New sealing treatment of microarc oxidation coating

Introduction

Microarc oxidation (MAO), also named plasma electrolytic oxidation, is an electrochemical surface treatment process developed from conventional anodisation. ¹ With this technology, a protective ceramic coating with high hardness, excellent wear and corrosion resistance and electrical insulation can be formed on the surfaces of some metals, such as Mg, Al, Ti, Zr or their alloys.^2–6 Notably, the MAO process builds high adhesion between the substrate and the coating due to the in situ growth mechanism; ⁷ it has been proven as one of the promising methods to form protective coatings on Mg materials.^8–12 Therefore, with the advantages of simple and environmentally friendly preparing process, relative low costs and outstanding performances, MAO has been increasingly attracting attention in recent decades.

As an anodising process, MAO combines electrochemical oxidation with a high voltage sparking discharge process to form a ceramic coating on some metal surfaces.^13–15 The thickness of the coating keeps increasing with dielectric breakdown and sparking discharge. However, the sparking discharge, which directly contributes to the growth of the MAO coating, will lead to a porous structure of the prepared coating. Therefore, it was believed that the microdefects with micrometre and submicrometre sizes are inevitable during the MAO process, which largely limits the wide application of MAO on metals or alloys in extreme conditions. When the alloys were posed to the corrosive environment, these micropores will act as transportation channels for the corrosive agents that reach the metal substrate; therefore, these defects are seen as the preferential sites initiating the corrosion in the metal substrate.^16,17 In order to overcome the critical problem of the MAO coating, a post-sealing process is necessary to increase the anticorrosive performances of the MAO treated alloys when they were exposed to extreme corrosive environments. Conventional sealing techniques of anodisation coatings, such as boiling water sealing or chromate sealing, however are generally not applicable as post-sealing process for the MAO coatings due to the evident microstructure difference of the two coatings. Thus, new sealing techniques need to be developed for MAO coatings.

Silanisation, as a new coating technology, is proposed to protect the metals or alloys from corrosion by forming a thin organic layer on the surfaces of the metals or alloys.^18–26 The essence of the silanisation is the formation of metallosiloxane bonds (Me–O–Si) via the condensation of the silanol groups (SiOH, derived from Si–O–R hydrolysis) on the hydrated metal surface (MeOH). The performing of Me–O–Si covalent bonds lead to a good anchoring of the silane layer to the underlying metal or alloy substrates. ¹⁸ However, the durability of the silane layer is still a bottleneck problem because the layer thickness is not able to satisfy the anticorrosive requirements when the metals or alloys are facing extreme corrosive environment. ²¹ Therefore, in most cases, silane layer only serves as a primer coating rather than an anticorrosive coating. Silane coupling agents are also applied as a surface modification approach on inorganic particles, such as Al₂O₃, TiO₂ and SiO₂.^27–30 Siloxane groups in the silane and hydroxyl groups on the surface of oxide particles can generate hydrolysis condensation reaction, which modifies the chemical properties of the particle surface. The MAO coating is mainly composed of inorganic oxides with hydroxyl groups, which suggests the possibility of sealing treatment by silanisation.

Recently, a MAO–NiP coating was treated by silanisation, and a positive performance was observed; ³¹ others have validated the feasibility but have not performed the silanisation on bulk oxide substrate. To the best of our knowledge, MAO and silanisation, both having their advantages and disadvantages, were not conjugated to prepare an anticorrosive coating. In this study, silanisation was introduced as a sealing process of the MAO coating; thus, these two complementary surface treatment techniques were firstly combined to prepare an anticorrosive coating on Mg AZ91D alloy. The main attention was focused on the effectiveness and durability of the silanisation sealing treatment and the improvements of the corrosion resistance performances.

Experimental

A commercial Mg alloy (AZ91D, Mg–8·82Al–0·62Zn–0·11Mn) was used as raw material in the current study. Square specimens with size of 50×50×3 mm were cut from the as received ingots, polished with sandpapers and followed by degreasing ultrasonically in acetone. The pretreated samples were subsequently immersed in the electrolyte for MAO treating. The electrolyte consisted of sodium silicate, potassium fluoride, potassium hydroxide and deionized water. The MAO system was composed by an AC pulse power supply, an electrolyte cell and a stirring system. The specimen and stainless steel sheet were used as the anode and the cathode respectively. A current density of 60 mA cm⁻² was preset and kept constant during the MAO process. The coated samples were rinsed with deionised water and then dried in air.

Silanisation treatment was applied after the MAO coating process to seal the natural micropores. Specimens after the MAO coating process were first immersed in a silane bath for 2 min followed by air flux drying. The air dried samples were cured at 150°C for 1 h in an oven. The silane bath used for sealing was a hydroalcoholic solution of γ-glycidoxy-propyl-trimethoxy-silane (Glymo) (12·5% silane/75% deionised water/12·5% methanol, vol.-%). The solution was regulated to pH 4 by dropping in 36% acetic acid, stirred for 60 min and subsequently hydrolysed for 48 h or longer before the sealing treatment.

The surface morphologies of the alloy after MAO with or without the silanisation treatment were observed on a scanning electron microscopy (SEM). Corrosion resistance was evaluated by potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS) in a 3·5%NaCl solution. Electrochemical tests were performed in a three-electrode cell, using a platinum sheet with size of 20 mm×20 mm as the counter electrode and an Ag/AgCl (saturated KCl) electrode as the reference electrode. The potentiodynamic polarisation curves were measured with a scan rate of 0·333 mV s⁻¹. The EIS was measured over a frequency ranging from 100 kHz to 10 MHz using sinusoidal voltage signal with 10 mV amplitude, and the experimental data were analysed by ZSimpWin software.

Results and discussion

The surface morphologies of the alloy after MAO with or without the silanisation treatment are shown in Fig. 1. The as prepared MAO coating without silanisation exhibits a typical porous morphology, as shown in Fig. 1a. The micrometre scale pores are now widely accepted as a result of the dielectric breakdown and the sparking discharge during the MAO process. Some microcracks can also be observed on the surface, which is supposed to relate with intense discharges and the arising of the residual stress. ³² Figure 1b and c shows that microcracks and micropores on the surface are both well sealed with the silanisation treatment. Notably, however, a few micropores are still detectable on the surface with the silanisation process; and these vestigial micropores are larger in size compared with the sealed ones. This phenomenon demonstrates that the current silanised coating still has a potential capability to improve its anticorrosion performances.

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Images (SEM) of MAO coating on Mg alloy AZ91D: a before sealing treatment; b after sealing treatment; c magnifying micrograph after sealing treatment

The potentiodynamic polarisation curves of the samples tested in a 3·5%NaCl solution are shown in Fig. 2. For comparison, the bare AZ91D Mg alloy was also silanised, and the corrosion resistance was measured. After the silane layer has been formed on the AZ91D alloy, the cathodic reaction of hydrogen evolution was observed inhibited, which resulted in the decrease in corrosion current density by one order of magnitude compared with the bare AZ91D alloy. In addition, since the anodic dissolution reaction has little change, the presence of the silane layer is believed to influence the cathodic reaction more than the anodic reaction.

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Potentiodynamic polarisation curves tested in 3·5%NaCl solution of bare and MAO coated AZ91D alloys before and after silanisation treatment

After AZ91D alloy was treated by MAO, the corrosion current density decreased by four orders of magnitude, and the corrosion potential increased from −1·46 to −1·27 V. Thus, both anodic dissolution and cathodic reactions of the alloy could be greatly inhibited by adopting the MAO treatment. Furthermore, the presence of MAO coatings restrains the anodic reaction more effectively than the cathodic reaction because the corrosion current density I_corr decreases, while the corrosion potential E_corr moves towards a more positive direction by 190 mV. When silanisation treatment was performed on the AZ91D alloy surfaces after MAO treatment, the corrosion resistance is observed to be further enhanced. The corrosion current density of the silanised MAO coating is about one order of magnitude lower than the unsealed MAO coating. Furthermore, the anodic current of the silanised MAO coating increases more slowly than the unsealed one, as shown in Fig. 2. The combination of MAO and silanisation treatment leads to a decrease in corrosion current density from 6·2×10⁻⁵ to 4·2×10⁻¹⁰ A cm⁻², exhibiting an excellent protective property. The sealing treatment via silanisation is thus found to further improve the protective property of MAO coating on Mg alloy, which reveals that the conjugation of the MAO and the silanisation treatment is an effective approach to increase the anticorrosive properties of the alloy.

The EIS was used to evaluate the corrosion resistance and to further investigate the anticorrosion mechanism of the MAO coatings with or without the silanisation treatment. The EIS diagrams of the alloy after MAO process with or without silanisation treatment are presented in Fig. 3. A common feature on the Nyquist plots are observed in the unsealed and sealed samples, i.e. three semicircles existing in the measured frequency range: a high frequency capacitive semicircle, a mid frequency capacitive semicircle and a low frequency inductive semicircle. The size of capacitive semicircles for the sample with silanisation is much larger than for the one without silanisation treatment, which suggests the corrosion resistance enhancement of the sealed MAO coating. In addition, the size of capacitive semicircles decreases with the increasing immersion time within 24 h, and then increases to some extent, which is believed related with the barrier action of some corrosion products.

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Electrochemical impedance spectroscopy diagrams of MAO coatings a before and b after silanisation treatment as function of immersion time in 3·5%NaCl solution

The impedance modulus at low frequency |Z|_{0·01 Hz} is usually used to evaluate the stability and corrosion resistance of coatings. As shown in Fig. 4a, the impedance modulus at low frequency |Z|_{0·01 Hz} increased from 1·2×10⁵ to 2·3×10⁷ Ω cm² at immersion time of 1 h after the MAO coating is sealed by silanisation treatment. This increase indicates that the corrosion resistance of MAO coating can be greatly improved via silanisation treatment.

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a impedance modulus at 0·01 Hz and b charge transfer resistance of MAO coatings before and after silanisation treatment obtained from EIS data fitting

For the coated samples, the charge transfer reactions occur once the corrosive agent reached the coating/substrate interfaces and reacted with the metal substrate. The high charge transfer resistance R_t means a high electrode reaction resistance and a low corrosion rate. The R_t of different samples obtained by equivalent circuit fitting is shown in Fig. 4b. The Q_dl R_t component, the charge transfer resistance R_t of the AZ91D alloy in parallel with the capacitance of double layer Q_dl, was used to characterise the mid frequency capacitive behaviour. It was attributed to the charge transfer reactions that took place on the alloy surface. Figure 4b indicates that R_t of the sealed MAO coating is three orders of magnitude higher than that of the coating without silanisation treatment when the immersion lasted for 1 h. The great difference in R_t suggests that the improvement of the corrosion resistance is basically attributed to the penetration inhibiting the corrosive agent. The MAO coating with silanisation serves as a more effective physical barrier delaying the transport of the aggressive agents to the surface of the substrate comparing with the as prepared unsealed one.

However, it also can be found from Fig. 4 that both the impedance modulus and charge transfer resistance of the MAO coating with silanisatoin decline quickly when exposed to the immersion and then reach a relatively stable state. It indicates that the inhibiting ability of the silane sealing layer on the MAO coating weakens when exposed to NaCl solution, especially at the initial immersion stage. Nevertheless, the sealed MAO coating still presents an advantage over the unsealed one. The silanisation is thus proven a potential sealing treatment method. However, the integrity, the compactability as well as the stability of the silane sealing layer need to be further improved by optimising the process. Such efforts, including performing a surface pretreatment on the MAO coating and introducing electrochemical assisted deposition in the silanisation process, are now in progress.

Conclusion

The combination of MAO and silanisation is proposed as a promising approach to prepare a high efficiency anticorrosive coating on AZ91D alloy surface. The microcracks and micropores in MAO coating can be sealed via the silanisation treatment, and the anticorrosive property of the MAO coating is thus improved. The combination of MAO and silanisation decreases the corrosion current density of AZ91D alloy by five orders of magnitude or higher. The sealed MAO coating can inhibit the penetration of the corrosive agent into the coating/substrate interfaces and restrain the charge transfer, which consequently enhance the corrosion resistance of AZ91D alloy greatly. However, the silanisation process optimisation is still a challenge in the future to prevent the rapid decline of protective effectiveness. In addition, the feasibility of the conjugation of MAO and silanisation on other metals is also necessary to be investigated.