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. 2018 Jun 29;11(7):1114.
doi: 10.3390/ma11071114.

Fabrication of Sputtered Ce/La, La/Ce Oxide Bilayers on AA6061 and AA7075 Aluminum Alloys for the Development of Corrosion Protective Coatings

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Fabrication of Sputtered Ce/La, La/Ce Oxide Bilayers on AA6061 and AA7075 Aluminum Alloys for the Development of Corrosion Protective Coatings

Silvia B Brachetti-Sibaja et al. Materials (Basel). .
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Abstract

This work provides a comparative study on the corrosion protection efficiency of Ce, La films as well as Ce/La and La/Ce oxide bilayered coatings deposited onto AA7075 and AA6061 substrates by the radio frequency (RF) magnetron sputtering technique. The coating thickness ranged approximately from 12 to 835 nm, which changed with the deposition parameters and substrate composition. The relationship between microstructure, roughness and electrochemical performance is examined. The reactivity and crystallinity of rare earth (RE) films can be tailored by adjusting the sputtering parameters. Sputtered La films with thickness ca. 390 nm and average roughness of 66 nm showed the best corrosion protection properties in chloride medium as determined by potentiodynamic curves and electrochemical impedance spectroscopy (EIS). The method to obtain RE bilayered coatings, i.e., La/Ce or Ce/La as well as the substrate composition and applied power conditioned their inhibition properties. The RE bilayered coatings displayed better barrier properties than Ce films, which were poorer than those featured by La films.

Keywords: CeO2; La2O3; bilayer; cerium oxide; corrosion; lanthanum oxide; magnetron sputtering; rare earth; thin film.

Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
XRD patterns of sputtered (a) lanthanum coatings at constant power P = 60 W, (b) lanthanum coatings at constant power P = 80 W, (c) cerium coatings at constant power P = 60 W, (d) cerium coatings at constant power P = 80 W. The samples were deposited on glass substrates varying deposition time (25, 40 and 60 min) and substrate temperatures (80 and 200 °C).
Figure 2
Figure 2
Representative high resolution XPS spectra of as-obtained samples at P60T80t60: (a) La 3d, (b) La O1s, (c) Ce 3d, and (d) Ce O1s.
Figure 3
Figure 3
High Resolution Scanning Electron Microscopy (HRSEM) images of La coatings on AA6061 aluminum alloy substrates: (a) P60T80t25, (b) P60T80t40, (c) P60T80t60, (d) P60T200t25, (e) P60T200t40, (f) P60T200t60, (g) P80T80t25, (h) P80T80t40, (i) P80T80t60, (j) P80T200t25, (k) P80T200t40, and (l) P80T200t60.
Figure 4
Figure 4
High Resolution Scanning Electron Microscopy (HRSEM) images of Ce coatings on AA6061 aluminum alloy substrates: (a) P60T80t25, (b) P60T80t40, (c) P60T80t60, (d) P60T200t25, (e) P60T200t40, (f) P60T200t60, (g) P80T80t25, (h) P80T80t40, (i) P80T80t60, (j) P80T200t25, (k) P80T200t40, and (l) P80T200t60.
Figure 5
Figure 5
Representative 3D images of sputtered bilayered films at 80 °C and 60 min: (a) Ce80W/La60W, (AA6061), (b) Ce80W/La60W (AA7075), (c) La60W/Ce80W, (AA6061), and (d) La60W/Ce80W (AA7075). The section analysis is also shown as a height profile.
Figure 6
Figure 6
Typical High Resolution Scanning Electron Microscopy (HRSEM) images of oxide sputtered bilayered coatings, their corresponding comparison with pure coatings and bare aluminum. (a) Uncoated AA7075, (b) uncoated AA6061, (c) AA7075/Ce (P80T80t60), (d) AA6061/Ce (P80T80t60), (e) AA7075/La (P60T80t60), (f) AA6061/La (P60T80t60), (g) AA7075/Ce/La (P80T80t60/P60T80t60), (h) AA6061/Ce/La (P80T80t60/P60T80t60), (i) AA7075/La/Ce (P60T80t60/P80T80t60), and (j) AA6061/La/Ce (P60T80t60/P80T80t60).
Figure 7
Figure 7
Growth film pathway (Stranski–Krastanov) characteristic of bilayered RE coatings.
Figure 8
Figure 8
XRD of RE oxide bilayered coatings: (a) Ce(P80T80t60)/La(P60T80t60) and, (b) La(P60T80t60)/Ce(P80T80t60) onto Si (100) substrates.
Figure 9
Figure 9
Polarization curves of RE coatings synthesized at 60 or 80 W (T = 80 °C and t = 60 min) and bilayered coatings on: (a) AA7075 and (b) AA6061 aluminum alloys.
Figure 10
Figure 10
Nyquist and Bode spectra of sputtered coatings on (ac) AA7075 and (df) AA6061 evaluated in a 3 wt % NaCl solution.
Figure 11
Figure 11
Equivalent circuits used for fitted experimental EIS data of RE coated metallic substrates. (a) bare substrates (b) coated specimens.

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