Ni-P-ZrO2 electroless coatings on AZ31 magnesium alloy with improved corrosion resistance

Surface & Coatings Technology 261 (2015) 161–166

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Double-layered Ni-P/Ni-P-ZrO2 electroless coatings on AZ31 magnesium alloy with improved corrosion resistance Xin Shu a,b,⁎, Yuxin Wang b, Chuming Liu a, Abdullah Aljaafari c, Wei Gao b a b c

School of Materials Science and Engineering, Central South University, Chang Sha 410083, China Department of Chemical and Materials Engineering, University of Auckland, PB 92019, Auckland 1142, New Zealand College of Science, King Feisal University, Hofuf, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 28 July 2014 Accepted in revised form 15 November 2014 Available online 25 November 2014 Keywords: Sol-enhanced electroless deposition Ni-P-ZrO2 composite coating AZ31 magnesium alloy

a b s t r a c t The double-layered electroless deposition of Ni-P/Ni-P-XZrO2 coatings (where X refers to the amount of ZrO2 sol (mL/L) added to the plating solution) on AZ31 magnesium alloy was investigated. The inner Ni-P layer contained high phosphorus content while the outer layer was a sol-enhanced Ni-P-ZrO2 composite coating with a lower phosphorus content. The morphology, phosphorus content, hardness and corrosion resistance of the coating were investigated. The results show that the corrosion resistance of coating has been improved greatly due to the high phosphorous inner coating layer. The corrosion potential moves positively to more than 1000 mV, and the corrosion current density is reduced by one order of magnitude. Double-layered Ni-P/Ni-P-XZrO2 coatings can resists salt spray test for more than 480 h. In addition, the hardness of coating is improved significantly from 640 HV to 820 HV due to the sol-enhanced nano ZrO2 co-deposited in the outer layer. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Magnesium alloys, as the lightest structural metallic materials, have received increasing interest for applications in automotive, aircraft, aerospace and electronic device industries due to its unique properties such as low density, high specific strength, good thermal conductivity, high dimensional stability and good electromagnetic shielding [1–3]. However, the wide application of magnesium alloys is hindered by their high chemical reactivity, which makes most magnesium alloys highly susceptible to chemical attack in a corrosive environment [4–8]. Many different types of protective coatings have been developed for Mg alloys, for example, chemical conversion coatings [9], plasma electrolytic oxidation (PEO) coatings [10,11] and electrochemical coatings [12]. Among all these coating technologies, electroless Ni-P coatings possess high corrosion and wear resistance and have been successfully utilized to protect various substrate materials including magnesium alloys [13–15]. The properties of electroless Ni-P coating are dependent on the phosphorus content [14]. It is generally accepted that high P coating possess relative high anticorrosion property while medium and low P containing coatings have relative high hardness [16,17]. A combination of different coatings can maximize the functionality of coating by utilizing the differing properties of each layer. For instance, Zhang deposited Ni-P/Ni-B duplex coatings on AZ91D magnesium alloy with higher hardness and better corrosion resistance [18]. Narayanan made ⁎ Corresponding author at: Department of Chemical and Materials Engineering, University of Auckland, PB 92019, Auckland 1142, New Zealand. E-mail address: [email protected] (X. Shu).

http://dx.doi.org/10.1016/j.surfcoat.2014.11.040 0257-8972/© 2014 Elsevier B.V. All rights reserved.

graded electroless Ni-P coating with low, medium and high P content to improve corrosion resistance [19]. Gu used multilayer coating to protect AZ91D magnesium alloy and obtained improved corrosion performance [20]. For electroless Ni-P coating, heat treatment can significantly increase the hardness while the corrosion resistance is negatively affected due to nickel phosphide precipitation and Ni crystallization [17]. Ni crystallization results in shrinkage of coating, which will interfere with the protective nature of electroless Ni-P coating on Mg alloy. This is due to the electroless Ni-P coating being a barrier coating on Mg alloy substrate and the shrinkage creating crevices via which chemical and electrochemical attack of the substrate may occur. Another drawback specific to magnesium alloy substrate is that heat treatment will influence the microstructure of substrate if the temperature rises above 473 K [21]. Consequently, significant magnesium alloy research in the last decades has focused on methods to improve the mechanical properties of the Ni-P coatings while avoiding heat treatment and keeping a high corrosion resistance. Recently, we developed a novel sol-enhanced electro and electroless plating technology [22–25]. A small amount of oxide containing sol has been added into the traditional electrolyte to produce in situ composite coatings with highly dispersive nanoparticles. By virtue of this novel method, the mechanical properties of coatings have been significantly improved. For example, the sol-enhanced nanocomposite Ni-P-TiO2 coating achieves hardness greater than 1000 HV without heat treatment. This paper reports our work on developing a duplex Ni-P/Ni-P-ZrO2 coating on AZ31 substrate with improved mechanical and corrosion

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Table 1 Bath composition and electroless plating parameters. Bath A (inner layer bath)

Bath B (outer layer bath)

Bath constituents

Quantity

Conditions

Bath constituents

Quantity

Conditions

NiCO3 · 2Ni(OH)2•4H2O NaH2PO2•H2O Lactic acid Citric acid HF (40%) Pb2+

15 g/L 30 g/L 10–20 mL/L 10–20 g/L 30–50 mL/L 1 mg/L

pH 4.5–5.0 Agitation: 200 rpm Temperature: 80 °C Time: 120 min

NiSO4•6H2O NaH2PO2•H2O CH3COONa NH4HF2 Thiourea ZrO2 sol

15 g/L 14 g/L 10–15 g/L 5–10 g/L 1 mg/L variable

pH 6.0–6.4 Agitation: 200 rpm Temperature: 80 °C Time: 90 min

properties. A high corrosion-resistant Ni-P coating is deposited as the inner layer and the sol-enhanced coating technology is utilized to form Ni-P-ZrO2 coating as the outer layer. The effect of sol concentration on the microstructure, microhardness and corrosion property of the coatings are discussed. 2. Experimental Commercial magnesium alloy AZ31 plates with dimension of 40 × 25 × 3 mm3 were used as substrates. Specimens were mechanically polished by SiC papers to a grit of #1200. Before electroless plating, the specimens were cleaned at 60 °C for 10 min in a solution containing 50 g/L NaOH and 10 g/L Na3PO4 · 12H2O, acid washed in 110 mL/L HNO3 and 125 g/L CrO3 solution for 40 s at room temperature (about 25 °C), then activated by 38.5 vol.% HF for 10 min at room temperature (about 25 °C). A water rinse was performed between each step and prior to the electroless plating. Two consecutive electroless processes were carried out in two different plating baths designated as bath A and bath B. Details of the bath composition and the operation parameters of these baths are listed in Table 1. In bath A, basic nickel carbonate is the nickel source. Higher sodium hypophosphite concentration and lower pH were used to access higher phosphorus content of the inner layer coating. All specimens were first plated for 2 h in electroless bath A to deposit high phosphorus inner layer. Specimens were then plated in electroless bath B with differing amount of ZrO2 sol in order to improve the hardness of outer layer. The

transparent ZrO2 sol used in this study was prepared using two steps; details of the preparation process may be found elsewhere [26]. ZrO2 sol was added to Bath B prior to heating the solution under 200 rpm constant agitation. For convenience of reporting, we use the nomenclature duplex Ni-P/ Ni-P-XZrO2 coating where X refers to the amount of sol (mL/L) in the outer coating solution to represent the different double-layer coatings. For instance, duplex Ni-P/Ni-P coating means coating bath B did not have ZrO2 sol; duplex Ni-P/Ni-P-25ZrO2 coating means 25 mL/L ZrO2 sol was added in the coating bath B. For comparison, single-layer Ni-P coating deposited in bath A was prepared which designated as inner Ni-P coating. The morphologies and elemental composition of the duplex Ni-P/NiP-XZrO2 coating were inspected by FEI Quanta 200 F FEG ESEM with energy-dispersive spectroscopy (EDS) system. The crystal structure of coating was studied using Bruker D2 Phaser X-ray diffraction (XRD) with Cu target. The Vickers microhardness of sample surface was measured by using a load of 50 g with a loading time of 15 s. Nine measurements were taken on each sample and the average value was reported. Corrosion resistance was evaluated by the electrochemical method; salt fog spray test and porosity test. The electrochemical tests were carried out by CHI604D electrochemical workstation using a classical three-electrode cell with platinum as the counter electrode; Ag/AgCl as reference electrode; and the coating samples with 1 cm2 exposed area as the working electrode. All samples for electrochemical test were left standing for 600 s prior the test to obtain stable open circle

Fig. 1. Surface morphology of Ni-P coatings: (a) monolayer Ni-P coating, (b) duplex Ni-P/Ni-P coating, (c) duplex Ni-P/Ni-P-25ZrO2 coating, (d) duplex Ni-P/Ni-P-50ZrO2 coating.

X. Shu et al. / Surface & Coatings Technology 261 (2015) 161–166

potential (OCP). Potentiodynamic polarization was measured at a scan rate of 1 mVs− 1 at room temperature (about 25 °C) in 3.5 wt.% NaCl solution. All the potentials in this report are referred to the Ag/AgCl electrode. The salt spray test was conducted according to the ASTM B117 standard (5 wt.% NaCl continuous spray at 35 °C). The variation of porosity of the inner layer Ni-P coating with respect to coating time was tested by a simple method explained in ref. [27] and briefly outlined here. A 1 cm2 filter paper was soaked with solution mainly containing NaCl (10 g/L) and phenolphthalein (0.1 g/L) dissolved in distilled water. Then the filter paper was pasted onto the nickel coating for 10 min. After taking the filter paper away, red spots or red areas were noted on the surface of the coating. The relative porosity of coating is evaluated using the ratio of red spot area to the total area covered by the filter paper. The mechanism of this method is discussed in ref. [28]. 3. Results and discussion 3.1. Surface and cross-sectional morphologies of coatings Surface morphologies of the coatings are shown in Fig. 1. All coatings show a typical nodular structure which is common for electroless Ni-P coating. The nodular size of the monolayer Ni-P coating is smaller than the duplex coatings. This is possibly due to the differing amounts of phosphorous in two layers. According to Ni-P binary phase diagram [29],

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phosphorous has very low solid solubility in Ni crystal at room temperature. During electroless plating, phosphorous tends to gather at the grain boundaries of Ni [30]. Consequently, the grain size of high phosphorous electroless Ni-P coating is so small that it exhibits an amorphous structure. The deposited phosphorus inhibits the growth of Ni grains. Thus, high phosphorous inner layer show small nodular size than low phosphorous outer layer due to smaller grain size and increased nucleation sites of Ni grain. A subset of the investigated surface morphologies for duplex coatings with different sol doping concentration is shown in Fig. 1 (b–d). Similar surface morphologies existed for sol concentrations of less than 25 mL/L ZrO2. However, when the sol doping concentration in the bath exceeded 25 mL/L, small pores appear on the surface. These are due to ZrO2 particles being washed away by ultrasonic cleaning post plating, as shown in Fig. 1(d). Two reactions occur simultaneously in the sol-enhanced electroless plating process; these are briefly outlined below. One is the redox reaction of Ni2+ and H2PO− 2 to form Ni-P coating, which can be briefly expressed as follows [31]: 2þ

Ni

þ 2H2 PO2

−

catalyst

þ 2H2 O → Ni þ 2H2 PO3

−

þ

þ 2H þ H2

The other is the hydrolysis and condensation of Zr containing sol to form ZrO2 nanoparticles; these particles are incorporated in situ as they are created in the Ni-P coating matrix to form highly dispersive

Fig. 2. Cross-sectional image and line scan of duplex coatings: (a) duplex Ni-P/Ni-P coating, (b) duplex Ni-P/Ni-P-5ZrO2 coating, (c) duplex Ni-P/Ni-P-20ZrO2 coating, (d) duplex Ni-P/Ni-P25ZrO2 coating, (e) duplex Ni-P/Ni-P-50ZrO2 coating, (f) element line scan of Ni-P/Ni-P-50ZrO2 coating.

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Fig. 3. XRD spectra of Ni-P coatings. Fig. 5. Microhardness of duplex Ni-P/Ni-P-XZrO2 coatings.

composite coating. In our previous work [26], the size of the ZrO2 nanoparticles was found to be around 20 nm. If the sol concentration in the coating bath exceeds a certain level, the nanoparticles will tend to agglomerate, as shown in Fig. 2(e). The simplified hydrolysis reaction [32]:

This experiment used a transparent ZrO2 sol utilizing ethanol as solvent. The condensation process of organic metal macromolecule ions started before the completion of hydrolysis, so the formation of ordered structure was hindered. The as-synthesized ZrO2 nanoparticles were amorphous in the electrolyte [33]. When ZrO2 sol was added into the electroless plating solution, the concentrated hydrate Ni ions in the solution destabilized the sol, leading to polymerization of sol. Once the nanoparticles formed in the electrolyte, some of them were immediately adsorbed onto the freshly deposited surface. The ethanol and diethanolamine (DEA) probably also contributed to the dispersion of the ion-adsorbed ZrO2 nanoparticles [34].

The cross-sectional image of coatings and a qualitative elemental distribution line scan are shown in Fig. 2. The total thickness of the coating is about 40 μm with ~ 15 μm inner layer and ~ 25 μm outer layer. There is no significant difference on thickness of coatings with differing amounts of sol addition. The interface between substrate and inner layer coating is very rough due to grain boundary attack by acid pickling treatment. This can enhance the coating adherence because certain surface roughness improves mechanical interlocking between coating and substrate. The interface between two layers is uniform and clean without any blended impurities. Mechanical breakage of coated duplex Ni-P/Ni-P-25ZrO2 samples show the coating failure is between substrate and inner layer coating. This indicates a good adherence between two layers. No apparent difference on the coating morphology is shown between sol-free and sol-added coatings. EDS results indicate that phosphorus content of outer layer is not influenced by sol doping. There are some tiny black spots deposited in the coating as pointed by an arrow in Figs. 2(c) and (d). When increasing the added sol to 50 mL/L, larger precipitate areas were distributed randomly in the coating as shown in Fig. 2(e). The line scans results of duplex Ni-P/Ni-P-ZrO2 coating are presented in Fig. 2(f), showing that the precipitate is rich in Zr. The quantitative analysis indicates 19.1 wt.% Zr in the black precipitate of scan area in Fig. 2(f). The rest is Ni, P, O and some other impurities. However, the dispersion strengthening effect on the coating mainly depends on the particle size and distribution in the coatings. Large particles are likely to have limited effect in improving the coating hardness. Fine nanoparticles with a uniform distribution

Fig. 4. XRD spectra of duplex coatings.

Fig. 6. Polarization curve of coatings in 3.5% NaCl solution.

H2 O

H2 O

‐ROH

‐ROH

ZrðORÞ4 → ZrðOHÞðORÞ3 → ZrðOHÞ2 ðORÞ2 H2 O

H2 O

‐ROH

‐ROH

→ ZrðOHÞ3 ðORÞ → ZrðOHÞ4 Here − R represents − CH2CH2CH3 as the precursor in current work is Zirconium (IV) propoxide. The simplified condensation reaction: ‐H2 O

ZrðOHÞ4 → ZrO2

X. Shu et al. / Surface & Coatings Technology 261 (2015) 161–166 Table 2 Corrosion potential and corrosion current density obtained from polarization curves. Sample

Ecorr (V) vs. Ag/AgCl

icorr (A · cm−2)

AZ31 substrate Monolayer Ni-P coating Duplex Ni-P/Ni-P coating Duplex Ni-P/Ni-P-25ZrO2 coating Duplex Ni-P/Ni-P-50ZrO2 coating

−1.458 −0.178 −0.242 −0.321 −0.303

6.676 1.243 1.054 3.741 1.028

× × × × ×

10−6 10−6 10−6 10−7 10−6

have significant strengthening effect according to the Orowan mechanism [35]. The double-layered structure with different phosphorus content shows obvious contrast under solid-state backscattered detector (SSD mode). EDS results show that phosphorus contents in the inner and outer layer layers are 11.8 wt.% and 5.0 wt.%, respectively. Phosphorus gradient of two layers can be clearly seen in Fig. 2(f) as shown by the arrow. It is of great importance to control the microstructure and composition of different coating layers in order to optimize the function of each layer. 3.2. Phase structures of coatings XRD patterns of monolayer coating and duplex Ni-P/Ni-P coating are shown in Fig. 3. Monolayer Ni-P coating with high P content shows a wide peak of Ni (111), which is an evidence of amorphous structure. Duplex Ni-P/Ni-P coating presents a sharp and high intensity peak, indicating the crystal structure with Ni (111) texture of the outer layer. Coating layers with different P content exhibit different crystal structures, in agreement with the EDS results of coating layers. XRD results of duplex coatings with different amount of sol additions are shown in Fig. 4. Although sol doping does not change the morphology of the deposit, it affects the crystallographic characteristics of the coatings. The nickel grains are more randomly orientated with sol addition. It was reported that Ni (200) and Ni (220) peaks attributed to the differing P content in the coating [36,37]. In the current work, P content in coatings with different concentrations of sol are almost identical according to EDS analyses. The Ni (111) peak intensity (not shown in the picture) is similar for all deposits. The texture of deposit is weakened by sol addition [22]. In current work, XRD patterns exhibited no ZrO2 peak. This is possibly due to the following reasons. First, the very small amount of ZrO2 content compared with Ni. Second, the ZrO2 nanoparticles formed by sol-gel process are amorphous in nature. Thus, it is likely that in current work the ZrO2 nanoparticles formed in situ via hydrolysis and condensation reactions are amorphous as well. Consequently, there is no ZrO2 peak detected by XRD.

low phosphorus Ni-P coatings. When the amount of sol added in the bath is between 5 and 15 mL/L, the hardness of coatings is only increased to ~ 720 HV, probably due to the insufficient addition of ZrO2 in the coating. With a higher concentration of sol, nano ZrO2 particles played a significant role and the hardness increased sharply to ~ 820 HV. However, further increase sol content to 50 mL/L decrease the hardness to ~730 HV. This can be explained by the larger agglomerated ZrO2 particles in the coating lessening the dispersion strengthening effect. It is proposed here that there exist two opposite effects of increasing the amount of sol in coating solution: more ZrO2 nano particles will be formed in the solutions as well as deposited in Ni-P coating; on the other hand, the agglomeration effect dominates if too much ZrO2 is used. Thus, an optimum concentration of ZrO2 sol is found. 3.4. Corrosion performance Fig. 6 shows the electrochemical polarization curves of AZ31 Mg alloy substrate, monolayer Ni-P coating and duplex Ni-P/Ni-P-XZrO2 coatings in 3.5 wt.% NaCl aqueous solution at room temperature (about 25 °C). It can be seen that all coatings show great positive shifts in corrosion potential and evident decrease in corrosion current density in comparison with the AZ31 alloy substrate. Corrosion potential and corrosion current density were summarized in Table 2. Monolayer Ni-P coating with high P content has the most positive potential and is believed to have the best corrosion resistance. It should be noted that the corrosion potential of the sol-enhanced coating was slightly more negative than the sol-free coating. However, the passivation plateau of the sol-enhanced coating was much wider and the corrosion current density was decreased more in the passive state. When the potential was more positive, the passive film broke down and the Ni in the coating was dissolved [38]. The salt fog spray test results of the substrate and coatings are summarized in Table 3. The surface of AZ31 magnesium alloy deteriorates within 24 h and form black-rough rust. Single Ni-P coating failed after 86 h. All duplex coatings show very good resistance to the salt fog attack regardless of sol doping. Only a black corrosion spot appeared on the surface of duplex Ni-P/Ni-P-15ZrO2 coating after 220 h. Permeation into the substrate does not happen till the end of salt spray test. The coating surface is still in good condition after more than 480 h exposure in salt fog chamber. It is believed that the observed black spot is a pinhole probably formed due to H2 release during electroless plating in bath B. Due to anodic magnesium substrate and cathodic coating, coating the sufficient thickness of pore-free coating is essential to provide a physical barrier. Any porosity in coatings will influence the service lifetime of the electroless nickel-plated magnesium [39]. The porosity and thickness of the inner layer Ni-P coating with different coating time are summarized in Fig. 7. The coating grows linearly with coating

3.3. Coating hardness Fig. 5 shows the microhardness of duplex Ni-P/Ni-P-XZrO2 coatings as a function of sol concentration in coating bath. The microhardness of coating without sol addition is about 640 HV, which is common for

Table 3 Salt spray time and failure type. Sample

Time (h)

AZ31 substrate

24

Monolayer Ni-P coating

86

Duplex Ni-P/Ni-P coating

480

Duplex Ni-P/Ni-P-25ZrO2 coating

480

Duplex Ni-P/Ni-P-50ZrO2 coating

480

Failure type Uniform corrosion with black and rough corrosion product Penetrating pitting corrosion with coating pilling off Controlled pitting corrosion with substrate unaffected Controlled pitting corrosion with substrate unaffected Controlled pitting corrosion with substrate unaffected

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Fig. 7. Porosity and thickness of inner layer Ni-P coating.

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hardness of the coating improved significantly by adding a suitable amount of Zr containing sol. This controllable duplex electroless coating system possesses excellent anticorrosion and mechanical properties. Further research will be focused on the development of its real applications.

Acknowledgments

Fig. 8. Schematic diagram of dual layer structure to block corrosion media.

time. However, the coverage of the substrate by the coating is not complete until 80 min of plating. This is due to the low plating rate of this coating bath, which combines a low PH with the use of strong complexing agent to ensure the high phosphorous content of the coating. Only one red spot was found in the sample plated for 80 min. For the coatings plated for periods greater than 80 min, no red area was observed. It is assumed that that relatively low plating speed minimizes defects formed in the coating due to H2 release. To make the coating even more corrosion resistance, the second layer was applied. Dual structure effectively blocks the permeation of corrosive media to substrate. As shown schematic in Fig. 8, even if penetrating holes developed in the outer layer, the chance of the corrosive media reaching the substrate is small due to mismatch of coating defects in two layers. The main aim of dual coating is to enhance the corrosion performance. High phosphorous inner layer, due to its amorphous nature, is believed to have a compact structure and less diffusion paths such as grain boundaries and thus show good corrosion performance [40]. Relatively, the low deposition rate of high P inner layer reduced the chance of pinholes caused by H2 release. On the other hand, sol-enhanced low P outer layer coating can provide satisfactory mechanical properties. Hard outer layer provides excellent mechanical protection of the coating system and allows the relatively soft inner layer to be preserved for a longer period, increasing the service life of a duplex-coated part, thus further expanding the application of Ni-P coatings. Some works have been conducted on dual or multilayer Ni-P coatings to provide good corrosion resistance. While most researchers believe that outer layer coating should be more corrosion resistant because it shows better corrosion behavior under electrochemical experiment, we suggest that inner layer can be more corrosion resistance. For long service life of coated components, the combination of more corrosion resistance inner layer and mechanically stronger outer layer will protect substrate from a harsh environment. The current work shows that the combinations of slightly different coatings possess very good corrosion resistance, and the strong outer layer provides the coating system with superior mechanical properties.

This work is part of a Marsden Project supported by the Royal Society of NZ. The authors would like to thank the technical staff in Department of Chemical and Materials Engineering and Research Centre for Surface and Materials Science for various assistances. The authors also acknowledge the support from Glen Slater and Chris Goode. Xin Shu is supported by The China Scholarship Council (CSC).

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