Sol–gel coatings of low sintering temperature for corrosion protection of ZE41 magnesium alloy

Abstract

Silica coatings have been obtained through the organic sol–gel route on magnesium–zinc alloys (ZE41) using low sintering temperatures for their corrosion protection. Tetraethoxysilane (TEOS) was used as alkoxide precursor, and the coatings, monolayer and 3-layers, were deposited by dip-coating technique at a controlled extraction speed of 35 cm/min. Temperatures of 135 °C were applied for several hours for coating densification avoiding mechanical deterioration of the magnesium alloy. To optimize the corrosion behaviour of the coatings different concentrations of sol solution were investigated. The corrosion resistance of the different coatings were tested by immersion in 3.5 wt.% NaCl aqueous solution and by electrochemical corrosion techniques. Low temperature silica coatings provided corrosion protection for up to 7 days without degrading the mechanical properties of the magnesium alloy used.

Introduction

Magnesium alloys are the lightest of all metals used as the basis for constructional alloys. This lightness, high strength to weight ratio and its proven versatility makes this material a great choice for application in aerospace, transportation and in civil and military applications [1]. Among all the magnesium alloys, the ZE41 is a well proven magnesium alloy that contains zinc, rare earth and zirconium and that has adequate mechanical properties at room and elevated temperatures, up to 150 °C, due to solution and precipitation hardening.

Magnesium alloys are intrinsically highly reactive and hence have insufficient corrosion resistance, which limit an extensive implementation of these alloys. Surface modification is often required to improve its corrosion resistance. To protect magnesium alloys from the corrosion, different surface treatments, such as plating, anodizing [2], [3], composite coatings [4], chemical conversion treatments [5], thermal spray coatings [6], [7] and even low temperature fabrication of intermetallic coatings in molten salts [8] are currently applied.

Sol–gel route is of particular interest when echo-friendly industrial technique has to be used. Some advantages of the sol–gel coating technique are: higher adhesion with metallic substrates, possibility of multilayer deposition, absence of defects after optimisation of the process, high purity of the final coating, possibility of coating in complex shapes, and availability of producing thin films without the need of machining or melting. It has been successfully used to improve the corrosion behaviour of some materials such as aluminium alloys [9], steels [10] and other metals [11]. Authors have proved the excellent corrosion improvement of aluminium matrix composites when sol–gel silica coatings are effectively deposited on them [12].

Some reports have been published on the protection of magnesium alloys by using sol–gel coatings. Tan et al. [13] used multilayered sol–gel coatings for corrosion protection of magnesium alloys using a first anodizing layer prior to sol–gel coating, to enhance the adhesion of the ceramic coating to the substrate. Phani et al. [14] improved the corrosion behaviour of the AZ91D magnesium alloy in salt spray testing by means of ZrO₂–CeO₂ coatings using the organic sol–gel route. There are also studies on the deposition of hybrid coatings sintered at low temperature, but the corrosion resistance obtained only lasted hours in the aggressive test environment. To increase the time of the protective behaviour of these hybrid coatings self-healing effect is promoted by the addition of TiO₂–CeO₂ particles [16]. To achieve corrosion resistance for longer times, purely inorganic sol–gel coatings sintered at high temperatures (400–500 °C) have been used. Under these conditions, the coating was compact and acted as a physical barrier between the electrolyte and the metallic substrate, providing protection for up to 24 h [17].

One of the main problems for coating magnesium alloy substrates with silica layers is the difference between their coefficients of thermal expansion (CTE): 26 × 10^− 6 K^− 1 for magnesium alloys and 0.55 × 10^− 6 K^− 1 for silica [14]. The use of high sintering temperatures could allow obtaining higher compaction of the layers but because of the CTE mismatch also increases the stress at the ceramic layer deposited in the surface of the magnesium alloy substrate and may result in the subsequent cracking of the coating.

In a previous work, authors reported the effective corrosion protection of ZE41 magnesium alloy up to 168 h of immersion in 3.5 wt.% NaCl aqueous solution by the use of sol–gel silica coatings sintered at 400 °C and 500 °C [16]. The major drawback of the use of high temperatures, when magnesium–zinc ZE41 alloy is used as substrate, is the reduction in its mechanical properties; in particular hardness reduces by 19%.

In this work, we report the deposition of pure inorganic sol–gel silica coatings on ZE41 magnesium–zinc alloy substrates using the dip-coating technique for corrosion protection and the effects of processing variables such as sol concentration, number of coating layers deposited and sintering thermal treatment. Finally, after optimization of the deposition parameters, two of the coatings tested were homogeneous and free of cracking; one of them was monolayer using a dense sol–gel solution (D500) and sintered at high temperature (500 °C) and the other one was multilayered and sintered at low temperature (135 °C) using a less dense solution (3 L135). These coatings provided corrosion resistance to ZE41 substrates for up to seven days of immersion in simulated sea water with minor lost of the mechanical properties of the substrate because of the low sintering temperatures used (135 °C).