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.
Highlights
The aluminum anodization process is an energy intensive process
This conclusion is consistent with observed significant incorporation of chromium into anodic oxides on aluminum sealed in chromate-containing sealing baths, in the light of reports [78,87] that hydrothermal sealing leads to increase in the proportion of octahedral sites in anodic oxides on aluminum
Most current industrial technology solutions for sealing anodized aluminum are in need of improvements in their energy efficiencies and the environmental impacts
Summary
The aluminum anodization process is an energy intensive process. Besides the anodization step, the sealing step when it involves high temperature sealing in hot water (often at temperatures ≥ 95 ◦C) is arguably the second highest energy consuming step. The efficacy of hot-water sealing treatment is based on its ability to promote hydration of the porous aluminum oxide and barrier layers, producing a crystalline hydrate phase (boehmite) which fills the pores [4]. Hydrothermal sealing of anodized aluminum surface in deionized boiling water (T > 95 ◦C) is reported to proceed at rates in the range of about 2 min/μm [5,6], which translates to long sealing times, typically several minutes. The high energy requirement of maintaining the sealing bath at temperatures ≥ 95 ◦C over several minutes, and the high-water quality requirement of the hydrothermal sealing process have jointly driven research efforts towards the development of mid-temperature and room temperature. 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
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