Formation of Anodic Films on Magnesium Alloys in an Alkaline Phosphate Electrolyte

Abstract

The development of anodic films at a constant current density of 10 mA cm−2 has been studied for Mg-W alloys, containing 0.4 and 1.0 atom % W, in 3 M ammonium hydroxide/0.05 M ammonium phosphate electrolyte at 293 K. The structure, morphology, and composition of the films were determined by X-ray diffraction, scanning electron microscopy, atomic force microscopy, glow discharge optical emission spectroscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. During anodizing to about 50 V, a relatively smooth film develops. Cross-sectional atomic force microscopy suggests the film may be finely porous. With an increase in voltage, the film transforms gradually to a coarse, porous morphology due to dielectric breakdown. The transformation coincides with a reduction in slope of the voltage-time response. Significant regions of both types of film morphology coexist during anodizing to 150 V. With further anodizing, a porous film, developed in the presence of sparking above about 270 V, eventually covers all of the macroscopic surface. Magnesium, phosphorus, oxygen, hydrogen, and nitrogen species are present throughout the film thickness at the resolution of the measurements. However, the films are initially free of tungsten species due to enrichment of tungsten in the alloy. The O:Mg atomic ratio is in the range 1.7 to 2.1, reducing with an increase in voltage for films formed up to 220 V, consistent with films composed primarily of hydroxide or oxyhydroxide. The P:O atomic ratio increases with an increase in voltage, starting at about 0.03 for voltages below 50 V and reaching 0.29 at 330 V. The films form at a reduced efficiency, typically about 40-50%. © 2001 The Electrochemical Society. All rights reserved.

Anodizing Treatments for Magnesium Alloys and Their Effect on Corrosion Resistance in Various Environments

“…[10] Compared to the films on aluminium and tantalum, studies on the films on magnesium formed below the dielectric breakdown voltage are relatively rare. Figure 3 shows a typical voltage-time response at low current anodization (10 mA cm -2 ) for the two alloys Mg-1.0 at.% W and Mg-0.4 at.% W. [11] The voltage-time response, which was similar for both alloys, revealed several features that were highly reproducible. The voltage initially rose rapidly, within 1 s, to about 10 V, probably due to the influence of the air-formed film on the alloy.…”

Effect of current density and behaviour of second phases in anodizing of a Mg-Zn-RE alloy in a fluoride/glycerol/water electrolyte

“…In contrast, barrier films, containing organic and phosphate species, have been grown in amine/ ethylene glycol and hot phosphate/glycerol electrolytes, respectively [10,11], and low-voltage films that contain electrolyte components have been formed in an ionic fluid [12]. Other work has employed a variety of aqueous electrolytes, revealing films containing Mg(OH) 2 [13], MgF 2 and Mg x+y/ 2 O x (OH) y [14], MgO and MgAl 2 O 4 [15,16] hydroxide or oxyhydroxide [17] and MgF 2 [18].…”

Influence of chloride ion concentration on the electrochemical corrosion behaviour of plasma electrolytic oxidation coated AM50 magnesium alloy

“…Even though PEO of magnesium alloys can be accomplished in many different electrolytes, silicate and phosphate based electrolytes are the most widely employed electrolytes even today [19][20][21][22][23][24]. In our previous work [21], the electrochemical behaviour of PEO coated AM50 magnesium alloy produced from silicate and phosphate electrolytes in 0.1 M NaCl solution was reported.…”