Surface characterization of plasma sprayed oxide coatings exposed to aqueous environments M' Harjua,

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Surface characterization of plasma sprayed oxide
coatings exposed to aqueous environments M.
Harjua, T. Mäntyläa, M. Järnb, J. Halmeb, S.
Arevab, J. B. Rosenholmb aTampere University
of Technology, P.O. Box 589, 33101 Tampere,
FinlandbDepartment of Physical Chemistry, Åbo
Akademi University, Porthansgatan 3-5, FIN-20500
Åbo, Finland
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Problematics of all fouling systems
Need to know properties and interactions of the
surface, fouling agent the environment
Even system composed of simple environment,
simple fouling agent and simple surface may form
complicated fouling system!!!
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Objectives 1) Characterize dissolution
behaviour of the plasma sprayed coatings at mild
aqueous conditions (pH4-9) 2) Characterize
surface charging of the coatings (zeta
potential) 3) Characterize influence of
dissolution on surface charging
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• Experiments
• crushed coating materials (Al2O3, TiO2, Cr2O3)
  were exposed to 10-3M NaCl electrolytes at pH4,
  7 or 9 (No CO2 uptake control)
• The solid content was 0.02 mass-
• Measured parameters
• Electrolyte conductivity, Redox potential,
  concentrations of dissolved species, amounts of
  added acid or base
• Zeta-potentials of sample powders
• Test period two weeks

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Plasma spraying torch
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Typical coating structure
solidified oxide splats
pores
microcracks
heterogeneous structure
Plasma sprayed Cr2O3 splats on plasma sprayed
Al2O3
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Sample materials
Change in stoichiometry
Change in crystallographic structure
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Results 1) subtracted conductivities
negative values powders or dissolved species
bind ions (H) from electrolyte!
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Results 2) concentrations of dissolved species
Clear dissolution of g-Al2O3 at pH4
Mild dissolution of g-Al2O3 at pH9
No clear dissolution of Cr2O3 or TiO2
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Results 3) Required acid/base addition to
maintain pH constant
Bigger HCl additions needed for Al2O3 than for
other oxides at pH4
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Results 4) Zeta potentials of sample powders
Increase in absolute value of zeta for TiO2
Change in zeta-potential sign for Al2O3
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Results 5) Estimated IEP as a function of
exposure time
Literature values
(Rough estimation)
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• Conclusions
• TiO2 and Cr2O3 no dissolution, small changes in
  surface charging
• Al2O3
• dissolution at pH9 and especially at pH4
• negative subtracted conductivities possibly
  due to chemical reactions between H and
  dissolved species
• drift in IEP may be due to formation of transient
  amorphous hydrated phase and finally formation of
  Bayerite g-Al2O3 3H2O 2b-Al(OH)3

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Acknowledgements This study was done
within the National Technology Programme, Clean
Surfaces, funded by the National Technology
Agency (Tekes) of Finland.