Nanocomposite coatings for corrosion protection of stainless steel

Name: Lijia YangSupervisor: Igor Zhitomirsky

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OutlineIntroductionLiterature reviewProblem statementObjectivesApproach and methodologyExperiment methodsResults and discussionConclusionsAcknowledgments

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IntroductionCeramic coatings for corrosion protection• Chemical stability• Wear resistance• Good corrosion protection• Advanced mechanical properties

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• EPD (Electrophoretic deposition) 1. Particles suspended in a liquid are forced to move

toward an electrode by applying an electric field2. The particles collect at one of the electrode and

form a coherent deposit on it.

• ELD (Electrolytic deposition)In ELD, nanoparticles are produced from solutions of metal salts

Charged ceramic particles

Ions or complexes

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Comparison• EPD is an important tool for the preparation of

thick ceramic films, while ELD enables the formation of nanostructured thin ceramic films.

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Electrophoretic mobility of ceramic particlesVelocity ν of a particle in an electric field Eμis the electrophoretic mobilityμof a rigid colloidal particle can be derived from the equation

a the particle radius1/κthe Debye length

εthe dielectric constant of solventThe function f(κa) increases from 1 for κa<<1 to 1.5 for κa>>1. For particles that are much smaller than the Debye length:

For particles that are large compared with 1/κ:

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Electrophoretic mobility of polyelectrolyte• The electrophoretic mobility of a spherical polyelectrolyte, in

which fixed charges are distributed at a uniform density ρfix, can be derived from the equation:

• ηthe viscosity

• When a rigid particle is coated by a layer of polyelectrolyte ofthickness d, the general mobility expression is given by

where d is the thickness of polyelectrolyte layer, ψ0 is the potential at the boundary between the polyelectrolyte and the surrounding solution, and ψDON is the Donnan potential

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Mechanism of electrolytic depositionELD: Mixed solvent(containing organic solvent and water) or

aqueous solventPrecipitation of particles in high pH region at the electrode surface• Electrosynthesis results in the accumulation of colloidal particles

near the electrode. • Formation of a deposit is caused by flocculation

Cathodic reactions

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Mechanism of electrophoreticdepositionFormation of coating is caused by motion of charged particles under electric field.

Solvents should be inert with respect to the powder. Organic liquids are superior to water as a suspension medium for EPDRelatively high electric fields (50-100 V/cm can be applied) without significant gas evolution at electrode Non-aqueous solvents prevent the deposit from hydrating.Typical organic solvents:ethanol, acetylacetone

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Particles interaction at the electrodesurface and film formation• Understanding of EPD and ELD deposition mechanisms have

come from the application of the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability.

• The DLVO theory considered two main forces1. double-layer repulsion 2.vander Waals’ attraction

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Particles interaction at the electrode surface and film formationOther forcesCoulombic attraction (in ionic solutions) exists

between highly charged colloidal particles, in addition to coulombic repulsion

Forces of other origins can also act between the particles near the electrode. Long-range attractions between similarly charged colloidal particles close to electrode surfaces have also been observed

Current gradients, caused by the shielding effect of individual particles, generated fluid flow, which in turn resulted in surface aggregation of the particles.

Coagulation of colloidal particles near the cathode could be enhanced by electrolyte and electric field.

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Cerium oxideCerium oxide (CeO2) is an effective corrosion inhibitor, especially in marine environments. Conventional: chromium compounds

Advantage of cerium: low toxicity and relatively abundance in natureFull immersion test plots (sample weight loss versus time):

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Electrosynthesis mechanism in ELD• ELD of CeO2 film was performed galvanostatically from

CeCl3solutions.• The mechanism of electrosynthesis of CeO2 from aerated

CeCl3 solutions involved OH- generation in electrode reactions and pH increase at the cathode

• Precipitation of insoluble CeO2 particles: coagulation and filmformation

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Problem statement - Cracking in ELDThin electrolytic deposits (0.1–0.2 μ m) were crack free and adhered well to the substrates. However, significant cracking was observed after air drying when deposit thickness exceeded ~0.2-0.3 μm.Shrinkage during air dying is the main reason for cracks in ELD. Sintering results in additional cracking.

0.01M CeCl3 +0.025 MH2O2 solution

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Problem statement- Cracking in EPD

Deposits have to be sintered at much higher temperatures (compared to ELD) to form dense and thick coating.

Usually metallic substrates can not be used under such high temperatures. Chemical reactions between substrates and coatings during sintering can be expected.

Shrinkage results in significant cracking, which may be up to 15-20% of deposit volume or even more.

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Problem statementElectrically neutral binders

A binder is added to suspensions or solutions in order to increase the adherence and strength of the deposited material and prevent cracking.

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Objective

• Development of composite cerium oxide –polymer coatings using room temperature processing

• Development of mechanisms for deposition of advanced charged polymers

• Development of composite coatings using EPD and ELD of cerium oxide

• Investigation of microstructure, properties and corrosion protection

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Approach and methodology• The use of water insoluble polymers with

high corrosion resistance

• PPY2

• PVPBM

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Approach and methodologySuggested protonation and deposition mechanisms

Insoluble in water

Soluble in water, charged

Soluble in water, charged

Base generation at electrode

Insoluble in water deposit

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Approach and methodology

Combined method based on ELD of CeO2 and EPD of polymer

pH increase at the cathode surface

Combined method based on EPD of CeO2 particles and EPD of polymer

CeO2H+ + OH- → CeO2 + H2O

CeO2PVPH+ + OH- → CeO2 PVP + H2O

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Experimental methods Preparation of deposits: EPD ELD

Testing methods:QCM (quartz crystal microbalance)

SEMTGA (thermogravimetric analysis ) and DTA (differential thermalanalysis)XRDCorrosion studies using electrochemical methods

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Experimental resultsEPD of PVP2 – proof of conceptSEM images of cross section and surface

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Experimental resultsEPD of PVPBM –proof of concept

QCM data

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Experimental resultsProof of concept

Benzoic acid was used for electrostatic stabilization and cathodic deposition of CeO2 particles

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Co-polymer as binder polymer

Structure

Similar to PVP, the electronegative nitrogen atoms in the pyridine rings that can be protonated in the presence of an acid.

QCM

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EPD of CeO2 without binderDeposition yield & surface morphology (cracks)

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ELD of CeO2 without binder

In both methods, cracks are obvious without addition of polymers

Deposition yield (using QCM) and surface morphology (porosity, cracks)

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ELD and EPD of CeO2 in the presence of PVP2 Proof of concept and Effect of polymer binder

SEM images of films prepared from 1g/L PVP solution containing (a) 5mM CeCl3 (b) 10g/L CeO2

(a) (b)

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EPD of CeO2 in the presence of PVPBMProof of concept and Effect of polymer binder

SEM images of films prepared from 1g/L PVPBM solution containing 10g/L CeO2

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TGA/DTA analysisTGA and DTA data for (a) pure PVP (b) deposit from 1g/L PVP solution containing 5mM CeCl3 (c) deposit from 1g/L PVP solution containing 10g/L CeO2

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XRD analysisX-ray diffraction patterns for (a) composite deposit from 1g/L PVP solution containing 5mM CeCl3 (b) composite deposit from 1g/L PVP solution containing 10g/L CeO2

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Tafel testTafel plots for (a) uncoated stainless steel (b) 1g/L PVP solution (c) 1g/L PVP solution containing 5mM CeCl3 (d)1g/L PVP solution containing 10g/L CeO2

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TGA/DTA analysisTGA and DTA data for (a) pure co-polymer (b) deposit from 2g/L co-polymer solution containing 10g/L CeO2

(a) (b)

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XRD analysisX-ray diffraction pattern for deposit from 2g/L co-polymer solution containing 10g/L CeO2

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Tafel testTafel plots for (a) uncoated stainless steel (b) 1g/L PVPBM solution containing 10g/L CeO2

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Conclusions

Nanostructured CeO2 films were prepared by ELD and EPDmethods.

The deposition yield can be controlled by the variation of deposition voltage, current density, and deposition time.

Electrochemical methods were developed for deposition of PVP2, CeO2, and the combination of them.

Use of PVP allows the formation of crack-free PVP-CeO2composite films.

Corrosion protection and mechanical properties are improved in such composite films.

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Acknowledgement• Dr. Zhitomirsky• Group members in lab• Staff for sample tests