Characteristics and corrosion studies of zinc–manganese phosphate coatings on magnesium–lithium alloy

Abstract

A zinc–manganese based conversion coating on magnesium–lithium alloy has been prepared from a phosphate solution at different pH values. The surface morphology and composition of the coating were characterized by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Corrosion resistance of the coating was studied by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and corrosion weight loss measurement. The experimental results indicated that the coating had large crystals that were homogeneous and ordered. The coating consists of Zn, Zn₃(PO₄)₂ and MnHPO₄. The corrosion resistance of the magnesium–lithium alloy was improved by the phosphate conversion treatment.

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1. Introduction

Magnesium–lithium alloys are the lightest structural metallic alloys currently available.^1,2 They possess high specific strength, excellent formability, impact properties, damping capacity and resistivity to energetic particle penetration. The addition of lithium to magnesium causes a considerable weight reduction compared to conventional magnesium alloys. Undoubtedly, they are the most promising materials in lightweight engineering applications, such as in the aerospace, weapons, automotive and electronics industries, among others.³ Magnesium–lithium alloys have many advantages: high electrical and thermal conductivity, good ratio of strength to weight, high ability to dampen shock waves, easily formed at room temperature, and so on.⁴ However, Mg–Li alloys have some disadvantages, particularly their poor corrosion resistance and wear. The corrosion of Mg–Li alloys is more serious than other Mg alloys because Li possesses high chemical activity, which greatly hinders the use of Mg–Li alloys in practice. Therefore, it is urgently necessary to find a suitable protection method to improve the corrosion resistance of magnesium–lithium alloys.

A number of methods are available to protect magnesium alloys against corrosion, including conversion coating,⁵ anodizing,⁶ vapor-phase processes,⁷ electroless plating,⁸ organic treatment, diamond-like carbon (DLC) coating⁹ and laser surface modification technique.¹⁰ Among them, the conversion coating treatment is widely used because of its comparatively cheap and easy operation. Conversion coating based on chromate ions is the traditional method. However, chromate is unfriendly to the environment. The use of chromate solution is being progressively restricted due to the high toxicity of hexavalent chromium compounds.¹¹ Recently, a number of chromate-free conversion coatings have been developed, such as molybdate conversion coatings, rare earth conversion coatings and stannate conversion coatings.^12–14 Several phosphate conversion coatings on magnesium alloys have been obtained. Mosiałek et al.¹⁵ prepared conversion coatings on the AZ81 magnesium alloy by using different compositions of phosphate-permanganate baths. Zeng et al.¹⁶ prepared zinc phosphate coating and zinc–calcium phosphate coating on the surface of AZ31 Mg alloy in phosphate baths. Zn–Mn phosphate coatings are currently used on steel, conferring better corrosion resistance. Nikdehghan et al.¹⁷ investigated the corrosion resistance of Zn–Mn phosphate coatings on the steel surface. However, little information about zinc–manganese phosphate conversion coating treatment on magnesium–lithium alloys has been reported.

The aim of this study is to develop the zinc–manganese phosphate conversion coating on magnesium–lithium alloy. The process only employs Zn(H₂PO₄)₂ and Mn(NO₃)₂ as the essential precursors, which makes the coating process easier and greener. Effect of pH on the corrosion resistance of the zinc–manganese phosphate conversion coating is investigated. The morphology and composition of this coating are also examined.

2. Experiments

2.1. Preparation of conversion coating

Mg-8.5Li alloy is employed for this study. Its chemical composition is as follows (in wt%): Al 3.2, Li 8.5, Y 1.2, Ce 1.2 and Mg balance. All samples are from the same kind of casting and prepared from Mg–Li alloys with the size of 15 mm × 15 mm × 7 mm.

Prior to formation of conversion coating, specimens were successively polished with 800#, 1500#, 2000# and 3000# abrasive paper to obtain a smooth surface and pretreated with alkaline degreasing and acid pickling. Samples were then treated in phosphate solution and rinsed twice in flowing distilled water between each step to remove all the contaminants. The phosphate bath for deposition of conversion coating contained 50 g L⁻¹ Zn(H₂PO₄)₂, 20 g L⁻¹ NaH₂PO₄, 30 g L⁻¹ 50% Mn(NO₃)₂, 5 g L⁻¹ C₆H₈O₇, and 0.2 g L⁻¹ C₁₈H₂₉NaO₃S, with the pH value adjusted by H₃PO₄ to 1.8–2.6. The temperature was 45 °C and deposition time was 15 min. Samples were then dried in hot air.

2.2. Characterization of the coating

The morphology of the conversion coating was examined by JSM-6480A scanning electron microscopy (SEM), and the energy spectrum images were analysed using energy dispersion spectroscopy (EDS).

The chemical composition of the coating was probed using Physical Electronics PHI 5700 EICA X-ray photoelectron spectroscopy (XPS) with Al Kα(1486.6 eV) monochromatic source. Data was taken after 120 s of ion etching. All energy values were corrected according to the adventitious C 1s signal, which was set at 284.62 eV. The data were analyzed with XPSPEAK 4.1 software.

The crystal structure of the alloy and zinc–manganese phosphate conversion coating were determined by X-ray diffraction (XRD) with a glancing incident angle of 2°. The detected angle was from 10° to 80° at a speed of 15° min⁻¹. The XRD pattern was analyzed with MDI Jade 5.0 software.

The corrosion resistance of zinc–manganese phosphate conversion coating was studied by electrochemical tests and weight loss measurement. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were conducted using a commercial-model CHI760B electrochemical workstation in a three-electrode system with the sample (1 cm × 1 cm surface area) as working electrode (WE), saturated calomel electrode as reference electrode (RE) and a platinum sheet with 1 cm × 1 cm surface area as counter electrode (CE). The corrosive medium was 3.5 wt% NaCl solution. The potentiodynamic polarization curves were tested at a scan rate of 0.01 V s⁻¹. The corrosion potential and corrosion current density were calculated by Tafel extrapolation method using the CHI version 6.28 software (by CH Instruments). The EIS measurements were carried out at open circuit potential, and the frequency range was between 0.01 Hz and 100 000 Hz. All of the measurements were carried out at room temperature.

The weight loss experiments were carried out using Mg–8.5Li alloy with phosphate conversion coatings. The samples were firstly dried and weighed (m₁), and then suspended in 100 mL solution of 3.5 wt% NaCl for 4 h at 25 °C. Secondly, each sample was washed with distilled water, then acetone in an ultrasonic bath, then dried and weighed again (m₂). Weight loss measurements were made in triplicate, and the loss of weight was calculated by taking an average of these values.

3. Results and discussion

3.1. Surface morphology and chemical composition of the conversion coatings

After phosphate conversion, a smooth gray coating formed on the Mg–Li alloy surface. Fig. 1 shows the surface morphology of Mg–Li alloy after treatment, during which it is immersed in zinc–manganese conversion solution at 45 °C and pH 2.0. The coating nearly covers the entire surface of the alloy. The conversion coating consists of crystal clusters with some cracks. It can be observed that the crystal clusters embrace together compactly in the joints. The formation of cracks is possibly due to the release of hydrogen via the chemical reaction during conversion. It is obvious that the compact crystal clusters can afford better protection than rare earth conversion coating and molybdate/permanganate conversion coating, which has a dry-mud shape.

Fig. 1 Surface morphology of zinc–manganese phosphate conversion coating on Mg–Li alloy (a) 200×, (b) 1000×.

The cross-sections of the coating at different pH levels have been analyzed as shown in Fig. 2. Fig. 2 shows that the substrate is corroded, and part of the conversion coating is not completed. The conversion coatings present crystal clusters, and because the Mg–Li alloy is reactive, the incompletely coated Mg–Li alloys are damaged by the corrosion ion when the samples are polished. The pH influences growth of the coating. The coating prepared at pH 2.0 is the thickest, which indicates that the coating at pH 2.0 may have a certain protective effect on the substrate.

Fig. 2 The cross-section view of the coating at (a) pH 1.8, (b) pH 2.0, (c) pH 2.2, (d) pH 2.4, and (e) pH 2.6.

XPS spectroscopy is a material characterization technique that is widely used to investigate the chemical composition of materials. The XPS results of the Zn–Mn phosphate conversion coatings are displayed in Fig. 3.

Fig. 3 XPS analyses of the Zn–Mn phosphate conversion coating.

Fig. 3a shows the survey spectra of the Zn–Mn phosphate conversion coatings obtained by XPS, and Table 1 shows the detailed XPS results of surface atomic compositions of the conversion coatings. The coating predominantly contains Mg, O, P, Mn and Zn elements. The XPS high-resolution scanning spectra of Mg 1s, P 2p, Mn 2p, Zn 2p and O 1s are depicted in Fig. 3b–f, respectively.

Table 1 XPS results of surface element compositions of Zn–Mn phosphate conversion coating

Element	Al	P	O	Mn	Zn	Mg
Atomic%	2.98	15.35	64.69	0.46	3.38	13.16

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The XPS spectrum of Mg 1s displayed in Fig. 3b indicates that the peaks at 1303.90 eV and 1302.7 eV correspond to MgO and Mg(OH)₂. Fig. 3c gives the XPS data of Mn 2p spectra peaks at 643.5 eV and 642.5 eV. The result proves that the conversion coating comprises the compounds Mn₃(PO₄)₂ and MnHPO₄. Fig. 3d presents the high resolution XPS spectrum of O 1s, which is divided into peaks at 532.5 eV, 531.2 eV, 531.3 eV and 531.7 eV. The peaks at 531.2 eV and 532.5 eV are attributed to PO₄³⁻ and HPO₄²⁻. The peaks at 531.7 eV and 531.3 eV correspond to O 1s in MgO and Mg(OH)₂. Fig. 3e shows the high resolution spectrum of P 2p, which can be divided into three peaks: 133.8 eV, 132.9 eV and 132.1 eV. The peak at 133.8 eV corresponds to MnHPO₄, and the peaks at 132.9 eV and 132.1 are attributed to the PO₄³⁻ of Zn₃(PO₄)₂ and Mn₃(PO₄)₂, respectively. Fig. 3f gives the XPS data of Zn 2p spectra peaks at 1021.2 eV and 1021.9 eV, which can be attributed to hopeite and Zn. It can be found that zinc takes the form of elementary Zn and Zn₃(PO₄)₂. The XPS results indicate that the Zn–Mn phosphate conversion coatings consiste of MgO, Mg(OH)₂, Zn₃(PO₄)₂, Zn, MnHPO₄ and Mn₃(PO₄).

The structure of conversion film was characterized by XRD as shown in Fig. 4. The diffraction peaks indicate the existence of Zn, Zn₃(PO₄)₂ and MnHPO₄. According to the intensity and quantity of the diffraction peaks, it can be speculated that the conversion film contains Zn, Zn₃(PO₄)₂, and MnHPO₄. The XRD results show great accordance with the XPS analysis.

Fig. 4 XRD pattern of the phosphate conversion film.

The possible reaction mechanism for the deposition of Zn–Mn phosphate conversion coatings may be explained as follows.

The surface of Mg–Li alloy in the acidic phosphate solution was dissolved to produce Mg²⁺ and Li⁺, accompanied by the hydrogen evolution reaction, resulting in the greatly increased concentration of OH⁻ at the interface of the Mg–Li alloy substrate and the phosphate solution.

Mg − 2e⁻ → Mg²⁺ (1)

Li − e⁻ → Li⁺ (2)

2H₂O + 2e⁻ → 2OH⁻ + H₂ (3)

Then, H₂PO₄⁻ reacts with OH⁻ to form HPO₄²⁻ and PO₄³⁻. In the acidic phosphate solution, most of the HPO₄²⁻ ions preferentially bonded with Mn⁺ to form insoluble MnHPO₄, and most of the PO₄³⁻ ions preferentially bonded with Zn²⁺ to form insoluble Zn₃(PO₄)₂ and Mn₃(PO₄)₂.

H₃PO₄ + OH⁻ → H₂PO₄⁻ + H₂O (4)

H₂PO₄⁻ + OH⁻ → HPO₄²⁻ + H₂O (5)

HPO₄²⁻ + OH⁻ → PO₄³⁻ + H₂O (6)

2PO₄³⁻ + 3Zn²⁺ → Zn₃(PO₄)₂(s) (7)

HPO₄²⁻ + Mn²⁺ → MnHPO₄(s) (8)

2PO₄³⁻ + 3Mn²⁺ → Mn₃(PO₄)₂(s) (9)

3Mg²⁺ + 2PO₄³⁻ → Mg₃(PO₄)₂ (10)

A handful of the Zn²⁺ ions reduced to elemental Zn, which deposited on the conversion film, accompanying the insoluble phosphate. In the acidic phosphate solution, the Zn²⁺ ions from Zn(H₂PO₄)₂ reduced to metallic zinc on the substrate. The reactions are as follows:

Mg + Zn²⁺ → Zn(s) + Mg²⁺ (11)

2Li + Zn²⁺ → Zn(s) + 2Li⁺ (12)

When the phosphate film nearly covers all over the magnesium–lithium alloy surface, the phosphate reaction stops.

Energy dispersive spectra of the phosphate coating are presented in Fig. 5. Fig. 5a shows the surface morphology (×250) of the Zn–Mn phosphate conversion coatings prepared at pH 2.6 and 45 °C. From the microcosmic aspect, elemental mapping (Fig. 5b) was used to determine the elemental distribution of O, Mg, P, Zn, Li and the trace Mn. It can be seen that O, Mg, P, Zn, Li and Mn distribute homogeneously on the surface. Mg and Li belong to the dominant component substrate. It further indicates that both substrate and electrolyte make a contribution to the formation of coating. This is consistent with the XPS result.

Fig. 5 Surface morphology and elemental mapping of the Zn–Mn phosphate coating on the magnesium–lithium alloy: (a) conversion coating treatment under the conditions of 45 °C and pH 2.6, (b) elemental mapping.

3.2. Corrosion resistance of conversion coating

3.2.1. Electrochemistry characterization. Fig. 6 shows the potentiodynamic polarization curves of bare Mg–Li alloy and Mg–Li alloy with Zn–Mn phosphate conversion coating at various pH values in 3.5 wt% NaCl solution. The results for corrosion potential (E_corr) and corrosion current density (I_corr) are listed in Table 2. It clearly shows that the corrosion resistance of the specimen with Zn–Mn phosphate conversion coating is enhanced when pH is 2.0. The specimen with conversion coating shows corrosion potential (V_SCE) that is much more positive than that of the bare alloy (V_SCE). The corrosion current density of the conversion coating is 5.004 × 10⁻⁶ A cm⁻² at pH 2.0, which is one order lower than that of molybdate conversion coating (1.843 × 10⁻⁵ A cm⁻²).¹² Thereby, the corrosion resistance of Zn–Mn phosphate conversion coatings is better than that of the molybdate conversion coatings. Zn–Mn phosphate conversion coatings can provide good protection for the Mg–Li alloy (1.294 × 10⁻⁴ A cm⁻²). These results explain that the samples coated at pH 2.0 have better corrosion resistance than the substrate. In the polarization curves, the cathode reaction corresponds to the formation of hydrogen, and the active dissolution reaction of anodic control is the most important feature which relates to the corrosion resistance.¹⁵

Fig. 6 Potentiodynamic polarization curves of Mg–Li alloys at various pH values in 3.5 wt% NaCl solution.

Table 2 Polarization curve parameters

Samples	Icorr (A cm^−2)	Ecorr (VSCE)
Bare Mg–Li alloy	1.294 × 10^−4	−1.571
pH = 1.8	8.837 × 10^−5	−1.366
pH = 2.0	5.004 × 10^−6	−0.370
pH = 2.2	6.401 × 10^−5	−1.517
pH = 2.4	8.180 × 10^−5	−1.520
pH = 2.6	1.058 × 10^−4	−1.071

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pH influences the growth and thickness of Zn–Mn phosphate coatings, which leads to different corrosion current densities. With decreasing pH value, the corrosion current density of conversion coatings decreases; it reaches minimum at pH 2.0. However, at pH 1.8, the phosphating reaction is violent; too much hydrogen on the Mg–Li alloy surface prevents the phosphide from depositing on the substrate easily; the phosphate conversion coating becomes loose, porous, thin and non-uniform, thus increasing the corrosion current density. When pH value increases, the reaction rate lowers, and it becomes more difficult for phosphide to deposit on the substrate; thus the corrosion current density increases.

The electrochemical impedance spectroscopy of bare Mg–8.5Li alloy and the alloy with phosphate conversion coatings were recorded at open circuit potential in 3.5 wt% NaCl solution. The Nyquist plots are shown in Fig. 7. All the samples contain a single capacitive semicircle according to the plot. The diameter of the capacitance loop of the phosphate samples coated in the pH 2.0 solution is larger than that of the samples coated at other pH values, which indicates better corrosion resistance of the phosphate conversion coating. The diameter of the capacitance loop when the pH value of the Zn–Mn phosphate conversion coating is 2.0 is bigger than that of the molybdate conversion coating¹² and Zn phosphate coating.¹⁶ This indicates that the phosphate conversion coating obtained at pH 2.0 presents the most active behavior, which agrees with the potentiodynamic polarization results.

Fig. 7 EIS plots of Mg–Li alloys prepared at various pH values in 3.5% NaCl solution.

3.2.2. Corrosion weight loss measurement. Corrosion weight loss is an average measurement determined by change of the quality of samples before and after corrosion. Corrosion rate can be calculated from the equation: v = (m₁ − m₂)/st, where m is the weight before and after exposure to corrosion media, (m₁ − m₂) is the weight loss in grams, s is the total surface area in m², t is the time of exposure in h, and v is the corrosion rate in g (m⁻² h⁻¹).

The weight loss method is the most fundamental measurement used to quantify corrosion attack in laboratory experiments. Fig. 8 shows the corrosion rate of phosphate conversion coatings at different pH values in 3.5 wt% NaCl solution at room temperature for 4 h. The sample weights decreased after immersion, which indicated dissolution of the coating. Based on the weight loss data, it is observed that the phosphate conversion coating in the solution at pH 2.0 has a lower corrosion rate, and the order is in accord with the EIS result.

Fig. 8 The change of the corrosion rate at pH value of the substrate and phosphate conversion coatings in 3.5% NaCl solution.

4. Conclusions

A conversion coating with uniform and block-like morphology was formed on the Mg–Li alloy in phosphate solution. The phosphate conversion coating mainly consists of Zn, Zn₃(PO₄)₂ and MnHPO₄. The corrosion resistance of the magnesium–lithium alloy has been improved by the phosphate conversion treatment and is better than the molybdate conversion coatings. When the pH value is 2.0, the coating obviously reduces the corrosion current density, and the conversion coating presents the best corrosion resistance behavior.

Acknowledgements

The authors gratefully acknowledge the financial support of the Fundamental Research Funds for the Central Universities (HEUCF20130910006), Heilongjiang Province Science Foundation for Youths (QC2012C068) and Innovation Talent Research Fund of Harbin Science and Technology (RC2014QN015006).

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