The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

Abstract

Increasing demands for environmental accountability and energy efficiency in industrial practice necessitates significant modification(s) of existing technologies and development of new ones to meet the stringent sustainability demands of the future. Generally, development of required new technologies and appropriate modifications of existing ones need to be premised on in-depth appreciation of existing technologies, their limitations, and desired ideal products or processes. In the light of these, published literature mostly in the past 30 years on the sealing process; the second highest energy consuming step in aluminum anodization and a step with significant environmental impacts has been critical reviewed in this systematic review. Emphasis have been placed on the need to reduce both the energy input in the anodization process and environmental implications. The implications of the nano-porous structure of the anodic oxide on mass transport and chemical reactivity of relevant species during the sealing process is highlighted with a focus on exploiting these peculiarities, in improving the quality of sealed products. In addition, perspective is provided on plausible approaches and important factors to be considered in developing sealing procedures that can minimize the energy input and environmental impact of the sealing step, and ensure a more sustainable aluminum anodization process/industry.Coatings 2020, 10, 226 2 of 55 sealing methods. Furthermore, the applicability of another hitherto popular sealing process; chromate sealing, is currently limited to essential parts in the aerospace industry due to toxicological, health, and environmental implications traced to Cr(VI) employed in the process [7][8][9][10][11][12][13][14][15][16]. On the other hand, the advantages of another industrially utilized sealing process; the nickel fluoride (cold) sealing process is limited by the toxicity of nickel salts which narrows its range of application, and introduces added costs due to post-sealing wastewater treatments and management [17][18][19][20]. Further efforts at sealing anodized aluminum at temperatures lower than that used in hydrothermal (high temperature) sealing, have led to much variety in the chemical constitution and operating temperatures of sealing baths [21]. On the basis of temperature at which the sealing step is carried out, sealing can be classified into three major categories; high temperature, mid-temperature, and room-temperature or cold sealing. In this work sealing at temperatures from 0 to 40 • C is classified as low temperature sealing, from ≥ 40 • C to 70 • C as intermediate or mid temperature sealing, and sealing at temperatures > 70 • C as high temperature sealing.

Infrared Absorption Study of Zn–S Hybrid and ZnS Ultrathin Films Deposited on Porous AAO Ceramic Support

“…It can be noticed that the concentration of Zn, S, and C elements evidently increases with the increase in the ZnS layer thickness, except the C content in the samples after annealing, which stays on a similar level (~3 at.%). As the amount of all elements adds up to 100% in the EDS analysis, the at.% concentration of all other elements (Al, O, and P; P is present in the AAO matrix due to the incorporation of PO 4 3− ions during anodization in 0.1 M H 3 PO 4 electrolyte [27,28]) tends to decrease with the increase in Zn, S, and C elements. As observed previously [24], before annealing the amount of S is about two times higher than the amount of Zn.…”

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

“…Several concepts, including the ones mentioned above but also others, are being considered and up to now none of them has imposed itself as the new industrial standard. The reader is referred to the recent systematic review published by Ofoegbu et al [155] for further detailed information.…”

Investigation of Coating Thickness, Hardness and Wear Resistance of Aluminum 6061 by Anodizing Treatment

“…Based on Figure 11, the highest hardness is indeed achieved by specimen number 2, however, a significant increase in hardness is achieved in specimens number 7, 8, and 9, which used 100% HNO3 solution. However, using this type of solution produces a rough surface, which is not a good result for the following processing.The increase in hardness in the anodizing treatment is influenced by the immersion time and the amount of voltage used [10].…”