Structure and composition of anodic films formed on binary Mg–Al alloys in KOH–aluminate solutions under continuous sparking

doi:10.1016/S0010-938X(00)00116-5

Corrosion Science

Volume 43, Issue 7, July 2001, Pages 1295-1307

https://doi.org/10.1016/S0010-938X(00)00116-5 Get rights and content

Abstract

Anodizing of binary Mg–Al alloys (2–12% at Al) and intermetallic Mg₁₇Al₁₂ was studied in a solution containing 3 M KOH+0.6 M KF+0.21 M Na₃PO₄, with and without addition of 0.4 M and 1.1 M aluminate. The anodic films were formed under continuous breakdown and anodizing was accompanied by intensive sparking and oxygen evolution. The films formed on Mg–Al alloys in a bath containing 1.1 M aluminate were composed of MgAl₂O₄ spinel and that on intermetallic Mg₁₇Al₂ of γ-Al₂O₃ and MgAl₂O₄.

Introduction

Anodic coatings available for magnesium and its alloys are usually formed at high voltages and anodizing is often accompanied by breakdown phenomenon. The formation of anodic films under continuous breakdown provided elaboration of some commercial anodizing processes [1]. However, the anodic coatings formed on magnesium by these processes provide limited corrosion protection and are mainly used as undercoatings for organic finishes [1], [2], [3].

Pure magnesium has limited application and is usually alloyed with aluminum, manganese and zinc. Anodizing of magnesium alloys was studied in several works. Takaya et al. [4], [5] studied anodizing of magnesium–manganese alloy in alkaline solutions. They showed that anodic films consisted mainly of Mg and O, however, incorporation of some electrolytic species was also detected. Timonova et al. [6], [7] studied high voltage anodizing of magnesium–aluminum alloys in cromate–fluoride solutions. The anodic films formed in these solutions were crystalline and mainly composed of chromium oxide and magnesium dichromate. Ono et al. [8], [9] showed that the anodic films formed on magnesium and magnesium–aluminum alloys at low voltages in similar electrolyte were mainly amorphous and had the structure resemble the one of anodic porous alumina. Zozulin and Bartak [10] studied corrosion properties of anodic coatings formed in acid fluoride solutions. Sharma et al. [11], [12], employed anodic coatings on magnesium–aluminum alloy for thermal control applications. Hagans [13] studied anodizing of magnesium–aluminum alloys in borate solution. He demonstrated that increase in aluminum content in the alloy was beneficial for passivation of the alloy. The chemical composition of the films formed in this solution showed that the films were enriched with aluminum compared with the base alloy.

To improve corrosion properties of anodic films on magnesium, we carried out a study of magnesium anodizing in KOH–aluminate solution [14], [15], [16]. Recently, we reported [16] that anodic films highly enriched in aluminum can be formed on pure magnesium under continuous sparking. The anodic coatings formed in a bath containing 1.1 M aluminate were composed of single magnesium–aluminum oxide spinel-MgAl₂O₄ which is considered as beneficial for the corrosion resistance of anodic films. The present work was undertaken to investigate the morphology, structure and composition of anodic films formed on Mg–Al alloys. Commercial Mg–Al alloys usually contain number of other alloying elements. Therefore, to reveal the effect of aluminum, this work is focused on anodizing of binary Mg–Al alloys. These alloys in the as-cast conditions have two-phase microstructure-α-Mg, which is a solid solution of aluminum in magnesium and β-phase, which is intermetallic Mg₁₇Al₁₂. Anodizing of the separate phases and binary Mg–Al alloys in the as-cast condition was studied.

Section snippets

Experimental

Binary Mg–Al alloys containing 2, 5, 8 and 12 at.% of Al and intermetallic Mg₁₇Al₁₂ (44.5 at.% Al) were supplied by Alcan International Ltd. To obtain single-phase microstructure, the alloys were given a solution heat treatment at 420°C for 24 h. The samples were mounted in a room temperature curing epoxy leaving 1 cm² of the surface exposed. After polishing to 600-grit alumina, the samples were carefully degreased with acetone and water-rinsed. The experiments were performed in a bath

Results and discussion

Fig. 1 shows voltage transients obtained during anodizing of Mg–Al alloys and intermetallic Mg₁₇Al₁₂ in a bath containing 1.1 M aluminate at a current density of 15 mA/cm². The voltage increased linearly until breakdown. The breakdown voltage was 58, 60, 61 and 80 V for Mg–2%Al, Mg–5%Al, Mg–8%Al and intermetallic Mg₁₇Al₁₂, respectively. Intensive sparking appeared on the surface of the alloy at this voltage accompanied by oscillation of voltage and gas evolution. Sparks rapidly scanned over the

Conclusions

Thick anodic films can be formed on Mg–Al alloys under continuous breakdown conditions. The formation of the film is accompanied by voltage oscillation and intensive oxygen evolution. The growth of anodic films is non-uniform. For two-phase alloys the process starts on α-Mg and continues on β-phase (Mg₁₇Al₁₂) when the voltage exceeds 80 V. A uniform anodic film is formed on the alloy substrate when the anodizing voltage reaches 120 V. Anodic films are porous and contain high amount of aluminum.

Acknowledgments

This work was performed by a grant from Dead Sea Works Ltd. The single-phase samples were donated by Alcan International with the help of Dr. Roland Timsit. We are grateful for his help.

References (18)

O. Khaselev et al.
Corros. Sci.
(1998)
E.F. Emley, Principles of Magnesium Technology. Pergamon Press, London, New York,...
R. Ross (Ed.), Handbook of Metal Treatments and Testing, second ed. Chapman & Hall, New York,...
G.G. Perrault, in: A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, vol. 8, New York, M. Dekker,...
M. Takaya
J. Metal Finishing Soc. Jpn
(1998)
M. Takaya
Gypsum & Lime
(1989)
M.A. Timonova et al.
Zasch. Metal.
(1968)
M.A. Timonova et al.
Metal. Nemetal. Pokrytia Legk. Metal. Splavov
(1972)
S. Ono et al.
J. Electrochem. Soc.
(1996)

There are more references available in the full text version of this article.

Cited by (213)

Corrosion behavior and incorporation mechanism of Y<inf>2</inf>O<inf>3</inf>-TiO<inf>2</inf> composite coatings fabricated on TC4 titanium alloy by plasma electrolytic oxidation
2024, Chemical Physics Letters
The anti-corrosion of titanium alloys is a topic of concern in many fields. In our work, the Y₂O₃-TiO₂ composite coatings were fabricated on the TC4 titanium alloy by plasma electrolytic oxidation (PEO). The reaction was conducted in an electrolyte system made up of Na₂SiO₃, KOH, and (NaPO₃)₆. The effects of additive Y(NO₃)₃ in the electrolyte on the microstructure, chemical component, phase structure, thickness, hydrophobicity, and corrosion resistance of the composite coatings were studied. The results show that the composite coatings prepared by PEO reaction exhibit preferable corrosion resistance. The addition of Y(NO₃)₃ is beneficial for thickening the coatings, reducing their porosity and pore size, and further improving their hydrophobicity and corrosion resistance. Furthermore, the incorporation mechanism of Y₂O₃ was discussed based on thermodynamics theory.
In-situ construction of grass-like hybrid architecture responsible for extraordinary corrosion performance: Experimental and theoretical approach
2024, Nano Materials Science
Despite the engineering potential by the co-existence of inorganic and organic substances to protect vulnerable metallic materials from corrosive environments, both their interaction and in-situ formation mechanism to induce the nature-inspired composite remained less understood. The present work used three distinctive mercapto-benzazole (MB) compounds working as corrosion inhibitors, such as 2-mercaptobenzoxazole (MBO), 2-mercaptobenzothiazole (MBT), and 2-mercaptobenzimidazole (MBI) in a bid to understand how the geometrical structure arising from O, S, and N atoms affected the interaction toward inorganic layer. MB compounds that were used here to control the corrosion kinetics would be interacted readily with the pre-existing MgO layer fabricated by plasma electrolysis. This phenomenon triggered the nucleation of the root network since MB compounds were seen to be adsorbed actively on the defective surface through the active sites in MB compound. Then, the molecule with twin donor atoms adjacent to the mercapto-sites affected the facile growth of the grass-like structures with ‘uniform’ distribution via molecular self-assembly, which showed better corrosion performance than those with having dissimilar donor atoms with the inhibition efficiency (η) of 97% approximately. The formation mechanism underlying nucleation and growth behavior of MB molecule was discussed concerning the theoretical calculation of density functional theory.
In-situ LDHs growth on PEO coatings on AZ31 magnesium alloy for active protection: Roles of PEO composition and conversion solution
2023, Journal of Magnesium and Alloys
In this work, plasma electrolytic oxidation (PEO) coatings were produced on magnesium alloy AZ31 in aluminate, silicate and phosphate- based electrolytes, and followed by hydrothermal treatments in order to synthesis layered double hydroxides (LDHs) based nanocontainers. LDHs synthesis was done in three different growth solutions (deionized water, sodium nitrate and aluminum nitrate containing solution). In frame of this work it was shown, that it was difficult to form LDHs on Si-based PEO coating, due to more stable silicate phases in comparison with aluminate and phosphate phases in respective PEO coatings. The obtained hybrid LDH/PEO coatings were characterized using SEM, EDS and GDOES, and then the corrosion protection was further investigated by EIS. Based on the obtained results, it was confirmed that, the hydrothermal treatments in Al³⁺ containing solution played an important role on overall corrosion resistance for phosphate and silicate-based PEO coatings, but not for Al-based PEO coatings.
Instant micro-arc oxidation constructing the ultrafine nanoparticles as high-performance catalyst and mechanism study
2023, Materials Chemistry and Physics
The instant synthesis of ultrafine, clean surface Ru nanoparticle (NP) catalysts embedded on a porous MgO support was achieved by instant micro-arc oxidation (IMAO). The high temperature from the micro-arc discharge facilitates the rapid decomposition of the catalyst precursor RuCl₃ into homogeneous nanoparticles, while the fast quenching process ensures instantaneous encapsulation of the formed NPs by the MgO support. The obtained Ru/MgO catalyst exhibited attractive catalytic behaviour for sodium borohydride and ammonia borane hydrolysis, with activities of 100,917 mL min⁻¹·g⁻¹ and 2336 mol min⁻¹·mol⁻¹, respectively. Moreover, this catalyst exhibited an excellent sintering resistance in high-temperature environment that was well retained after 40 cycles, indicating superior recyclability. This work provides a novel pathway for the direct and rapid synthesis of high-quality nanocatalyst and can be generalized to prepare other supported metal nanocatalyst (e.g. Pd/MgO).
Growth behavior and corrosion resistance of graphene oxide/MgAl Layered double hydroxide coating grown on micro-arc oxidation film of magnesium alloys
2023, Journal of Industrial and Engineering Chemistry
The MgAl Layered double hydroxides (LDHs)/graphene oxide (GO) composite film was fabricated in situ on the micro-arc oxidation (MAO) film (MAO/LG) of AZ31 magnesium alloy in one step by hydrothermal chemical conversion method. The growth behavior and corrosion properties of LG films were investigated. The results show that the chemical structure, composition and morphology of the LG film will gradually change with the extension of the hydrothermal treatment time based on using the self-dissolved cation (Mg²⁺ and Al³⁺) in the MAO film as the source of growing LDHs without introducing any external metal cations. The potentiodynamic polarization showed that the MAO/LG-12 h coating (1.28 × 10⁻⁹ A/cm²) after in situ growth for 12 h was more corrosion resistant than the MAO coating (8.49 × 10⁻⁶ A/cm²). The synergistic effect of GO and LDHs can significantly improve the corrosion protection ability of MAO coating. In addition, the transition process of the growth behavior of the LG film layer on the MAO coating is explained in detail.
Synergistic effect of graphene oxide/ ternary Mg-Al-La layered double hydroxide for dual self-healing corrosion protection of micro-arc oxide coating of magnesium alloy
2022, Colloids and Surfaces A: Physicochemical and Engineering Aspects
Micro-arc oxidation (MAO) coatings have good corrosion protection properties and can effectively inhibit the corrosion of magnesium alloy surfaces in salt environments. This study aimed to achieve and improve the dual self-healing and corrosion resistance of MAO coating by in-situ growth to form a hybrid coating of vanadate (V₂O₇^4-) intercalated a graphene oxide (GO)-doped ternary Mg-Al-La layered double hydroxides (LDHs) (ternary L/G-V coating). The corrosion resistance and self-healing properties were verified by electrochemical experiments and scratch tests. Due to the synergistic effect of the strong barrier properties of GO and LDHs, the ternary L/G-V coating exhibits excellent anticorrosion ability. Interestingly, the ternary L/G-V coating also exhibits self-healing properties when corrosion ions reach the metal/coating interface due to the release of interlayer anionic corrosion inhibitor (vanadate) and rare earth cationic corrosion inhibitor (lanthanum) of LDHs. It makes up for coating defects in time and realizes the dual self-healing function of corrosion protection of MAO coating.

View all citing articles on Scopus

View full text

Structure and composition of anodic films formed on binary Mg–Al alloys in KOH–aluminate solutions under continuous sparking

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Corros. Sci.

J. Metal Finishing Soc. Jpn

Gypsum & Lime

Zasch. Metal.

Metal. Nemetal. Pokrytia Legk. Metal. Splavov

J. Electrochem. Soc.