Effect of sealing on the morphology of anodized aluminum oxide

Abstract

Ultra-small angle x-ray scattering (USAXS), small-angle neutron scattering (SANS), x-ray reflectometry (XRR) and neutron reflectometry (NR) were used to probe structure evolution induced by sealing of anodized aluminum. While cold nickel acetate sealing and hot-water sealing decrease pore size, these methods do not alter the cylindrical porous framework of the anodic aluminum oxide layer. Hot nickel acetate both fills the pores and deposits on the air surface (air-oxide interface), leading to low porosity and small mean pore radius (39 Å). Electrochemical impedance spectroscopy and direct current polarization show that samples sealed by hot nickel acetate outperform all other sealing methods. Recent studies on new sealants and sealing processes include cold nickel acetate sealing, [13] sodium silicate sealing,[18] nickel fluoride sealing, [10, 18] Cr 2 O 3 sealing,[19] sodium acetate sealing, [12, 14] cerium acetate sealing, [17, 20] cerium nitrate and yttrium sulfate sealing, [9]sol-gel sealing, [16] and even an expensive sealing process using polytetrafluroethylene (PTFE).[19] In spite of these efforts to improve the performance, more convenient and effective processes are still needed. [19] In developing the new sealing methods, DC polarization (DCP) [10,19,21,22] and electrochemical impedance spectroscopy (EIS) [13, 15-17, 20, 23-25] have been used to compare the corrosion performance between different methods. For hydrothermal sealing it is believed that boehmite (AlOOH) is produced at temperatures above 80 ºC, while less soluble than hydrargillite (Al(OH) 3 ) that is formed at low temperatures. For cold sealing methods, however, the mechanism has not been completely elucidated.[18] The structural alternation induced by sealing and the relationship between pore evolution and corrosion resistance have not been investigated.In this work, we use x-ray reflectivity (XRR), neutron reflectivity (NR), ultra-small-angle x-ray scattering (USAXS) and small-angle neutron scattering (SANS) to investigate the morphological changes induced by sealing. Cold nickel acetate sealing is of particular interest as it has been reported to be a promising alternative to conventional hot water sealing. [10,13,17,20] Our results, however, show that both Ni and high temperature are required for effective corrosion protection.

Effect of NaAlO2 sealing on corrosion resistance of 2024 aluminum alloy anodized film

“…When immersing the sample in NaCl solution, the electrolyte can easily pass through the porous layer, thus only the information of barrier layers can be obtained during impedance spectroscopy . In the case of the sealed anodized sample, the nano‐pores are almost filled due to the volume expansion of the alumina or the formation of products in the sealing process . Therefore, the structure of the porous layer changes, such as a decrease in the radius of the pores and an increase in the thickness of the pore walls.…”

“…Cr(VI) used for dichromate sealing is limited by the high toxicity and nickel salts are not only expensive but also contain toxic. Thus, they cannot be used in the food industry . Up to date, some green sealing methods have been proposed, including sol‐gel sealing, organic acid sealing, and so on.…”

Neutron Reflectometry for Studying Corrosion and Corrosion Inhibition

“…Interestingly, the authors found that the modification of conditions for the reflectivity measurements (e.g., limited currents) led to a different structure to that observed with the other techniques where the current was unlimited. Whereas the SAXS and SANS results showed varying penetration of the different sealing agents, the AA films studied by NR appeared to have their pores capped during the anodisation process [89]. Their work demonstrated the high desirability of combining a range of complementary techniques to study a corrosion system.…”

Sealing Holes of an Anodic Aluminum Oxide Alloy by Nanotitania for Enhanced Corrosion Resistance

“…For instance, nanotitania can be used to seal aluminum oxide for corrosion resistance and UV-protection by virtue of the commonly-known physiochemical stability and UVshielding performance of the nanotitania [42,43]. However, it remains a great challenge to use nanotitania as a hole sealing material for AAO film [23][24][25][26][27][28][29][30][31][32][33][34] This study aims to grow a layer of nanotitania film for the purpose of sealing the holes of the AAO film on the aluminum alloy.…”

“…Nanotitania has been regarded as the most promising semiconductor catalyst and environmentally friendly photocatalytic material, owing to its impressive photocatalysis, hydrophilicity, high electron-hole counter potential, and good physiochemical stability, low cost, non-toxicity and ready availability [19][20][21][22][23][24]. As a consequence, nanotitania has been extensively explored across various scientific disciplines [21,22,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41].…”