Many surface treatment methods are used to improve the corrosion resistance of magnesium alloys. LDH (layered double hydroxides) conversion coatings are currently found in the most environmentally friendly and pollution-free coatings of magnesium alloy. In this study, the CO2 pressurization method was applied to the preparation of LDH coating on magnesium alloy for the first time. The effect of CO2 pressurization on the formation and corrosion resistance of LDH coating on AZ91D alloy was investigated. The hardness and adhesion were significantly higher on LDH coating in the case of CO2 pressurization than it is in atmospheric pressure. The surface and cross-sectional morphologies show that LDH coating is more compact in the case of CO2 pressurization than with atmospheric pressure. The results of the polarization curve, hydrogen evolution, and immersion tests indicate that the corrosion resistance of the LDH coating prepared by the CO2 pressurization method was significantly improved.
Magnesium and magnesium alloys are a “green engineering material” in the 21st century, having a wide range of application prospects, such as automotive, in aerospace, portable electronic devices, and in medicine. This is due to their superior strength-weight ratio, dimensional stability, light weight, recycling ability, and other excellent properties [
Layered double hydroxides (LDH) are environmentally friendly intercalation compounds. They are represented by a general formula of [
In this study, considering the time consuming and weak antipollution properties owing to the complex process preparation of LDH conversion coatings for former process, a new method, CO2 pressurization method, for preparing LDH conversion coatings was proposed. The introduction of a CO2 pressurization method is based on the previous studies of Uan et al. [
For the purposes of this paper, AZ91D magnesium alloy was selected as the object material to be studied. It is composed of 8.8 wt.% Al, 0.69 wt.% Zn, 0.212 wt.% Mn, 0.02 wt.% Si, 0.002 wt.% Cu, 0.005 wt.% Fe, and 0.001 wt.% Ni. The AZ91D magnesium alloy ingot was cut into 20mm×12mm×6mm samples, each of which was ground with 1000#-2000#-mesh SiC abrasive paper and ultrasonically cleaned in anhydrous ethanol.
All of the reagents/reactants used were clean and nonpolluting. In a typical preparation, the CO2 was introduced in deionized water at room temperature with the flow rate of 1 dm3/min for 20 min, in order to form the
Preparation parameters for three kinds of LDH conversion coatings.
Parameters | Temperature, °C | pH | Time, h | Pressure, MPa |
---|---|---|---|---|
1 [ | 50 | 4.3 | 24 | 0.1 |
2 [ | 50 | 4.3/11.5 | 4 | 0.1 |
3 CO2_3MPa_0.5h | 50 | 4.3 | 0.5 | 3 |
The first LDH coating was prepared by a one-step immersion method. The specimens were statically immersed in the bath at 50°C for a particular period for 24h, denoted above as CO2_24h treatment [
Operations for the preparation of conversion coating with CO2 pressurization method.
The HVS-5 digital Vickers hardness was used to test the Vickers hardness, with 5Kgf being loaded for 10s. The QFH type paint film was used to test the adhesion of the LDH coating, with the test results of the LDH coating being observed according to ISO2409-1974.
The surface and cross-sectional morphologies of LDH coating were observed by a Philips XL30 and JEOL JSM-6700F SEM, respectively. The microstructure was analyzed with X-ray diffraction (GAXRD) at Cu k
Potentiodynamic polarization measurements of AZ91D alloy with and without LDH coating were performed in an electrochemical workstation (Zennium, Zahner) with a three-electrode cell, using a platinum foil as the counter electrode and a saturated calomel electrode (SCE, saturated KCl) as the reference electrode in aerated 3.5wt% NaCl solution. The corrosion of magnesium alloy is mainly shown as the hydrogen evolution of the cathode. In order to avoid the influence of the cathode process on the whole electrochemical testing process, the anodic and cathodic polarization curves of the specimens were measured from the open circuit potential (OCP) to the anodic and cathodic side in the 300mv range, with a scan rate of 0.333mV·s−1, respectively. The above measurements were repeated at least five times. Hydrogen evolution data were measured by collecting hydrogen from the reaction in a hydrogen collector. The samples were placed in a beaker containing 3.5wt% NaCl solution and in a water bath pot with a constant temperature (30±1°C). The burette was connected to the funnel, inverted into the solution, perpendicular to the sample to be tested, with it being noted that the top of the burette should be fully immersed in the solution. The hydrogen bubble produced by the magnesium alloy corrosion was introduced into the burette through the funnel so that the hydrogen evolution rate of the magnesium alloy and the film could be determined by the change of the reading on the burette after the hydrogen is collected. All of the hydrogen measurements were repeated at least three times. The immersion test was conducted to determine the corrosion rate of the AZ91D alloy with different LDH coating for 120 hours, with the macroscopic corrosion morphologies being obtained using a digital camera. For the immersion test, all measurements were repeated at least three times at 30 ± 1°C.
The effect of the CO2 pressurization method on the hardness of the magnesium alloy surface is shown in Figure
Hardness test results of the AZ91D alloy with and without LDH conversion coating.
The surface hardness was improved by the surface treatment and the hardness increased even more of the CO2_3MPa_0.5h LDH coating. The macroscopic morphologies of the three specimens with different surface treatments after adhesion test are shown in Figure
Adhesion test of conversion coatings with various process on AZ91D alloys: (a) CO2_24h, (b) CO2_2h/pH11.5_2h, and (c) CO2_3MPa_0.5h.
The results of the adhesion test are shown in Table
Adhesion test results of LDH conversion coatings with various processes.
Adhesions | Adhesion klass | Affected rate |
---|---|---|
1 CO2_24h | 1 | <5% |
2CO2_2h/pH11.5_2h | 1 | <5% |
3 CO2_3MPa_0.5h | 0 | Almost never off |
The surface and cross-sectional morphologies of LDH coating are shown in Figures
Morphologies of conversion coatings with various processes on AZ91D alloys: (a) unhandled-AZ91D, (b) CO2_24h, (c) CO2_2h/pH11.5_2h, and (d) CO2_3MPa_0.5h.
Cross-sectional microstructures of conversion coatings with various processes on AZ91D alloys: (a) unhandled-AZ91D, (b) CO2_24h, (c) CO2_2h/pH11.5_2h, and (d) CO2_3MPa_0.5h.
Opposed to this, CO2_24h and CO2_2h/pH11.5_2h show a large number of microcracks on the LDH coating, with the possibility that there are some cracks reaching the interface between the coating and the substrate.
The XRD patterns of the cast and different conversion coatings of AZ91D alloy are shown in Figure
GAXRD patterns of the AZ91D alloy with and without conversion coating.
The potentiodynamic polarization curves of the AZ91D alloy with and without conversion coatings are shown in Figure
Polarization curves of the AZ91D alloy with and without conversion coating conversion coatings tested in 3.5wt.% NaCl solution.
The corresponding electrochemical parameters, including corrosion potential (
Electrochemical test results of the AZ91D alloy with and without LDH conversion coating.
Samples | AZ91D | CO2_2h/pH11.5_2h | CO2_24h | CO2_3Mpa_0.5h |
---|---|---|---|---|
| 1.41(±0.059) | -1.36(±0.026) | 1.34(±0.054) | -1.36(±0.034) |
| 83.62(±1.67) | 15.81(±1.69) | 17.34(±1.78) | 8.92(±1.63) |
efficiency% | - | 81.1% | 79.3% | 89.3% |
The hydrogen evolution rate (HER) of the AZ91D alloy with and without conversion coating is shown in Figure
Hydrogen Evolution Curve of the AZ91D alloy with and without conversion coating in 0.6M NaCl solution.
Optical corrosion morphologies of the AZ91D alloy with and without conversion coating of immersing test in 0.6M NaCl: (a1) nonimmersed original sample, (a2) original sample immersed for 24h, (a3) original sample immersed for 48h, (a4) original sample immersed for 72h, (a5) original sample immersed for 120h, (b1) nonimmersed CO2_24h sample, (b2) CO2_24h sample immersed for 24h, (b3) CO2_24h sample immersed for 48h, (b4) CO2_24h sample immersed for 72h, (b5) CO2_24h sample immersed for 120h, (c1) nonimmersed CO2_2h /pH11.5_2h sample, (c2) CO2_2h/pH11.5_2h sample immersed for 24h, (c3) CO2_2h /pH11.5_2h sample immersed for 48h, (c4) CO2_2h /pH11.5_2h sample immersed for 72h, (c5) CO2_2h /pH11.5_2h sample immersed for 120h, (d1) nonimmersed CO2_3MPa_0.5h sample, (d2) CO2_3MPa_0.5h sample immersed for 24h, (d3) CO2_3MPa_0.5h sample immersed for 48h, (d4) CO2_3MPa_0.5h sample immersed for 72h, and (d5) CO2_3MPa_0.5h sample immersed for120h.
According to the Butler-Volmer equation, the corrosion current density (
Additionally, the percentage of the surface area rusted for each specimen is estimated by using the visual examples according to ASTM D610-08. It is evident that the bare AZ91D alloy underwent severe attack rust grade 3G. Meanwhile, only several corroded spots are observed on the surfaces of the CO2_3MPa_0.5h, CO2_2h/pH11.5_2h, and CO2_24h coated specimens, where the corresponding rust grades were 7G, 7G, and 6G, respectively. The results of the immersion test are in good agreement with those of the polarization curve and HER, indicating the improvement of the corrosion resistance of AZ91D alloy after CO2_3MPa_0.5h conversion treatment.
According to the above results, the anticorrosion performance of LDH coating can be ranked in the following decreasing series: CO2_3MPa_0.5h ≈ CO2_2h/pH11.5_2h > CO2_24h. However, the preparation efficiency of CO2_3MPa_0.5h coating is 8 and 48 times higher than that of the CO2_2h/pH11.5_2h and CO2_24h coatings. Considering the corrosion resistance and preparation efficiency of the three coatings, the performance of CO2_3MPa_0.5h coating is superior to that of the CO2_2h/pH11.5_2h and CO2_24h coatings.
The film-forming process of the LDH conversion coating is a kind of physical and chemical processes. The reaction process of the AZ91D magnesium alloy matrix material mainly includes an electrochemical reaction, an ionization reaction, and a coating-forming reaction in carbonate solution. Its specific chemical reaction equation is shown in formulae (
Due to the CO2 pressurization, the CO2 solubility in the solution increased, with the proportion of carbonate ions in the solution also increasing, resulting in an increase in the hydrogen ion concentration increased and promoting the electrochemical reaction to the right, thus accelerating the dissolution of the aluminium and magnesium ions. Further due to the CO2 pressurization, the ionization is promoted to a positive reaction, accelerating, increasing the hydroxyl ions in the solution, and at the same time increasing the ionization reaction to the right, with the concentration of carbonate ion and bicarbonate ion in the solution being increased. The acceleration of the electrochemical reaction and ionization promoted the film-forming of the magnesium alloy matrix and the increase of the magnesium ions, aluminium ion, carbonate ions, and hydroxyl ions, promoting the film to a positive reaction and eventually improving the film-forming reaction rate. The film-forming process is shown in Figure
The film-forming process of the LDH conversion coating on the AZ91D magnesium alloy matrix material: (a) morphologies of the 10min sample, (b) morphologies of the 20min sample, (c) morphologies of the 30min sample, and (d) cross-sectional of conversion coating-forming process.
In the initial stages, the conversion coating was very thin, with a small angular cavity or pit, such as a honeycomb cell, as can be seen in Figure
The CO2 pressurization method was first applied to the preparation of LDH coating on AZ91D alloy. The conversion coating first became in the
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The authors hereby declare that there are no conflicts of interest regarding the publication of this paper.
The authors wish to acknowledge the financial support of the National program for the Young Top-notch Professionals, the National Natural Science Foundation of China (nos. 51531007, 51771050, and 51705038), and Foundation of Young Scholars in HLJIT (2014QJ12).