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Dissertation_Xiaolei Guo - Final.pdf (4.85 MB)

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Corrosion inhibition of aluminum alloy 2024-T3 based on smart coatings, hybrid corrosion inhibitors, and organic conversion coatings

Guo, Xiaolei
http://orcid.org/0000-0002-8074-7205
http://rave.ohiolink.edu/etdc/view?acc_num=osu1461188604

Abstract Details

2016, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
In this dissertation, several novel corrosion inhibition strategies have been developed, including application of smart coatings with self-healing capabilities, utilization of organic/inorganic hybrid corrosion inhibitors, and employment of novel organic conversion coatings. In the first part of this dissertation, a one-step method is presented for encapsulating a corrosion inhibitor, sodium metavanadate (NaVO3), relevant to protection of AA2024-T3, into hollow microparticles. The surface morphologies and core-shell structure of as-fabricated microparticles were characterized by using scanning electron microscopy and confocal microscopy. A release study showed that the encapsulated NaVO3 was continuously released from the microparticles over time. Potentiodynamic polarization scans indicated that the released NaVO3 noticeably inhibited corrosion on an AA2024-T3 surface by discouraging the oxygen reduction reaction. A smart coating with self-healing capability was developed by dispersing these microparticles in a polyvinyl butyral (PVB) coating. The improvement of corrosion inhibition by the NaVO3 microencapsulation described herein was validated through electrochemical methods and salt spray/immersion tests. The results indicated that the direct introduction of NaVO3 in the PVB coating impaired the barrier properties of the coating. However, by entrapping NaVO3 inmicroparticles, the corrosion resistance of the embedded coating was remarkably improved. In separate work, a strong synergistic corrosion inhibition effect was observed when phytate and molybdate were combined and applied onto a AA2024-T3 substrate. At pH 5, it was found that the optimum condition as revealed by free corrosion tests in 3.5 % NaCl solution with 1 mM phytate and 10 mM molybdate. The inhibition mechanism was explored by electrochemical, microscopic, and surface analytical techniques. XPS, SEM-EDS, and Raman results show that both phytate and molybdate exist on the metal surface treated at pH 5, and both pit initiation and pit growth were suppressed. While at pH 8 only phytate was detected and only pit initiation was reduced. It is therefore hypothesized that phytate suppresses pit initiation while molybdate-phytate complex plays a major role in reducing both pit initiation and pit growth. The complexation between phytate and molybdate at pH 5 was validated by UV-vis spectroscopy, but no interaction was detected between these two species at pH 8. Combined with surface analysis, this supports the idea that molybdate is entrapped and incorporated into the phytate film on the substrate at pH 5. Titration study results suggest that a buffering effect is offered by the combination of phytate and molybdate around pH 5 and may prevent local acidification/alkalinization and thereby reducing localized corrosion. Furthermore, a novel and "green" organic conversion coating has been developed based on an organic molecule, N-Benzoyl-N-phenylhydroxylamine (BPHA), to improve the corrosion resistance of AA2024-T3. Coatings are applied to clean surfaces by simple immersion in solutions containing 0.5 mM BPHA. After the coating forms, persistent andsignificant corrosion resistance is imparted to the underlying substrate as validated by potentiodynamic polarization, electrochemical impedance spectroscopy and ASTM B117 salt spray exposure testing. Potentiodynamic polarization was performed to compare the corrosion resistance of BPHA conversion coated substrates with samples treated in the same concentration of other corrosion inhibitors including sodium vanadate, sodium silicate, benzotriazole, and cerium chloride. It was found that the BPHA conversion coating provided the best corrosion protection due to substantial anodic and cathodic inhibition. BPHA-coated samples also showed excellent corrosion resistance under B117 salt spray exposure conditions during a 168 h exposure period. The chemistry of conversion-coated surfaces was studied by Raman spectroscopy and x-ray photoelectron spectroscopy (XPS), which revealed the formation of a stable aluminum oxide-BPHA complex characterized by a keto-to-iminol conversion of the BPHA molecule. The formation of this complex appears to be closely related to the high degree of corrosion resistance conferred by BPHA treatment. The conversion treatment resulted in a thick surface film with a three-layer structure, which was revealed by focus ion beam-scanning transmission electron microscopy (FIB-STEM) analysis. As suggested by previous studies, hydroxamic acids can form stable surface films on metal oxide surfaces with isoelectric points (IEP) greater than the pKa of the acid. This hypothesis is validated in this study using three hydroxamic acids: N-Benzoyl-N-phenylhydroxylamine (BPHA), benzhydroxamic acid (BHA), and salicylhydroxamic acid (SHA), with the following relative pKa values: SHA < BPHA < BHA. Based on this hypothesis, the binding strengths of the hydroxamic acid-metal oxides complexes can beranked as SHA > BPHA > BHA. XPS and Raman spectroscopy both reveal the existence of the three hydroxamic acids on AA2024-T3 substrates after conversion treatments. It is also found that water aging time of AA2024-T3 substrates markedly affects the degree of adsorption and deprotonation of hydroxamic acids, probably through a tailoring of the isoelectric points of the metal oxides. The corrosion inhibition efficacies of the three hydroxamic acids films were assessed by potentiodynamic polarization and electrochemical impedance spectroscopy. The degree of corrosion inhibition is ranked as BPHA > BHA > SHA, which conflicts with the order of the complex stability, but agrees well with the order of hydrophobicity. Therefore it is suggested here that although the pKa value of a hydroxamic acid is critical in determining the binding strength of the hydroxamic acid-metal oxide complex, the efficacy of corrosion inhibition may be governed by the hydrophobicity of ligands rather than the stability constant of the complex.
Rudolph Buchheit (Advisor)
Gerald Frankel (Committee Member)
Jenifer Locke (Committee Member)
220 p.
Chemistry; Engineering; Materials Science
Corrosion, Aluminum Alloy, Corrosion Inhibitors, Microencapsulation, Organic Conversion Coatings, Smart Coatings

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Citations

  • Guo, X. (2016). Corrosion inhibition of aluminum alloy 2024-T3 based on smart coatings, hybrid corrosion inhibitors, and organic conversion coatings [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461188604

    APA Style (7th edition)

  • Guo, Xiaolei. Corrosion inhibition of aluminum alloy 2024-T3 based on smart coatings, hybrid corrosion inhibitors, and organic conversion coatings. 2016. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1461188604.

    MLA Style (8th edition)

  • Guo, Xiaolei. "Corrosion inhibition of aluminum alloy 2024-T3 based on smart coatings, hybrid corrosion inhibitors, and organic conversion coatings." Doctoral dissertation, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461188604

    Chicago Manual of Style (17th edition)

osu1461188604
1,139
© 2016, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.