Corrosion Resistance of Zinc Phosphate Conversion Coatings on AZ91D Surface

Abstract

Magnesium alloys have been widely applied in many fields, because of their high strength-to-weight ratio. However, magnesium alloys have high chemical activity and are easily corroded. The poor corrosion resistance of magnesium alloys greatly limits its further application. In this paper, the zinc phosphate conversion coatings were prepared on the surface of AZ91D magnesium alloys. Nano-zinc oxide was the source of zinc and the zinc phosphate conversion coatings were prepared by the given process: 1.25 g/L NaNO3, 3 g/L C6H8O7 H2O, 2.5 g/L NaF, 5.5 g/L ZnO, 12.5 mL/L H3PO4, reaction temperature 50°C, reaction for 30 minutes. The full immersion uniform corrosion test was conducted for the fabricated coatings. The morphology and composition of corrosion in different corrosion stages were characterized by XRD, SEM and other microscopies. The results showed that: (1) the corrosion process of the conversion coatings could be divided into three stages: the dissolution of the conversion coatings, the corrosion of the matrix and the deposition of insoluble matter; (2) XRD analysis and other methods found that the pine-needle magnesium oxychloride compounds were formed in the process of immersion firstly, and it was dissolved into Mg(OH)2 over time; (3) With the extension of immersion time, Mg(OH)2 increased continuously and played a major role in corrosion prevention. The deposited Mg(OH)2 was divided into two layers. In the initial deposition stage, it was mainly evenly dispersed on the surface of the alloy to form a tightly arranged inner layer. Afterwards, the crystals of Mg(OH)2 agglomerated and formed a sphere, becoming the outer layers.

Investigating the Concurrent Effect of Cerium/Hydroxyapatite Coatings on Mg-Based Implant for Enhancing Corrosion Performance and In-Vitro Activity

“…One of the factors limiting the use of this metal is its weak corrosion resistance [11][12][13]. In fact, magnesium metal with a standard potential of −2.34 V vs. NHE (normal hydrogen electrode), when exposed to a corrosive environment, shows weak corrosion resistance and quickly disappears [14][15][16][17].…”

Effect of Surfactants on the Corrosion Protectability of Calcium Phosphate Conversion Coatings on Duplex Structured Mg-8Li (in Wt.%) Alloy

“…Due to the simplicity in operation and low prices, phosphate conversion coatings being an alternative to chromate conversion coatings are developed [31,32,[37][38][39][40]. Through preparing a manganese phosphate conversion coating (MnPCC) on the Mg-10Li-1Zn alloy (in wt.%) and measuring its corrosion resistance, Zhang et al reported that the MnPCC of Mg alloys had the better corrosion resistance than chromate conversion coatings [33].…”

One-Pot Preparation of Silica Fibers from Rice Straw for Efficiency Removal of Heavy Metal Ions

“…This attributes to the decomposition of cellulose and lignin through the dehydration on the polymer side chain during the calcination process. In addition, the peak at 440 °C corresponds to the decomposition of the main chain of cellulose and lignin [28]. Therefore, the slow calcinations of the rice straw undergo a stepby-step thermal decomposition, which makes it fully take advantage of the organism fibril structures as the templates to form the silica fibers.…”