Abstract
In this paper, the effect of zinc dipping pretreatment on corrosion resistance of electroless nickel plating on magnesium alloy was studied. The results showed that the corrosion resistance of nickel coating was influenced greatly by the complexing agent and main salt in the zinc dipping solution, and the formula and technological conditions of zinc dipping were optimized by the orthogonal test. The optimum formulation and process conditions were as follows: C4H6O5 40 g L−1, ZnSO4·7H2O 40 g L−1, NaF 4 g L−1, pH 9, temperature 90°C, time 10 min. The corrosion resistance, morphology and phase composition of the nickel coating prepared under the optimum conditions were analysed by SEM, XRD and electrochemical workstation. The results showed that the self-corrosion potential of the nickel coating is 0.87 V higher than the magnesium alloy substrate, the self-corrosion current reduces by two orders of magnitude and the magnesium alloy substrate is completely covered. The corrosion resistance of magnesium alloy was effectively enhanced.
Introduction
The advantages and properties of magnesium alloys as structural materials have been known for a long time, but they have not been widely used. The main reason is the relatively high activity of magnesium, which leads to poor corrosion and wear resistance of magnesium alloys [1-5]. Magnesium alloy surface treatment is an effective method to improve the performance of magnesium alloy application. The key to magnesium alloy surface treatment technology is to pretreat the surface. The correct pretreatment method can obtain good corrosion resistance and wear resistant coating [6-8]. If the corrosion and wear resistance of magnesium alloys are improved, magnesium alloys will be widely used in automotive, aerospace and electronic industries.
Electroless plating can form cathodic protective coatings with uniform thickness, corrosion resistance and wear resistance on the surface of magnesium alloys, which is one of the important methods for surface treatment of magnesium alloys [9-13]. The standard electrode potential of magnesium and nickel was −2.36 and −0.25 V, respectively. The difference between them is so great that it is difficult for a nickel to deposit directly on the surface of magnesium [14]. Therefore, a pre-impregnated layer as medium coating is necessary, and the potential of the pre-impregnated layer such as the conversion coating is between that of magnesium and nickel [15]. The standard electrode potential of zinc was −0.76 V; adding a good adhesion layer of zinc between magnesium and nickel is beneficial to the deposition of metallic nickel [16-18]. The zinc coating can be well combined with magnesium and subsequent nickel coating, and can cover the defects of the surface of the magnesium alloy substrate, which is beneficial to obtain ideal coating. Therefore, it is of great significance to study the pretreatment of zinc dipping.
In this paper, the pretreatment solution of zinc dipping containing four different complexing agents was studied. The optimum complexing agent was determined by observing the cross-section morphology of zinc immersion solution after electroless plating by SEM. The optimum concentration of the main salt was determined by a single factor experiment. Finally, the optimum formula and the optimum technological conditions of zinc dipping solution were optimized by the orthogonal experiment.
Experimental
Formulations of different Zn dipping solutions.
The microstructure of electroless nickel coating was analysed by a VEGA3 XMU scanning electron microscope. The analysis of chemical nickel plating composition was carried out using a SHIMADZU 6100 XRD diffractometer. Potentiodynamic polarization curves were obtained by a CHI660E electrochemical workstation: the saturated calomel electrode as the reference electrode, platinum sheet as the auxiliary electrode, magnesium alloy as the working electrode, the working area is 1 cm2, with 3.5% NaCl demonized water solution as the test solution. The scanning rate is 0.005 V s−1, the scanning frequency is 100,000–0.1 Hz, and the non-working area is covered with epoxy resin. In order to study the influence of some factors in the pretreatment process of magnesium electroless nickel plating, the pretreatment conditions were optimized, and the bonding strength of the coating was used as the evaluation index. The evaluation level of the bonding force was: good bonding strength – 5, better bonding strength – 4, the general strength – 3, poorer binding – 2 and poor bonding – 1.
The corrosion test was carried out by static soaking. The corrosive medium was 3.5 wt-% NaCl solution with distilled water as the solvent. The untreated magnesium alloy sample and the electroless nickel-plated sample were respectively immersed in 3.5 wt-% NaCl solution for different times, and then taken out, the quality of samples before and after corrosion was analysed. The corrosion rate calculation formula is as follows:
In the above formula
is the poor quality of the film before and after corrosion (g);
is the surface area of the sample (m2) and
is the corrosion time (h). The experimental data are the average of the three experimental results.
List of different factors and levels.
Results and discussion
The choice of complexing agent
Figure 1 is cross-section morphology of the electroless nickel-plating coating under different complexing agents. As can be seen from Figure 1(a), the coating thickness is about 20 μm, the right side of the coating is relatively flat, the left side of the coating is defect, the thickness of the zinc dipping layer between the coating and the substrate is uneven and there are voids, which makes the adhesion between the coating and the substrate is poor. Figure 1(b) has a uniform and compact coating, the thickness of coating is about 30 μm, but the zinc dipping layer is granular and coarse, with a maximum void size about 25 μm. The coating is easy to fall off. The coating of Figure 1(c) is uniform and compact, the thickness is close to 30 μm, and the zinc dipping layer is loose and massive. Compared with Figure 1(d), the situation is improved, but the bonding force is still poor. Figure 1(d) showed that the zinc dipping layer is embedded in the substrate, the zinc dipping layer between the coating and the substrate is even and there are no voids. The thickness of the coating is about 22 μm. To sum up, C4H6O5 was selected as the complexing agent in this paper.
Cross-section morphology of the electroless nickel-plating coating under different complexing agents (a: sodium pyrophosphate, b: potassium sodium tartrate, c: citric acid and d: malic acid).
Selection of malic acid concentration
The complexing agent helps to obtain a zinc immersion layer with good bonding force, which can quickly react with the surface oxide film and hydroxide of the magnesium substrate to dissolve into a water-soluble complex.
Figure 2 is the cross-section of the electroless nickel-plating coating under different concentrations of malic acid. From Figure 2, it can be seen that the concentration of malic acid has a wide range. The conversion film has good adhesion with the substrate. When the concentration of malic acid is 50 g L−1, the binding force between the substrate and the coating is better. Therefore, the optimum concentration of malic acid in zinc dipping solution is 50 g L−1.
Cross-section morphology of the electroless nickel-plating coating under different malic acid concentrations (a: 30 g L−1, b: 40 g L−1, c: 50 g L−1 and d: 60 g L−1).
Effect of ZnSO4·7H2O on electroless nickel plating
ZnSO4·7H2O mainly provides Zn2+ in the process of zinc dipping. When the Zn2+ concentration is too high, the zinc immersion film is loose and rough, and the bonding strength is not too high, which is mainly due to the deposition rate being too fast. When the concentration of Zn2+ is too low, the deposition rate of the film is very low, but the film is dense and the bonding strength is high. Therefore, choosing the appropriate concentration of Zn2+ is critical to the corrosion resistance of the coating.
Figure 3 is a broken line graph of electroless nickel plating thickness under different concentrations of ZnSO4·7H2O. It can be seen from the diagram that when the concentration of ZnSO4·7H2O is less than 30 g L−1, the thickness of the coating increases with the increase of ZnSO4·7H2O concentration. When the concentration of ZnSO4·7H2O is 30 g L−1, the thickness of the coating reaches the maximum. When the concentration of ZnSO4·7H2O is higher than 30 g L−1, the thickness of the coating decreases with the increase of ZnSO4·7H2O concentration. Figure 4 shows the surface morphology and cross-section morphology of the electroless nickel plating when the ZnSO4·7H2O concentration is 30 g L−1. It can be seen from Figure 4 that the morphology and adhesion of the coating are better. Therefore, the optimum concentration of ZnSO4·7H2O is 30 g L−1 in the zinc dipping solution.
Electroless nickel-plating coating thickness under different concentrations of ZnSO4·7H2O. Surface morphology and cross-section morphology of the electroless nickel-plating coating when the concentration of ZnSO4·7H2O is 30 g L−1.
Orthogonal experiment
Orthogonal experimental table.
Range analysis
The range is also called the range error, which represents the difference between the maximum and minimum values. Generally speaking, the range of each column is not equal, which indicates that the effect of the level change of each factor has distinct effects on the test results. The greater range indicates that the level of this factor has the greatest impact on the test results.
Range analysis of different factors.
Verification experiment
Through analysis of the orthogonal experiment, optimized formula and optimal process conditions are as follows: C4H6O5 40 g L−1, ZnSO4·7H2O 40 g L−1, NaF 4 g L−1, pH 9 and temperature 90°C, time 10 min.
Figure 5 shows the surface morphology and cross-section morphology of the electroless nickel coating prepared under the optimum conditions. It can be seen from Figure 5 that the electroless nickel coating prepared under this condition has a smooth surface morphology, uniform particle size, all the coating covers the substrate, and the adhesion between the film and the substrate is better. The optimum formula and process conditions of the zinc dipping solution obtained by the orthogonal experiment was verified.
Surface morphology and cross-section morphology of the electroless nickel coating prepared under the optimum conditions (a: surface morphology and b: cross-section morphology).
Corrosion rate analysis
Figure 6 is the corrosion rate curve of the AZ91D magnesium alloy immersed in 3.5 wt-% NaCl solution. It can be seen from Figure 6 that the corrosion rate of magnesium alloys decreases gradually from 0.627 mg cm−2 h−1–0.463 mg cm−2 h−1 as the immersion time goes on. The corrosion rate curves of the AZ91D magnesium alloy after coating treatment in 3.5 wt-% NaCl solution for different time are shown in Figure 7. The corrosion rate of nickel coating in solution increased from 0.0471–0.1209 mg cm−2 h−1 with the increase of immersion time. Therefore, the corrosion rate of magnesium alloys after electroless nickel plating is much lower than that of untreated magnesium alloys. At the same time, after electroless nickel plating treatment, the corrosion rate is relatively slow, indicating that electroless nickel coating can enhance the corrosion resistance of magnesium alloy.
Corrosion rate curve for the AZ91D magnesium alloy. Corrosion rate curve for electroless nickel plating after immersing in 3.5wt-% NaCl solution.
Phase composition of the electroless nickel coating
Figure 8 shows the XRD analysis of the magnesium substrates with electroless nickel coating. It can be seen from Figure 8 that the typical characteristic diffraction peak is at 2θ = 45°, with a broad peak, which indicates that the coating belongs to an amorphous structure. The peak value of other materials is weak and the substrate material can hardly be detected, indicating that the coating is thick and dense.
The XRD patterns of electroless nickel plating.
Potentiodynamic polarisation
The polarization curve is a commonly used electrochemical method to study the corrosion mechanism of metal. It can obtain the conventional electrochemical characteristics of the AZ91D magnesium alloy, such as self-corrosion current density, self-corrosion potential and so on. To obtain the corrosion current, the Tafel curve extrapolation method can be used to treat the polarization curve. The Tafel line extrapolation formula is as follows:
In the above formula,
is the slope of the anodic polarization curve,
is the slope of cathodic polarization curve and
is the corrosion current density.
In this test, the sample was placed in 3.5 wt-% NaCl solution, and the dynamic potential polarization curve after corrosion was tested to further study the corrosion resistance of the electroless nickel plating layer on the surface of the magnesium alloy. Figure 9 shows the polarization curves of the electroless nickel-plating coating and the Mg alloy substrate. It can be seen from Figure 9 that the self-corrosion potential of the substrate was –1.471 V. After electroless nickel, the self-corrosion potential of the magnesium alloy was distinctly shifted to the right side and the corrosion potential was –0.602 V, compared with the magnesium alloy substrate improving to 0.869 V. Corrosion current decreased from 1.24 × 10−4–1.26 × 10−6 A cm−2, the self-corrosion current density decreased by two orders of magnitude, the passivation range of the coating was –0.6–0.2 V. It can be clearly seen that the nickel coating plays a protective role on the magnesium alloy substrate. Through the polarization curves analysis, it was found that the corrosion resistance of magnesium alloy after electroless nickel-plating was enhanced than the substrate.
Polarization curves of the electroless nickel-plating coating and the Mg alloy substrate.
Conclusions
This paper demonstrated that it is feasible to electroless nickel on the AZ91D magnesium alloy, the main conclusions are summarised as follows:
The optimum complexing agent for magnesium alloy zinc pretreatment solution is malic acid. A suitable orthogonal test optimised for Mg alloy electroless nickel plating was developed, the optimum formulation and process conditions were as follows: C4H6O5 40 g L−1, ZnSO4·7H2O 40 g L−1, NaF 4 g L−1, pH 9, temperature 90°C and time 10 min. The longer the immersion time of the AZ91D magnesium alloy sample in corrosion solution, the lower the corrosion rate of the AZ91D magnesium alloy. When the AZ91D magnesium alloy is treated by electroless nickel plating, the longer the immersion time, the more increases in corrosion rate, and finally stabilizes after 72 h. The corrosion resistance of the AZ91D magnesium alloy can be improved greatly by electroless nickel treatment. Compared with the magnesium alloy substrate, the self-corrosion potential of the electroless nickel-plating coating improved by 0.869 V. The SEM studies revealed that the Mg substrate was completely covered with nickel coating. The XRD showed that the coating was mainly made up of nickel.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.