Corrosion-Resistant Porous Alumina Formed via Anodizing Aluminum in Etidronic Acid and Its Pore-Sealing Behavior in Boiling Water

Abstract

Alkaline corrosion-resistant porous alumina was fabricated by anodizing aluminum in etidronic acid, and its hydration behavior during pore sealing in boiling water was investigated. High-purity aluminum plates were anodized in sulfuric, oxalic, citric, and etidronic acid solutions. Anodizing with etidronic acid caused stable growth of a uniform, porous alumina layer at a high voltage of approximately 200 V. This porous alumina possessed a thick barrier layer with an inner layer of pure aluminum oxide and exhibited a 10-fold increase in the corrosion resistance in a 2.5 M sodium hydroxide solution. When the porous alumina film formed by sulfuric acid anodizing was immersed in boiling water, plate-like hydroxide scales rapidly formed on the whole surface, and the pores were sealed within 10 min. In the case of etidronic acid, the hydroxides formed at the bottom of the pores in the initial stage of immersion, and the thickness of the hydroxide layer gradually increased with the immersion time. The porous layer was completely sealed by long-term immersion. Although the barrier layer was reduced to approximately 80% of its original size due to hydration, a thick barrier layer was still maintained at the bottom of the pores after immersion.

Incorporation of Ions into Nanostructured Anodic Oxides—Mechanism and Functionalities

“…Further research is therefore necessary for better understanding the incorporation of larger anions, as the resulting properties are of particular interest. For example, AAO formed in etidronic acid is more resistant to etching in 2.5 M NaOH, and pore sealing in boiling water is slower as compared with AAO prepared in the same conditions in sulfuric acid [97].…”

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

“…While most organic acids form barrier anodic oxide layers when used as sole anodizing electrolytes, porous anodic oxide layers are formed when they are combined with sulfuric acid [34]. The addition of organic acids to sulfuric acid electrolytes has been studied for different purposes [15,[31][32][33][34]45,83,[124][125][126][127]. In particular, Vignoli et al [34] studied the influence that adding different carboxylic acids to sulfuric acid has on the morphology, composition, and corrosion behavior of anodic oxide films on AA2024 alloy and compared their performance to CAA oxide films.…”

“…During a hydro-thermal sealing the anodized part is immersed in boiling water with or without additives to further enhance corrosion protection. Three steps take place during immersion: first the aluminum oxide dissolves into the hot water, then a gel-like aluminum hydroxide is deposited on the surface and finally pseudoboehmite is formed by condensation of the gel [31]. As previously mentioned, phosphates have the ability to suppress the alumina hydration mechanism [53].…”

Nanostructured Anodic Copper Oxides as Catalysts in Electrochemical and Photoelectrochemical Reactions

“…Anodic oxides are well-known for their applications in metals corrosion protection. Compact, adherent and hard oxide coatings prevent majority of aluminum alloys items and devices from the degradation [1][2][3]. This approach is widely applied in industry-for example: bodywork of aircrafts, aluminum alloys turbine blades, and smartphones are anodized.…”