Phytic acid conversion coating on AZ31B magnesium alloy

Sample preparation

The substrate material used in this study was AZ31B magnesium alloy (Mg–2·91%Al–0·85%Zn–0·4%Mn–0·21%Si) with the dimension of 7×4×1 mm. All sides of the samples were polished successively with grade 400, 800 and 1200 metallographic abrasive paper in turn, ultrasonically washed in ethanol for 5 min and then dried by air.

Phytic acid is a chemical reagent with purity⩾50%, and the other materials are all analytical reagent with purity⩾99%. The conversion coatings were prepared by immerging AZ31B magnesium alloy samples in the phytic acid solution containing a certain concentration under different pH values and different temperatures for different times. The pH values of conversion solutions were adjusted with triethylamine. The parameters of chemical conversion treatment were based on Table 1 and the optimum processing parameters of the phytic acid conversion coating on AZ31B magnesium were investigated through single factor experiment. The samples with a chemical conversion coating were taken out from the treatment solution, washed with distilled water and then dried at room temperature.

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Table 1

Parameters of conversion treatment for AZ31B magnesium alloy

Treatment object	Medium	pH value	Time/min	Temperature/°C	Concentration of phytic acid/g L−1
AZ31B	Water	1–6	10–120	20–80	1–10

Measurements

Magnesium is a very active metal element. The low potential makes it easy to react with phytic acid and water to produce hydrogen. When a magnesium alloy contacts with the solution, the main reaction in solution is as follows (1) (2) According to the reactions (1) and (2), 1M H₂ formed and escaped from the solution when 1M magnesium reacted. The H₂ can be collected in a container with scale to be measured. Therefore, the corrosion resistance of conversion coating can be evaluated in 3·5% sodium chloride solution through the hydrogen evolution rate after some time.14, 15

The surface morphology was observed using a JPK NanoWizard II atomic force microscope and a TESCAN VEGA II scanning electron microscope. The element compositions of coating were examined by an energy dispersive X-ray spectroscope. The chemical valence state of the elements in the conversion coating was analysed by an ESCALAB 250 X-ray photoelectron spectroscope. All such spectra were recorded using monochromated Al K_α radiation (1486·6 eV), with a pass energy of 30 eV. The shift in binding energy due to relative surface charging was corrected by using the C 1s level at 285·0 eV as an internal standard. The chemical nature of the coating was investigated by a NEXUS 470 Fourier transform infrared spectroscopeFTIR. The corrosion resistance of the phytic acid conversion coating in 3·5% sodium chloride solution was carried out using a CHI604C electrochemical workstation at room temperature. A three electrode cell with sample that surface area was 2·68 cm² as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum sheet as the counter electrode.

Analysis of processing parameters of phytic acid conversion coating

There are four factors influencing the formation of phytic acid conversion coating, i.e. pH value of the solution, treating time, treating temperature and solution concentration. The total of hydrogen evolved after 12 h immersion was used as evaluation standard of the corrosion resistance.14, 15 The effects of the above four factors on corrosion resistance of the conversion coating are shown in Fig. 1. It can be seen that the optimum processing parameters of phytic acid conversion coating on AZ31B magnesium alloy are pH value of 2, treating time of 40 min, treating temperature of 40°C and solution concentration of 4 g L⁻¹. Therefore, the morphology, composition, chemical valence state of the elements, chemical nature and corrosion resistance of phytic acid conversion coating formed under the optimum processing conditions were further investigated and the results were shown as follows.

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Figure 1

Effects of four factors on total of hydrogen evolved of conversion coating on AZ31B magnesium alloy after 12 h immersion: a pH value of solution; b treating time; c treating temperature; d solution concentration

Morphology and composition of conversion coatings

The surface morphology was observed using AFM and the results are shown in Fig. 2. It can be seen that the uniform nodular texture is exhibited on the surface of the conversion coating. The roughness of the conversion coating is small and the height difference is less than 3 μm. Figure 3 depicts the SEM surface micrographs and EDS spectra of AZ31B samples treated in phytic acid solution. The conversion coating is homogeneous except some fine cracks, approximately 1 μm in width, which formed because of hydrogen evolution and the higher inners tress.16, 17 The EDS results show that the coatings were composed of O, C, P, Mg, Al and Zn. It is concluded that P, C and partial O elements in the coating came from phytic acid during the coating formation process, and Mg, Al and Zn came from the AZ31B magnesium alloy. The EDS signals of O and P suggest the presence of phytic acid conversion coating on the surface of AZ31B sample. The conversion coating should be the reaction product of phytic acid and Mg, Al.14 Furthermore, it can be seen that O and P exist at some cracks, that is to say, the conversion coating may consist of two layers: the compact inner layer and the outer layer with some fine cracks. The fine cracks could improve the adhesive ability of organic conversion coating to the substrate and the outer paint coating.18

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Figure 2

Images (AFM) of conversion coating on AZ31B magnesium alloy

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Figure 3

Image (SEM) and EDS spectra of conversion coating on AZ31B magnesium alloy

To understand the compositions and chemical state of the elements in phytic acid conversion coating, XPS was also used to analyse the conversion coating and the result is shown in Fig. 4. It shows that O, C, P, Mg, Al and Zn elements are also in the coating. Figure 5 shows high resolution spectra of the major elements in the conversion coating. The typical O 1s peak can be consistently fitted by four nearly Gaussian distributions as shown in Fig. 5a. The first peak of the fitted O 1s centred at 532·1 eV corresponds to and in phytic acid, which indicates that partial O element came from phytic acid. The second peak for O 1s centred at 531·4 eV corresponds to hydroxide of Zn. The third peak for O 1s centred at 531·0 eV corresponds to oxide and hydroxide of Al. Another fitted peak for O 1s centred at 529·8 eV corresponds to oxide and hydroxide of Mg. The typical Mg 1s peak is shown in Fig. 5b, in which the Mg 2p signal is decomposed into three peaks at 1305·3, 1304·4 and 1303·4 eV corresponding to magnesium phytic acid, magnesium oxide and magnesium hydroxide respectively.17 Figure 5c reports that the spectrum of P 2p is decomposed into two peaks. The two fitted peaks for P 2p at 133·8 eV correspond to . Another fitted peak for P 2p at 132·6 eV correspond to . The peaks of C 1s (Fig. 5d) at 287·9 and 286·5 eV correspond to carbon dioxide and phytic acid radical on magnesium oxide respectively. The typical Al 2p peak is shown in Fig. 5e, in which the Al 2p signal is decomposed into three peaks at 74·9, 74·1 and 73·3 eV corresponding to aluminium phytic acid, aluminium oxide and aluminium hydroxide respectively. The peak of Zn 2p (Fig. 5f) at 1022·2 eV corresponds to Zn(OH)₂.

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Figure 4

X-ray photoelectron spectra of conversion coating on AZ31B magnesium alloy

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Figure 5

High resolution spectra of major elements in phytic acid conversion coating: a O 1s; b Mg 1s; c P 2p; d C 1s; e Al 2p; f Zn 2p

The chemical nature of the coating was investigated by FTIR and the result is shown in Fig. 6. It can be seen from Fig. 6 that there are four characteristic bands for the phytic acid conversion coating. The strong and wide absorption peak at 3434 cm⁻¹ was attributable to the expansion vibration of P–OH.14 The band at 1639 cm⁻¹ was assigned to , and the bands at 1083 and 540 cm⁻¹ were assigned to . The result further confirmed that the conversion coating was formed mainly by the deposition of the magnesium phytate compounds. The phytic acid conversion coating has the similar properties with organic paintcoat because of the hydroxyl groups and phosphate groups, which are beneficial to the combination of substrate and organic coating.17

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Figure 6

Fourier transform infrared spectra of conversion coating on AZ31B magnesium alloy

Corrosion behaviour

Figure 7 shows potentiodynamic polarisation curves of the treated and untreated samples in 3·5% sodium chloride solution. The results of corrosion potential E_corr and corrosion current density i_corr are listed in Table 2. The data clearly showed that the corrosion potential of the treated sample was higher 346 mV than that of the untreated sample. Furthermore, the corrosion current density of the treated sample was 4·656×10⁻⁶ A cm⁻², which decreases about three orders than that of the untreated sample (1·483×10⁻³ A cm⁻²). These results suggest that the treated sample showed better corrosion resistance and protection characteristic than the untreated sample.

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Figure 7

Potentiodynamic polarisation curves of treated and untreated samples in 3·5% sodium chloride solution

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Table 2

Corrosion potentials and corrosion current densities obtained from potentiodynamic polarisation curves

samples	Ecorr/V(SCE)	icorr /A cm−2
Treated sample	−1·263	4·656×10−6
Untreated sample	−1·609	1·483×10−3

The hydrogen evolution rate curves of the treated and untreated samples in 3·5% sodium chloride solution are shown in Fig. 8. The hydrogen evolution rate of the untreated sample increased rapidly with the immersion time before 4 h, and then it approximately maintained at a relatively constant value after 5 h, which is about 2000 mL h⁻¹ m⁻². The hydrogen evolution rate of the treated sample is less than that of the untreated sample because the phytic acid conversion coating is homogeneous, which is about 500 mL h⁻¹ m⁻² during immersion for 5–12 h. These results further suggest that the treated sample showed better corrosion resistance and protection characteristic than the untreated sample.

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Figure 8

Hydrogen evolution rate curves of treated and untreated samples