated three times under the same conditions, checking that they presented reasonable reproducibility.

Results and discussion

Morphology and composition analysis

The surface morphologies of the electroless Ni–P coating and Ni–Co alloy coating are displayed in Fig. 1. Figure 1a shows that the surface of the magnesium alloy is fully covered by the Ni–P coating, which is conducive to the following electrodeposition process. Many cauliflower-like particles are observed in the high magnification SEM images of the Ni–P coating, as shown in Fig. 1b. The typical EDS plot of the Ni–P coating shown in Fig. 2a reveals that the content of phosphorus element in the Ni–P coating is ∼5·6 wt-%. Figure 1c and d indicates that the Ni–Co alloy coating possesses uniform surface morphologies, and grain congeries are marked with a cone shaped structure. The typical EDS plot of the Ni–Co alloy coating is shown in Fig. 2b, from which the existence of a cobalt element in the alloy coating is observed. The EDS examination shows that the content of cobalt element in the alloy coating is ∼31 wt-%. From the cross-section morphologies, as shown in Fig. 3, it can be identified that the total thickness of the coatings is ∼30 μm. No obvious breaks or cracks are observed along the interfaces between the coatings and the magnesium alloy substrate. In addition, because there is no boundary between the Ni–P and Ni–Co alloy coatings, the thicknesses of different coatings cannot be differentiated from the cross-section morphologies, whereas EDS line scanning reveals that the thicknesses of the Ni–P and Ni–Co alloy coatings are approximately 11 and 19 μm respectively.

Figure 1

Surface morphologies of a, b electroless Ni–P coating and c, d Ni–Co alloy coating prepared on AZ91D magnesium alloy

Figure 2

Typical EDS plots of a Ni–P coating and b Ni–Co alloy coating prepared on AZ91D magnesium alloy

Figure 3

Cross-section morphologies of coatings prepared on AZ91D magnesium alloy

Microstructure analysis

The X-ray diffraction pattern of the Ni–Co alloy coating is shown in Fig. 4. It can be seen from Fig. 4 that the Ni–Co alloy coating exhibits a single phase of Ni matrix with face centred cubic crystal structure (PDF number 04-0850), and the peaks corresponding to the Co phase are not found in the spectra, which could be ascribed to the fact that the crystal lattice in the Ni matrix perhaps have been replaced by Co partly and formed solid solution.14 It is apparent that the Ni–Co alloy coating exhibits intense (1 1 1) preferred orientation and presents three acutely peaks centred 2θ at 44, 51 and 76°, indicating the crystal nature of the alloy coating.

Figure 4

X-ray diffraction pattern of Ni–Co alloy coating prepared on AZ91D magnesium alloy

Corrosion analysis

The anticorrosive properties of the bare magnesium alloy, pretreated magnesium alloy with electroless plating and Ni–Co alloy coating coated magnesium alloy in the simulated marine (neutral 3·5 wt-%NaCl solutions) environment and the industrial (0·5 mol L^–1 Na₂SO₄ solutions) environment are studied and compared using potentiodynamic polarisation technique (Fig. 5) and EIS method (Figs. 6 and 7).

Figure 5

Typical potentiodynamic polarisation curves of bare magnesium alloy, pretreated magnesium alloy and Ni–Co alloy coating coated magnesium alloy in two kinds of corrosive media: a 3·5 wt-%NaCl solutions; b 0·5 mol L^–1 Na₂SO₄ solutions

Figure 6

Typical Nyquist plots of bare magnesium alloy, pretreated magnesium alloy and Ni–Co alloy coating coated magnesium alloy in two kinds of corrosive media: a 3·5 wt-%NaCl solutions; b 0·5 mol L^–1 Na₂SO₄ solutions

Figure 7

Typical Bode plots of bare magnesium alloy, pretreated magnesium alloy and Ni–Co alloy coating coated magnesium alloy in two kinds of corrosive media: a 3·5 wt-%NaCl solutions; b 0·5 mol L^–1 Na₂SO₄ solutions

Potentiodynamic polarisation

The typical potentiodynamic polarisation curves of the different corrosive systems in neutral 3·5 wt-%NaCl solutions and 0·5 mol L^–1 Na₂SO₄ solutions are presented in Fig. 5a and b respectively. Different parameters, such as corrosion current density i_corr and corrosion potential E_corr, derived from Fig. 5 using the Tafel extrapolation, are summarised in Table 3.

Table 3

Electrochemical parameters calculated from potentiodynamic polarisation curves

Specimens	E_corr/mV(SCE)	i_corr/A cm⁻²
Bare magnesium alloy in neutral 3·5 wt-%NaCl solutions	−1485	3·1×10⁻⁵
Pretreated magnesium alloy in neutral 3·5 wt-%NaCl solutions	−372	1·5×10⁻⁶
Ni–Co coating coated in neutral 3·5 wt-%NaCl solutions	−284	2·6×10⁻⁷
Bare magnesium alloy in 0·5 mol L⁻¹ Na₂SO₄ solutions	−1521	6·3×10⁻⁶
Pretreated magnesium alloy in 0·5 mol L⁻¹ Na₂SO₄ solutions	−394	2·1×10⁻⁶
Ni–Co coating coated in 0·5 mol L⁻¹ Na₂SO₄ solutions	−285	2·2×10⁻⁷

From the polarisation tests in neutral 3·5 wt-%NaCl solutions, it can be found that, with pretreatment of the Ni–P coating on the surface of the bare magnesium alloy, the E_corr of corrosive system is 1113 mV nobler than that of the bare magnesium alloy, and the i_corr of the corrosive system is that of the bare magnesium alloy, suggesting a certain extent corrosion protection for magnesium alloy. With further electrodeposition of the Ni–Co alloy coating, the E_corr of the corrosive system distinctly shifts to a more positive direction up to −284 mV, which is 88 and 1201 mV nobler than that of the pretreated magnesium alloy and bare substrate respectively, and the i_corr of the corrosive system is reduced to 2·6×10⁻⁷ A cm⁻², which is approximately and that of the pretreated magnesium alloy and bare magnesium alloy respectively.

According to the typical potentiodynamic polarisation curves in 0·5 mol L^–1 Na₂SO₄ solutions, it can be educed that the Ni–Co alloy coating coated magnesium alloy exhibits the noblest E_corr with a value of −285 mV and the lowest i_corr with a value of 2·2×10⁻⁷ A cm⁻², while the bare magnesium alloy shows the lowest E_corr with a value of −1521 mV and the highest i_corr with a value of 6·3×10⁻⁶ A cm⁻², which suggests that the Ni–Co alloy coating coated magnesium alloy exhibits the best anticorrosive properties. The E_corr and i_corr of the pretreated magnesium alloy with electroless plating are measured to be −394 mV and 2·1×10⁻⁶ A cm⁻² respectively.

From the analysis mentioned above, it can be concluded that both the Ni–P coating as a pretreatment and the Ni–Co alloy coating could provide corrosion protection for AZ91D magnesium alloy, while nobler E_corr and much lower i_corr for the Ni–Co alloy coating indicate better corrosion protection for the magnesium alloy.

Electrochemical impedance spectroscopy

The typical Nyquist plots of the different corrosive systems in neutral 3·5 wt-%NaCl solutions and 0·5 mol L^–1 Na₂SO₄ solutions are presented in Fig. 6a and b respectively. A low frequency inductive loop, whose shape is characterised with a retractile real part, is observed in the Nyquist plots of the pretreated magnesium alloy in neutral 3·5 wt-%NaCl solutions and bare magnesium in two kinds of corrosive media. The appearance of the low frequency inductive loop may originate from the pitting corrosion on the electrode surface.17 It can be seen from Fig. 6a that the diameter of the capacitance loop of the Ni–Co alloy coating coated magnesium alloy is exceedingly larger than that of the pretreated magnesium alloy, indicating that the Ni–Co alloy coating could provide better and more effective corrosion protection for magnesium alloy compared with that of the pretreated magnesium with electroless plating.

The typical Bode plots of the different corrosive system in neutral 3·5 wt-%NaCl solutions and 0·5 mol L^–1 Na₂SO₄ solutions are presented in Fig. 7a and b. From the Bode plots (impedance modulus |Z| as a function of frequency), it can be detected that the impedance value of the Ni–Co alloy coating coated magnesium alloy in two kinds of corrosive media increases by approximately more than two orders of magnitude and one order of magnitude compared with that of the bare magnesium alloy and pretreated magnesium alloy respectively. This result indicates that the corrosive system of the Ni–Co alloy coating coated magnesium alloy possesses the best anticorrosive properties, and the Ni–Co alloy coating could provide the best corrosion protection for magnesium alloy. The changes of the Bode plots (phase angle as a function of frequency) indicate that there is only one time constant for the Ni–Co alloy coating coated magnesium alloy in two kinds of corrosive media and pretreated magnesium alloy in 0·5 mol L^–1 Na₂SO₄ solutions over the whole frequency range studied, which reflects one relaxation process, i.e. dissolution of working electrode during the corrosion process.

In order to further and quantificationally study the corrosion process of different corrosive systems, a more detailed interpretation of the EIS measurement is performed by fitting the experimental plots using the electrochemical equivalent circuit depicted in Fig. 8. The electrochemical equivalent circuit displayed in Fig. 8a is proposed to account for the EIS data of the pretreated magnesium alloy in neutral 3·5 wt-%NaCl solutions and bare magnesium in two kinds of corrosive media. Figure 8b is used to fit the EIS data of the Ni–Co coating coated magnesium alloy in two kinds of corrosive media and the pretreated magnesium alloy in 0·5 mol L^–1 Na₂SO₄ solutions. The circuit, which is presented in Fig. 8a, consists of parameters, namely, solution resistance R_s, charge transfer resistance R_ct, inductance resistance R_L, inductance L and a constant phase element (CPE) that replaces the capacitance of the double layer C_dl. The circuit, which is displayed in Fig. 8b, is composed of parameters, namely, solution resistance R_s, charge transfer resistance R_ct and a CPE. A CPE replaces the capacitance of the double layer C_dl due to the roughness and inhomogeneity of the electrode surface, as reported elsewhere.18, 19 The impedance of the CPE is given by the following equation18 (1) where Y₀ is a constant that is independent of frequency, ω is the angular frequency, j = (−1)^1/2 and n is an exponential index, which represents a dispersion of relaxation. When n = 0, CPE acts as a pure resistor; when n = 1, CPE represents an ideal capacitor; and when n = −1, CPE possesses the characteristic of inductance. The impedance of L could be described as follows (2) The charge transfer resistance R_ct is the resistance offered by the metal atom to get ionised when in contact with the electrolyte. The higher the charge transfer resistance, the better the corrosion resistance. The fitted values of R_ct with regard to the bare magnesium alloy, pretreated magnesium alloy and Ni–Co alloy coating coated magnesium alloy in neutral 3·5 wt-%NaCl solutions are 1712, 17 800 and 250 700 Ω cm² respectively. The fitted values of R_ct with respect to the bare magnesium alloy, pretreated magnesium alloy and Ni–Co alloy coating coated magnesium alloy in 0·5 mol L^–1 Na₂SO₄ solutions are 1069, 12 430 and 180 900 Ω cm² respectively. It can be educed from the values of R_ct mentioned above that the values of R_ct of the pretreated magnesium alloy are approximately 10 and 12 times higher than that of bare magnesium alloy in neutral 3·5 wt-%NaCl solutions and 0·5 mol L^–1 Na₂SO₄ solutions respectively, which suggests a certain extent improvement in the anticorrosive properties of magnesium alloy with the pretreatment of electroless plating. After the further electrodeposition of the Ni–Co alloy coating, the impedance value of the corrosive system increases by more than two orders of magnitude compared with that of the bare magnesium alloy in two kinds of corrosive media, indicating the notable enhancement of the anticorrosive properties of the AZ91D magnesium alloy.

Figure 8

Electrochemical equivalent circuits used for fitting EIS data of a pretreated magnesium alloy in neutral 3·5 wt-%NaCl solutions and bare magnesium in two kinds of corrosive media and b Ni–Co coating coated magnesium alloy in two kinds of corrosive media and pretreated magnesium alloy in 0·5 mol L^–1 Na₂SO₄ solutions

From the corrosion analysis mentioned above, it can be seen that both the pretreatment of electroless plating and the electrodeposition of Ni–Co alloy coating could provide corrosion protection for the AZ91D magnesium alloy in the simulated marine (chloride) and industrial (sulphate) environments, and the effect of the former is limited, which is consistent with the potentiodynamic polarisation analysis. The electrodeposition of the Ni–Co alloy coating on the pretreated magnesium alloy could remarkably enhance the anticorrosive properties of the AZ91D magnesium alloy.

Conclusion

In summary, the following conclusions can be drawn from the present investigation:

The protective Ni–Co alloy coating was successfully applied to the corrosion protection of the AZ91D magnesium alloy.

The environmental SEM and EDS analysis showed that the Ni–Co alloy coating possessed cone shaped grain congeries, and the content of cobalt element in the alloy coating was ∼31 wt-%.

The X-ray diffraction analysis indicated that the Ni–Co alloy coating presented a single phase of Ni matrix with face centred cubic crystal structure and had formed the solid solution.

The results of electrochemical corrosion tests showed that the Ni–Co alloy coating could provide good corrosion protection for the AZ91D magnesium alloy in the simulated marine (chloride) and industrial (sulphate) environments.

References

Mordike

, Ebert

: Mater. Sci. Eng. A, 2001, A302, 37–45.

Aghion

, Bronfin

, Eliezer

: J. Mater. Process. Technol., 2001, 117, 381–385.

Song

, Bowles

, StJohn

: Mater. Sci. Eng. A, 2004, A366, 74–86.

Liu

, Schlesinger

: Corros. Sci., 2009, 51, 1733–1737.

Zucchi

, Frignani

, Grassi

, Balbo

, Trabanelli

: Mater. Chem. Phys., 2008, 110, 263–268.

Yao

, Cui

, Jiang

, Wang

: Surf. Eng., 2008, 24, 355–357.

, Fu

, Li

, Gao

: Scr. Mater., 2008, 58, 504–507.

Zhang

, He

, Jiang

, Lian

: Surf. Coat. Technol., 2007, 201, 4594–4600.

, Lian

, Niu

, Jiang

, Dong

: Surf. Eng., 2007, 23, 56–61.

10.

Song

, Shan

, Han

: Electrochim. Acta, 2008, 53, 2135–2143.

11.

Gray

, Luan

: J. Alloys Compd, 2002, 336, 88–113.

12.

Tury

, Radnóczi

, Varsányi

: Surf. Coat. Technol., 2007, 202, 331–335.

13.

Hibbard

, Aust

, Erb

: Mater. Sci. Eng. A, 2006, A433, 195–202.

14.

Tury

, Lakatos-Varsányi

, Roy

: Surf. Coat. Technol., 2006, 200, 6713–6717.

15.

Tury

, Lakatos-Varsány

, Roy

: Appl. Surf. Sci., 2007, 253, 3103–3108.

16.

Liu

, Xiang

, Hu

, Ding

: Aerosp. Mater. Technol., 2001, 31, 21–25.

17.

, Gao

, Zhang

: Appl. Surf. Sci., 2006, 253, 2251–2256.

18.

Cao

, Zhang

: ‘An introduction of electrochemical impedance spectroscopy’, 1st edn, 26–27; 2002, Beijing, Science Press.

19.

Song

, Shan

, Han

: Electrochim. Acta, 2007, 53, 2009–2015.