Preparation and characterisation of composite double phosphate conversion coatings on AZ91D magnesium alloy

Abstract

A composite double zinc phosphate conversion coating was obtained via forming a hopeite crystal layer on an amorphous phosphate layer on the AZ91D magnesium alloy. The surface morphology, structure and phase composition of the composite coating were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). As a comparison, the single zinc phosphate coating and the bare substrate were also discussed. The phosphate crystal clusters can form on the amorphous layer densely and fine. According to the open circuit potential (OCP) and potentiodynamic polarisation curve tests, the composite phosphate conversion coating on the AZ91D magnesium alloy yielded the best protective effect compared with the single amorphous layer and crystal layer.

Laser Surface Engineering of Magnesium Alloys: A Review

“…Most of the developed conversion coatings cannot be used at elevated temperatures (above 65°C). [14][15][16][17] Anodizing results in the formation of brittle oxide layers that might not have sufficient mechanical strength for a wide range of applications. Also, most of the anodized surfaces do not offer sufficient protection as the single surface treatment, but they can form excellent presurfaces for other techniques like paints and organic coatings.…”

“…In contrast, a very low laser power or high scan speed can cause nonuniform distribution of Si and Al 2 O 3 particles on the surface because of poor melting and inadequate intermixing. Liu et al 58 17 . LSC on AZ91HP was also done using Al (33 wt.%)-Cu alloy to enhance the corrosion resistance of the surface.…”

“…LSC of ZE41 with Al-Si was studied by Volovitch et al 60 In this study, three different consecutive passes with overlapping zones were created with scan speed of 300 mm/min, 400 mm/min, and 500 mm/min. The different phases formed in the cladded surface were Al, Mg, Mg 17 61 performed a parametric study on LSC of AS21 (1.9-2.5%Al-0.15-0.2%Zn-0.35%Mn-0.7-1.2%Si-0.04%Cu-0.002%Ni) with Al/Si (41 wt.%) and WC (60 wt.%) powder. At a higher scan speed, clad thickness and clad width both decreased.…”

Microporosity formation in Mg alloys and its effect on protective coatings

“…4 However, one issue of Mg alloys is corrosion resistance, 5,6 and a number of surface treatment methods have been used to improve this. [7][8][9][10][11][12][13][14] One such treatment is chemical conversion, which has been extensively investigated and used in industrial applications because it can effectively prevent the corrosion of components in aqueous solutions. 2,5,9,14 For both protection and decorative applications, organic coatings on conversion coated Mg alloys have the advantage of low cost and simple processing.…”

“…[7][8][9][10][11][12][13][14] One such treatment is chemical conversion, which has been extensively investigated and used in industrial applications because it can effectively prevent the corrosion of components in aqueous solutions. 2,5,9,14 For both protection and decorative applications, organic coatings on conversion coated Mg alloys have the advantage of low cost and simple processing. 2,15 The most important aspect of an organic coating is adhesion to the substrate.…”