Study of gadolinium based protective coating for magnesium alloys

Preparation of samples

The gadolinium conversion coating was deposited on the substrate of die cast AZ91. The test samples (15×10×5 mm) were polished using waterproof abrasive paper from 360 to 2500 grits, then fine polished using 3·5 μm diamond paste, subsequently degreased with absolute ethanol in an ultrasonic bath for 15 min and, finally, dried by cold air at room temperature.

The optimal technical parameters determined by the orthogonal experiment involved the gadolinium nitrate, Gd (NO₃)₃, with concentrations of 5 g L⁻¹ and 20 mL L⁻¹ of hydrogen dioxide solution, H₂O₂ (30%; Fisher Scientific), used as the accelerant. The obtained solution had been mixed for 5 min with a magnetic stirrer before deposition. The samples were immersed in the conversion solution at 50°C for 20 min. After all of these treatments, the samples were thoroughly rinsed with deionised water and then dried in cold air.

Testing of nanomechanical properties

The load–displacement variation of coatings was measured by TriboIndenter nanoindentation tester. And nanohardness, elastic modulus and surface morphology in three dimensions were obtained in this way. Meanwhile, indentation depth and contact area were obtained while the pressure head impressed were achieved through software equipped in the system. All these data were substituted in empirical formula and, at last, values of residual stress in gadolinium conversion coatings were investigated.

Characterisation of microstructure and corrosion resistance

The micromorphology was observed by an FEI Quant200 SEM equipped with EDS. The elemental chemical states of the conversion coating were studied by XPS. The anticorrosion performance was evaluated by recording the corrosion potential and corrosion current density of the samples in 3·5% sodium chloride (NaCl) solution upon immersion, which was conducted on a CHI660B electrochemical workstation. The electrochemical cell used a classic three-electrode system that consists of a reference electrode (a saturated calomel electrode), a counter electrode (a platinum foil) and a working electrode. The potentiodynamic polarisation curves were performed at a scanning rate of 1 mV s⁻¹. EIS measurements were carried out on the corrosion potential in the frequency range between 0·01 and 100 000 Hz using a 10 mV amplitude perturbation.

Morphology and composition of conversion coatings

The morphology and composition of the gadolinium conversion coating are shown in Fig. 1. The conversion coating was compact, except for some homogeneous fine cracks and some randomly distributed elliptical white tesserae that can be found on the surface of the conversion coating. It can be confirmed from EDS that the white elliptical substances were mainly composed of O, Gd, Mg and Al with the atom percentages of 66·41, 22·75, 7·94 and 2·90 at-% respectively. The white particles might be small crystals of the oxide or hydroxide of gadolinium. The coexistence of Mg and Gd proved that the coating forming process involved the substrate after it dissolved.

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Figure 1

SEM morphology and EDS of surface for gadolinium conversion coating (magnified map is placed at lower right corner)

Figure 2 displays the cross-sectional morphology and elemental area profile of the conversion specimen. The morphology analysis of the cross-section shows that the conversion coating was compact and the thickness was approximately 4–5 μm. The results of the elemental area profile shows that Mg and Al concentrated mainly in the substrate, and simultaneously, and there was a little Mg in the coating that decreased as the thickness of the coating increased. The detection of gadolinium and oxygen by EDX suggested that the gadolinium conversion coating was successfully grown on the surface of AZ91 alloy by the gadolinium ion bath. The results indicate that there was a homogeneous gadolinium conversion coating formed on the surface of magnesium alloy that may be the product of an oxidation reaction between the solution containing Gd³⁺ and magnesium alloy. Meanwhile, the result also confirms that it was well combined with the substrate.

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Figure 2

SEM morphology and elemental area profile of cross-section of gadolinium conversion coating

Structure of conversion coatings

In Fig. 3, the XPS spectra of the gadolinium conversion coating are depicted, and the high resolution spectra of a single element are shown in Figs. 3. As shown in Fig. 3a, the Gd4d3/2 and Gd4d5/2 peaks are measured elaborately. They correspond to Gd₂O₃ and the binding energy of the oxides at 147·4 and 142·5 eV respectively. The binding energy of the two peaks is almost coincident with the reference value. The Mg 1s peak is assigned to a large amount of MgO and a little Mg(OH)₂, which were labelled in Fig. 3b. The typical O1s peak can be consistently fitted by three nearly Gaussian distributions, as is shown in Fig. 3c. The first peak of the fitted O1s centred at 529·9 eV corresponds to Gd₂O₃, and the second peak for O1s centred at 530·8 eV corresponds to an oxide of Al. Another fitted peak for O1s centred at 532·1 eV corresponds to the oxide and hydroxide of Mg at 531·9 eV. All these results reveal that the gadolinium conversion coating is mainly composed of Gd₂O₃, MgO, Mg (OH)₂ and Al₂O₃.

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Figure 3

XPS patterns of gadolinium conversion coating on magnesium alloy: (a) the high resolution of Gd, (b) the high resolution of Mg, (c) the high resolution of O

Corrosion resistance of conversion coating

The impedance spectra of the untreated magnesium electrode and the gadolinium treated electrode recorded in 3·5% NaCl solution are shown in Fig. 4. Only a capacitive loop appears in the EIS of the treated sample, but a capacitive loop in the high frequency range and an inductive loop in the intermediate frequency range appear in the EIS of the untreated sample. The total impedance value of the treated sample is 8·37×10³ Ω cm², and that of the untreated one is 1·3×10³ Ω cm².

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Figure 4

EIS spectra of treated and untreated samples in 3·5% NaCl aqueous solution

Figure 5 shows the potentiodynamic polarisation curves of the treated and untreated sample in 3·5% NaCl aqueous solution at room temperature. In a typical polarisation curve, lower corrosion densities correspond to lower corrosion rates and better corrosion resistance. The gadolinium conversion coating on the magnesium alloy surface decreases the corrosion current density I_corr by approximately two orders of magnitude compared with the substrate, partially blocking the cathodic reaction and shifting the polarisation curves towards lower current density values. It is obvious that the corrosion potential E_corr of the coating is approximately 150 mV higher than that of the substrate. These results demonstrate that the anticorrosion capability of the magnesium alloys can be increased through the gadolinium conversion treatment.

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Figure 5

Potentiodynamic polarisation curves of treated and untreated samples in 3·5% NaCl aqueous solution

Nanomechanical properties of conversion coatings

The formation of conversion coatings is coexistent with the dissolution of substrate metal and disposition of rare earth oxide ions. In initial state, dissolution is faster than disposition and this procedure brings about the increase in pH value near surface and then it boosts precipitation of rare earth oxides. Therefore, the content of rare earth oxides in conversion coating gradually increases with increasing times. It is beneficial to regulate internal stresses in conversion coating because of the component gradient. When obtained conversion coatings get dried, the interface between substrate and conversion coatings will shrink. However, contraction volume of the interface will be constrained by coating–substrate bonds. Therefore, certain interior stress emerges inevitably during formation of coatings. The coating cracks when stress is higher than binding force and the internal stress releases partly in this way. Finally, there is a new balance in coatings.

It is unsure about the influence of press-in load owing to few reference materials of nanomechanical properties concerning about conversion coating. Three types of loading (50, 100 and 150 μN) were employed to conduct nano indentation test. Curves of load–displacement are shown in Fig. 6. Indentation reference and mechanical properties are listed in Table 1. Load–displacement curves were influenced definitely by micropores and cracks which were associated with variation of thickness and microstructure of conversion coatings. For this reason, the repeatability of load–displacement curves was low.

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Figure 6

Load–displacement curves of gadolinium conversion coating under different loads

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Table 1

Nanomechanical properties of gadolinium conversion coating

Load/μN	hmax/nm	A/nm2	Er/GPa	H/GPa	hf/nm
50	269·3	332 326	5·3	0·15	236·2
100	310·1	420 242	6·3	0·24	250·7
150	337·8	463 406	4·99	0·32	215·2

According to Table 1, the in-press depths and projected areas added with the increasing in-press loads. And the nanohardness also increased slightly. The in-press depth was 267–337 nm and hardness and elastic modulus of conversion coating were 0·24 and 5·5 GPa on average. On the basis of the discussion, the residual stress in conversion coating was tensile stress. The residual stress of gadolinium conversion coating was 0·17 MPa and it was figured out by adopting Suresh model14 based results of indentation tests.

Discussion about gadolinium conversion coating forming mechanism

The following is the discussion about gadolinium conversion coating forming mechanism based on testing results. Dissolution is the first step for metal to achieve complete structure conversion coating in solution containing activator. Therefore, the initial reacting process of gadolinium conversion coating is similar to that of chromate one.

When Mg alloy is immersed into gadolinium solution, the natural oxide layer is soon dissolved and the substrate appears due to the acid solution. There are some potential differences among the main substrate phases due to the different contents of metal elements, such as Al and Zn, so many corrosion micro cells come into being on the surface of the substrate. Therefore, at the initial stage, the positive electrode of the substrate begins to dissolve, and generates a large amount of Mg²⁺. Meanwhile, hydrogen evolution happens at the negative electrode and generates OH⁻ (according to equations (1) and (2)). With the continuing reaction, the pH value of the solution gradually increases and the addition of H₂O₂ accelerates the increase of the pH value. When the concentration of OH⁻ reaches a certain value, Mg²⁺ and OH⁻ react and generate Mg(OH)₂, which deposits on the substrate surface (according to equations (3) and (4)). Therefore, the initial reactions are mainly between Mg²⁺ and OH⁻. The processes are as follows (1) (2) (3) (4) (5) (6) Generally chromate conversion film is mainly formed through the redox reaction between the substrate and hexavalent chromium, and consists of MgO.Cr₂O₃, CrO₃ and MgCrO₄. Gadolinium is a rare earth element, which shares similarities with cerium and lanthanum in subsequent reactions and layer growing mechanism, but it is quite different from chromate. With the continuing reaction (in equations (1)–(4)), the substrate is gradually covered by Mg(OH)₂ and the dissolution reaction is hindered. Then the following reactions are mainly between OH⁻ and Gd³⁺, Gd(OH)₃ and Mg(OH)₂ depositing together on the previous coating. According to Fig. 2, with the progressing reaction, the substrate is completely covered by the coating; thus, the deposition of Gd(OH)₃ begins to increase. With increasing coating thickness, the content of gadolinium also increases. After this series of reactions, the inner layer close to the substrate is mainly composed of Mg hydroxide, while the outer layer is mainly composed of gadolinium hydroxide. As the reactions continue, the concentrations of Gd³⁺ and OH⁻ gradually decreases, coating formation begin to slow down. When the substrate is taken out and dried, dehydration reaction turns hydroxide of Gd and Mg into oxide of Gd and Mg (according to equations (4)–(6)). And the whole reactions are shown in Fig. 7.

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Figure 7

Reaction mechanism sketch of gadolinium conversion coating

Therefore, gadolinium conversion coating can improve the corrosion resistance of Mg alloy from two aspects. On the one hand, Gd₂O₃ can improve the density of coating composed of Mg hydroxide or oxide only; on the other hand, due to the continuous reaction of gadolinium, coating thickness is improved, which strengthens shielding effect and hinders positive electrode dissolution.