Neutron reflectivity (NR) and X-ray reflectivity (XRR) were used to determine the structure and composition of trivalent chromium process (TCP) films on aluminum alloy AA-2024. Two deposition methods were developed: immersion and electroassisted (EA) deposition. The EA deposition method was designed to guarantee a well-defined film with a uniform structure and minimum contamination from Al and additives in the precursor solution. Quantitative NR and XRR analysis of the EA-TCP film confirmed linear growth as a function of deposition time. By analyzing both NR and XRR data on as-prepared and dried films, the film composition was determined to be Cr(2)O(3) x iH(2)O x x(ZrO(2) x jH(2)O) (i = 2.10 +/- 0.55, j = 1.60 +/- 0.45, and x = 0.85 +/- 0.14).

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.

Abstract. The present study investigates the formation mechanism of hollow SnO 2 nanofibers and the form of nanograin growth in nanofibers. SnO 2 hollow nanofibers were fabricated by directly annealing electrospun polyvinylpyrrolidone (PVP)/Sn precursor composite nanofibers. In this approach, an appropriate proportion of PVP/Sn precursor with co-solvents established a system to form core/shell PVP/Sn precursor structure, and then PVP was decomposed quickly which acted as sacrificial template to keep fibrous structure and there existed a Sn precursor/SnO 2 concentration gradient to form hollow SnO 2 nanofibers due to the Kirkendall effect and surface diffusion during the calcination process. This deduction was also confirmed by experimental observations using transmission electron microscopy. The study suggested that surface diffusion and lattice diffusion were both driving force for nanograin growth on the surface of SnO 2 nanofibers. As supporting evidence, the tetragonal rutile SnO 2 hollow nanofibers were also characterized by X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller analysis.

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In situ neutron reflectivity (NR) is used to observe the structure and evolution of a Trivalent Chromium Process (TCP) passive film on Al in a NaCl-D(2)O solution. Using a split liquid reflectivity cell we mimicked the corrosion process on the anodic sites in alloy AA 2024-T3 in a pitting scenario. The split cell separates the anodic and cathodic reactions, allowing NR observation of the corroding anodic surface under potential control. We observed the evolution of the TCP film on the Al anode and compared the degradation of the Al with and without TCP protection. When held at 100 mV above the open-circuit potential (OCP), unprotected aluminum dissolves at a rate of 120 Å/h. By contrast, TCP-coated Al is stable up to the pitting potential (200 mV above OCP). In the passive state D(2)O molecules penetrate the bulk TCP film by partially replacing the hydrate water. In spite of exchange of hydration water, the TCP film is stable and the underlying aluminum is fully protected. The passive character of the TCP film is due to a dense layer at the metal-TCP interface and/or to suppression of ion transport in the bulk film. As the pitting potential is approached the film swells and NaCl-D(2)O solution penetrates the TCP film. At this point, 50 vol % of the TCP film is occupied by bulk NaCl-D(2)O solution. Failure occurs by aluminum dissolution under the swollen TCP film as the imbibed solution contacts the Al metal. Further increase in potential leads to complete stripping of the TCP film.

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X-ray reflectivity (XRR) and neutron reflectivity (NR), as well as ultra-smallangle X-ray scattering (USAXS), are used to examine the in-plane and surfacenormal structure of anodic films formed on aluminium alloy AA2024 and pure aluminium. Aluminium and alloy films up to 3500 Å thick were deposited on Si wafers by electron beam evaporation of ingots. Porous anodic aluminium oxide (AAO) films are formed by polarizing at constant voltage up to 20 V noble to the open circuit potential. The voltage sweet spot (5 V) appropriate for constant-voltage anodization of such thin films was determined for both alloy and pure Al. In addition, a new concurrent voltage-and current-control protocol was developed to prepare films with larger pores (voltages higher than 5 V), but formed at a controlled current so that pore growth is slow enough to avoid stripping the aluminium substrate layer. USAXS shows that the pore size and interpore spacing are fixed in the first 10 s after initiation of anodization. Pores then grow linearly in time, at constant radius and interpore spacing. Using a combination of XRR and NR, the film density and degree of hydration of the films were determined from the ratio of scattering length densities. Assuming a chemical formula Al 2 O 3 ÁxH 2 O, it was found that x varies from 0.29 for the native oxide to 1.29 for AAO grown at 20 V under concurrent voltage and current control. The average AAO film density of the porous film at the air surface is 2.45 (20) g cm À3 . The density of the 'barrier' layer at the metal interface is 2.9 (4) g cm À3 , which indicates that this layer is also quite porous.