Novel composite films prepared by sol–gel technology for the corrosion protection of AZ91D magnesium alloy

Abstract

A novel composite films technology was developed to improve the anticorrosion performance of AZ91D magnesium alloy. The composite films were prepared via sol–gel method which is a simple application procedure within industry. The structure of xerogels and films, morphology of films were analyzed using thermo-gravimetric (TG) and differential thermal analysis (DTA), Fourier transformed infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scan electron microscopy (SEM). The anticorrosion performance of the sol–gel coated and uncoated samples in 3.5 wt.% NaCl neutral solution had been evaluated by electrochemical tests. The results demonstrated that the composite films “Ce inner film/Mg outer film” exhibited better corrosion resistance.

Introduction

Magnesium alloys have a number of advantageous physical and mechanical properties that make them attractive materials for many industrial applications, such as automotive, portable devices, aircraft, military equipment, orthopedic equipment, diving gear and sport goods. However, one of the main reasons limiting or even preventing larger scale use of magnesium alloys for various applications is their high corrosion susceptibility [1], [2], [3], [4], [5]. Therefore, improving the corrosion protection of Mg alloys without the loss of high strength/weight ratio is a real challenge that can lead to a break-through in many industrial areas. In recent years, a number of surface technologies have been employed to improve their corrosion resistance including chemical conversion coating [6], [7], anodizing [8], electrochemical plating [9], [10], [11], sol–gel process [12], [13], [14], [15], [16], [17], etc.

Among all the techniques, the sol–gel process has been demonstrated to be an effective and environment-friendly route to prepare films on metallic substrates at low cost [18], [19], [20], [21]. Some authors [12], [13], [14], [15], [16], [17] have reported the anticorrosion resistance of sol–gel thin films on magnesium alloys. Generally, the sol–gel process is widely used for the production of effective films starting from alkoxide precursors including Si, Al, Zr, Ce, etc., they are expensive and the process of hydrolysis and polycondensation reaction is not easy to control. While the sols using inorganic salts precursors such as Ce own a relatively low pH [22], [23], [24], [25] and magnesium alloys would be eroded by the sol, leading to poor coating adhesion and subsequent corrosion. In previous works [26], we developed a novel approach to prepare sol–gel films. In the novel sol–gel process, appropriate additive was used to stabilize and disperse uniformly the inorganic salts, the sols with appropriate pH could be applied directly on magnesium alloys. Compared with the traditional sol–gel process, the novel sol–gel technology offers important advantages such as lower cost, more environment-friendly and simpler application procedures easily adaptable within industry.

In recent years, composite films [15], [27], [28], [29] are of great interest in corrosion protection. Due to the structure limits of individual film, defects originated in individual film form a path for the corrosive medium to penetrate the substrate. Composite films decouple the defects on each individual film and the formation of such corrosive paths is minimized. In addition, film materials can be chosen to combine desired properties like low chemical reactivity and diffusion barriers.

The present work has two objectives: (i) To prepare Ce and Mg films on AZ91D Mg alloys using the novel sol–gel technologies, and study the characters of the both xerogels through TG–DTA, XRD and FT-IR measurements. (ii) To obtain suitable composite films on AZ91D Mg alloys via combination of single Ce and Mg films. The surface morphology and the corrosion resistance of the films were evaluated using SEM and electrochemical tests, respectively.