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
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)
Model the Scratch on Coating 2 Inhibitor Concentration (mol/m 3 ) 1.6 1.2.8.4 coated AA224 scratch Inhibitor is released and transports horizontally.2.4.6.8.1 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)
Model and Assumptions - Transport modes: diffusion and migration - Complex reaction system - Electrochemical reactions Al Al 3+ +3 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)
Electrochemical Boundary Conditions -2 Potential mv/sce -4-6 -8-1 224/224 224/V-HT Potential V/SCE -.2 -.4 -.6 -.8-1 1e-11e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 Current density (A/cm2) -1.2.1.1.1 1 1 Current density (A/m 2 ) AA224-T3 with/without exposure to V/HT in simulated scratch cell,.1 M NaCl Simulated kinetics model
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) http://www.virginia.edu/cese/research/crevicer/ 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
I(x) and ph Evolution Net Current Density (A/m 2 ).2.1. -.1 -.2.1 s 1 s 3 s 5 s.2.4.6.8.1 ph 14 12 1 8 6 4 2.1 s 1 s 3 s 5 s.2.4.6.8.1 25 µm scratch, 5 µm water layer,.1 M NaCl, release rate (A4)
Inhibitor Concentration Evolution and Protection of the Scratch 25 µm scratch Inhibitor Concentration (mol/m 3 ) 2 1.6 1.2.8.4 t.2.4.6.8.1.1 s 1 s 3 s 5 s Scratch inhibition (48%) Point B Vanadate Inhibitor Concentration (mol/m 3 ) 1.2.8.4. Point A Time to inhibit Point B 2 4 6 Time (s) Point A 25 µm scratch, 5 µm water layer,.1 M NaCl, release rate (A4)
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 -11 2 4 6 8 1 12 14 ph Scratch Protection Percent (%) 8 6 4 2 Increase release ph dependence A2=2 1-1 1 2 3 4 5 A1=2 1-11 Scratch Size (um) A5=2 1-6 A4=2 1-7 A3=2 1-8.1 M NaCl, 5 µm water layer, 5 seconds
Increased Water Layer Thickness Slows Inhibition 1.2 Vanadate Inhibitor Concentration (mol/m 3 ) 1.8.6.4.2 1 5 5 Water layer thickness (micron) 32% ph 12 1 8 6 Humid air: 1 µm water layer.2.4.6.8.1 4 2 1 5 5 Initial ph=7 Water layer thickness (micron) [Cl - ] decrease is <1% in 5 s.2.4.6.8.1 25 µm scratch,.1 M NaCl solution, release rate (A3), 5 seconds
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 http://www.virginia.edu/cese/research/crevicer/ 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