Nano-Engineering of Anodic Oxide Coatings for Anti-Corrosion and Anti-Biofouling

J. Lee, C-H Choi
Stevens Institute of Technology,
United States

Keywords: anodizing, corrosion, biofouling, self-healing

Summary:

Anodizing process has widely been used in industries for the protection and passivation of metallic substrates, growing an oxide layer. However, self-ordered nano-porous structures are generally formed during the anodizing process, which are typically filled with solid-state material for better protection and passivation. Nonetheless, the imbibition of corrosive liquids into the porous layer of anodic oxide is inevitable, due to the hydrophilic nature of the anodic oxide and sealing materials. The contact and imbibition of corrosive liquids can significantly be inhibited by hydrophobic coating on the nano-porous anodic oxide surface, which allows the retention of air within the pores. In addition, the porous nanostructures of anodic oxide with hydrophobic coating can enhance the anti-bacterial adhesion causing a wide range of serious persistent infections. However, the prevention of air depletion from the nanostructured surface still remains as a challenge for real applications. In addition, the major drawback of current passivation techniques for preventing corrosion is the lack of the ability to withstand any external damages or local defects, which leads to catastrophic corrosion. In order to overcome the limitations of current passivation techniques for anodic oxide, we have developed an oil-impregnated anodic oxide that has high-aspect-ratio dead-end nano-pores. Although the disconnected pores from one another would be advantageous for immobilizing the oil within the porous structure, the complete oil-impregnation into the nano-pores is challenging due to the high-aspect-ratio dead-end geometry of the pores on a nanoscale. We have developed a solvent exchange method for the complete oil-impregnation, which is critical for the enhanced durability and self-healing property. The complete oil-impregnation into a nano-porous anodic aluminum oxide (AAO) layer resulted in the superior inhibition of the penetration of corrosive media into the AAO layer and hence the corrosion of aluminum. In addition, the oil completely impregnated within the nano-porous AAO layer naturally permeates into defects or damaged areas and ultimately inhibits the exposure of the aluminum surface to the corrosive media, exhibiting an effective self-healing capability. In order to further enhance the retentivity of oil within the pores for more durable corrosion resistance and omniphobicity, we have designed various types of AAO nanostructures. The robust oil-impregnation has also enabled the nano-porous AAO surface to resist the adhesion of bacteria significantly (e.g., by 99.2% for E. coli K-12) compared to a bare aluminum. Recently, we have also successfully realized versatile oil-impregnated oxide coatings on stainless steel with anodizing processes, and demonstrated the superior corrosion resistance with self-healing capability. The anodizing and oil-impregnation processes that we have developed are scalable and readily applicable to current industrial manufacturing processes so as to provide the industry-relevant metallic substrates with multifunctional surface properties, such as anti-corrosion and anti-biofouling, with great durability and self-healing capability.