Abstract
Mg-rich primer (MRP) containing aluminum tri-polyphosphate functions via a galvanic mechanism to protect AZ91D alloy from corrosion in the 3 wt % NaCl solution. However, its protective performance can be strongly affected by the testing environment. Therefore, it is important to investigate the performance of the primer on magnesium alloys in an acid rain environment. In the present study, the protective performance of MRP with or without aluminum tri-polyphosphate was investigated via open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) in the simulated acid rain. Compared to the primer without aluminum tri-polyphosphate, the MRP containing aluminum tri-polyphosphate pigments exhibited better protective performance in the simulated acid rain condition. In the initial stage, the acidic condition prompted the aluminum tri-polyphosphate pigments to release phosphates and H+ to form magnesium phosphates on Mg particles, retarding their consumption rate. The Mg-rich primer with aluminum tri-polyphosphate can provide cathodic protection to AZ91D alloy for about 49 days in the simulated acid rain solution. Simultaneously, the corrosion products of Mg particles, magnesium oxides and phosphates, precipitated on the Mg particles and improved the stability of the primer. In addition, a protective film, consisting of magnesium oxides and phosphates, formed on the AZ91D substrate. Consequently, all these factors contributed to the long cathodic protection and improved corrosion resistance of MRP containing aluminum tri-polyphosphate in the simulated acid rain.
Highlights
Magnesium alloys are among the potential candidates for applications in aerospace, automobile and communication fields, etc., due to their properties of high specific strength, low density, good machinability and low elastic modulus [1,2]
The widespread applications of Mg alloys are limited by their poor corrosion resistance, which is mainly attributed to their high reacting activity [3,4]
The AZ91D alloy could be cathodically polarized by the Mg-rich primer (MRP) and the protective performance of the epoxy coatings for the substrate was significantly strengthened by Mg particles [14,15]
Summary
Magnesium alloys are among the potential candidates for applications in aerospace, automobile and communication fields, etc., due to their properties of high specific strength, low density, good machinability and low elastic modulus [1,2]. Organic coatings have better corrosion resistance than chemical conversion coatings [5,6]. A magnesium-rich primer (MRP) providing cathodic protection to AZ91D Mg alloy has been developed [14,15]. On the basis of above principle, MRP was developed by adding pure magnesium particles with diameters of 10–20 μm to epoxy coatings [14,15,18,19,20,21]. The AZ91D alloy could be cathodically polarized by the MRP and the protective performance of the epoxy coatings for the substrate was significantly strengthened by Mg particles [14,15]. The barrier effect of MRP is observed on the AZ91D substrate; the permeated water activates the Mg particles in the primer gradually
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