Modeling of Inhibitor Release from Epoxy Coating with Hydrotalcites Using Finite Element Method

Transcription

1 Modeling of Inhibitor Release from Epoxy Coating with Hydrotalcites Using Finite Element Method Hongwei Wang, 1 Hong Guan, 2 Francisco J. Presuel-Moreno, 1 Robert G. Kelly, 1 and Rudolph G. Buchheit 2 1 Center for Electrochemical Science and Engineering Department of Materials Science and Engineering University of Virginia, Charlottesville, VA 2294 USA 2 Fontana Corrosion Center Department of Materials Science and Engineering Ohio State University, Columbus, Ohio 4321 USA Acknowledgment: AFOSR Presented in the Corrosion Inhibitor Session of the 22 ECS Fall Meeting at Salt Lake City, Utah, USA October 22, 22

2 Inhibition by Ion Exchange Using Hydrotalcites Host layer: double metal hydroxide: Al-Mg, Al-Li, Al-Zn, Al-Ni... high temperature thermal stability anion selectivity exchange kinetics aggressive anions inhibiting anions Anion interlayer: OH -, CO 3 2-, NO 3-, VO 3-, V 1 O 28 6-, CrO 4 2-, Fe(CN) 6 3-, S 2 O 8 2-, MoO 4 2-,MnO 4 2-,... inhibitors sensing ions, e.g. ph hydrophilic inhibitor reservoir aggressive anions immobilized Hydrotalcites compound released inhibiting anions and immobilized aggressive anions [M 1-x M x (OH) 2 ] x+ (X) x/m.nh 2 O, where M=Zn(II), M =Al(III) and X=[V 1 O 28 ] 6- (decavanadate)

3 Model the Scratch on Coating 2 Inhibitor Concentration (mol/m 3 ) coated AA224 scratch Inhibitor is released and transports horizontally Sample Geometry J= (symmetry) AA 224 T3 Water layer Primer coating 25 µm 5 µm HT inhibitor particles.1 cm A B 2S 2 cm J= Aluminum Coated AA224 Clad Substrate AA224 High aluminum dissolution rate in scratch J = Electric Flux Figure not to scale (Figure not to scale)

4 Model and Assumptions - Transport modes: diffusion and migration - Complex reaction system - Electrochemical reactions Al Al e - O 2 +2H 2 O +4e - 4OH - - Chemical reactions/processes Al 3+ + yh 2 O = Al(OH) 3-y y +yh + V 6 O 6- = 28 (HT) V 6 O 6-28 (sol) Cl - (sol) = Cl- (HT) anodic reaction cathodic reaction hydrolysis inhibitor release chloride gettering - Mass balance (11 chemical species) - Electrical charge balance (electrochemical reaction) - Solution electroneutrality (Na + to neutralize)

5 Electrochemical Boundary Conditions -2 Potential mv/sce / /V-HT Potential V/SCE e-11e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 Current density (A/cm2) Current density (A/m 2 ) AA224-T3 with/without exposure to V/HT in simulated scratch cell,.1 M NaCl Simulated kinetics model

6 Modeling and Simulation System Development Numerical calculation Finite element method (ANSYS) Engineering software development C++ Object Oriented Programming Open source codes and executable files available on web (IT IS FREE) Encourage different researchers to use for the specific purposes 1994-present at University of Virginia lead by R.G. Kelly Computing facility PC is enough Super computer might be needed for the long term simulation

7 I(x) and ph Evolution Net Current Density (A/m 2 ) s 1 s 3 s 5 s ph s 1 s 3 s 5 s µm scratch, 5 µm water layer,.1 M NaCl, release rate (A4)

8 Inhibitor Concentration Evolution and Protection of the Scratch 25 µm scratch Inhibitor Concentration (mol/m 3 ) t s 1 s 3 s 5 s Scratch inhibition (48%) Point B Vanadate Inhibitor Concentration (mol/m 3 ) Point A Time to inhibit Point B Time (s) Point A 25 µm scratch, 5 µm water layer,.1 M NaCl, release rate (A4)

9 Effect of Scratch Size and Inhibitor Release ph Dependencies Y=A ph + C 1.E-3 1 Vanadate Inhibitor Release Rate (mol/m2/s) 1.E-4 1.E-5 1.E-6 1.E-7 1.E-8 1.E-9 1.E-1 1.E-11 Y=2e -6 ph+7e -11 Y=2e -7 ph+7e -11 Y=2e -8 ph+7e -11 Y=2e -1 ph+7e -11 Experimental: Y=2e -11 ph+7e ph Scratch Protection Percent (%) Increase release ph dependence A2= A1= Scratch Size (um) A5=2 1-6 A4=2 1-7 A3= M NaCl, 5 µm water layer, 5 seconds

10 Increased Water Layer Thickness Slows Inhibition 1.2 Vanadate Inhibitor Concentration (mol/m 3 ) Water layer thickness (micron) 32% ph Humid air: 1 µm water layer Initial ph=7 Water layer thickness (micron) [Cl - ] decrease is <1% in 5 s µm scratch,.1 M NaCl solution, release rate (A3), 5 seconds

11 Conclusions We have extended the occluded corrosion mass transport model to atmospheric exposure of multifunctional coatings to include: Anodic and cathodic reactions in a closed (open circuit) system Al 3+ hydrolysis ph-dependent inhibitor release and Cl - gettering Provides a tool for design parameter evaluation The ph dependency of the inhibitor release is the primary controlling factor for protection of a scratch. Larger ph dependencies are desirable Increases in water layer thickness have two compounding effects: Slow the ph increase over the coating Dilute the inhibitor concentration