Microstructure and Voltage-Current Characteristics of Anodic Films Formed on Magnesium in Electrolytes Containing Fluoride

Abstract

Formation behavior of anodic oxide films on magnesium in various electrolytes including fluoride was investigated with attention to the effects of anodizing voltage, pH and aluminum content. In the range of formation voltage between 2 V and 100 V, porous film was formed in alkaline fluoride solution associated with high current density at around 5 V and at breakdown voltage. The critical voltage of breakdown to allow maximum current flow was approximately 60 V and relatively independent on substrate purity. Barrier type films or semi-barrier type films, which were composed of hydrated outer layer and inner layer, were formed at the other voltages. A peculiar phenomenon of high current density at around 5 V, which may be caused by trans-passive state, was not observed for anodizing in acidic fluoride solutions such as Dow17 and ammonium fluoride. In the case of AZ91D, the critical voltage increased to 70 V and peculiar phenomenon at 5 V was not observed, so that only barrier films were formed at less than the critical breakdown voltage. When AlO À 2 ion was added in the electrolytes, the critical voltage remarkably increased and current density effectively decreased with increasing AlO À 2 content. The passivation effect of aluminum addition in the electrolytes is more remarkable than the addition in magnesium substrates. The depth profiles of constituent elements showed that aluminum migrated into oxide film to reach near oxide/substrate interface. Atomic ratio of aluminum to magnesium increased with increasing voltage to attain 0.42 at 80 V and crystalline MgAl 2 O 4 and MgO were found in the film.

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

“…[6] Ono et al reported similar breakdown potentials for various fluoride containing solutions. [7,8] They found that except at around 5V and breakdown voltages, a barrier type film or a semi-barrier film was formed in the alkaline fluoride solution. Above the breakdown potential, porous films were formed.…”

“…The atomic ratio of aluminium to magnesium increased with increasing voltage to attain 0.42 at 80 V and crystalline MgAl 2 O 4 and MgO were found in the film. [8] The anodic behaviour of Mg-Al-Zn alloy (AZ91D) under low potential in 3 M KOH solutions was studied with and without addition of 0.5-5 M Na 2 SiO 3 . [9] Anodic films incorporating silicon were formed during electrolysis, and the films formed under a constant potential of 4 V in 3 M KOH solution with Na 2 SiO 3 were uniform and thicker than the films formed without Na 2 SiO 3 .…”

“…[24] The relation between final current density after 10 min and formation voltage in 3 M KOH, 0.6 M KF, and 0.2 M Na 3 PO 4 (pH13) at 29.8°C has been studied by Ono and Masuko. [7,8] It showed a difference between AZ31, AZ91 and pure Magnesium. A peculiar phenomenon of high current density at around 5 V can be explained by a trans-passive state similar to that observed with sodium hydroxide solution.…”

“…6). [7,8] Addition of fluoride, aluminate, phospate or tetraborate to the electrolyte solution resulted in a mixed composition of various magnesium compounds in the anodized layer. [4] Aluminate was incorporated into the coating as a significant component, producing a coating several micrometer in thickness.…”

Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ80 Magnesium Alloy in Alkaline Aluminate Solution

“…5) Plasma electrolytic oxidation (PEO) has recently attracted attention to form ceramic-like coatings on aluminum, magnesium and titanium for corrosion and wear resistance. [6][7][8] Spark micro-discharge in PEO process modifies the structure, composition and morphology of the oxide layer; a thick, highly crystalline and melt-quenched high temperature oxide coating is formed by this process.…”