Experimental study of corrosion protection of a three-layer film on AZ31B Mg alloy
Highlights
► A coherent bilayer is formed by nanoparticles and plasma electrolytic oxidation film. ► A coherent bilayer has excellent barrier property. ► Galvanic corrosion of the three-layer film on Mg substrate is inhibited greatly. ► The three-layer film provides long-term immersion corrosion protection.
Introduction
It is well known that Mg alloy with poor corrosion resistance and high chemical reactivity limits its practical application in many domains. Among various surface treatment techniques of Mg alloy, EN plating is an effective way due to good chemical stability, high rigidity, good abrasion resistance and corrosion resistance [1], [2]. However, galvanic corrosion between the EN plating and the Mg alloy substrate seems still to be a serious problem due to the pores in the EN plating. Thus, developing a compact EN plating process on Mg alloy and Mg alloy with good barrier is an important task.
PEO can produce a ceramic film by in situ growth on the surface of Mg with excellent adhesion [3], [4], [5], [6], [7]. However, the porous PEO film as the barrier layer is unsuitable to provide long-term immersion protection for Mg alloy substrate. In recent years, combined technologies that produce multilayer, composite film for Mg alloy are a promising way. SANP technology is a new organic–inorganic hybrid system which can produce inorganic silica nanoparticles in an aqueous sol–gel process [8], [9], [10], [11], [12], [13], [14]. Our previous studies [15], [16] have proved that these nanoparticles can penetrate into the porous PEO film to form an integrated SANP + PEO film via organic crosslink agent. Accordingly, the SANP + PEO film will be a good choice as the barrier layer between the EN plating and the Mg alloy substrate.
The aim of this work is to prepare a three-layer film on AZ31 Mg alloy which consists of the PEO film as the bottom layer, the SANP film as the intermediate layer and the EN plating as the top layer. For the systematic study of the three-layer film property, another three films coated on Mg substrate are also prepared: a bilayer of PEO + EN film, single PEO film and single EN plating. Their properties are characterized by surface analysis measurements and electrochemical methods. It is our intention to clarify the protection property of the three-layer film against galvanic corrosion and long-term immersion corrosion through the studies mentioned above.
Section snippets
Preparation
AZ31 Mg alloy plates (50 × 50 × 1 mm) were used as the substrate material for the surface treatments with the following composition (wt.%): Al 2.5–3.5, Zn 0.6–1.4, Mn 0.2, Si ⩽ 0.1, Fe 0.005, Cu ⩽ 0.05, Ni 0.005 and Mg balance. The Mg plates were degreased ultrasonically in acetone, cleaned with distilled water, and then dried in air.
SANP solution (Table 1) was prepared by the hydrolysis of 3-glycidoxypropylp- trimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) in de-ionized water under stirring at
SANP solution characterization
Figure 1 shows that the TEM image of nanoparticles with a diameter of 20–25 nm after 60 days can still suspend stably in the SANP solution. Like most traditional sol–gel processes, the SANP processing also consists of hydrolysis reaction and condensation reaction. Our studies [16], [17], [18] indicate that the hydrolysis reaction takes place very fast in the high water concentration solution; while the condensation reaction is much too slow to neglect for most cases. Therefore, the nanoparticles
Conclusions
A composite film coated on AZ31B Mg alloy was studied which consisted of the bottom PEO film, intermediate SANP film and the top EN plating. The SANP solution contained large numbers of SiO2 nanoparticles with the structure of cyclic rings and epoxy functional groups, which penetrated into the porous PEO film to form a coherent SANP + PEO film. The electrochemical tests indicated that the galvanic corrosion was prone to happen on the surface of single EN plating; the PEO film on PE film inhibited
Acknowledgements
This work was supported by project supported by National Science and Technology Ministry (Grant No. 2011BAE22B05) and Material foundation and application technology of key projects (Grant No. A0920110028).
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