Formation Process of an LDHs Coating on Magnesium Alloy by a CO2 Pressurization Method

Abstract

The formation process of LDHs (layered double hydroxides) coating on magnesium alloy by the CO2 pressurization method was studied. The micro-structure was observed by OM, SEM and GAXRD. The weighted gain curve, apparent activation energy, and CO2 solubility curve were all calculated by equations. The potentiodynamic polarization curve, hydrogen evolution data, and immersion were analyzed by an electrochemical method. The results show that the LDHs coating was formed layer-by-layer. The formation positions were initially on the α-Mg phase, and then on the β-Mg17Al12 phase. It was found to be the most compact after 30 min. The LDHs coating began to appear to have severe cracks and holes over time. The formation process of the LDHs coating can be divided into three stages: a rapid growth stage (0–10 min), slow growth stage (10–20 min), and periodic growth stage (30 min, 1 h). The apparent activation energies in each of the three stages are 21.78, 31.86 and 34.92 kJ mol−1, respectively. The LDHs coating has a compact micro-structure and better anti-corrosion at a pressure of 3 MPa, a temperature of 50 °C and a time of 30 min. The CO2 pressurization promotes a formation reaction rate and achieves a high formation efficiency and good formation stability under the condition of zero pollution.

Effect of Temperature on Corrosion Resistance of Layered Double Hydroxides Conversion Coatings on Magnesium Alloys Based on a Closed-Cycle System

“…From the sample electrochemical test results, the E corr (1.36V ± 0.034) of the LDH conversion coating at 50 • C was higher than that of the AZ91D magnesium alloy (−1.41V ± 0.059) and tends to be more positive at 50 • C. The i corr (8.92 ± 1.37 µA•cm −2 ) of the LDHs conversion coating prepared at 50 • C was nearly one order of magnitude lower than that of the AZ91D magnesium alloy (83.62 ± 1.63 µA•cm −2 ). The E corr and i corr of samples with LDH conversion coating have a certain error but agree well with the published values [53][54][55].…”

“…The icorr (8.92 ± 1.37 μA•cm −2 ) of the LDHs conversion coating prepared at 50 °C was nearly one order of magnitude lower than that of the AZ91D magnesium alloy (83.62 ± 1.63 μA•cm −2 ). The Ecorr and icorr of samples with LDH conversion coating have a certain error but agree well with the published values [53][54][55]. The results indicate that the LDH conversion coating prepared at 50 °C can effectively enhance the corrosion resistance of the AZ91D magnesium alloy matrix.…”

“…The treatment solution can be recycled without affecting the formation effect. So, the author proposed a CO 2 pressurization method for the first time, and the effect of CO 2 pressure and treatment time on the corrosion resistance of LDH conversion coating was studied, and the formation mechanism of the micro-galvanic regions with different β-phase morphology on the magnesium alloy was proposed [53][54][55]. The author found that temperature is also an important factor under the closed-cycle system, and the microstructures of LDH conversion coatings are quite different at low and high temperatures.…”

Sol–Gel Synthesis and Characterization of Coatings of Mg-Al Layered Double Hydroxides

“…The urea-assisted synthetic approach was transferred for the fabrication of LDH coatings on a plasma electrolysis (PE) Al alloy coating [57]. Recently, formation processes for LDH coatings on Mg alloy or on alumina by the CO 2 pressurization and electrophoretic deposition methods, respectively, have been developed [58,59].…”

The Role of Ammonium Chloride in the Powder Thermal Diffusion Alloying Process on a Magnesium Alloy

“…To protect magnesium alloys from serious corrosion, various methods are raised. For example, magnesium alloys could be covered with coatings which are much more corrosion resistant [5][6][7].…”