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Mechanism of Passivation and Inhibition of Trivalent Chromium Process Coating on Aluminum Alloys

Dong, Xuecheng
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1335904469

Abstract Details

2012, PhD, University of Cincinnati, Engineering and Applied Science: Materials Science.
Chromium conversion coatings (CCC) based on Cr(VI) are widely used enhance corrosion resistance of aluminum alloys. However, the conventional CCC system consists of various Cr(VI) compounds, which are toxic and carcinogenic, leading to increasingly restricted usage. The Trivalent Chromium Process (TCP), for example, has proven to be a promising alternative to CCC. Although significant research on the application and performance of TCP has been done, questions such as the film microstructure, the nature of the TCP/substrate interface and the evolution of the TCP film in response to different environments have not been answered. Neutron reflectivity (NR) and x-ray reflectivity (XRR) are used to determine the structure and composition of TCP films on aluminum alloy 2024-T3. A new electro-assisted deposition method was designed. Quantitative NR and XRR analysis confirmed linear film growth. The film composition was determined to be Cr2O3¿¿¿¿iH2O¿¿¿¿x(ZrO2¿¿¿¿jH2O) (i =2.10 ¿¿¿¿ 0.55, j = 1.60 ¿¿¿¿ 0.45 and x = 0.85 ¿¿¿¿ 0.14). In-situ neutron reflectivity was used to observe the structure and evolution of a TCP passive film with a simplified formulation on Al in a NaCl-D2O solution. We observed the evolution of the TCP film on the Al anode and compared the degradation of the Al with and without TCP protection. A dramatic improvement in corrosion resistance of AA2024-T3 is achieved by anodic hardening TCP passive films with Ce(III). The anodic current density is suppressed by the factor of 500 after exposure in the presence of Ce(III) at potentials in metastable pitting region (-580 mV vs. SCE for 4 < pH < 8). Dynamic polarization analysis after step-by-step polarization reveals that Ce(III) increases both the open circuit potential and the pitting potential. In-situ NR is applied to investigate the evolution of structure and composition of a TCP passive film on pure Al under Ce(III) anodic hardening. We observed the formation of a Ce-enriched TCP layer at the film/solution interface during anodic polarization, which extends the pitting potential range and reduces the baseline corrosion current. Similar to the in-situ Ce(III) anodic hardening experiment, in-situ NR is used to investigate the evolution of structure and composition of a TCP passive film under cathodic polarization in the presence of Ce(III). We observed a film evolution at Al/TCP interface; while the in-situ current between the anode and the cathode process shows significant attenuation over 24 hours. Cathodic inhibition was verified by a separate potentiostatic electrochemical experiment. An evolution model based on the in-situ NR data is proposed to explain the cathodic inhibition process. These experiments are the first to measure the response of a passive film on Al in an active corrosion environment. The results dramatically alter the conventional wisdom regarding passive films. TCP film is not a water barrier. In fact the films rapidly imbibe water. Nevertheless the metal remains passive. Protection is due to a very thin barrier layer right at the metal/film interface. The presence of the water in the bulk film actually facilitates Ce(III) penetration, which leads to further improvement in corrosion protection.
Dale Schaefer, PhD (Committee Chair)
Vikram Kuppa, PhD (Committee Member)
Relva Buchanan, ScD (Committee Member)
Donglu Shi, PhD (Committee Member)
William Vanooij, PhD (Committee Member)
135 p.
Materials Science
aluminum; passivation; inhibition; neutron reflectivity; cerium; trivalent chromium;

Recommended Citations

Citations

  • Dong, X. (2012). Mechanism of Passivation and Inhibition of Trivalent Chromium Process Coating on Aluminum Alloys [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1335904469

    APA Style (7th edition)

  • Dong, Xuecheng. Mechanism of Passivation and Inhibition of Trivalent Chromium Process Coating on Aluminum Alloys. 2012. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1335904469.

    MLA Style (8th edition)

  • Dong, Xuecheng. "Mechanism of Passivation and Inhibition of Trivalent Chromium Process Coating on Aluminum Alloys." Doctoral dissertation, University of Cincinnati, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1335904469

    Chicago Manual of Style (17th edition)

ucin1335904469
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© 2012, all rights reserved.
This open access ETD is published by University of Cincinnati and OhioLINK.