corrosion inhibition of aluminum alloy 2024-t3 based on smart coatings, hybrid corrosion ...

220 Pages·2016·4.85 MB·English

CORROSION INHIBITION OF ALUMINUM ALLOY 2024-T3 BASED ON SMART
COATINGS, HYBRID CORROSION INHIBITIONS, AND ORGANIC CONVERSION
COATINGS

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy
in the Graduate School of The Ohio State University

By
Xiaolei Guo, M.S.
Graduate Program in Materials Science and Engineering

The Ohio State University
2016

Dissertation Committee:
Professor Rudolph G. Buchheit, Advisor
Professor Gerald S. Frankel
Professor Jenifer S. Warner-Locke
Copyrighted by
Xiaolei Guo
2016
ABSTRACT

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 (NaVO ), relevant to protection of AA2024-
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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 NaVO was
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continuously released from the microparticles over time. Potentiodynamic polarization
scans indicated that the released NaVO noticeably inhibited corrosion on an AA2024-T3
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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 NaVO microencapsulation
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described herein was validated through electrochemical methods and salt spray/immersion
tests. The results indicated that the direct introduction of NaVO in the PVB coating
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impaired the barrier properties of the coating. However, by entrapping NaVO in
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microparticles, 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 and
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significant 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 pK of the acid. This
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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 pK values: SHA < BPHA < BHA. Based on this
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hypothesis, the binding strengths of the hydroxamic acid-metal oxides complexes can be
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ranked 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 pK
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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.
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Dedicated to my wife Lingjun
And my parents

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ACKNOWLEDGEMENTS

I would like to express my deepest gratitude to my advisor, Dr. Rudolph Buchheit,
who offered me an opportunity to seek my Ph.D. professional in this outstanding corrosion
research group. His invaluable advice, gentle demeanor, and steadfast encouragement
navigate me through the course of my Ph.D. career, especially during the most difficult
time of my life. I also want to thank Dr. Gerald Frankel for his penetrating insights,
illuminating observations, and encouragement, honing my critical thinking skills. To Dr.
Jenifer Warner-Locke I am deeply grateful for her boundless support both in research and
personal aspects.
I am indebted to Dr. Belinda Hurley, who is always available to help me with my
experiments, and spent incredible amount of time to help improve my writings. I also want
to thank Dr. John Lannutti for his permission to use the electrospinning instrument. My
sincere thanks also go to Dr. Lisa Hommel, Dr. Gordon Renkes, Dr. Anthony Lutton, Mr.
Steve Bright, Mr. Ross Baldwin, and Mr. Ken Kushner, for their instruction and guidance
on the instrumental usage.
I would like to thank all FCC members, including Dr. Santiago Fajardo, Dr. Fan
Yang, Dr. Shanshan Wang, Dr. Zhicao Feng, Dr. I-Wen Huang, Jiheon Jun, Sara
Cantonwine, Mark Thomson, Benjamin Hanna, Xi Wang, Angeire Huggins, Weijie Mai,
and Dadi Zhang for being so generous in offering their assistance in my research. I also
acknowledge the help from previous FCC members, including Dr. Severine Cambier, Dr.
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Jinwook Seong, Dr. Jichao Li, Daniel T Kaminski, and Kerrie Holguin. I would also like
to thank Mrs. Christine Putnam for all of the administrative support till her retirement.
Support by the U.S. Department of Agriculture under Award number 2012-38202-
19288 and The Ohio State University is gratefully acknowledged.
I am also especially indebted to Dr. Binyang Du for enlightening me the first glance
of research as an undergraduate.
Last but the most important, I would like to extend my deepest gratitude to my
beloved wife Lingjun Tang, my father Mingduan Guo, and my mother Zuqiong Wu.
Without their support, I would not have been able to achieve this goal.

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VITA

2010 ...............................................................B.S., Polymer Science and Engineering,
Zhejiang University, China
2012 ...............................................................M.S., Materials Science and Engineering,
The Ohio State University
2012 to present ..............................................Ph.D., Materials Science and Engineering,
The Ohio State University

FIELDS OF STUDY

Major Field: Materials Science and Engineering

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