Corrosion resistance of anticorrosive film formed on Mg alloy by steam coating

摘要

Development of effective corrosion protection is an issue of major economic importance, because the market of metal and metal alloys are constantly growing and environmental issues are becoming more important. Magnesium and its alloys have been used as advanced structural and functional materials in aerospace, automobile, and railway industries because of excellent physical and mechanical properties such as low density, good electromagnetic shielding, and high strength/weight ratio. Their low corrosion resistance, however, hinders their use on a larger scale. To improve the corrosion resistance of the magnesium alloys, the development of a novel coating technology has been highly desirable. In this presentation, we report a novel preparation method of an anticorrosive Mg-Al layered double hydroxide/oriented magnesium hydroxide composite film on magnesium alloys by steam coating. The corrosion resistance of the composite film was also investigated.Magnesium alloy AZ31 was used as the substrate. The substrates were ultrasonically cleaned in absolute ethanol for 10 min. The cleaned magnesium alloys were set in the autoclave. The cleaned AZ31 substrates were introduced in a Teflon-lined autoclave with a 100 ml capacity. 20 mL of ultrapure water containing 10mM NaNO3 was located at the bottom of the autoclave to produce steam. The autoclave was heated to a temperature of 433 K, and then held at this temperature for 3 to 9 h, and was subsequently cooled naturally to room temperature, resulting in the formation of anticorrosive films on magnesium alloys. The resulting films were characterized by XRD, XPS, SEM, FT-IR, and electrochemical measurements.rnSome peaks at approximately 2θ = 18, 33, 38, 51, 58, 62, and 72°, which could be assigned to the 001, 100, 101, 102, 110, 111, and 201 reflections of brucite type Mg(OH)_2 were clearly observed in the XRD patterns. In addition, two peaks at around 2θ = 11 and 22o assigned to the 003 and 006 diffraction peaks were clearly observed, which could be assigned to Mg-Al LDH. These results indicates that the film formed on AZ31 by steam coating was composed of crystal Mg(OH)_2 and Mg-Al-LDH. The corrosion resistance of the film were investigated by electrochemical measurements. The potentiodynamic polarization curves of the film coated and uncoated AZ31 after immersion in the 5 wt% NaCl aqueous solution for 30 min revealed that the corrosion current density, jcorr, of the AZ31 coated with films decreased by more than four orders of magnitude as compared to that of the uncoated one. This indicates that the film coated AZ31 has a better corrosive resistance than uncoated one.