镁合金表面自愈合钼酸根水滑石涂层

Self-healing molybdate intercalated hydrotalcite coating on Mg alloy

曾荣昌

山东科技大学

Rong-Chang Zeng

Shandong Uniersity of Science and Technology

    我们采用共沉积和水热法在镁合金AZ31表面制备了钼酸根水滑石涂层，这种涂层具有纳米层状结构、离子交换和自愈合（self-healing）功能，有望成为一类对环境刺激发生响应的智能涂层（smart coating）。研究结果发表在《Journal of Materials  Chemistry A》(2014, 2, 13049–13057) 。

Abstract

A molybdate intercalated hydrotalcite (HT-MoO₄^2-
) coating with a nanosized lamellar structure was synthesized on AZ31 Mg alloy by a combination of the co-precipitation and hydrothermal processes. The characteristics of the coatings were investigated by SEM, EPMA, XRD, EDS and FT-IR. The corrosion resistance of the coatings was assessed by potentiodynamic polarization, electrochemical impedance spectrum, and hydrogen evolution. The results indicated that the HT-MoO₄^2- coating, characterized by interlocking plate-like nanostructures, ion-exchange and self-healing ability, has a potential to be a “smart” coating capable of responding to stimuli from the environment.

Self-healing mechanism

The self-healing process of the HT-MoO₄^2-
coating in the corrosive medium is demonstrated on the cross-sectional views of the coatings (Fig. 1).

Fig. 1a and b show the cross-sectional views of the coatings before and after 144 h of immersion, respectively. Fig. 1c and d designate the magnitude morphologies and their corresponding EDS spectra of the original and the immersed coating, respectively.

It was revealed in Fig. 10b that the coating contained two layers: the newly formed outer layer and the thinned inner HT-MoO₄^2-
coating after 144 h of immersion in NaCl solutions. It is also found that the coating morphology has been changed. The non-uniform hexagonal flakes of the HT-MoO₄^2-
coating (Fig. 1c) were changed into round and bar-like particles (Fig. 1d). The EDS spectrum in the inset of Fig. 1d indicates that the main component of the outer coating is Mg(OH)₂.

Fig. 1 The self-healing process of the HT-MoO₄^2-
coating demonstrated on the cross-sectional views
.

The XRD patterns of the original HT-MoO₄^2-
coated sample and the samples after different immersion times are shown in Fig. 2a. Obvious Mg(OH)₂ peaks appeared on the immersed samples, in addition to those of the HT-MoO₄^2-
layer. and the Mg substrate. With the extended immersion time, it can be seen from Fig. 2 that the intensity of Mg(OH)₂ peaks increased, while the intensity of HT-MoO₄^2-

peaks decreased, the peak at 22.5 nearly disappeared after the immersion of 12 days. But, it can be seen that the peaks of the HT-MoO₄^2-

coating on AZ31 Mg substrate still existed after a 12 days immersion test, which indicated that the HT-MoO₄^2-

coating had a good corrosion resistance. The peaks position of (003) were shied to a large angle of approximately 0.2 (Fig. 2b), indicating that the chloride ions were intercalated by ion exchange.

Fig. 2(a) XRD patterns of the original HT-MoO₄^2
- coated sample and immersed sample with different time. (b) Detail XRD patterns of (003).

The dissolution reaction of the Mg(OH)₂ film on the Mg alloy surface in chloride solution can be given as follows:

Mg(OH)₂ + Cl^- → Mg(OH)Cl + OH^-                        (1)

Mg(OH)Cl + Cl^- → MgCl₂ + OH^-                            (2)

The ion-exchange reaction of the HT-MoO₄^2- coating on the Mg alloy in chloride containing solution can be expressed as follows (Fig. 3):

HT-MoO₄^2- +2 Cl^- → HT-2Cl^- + MoO₄^2-                 (3)

Anodic reaction:

                                  Mg → Mg²⁺ + 2e                     (4)

Cathodic reaction:

                           2 H₂O + 2e^- → 2OH^- + H₂↑               (5)

The total reaction:

                       Mg + 2 H₂O → Mg(OH)₂ + H₂↑           (6)

The released MoO₄^2- ions can produce the following reactions:

                 MoO₄^2- + 8 H⁺ + 3e^- → Mo³⁺ + 4 H₂O        (7)

At the same time, Mo³⁺ ions also consume the OH^- ions and create the formation of Mo(OH)₃. The Mo(OH)₃ compound is quite unstable and has a tendency which can transform into more stable compounds:

                        Mo(OH)₃ + OH^- → Mo(OH)₄                   (8)

MoO₄^2- may react with the dissolved Mg²⁺ to form a protective deposition film. The deposition of MoO₄^2- can inhibit the expansion and spreading of pitting corrosion. The probable reaction can be given as follows

         Mg²⁺ + MoO₄^2- → [MgMoO₄ ]                              (9)

Fig. 3 Corrosion protection mechanism of the HT-MoO₄^2-
coating.

Conclusions
(1) Based on the ion exchange, the released MoO₄^2-
ions lead to the formation of a diffusion boundary layer.
(2) In the diffusion boundary layer, the released MoO₄^2-

ions greatly impair the adsorption of Cl^-
on the surface of the coating. Also, the released MoO₄^2-
with the ability for oxidation
and deposition can effectively reduce the damage of pitting corrosion to the substrate.
(3) In the HT-MoO₄^2-
coating, the coexistence of HT-MoO₄^2-
and HT-2Cl
can effectively block the penetration of aggressive ions, the corrosion pits can thus be healed by the Mg(OH)₂ layer and inhibit MoO₄^2-

.

This article has been published on Journal of Materials  Chemistry A (2014, 2, 13049–13057).