Formation of Bright White Plasma Electrolytic Oxidation Films with a Uniform Maze-Like Structure by Anodizing Aluminum in Ammonium Tetraborate Solutions

Abstract

Bright white plasma electrolytic oxidation (PEO) films with uniform maze-like structures were obtained by anodizing Al in an ammonium tetraborate solution. High-purity Al plates were galvanostatically anodized in 0.3-2.4 M ammonium tetraborate solutions at 303-343 K and 10-100 Am-2. A PEO film consisting of an outer porous layer consisting of amorphous alumina and crystalline alumina with α- and γ-phases and an inner amorphous barrier alumina layer was obtained on the Al surface. An extremely uneven PEO film with various pore sizes and many cracks was formed in a 0.3 M ammonium tetraborate solution, whereas a relatively uniform porous PEO film with similar pore sizes was obtained in 0.9-2.4 M solutions. This difference in the PEO film morphology was due to the plasma generation behavior while anodizing. The lightness of the PEO film increased with increasing anodizing time and PEO film thickness; thus, a bright white PEO film measuring 87.5 in lightness (L*) was formed on the Al surface. The water wettability of the PEO film exhibited weak hydrophilicity. Moreover, a superhydrophobic PEO film with a contact angle of 154° was easily fabricated by self-assembled monolayer modification. Similar bright white PEO coatings were successfully fabricated on various industrial alloys.

A Novel Polishing Process for Ultra-Smooth Aluminum Surfaces via Anodizing in Sodium Metaborate

“…Therefore, electrochemical polishing (electropolishing) based on anodic dissolution in several electrolyte solutions, such as perchloric acid and phosphoric acid, is widely used for surface polishing aluminum. [7][8][9][10][11][12][13][14][15][16][17] Although electropolishing is a powerful and simple method for achieving smooth aluminum surfaces on three-dimensional shapes, characteristic dimples or stripe patterns measuring several tens of nanometers in length and sub-tens of nanometers in depth typically remain on the surface after electropolishing, and these nanostructures depend on the crystallographic orientation of aluminum. [18][19][20][21][22][23][24][25] Hence, an additional process consisting of barrier oxide formation and subsequent oxide dissolution was developed for further smoothing.…”