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5.1 Task 1: Fundamental Studies of the Trivalent Chrome Process (TCP) 5.1.1 Objectives This task focused on trivalent chrome process (TCP) conversion coatings. A number of important issues related to TCP were addressed: 1. What is the formation mechanism, composition and structure of the TCP coating on AA2024? 2. How does the TCP coating inhibit the anodic and or cathodic kinetics of reactions occurring on the Al matrix and the Cu-rich intermetallics? 3. Does TCP provide active corrosion protection or is it simply a barrier? 4. Is there any evidence for transient formation of Cr(VI) species in the coating during formation and drying, or during immersion in electrolyte solutions? 5. What level of short-term chemical stability does the coating exhibit and is any benefit gained by aging (prolonged air drying)? 6. What are the structure and inhibitory properties of the TCP coating on AA6061 and 7075 alloys? 5.1.2 Background Chromate conversion coatings (CCCs) have long been employed in the surface finishing process for AA2024-T3 and other metal alloys for their excellent ability to resist localized corrosion and to promote paint adhesion. “Self-healing” is the predominant feature of CCCs, in which substrate metal exposed at a scratch or defect in the CCC is protected by the Cr(VI) species leached from the coating. However, due to the toxic and carcinogenic effects of hexavalent chromium compounds on the environment and human health, there have been increasingly stringent legislations regarding their use and waste disposal. Consequently, significant efforts have focused on chromate-free corrosion inhibitor systems that can provide comparable performance. The trivalent chrome process (TCP) developed at NAVAIR is currently one of the leading nonchromate conversion coatings on the market and has been shown to provide excellent corrosion protection and paint adhesion in standardized tests. The TCP process is a drop-in replacement for hexavalent chromate treatments and it contains no Cr(VI) in the coating bath and resulting film. The TCP coating baths are similar to commercial fluorozirconate/fluorotitanate-based conversion coating systems, but they also contain Cr(III) species, which improve the coating to provide better protection. There are currently several Cr(VI)-free products that meets the requirements according to Class 1A of MIL SPEC MIL-DTL-5541F, Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys. These different products are all based on Cr(III). However, before this program there was a lack of fundamental understanding of how TCP worked to provide corrosion protection. A number of questions addressed in this work are shown in the objectives section above. 5.1.3 Materials and Methods 5.1.3.1 Electrode Preparation and TCP Coating Procedure (Immersion). New AA2024-T3, AA6061 and AA7075 samples were used (1 cm 2 plates). Some of the testing was performed on unpolished plates, but most were initially polished to smooth and clean the surface. The polishing involved wet sanding with 1500 grit silicon carbide paper followed by a 20 min ultrasonic cleaning in ultrapure water. The sample was then polished using 0.05 µm alumina 14
powder, slurried with ultrapure water, on a felt polishing pad. This was followed by another 20 min ultrasonic cleaning in ultrapure water. Polishing was performed until the surface exhibited a mirror-like finish with no visible polishing striations. A polished or unpolished plate was then degreased for 10 min. in Turco 6849 TM (Henkel Corp., 20 % v/v) at 55 o C. This was followed by rinsing with tap water for 2 min. The sample was then deoxidized for 2 min. in Turco Liquid Smut-Go NC TM (Henkel Corp., 20% v/v) at room temperature (19-23 o C). This was again followed by rinsing with tap water for 2 min. Finally, the cleaned and deoxidized sample was coated with TCP for 10 min. (Alodine TM T5900, Henkel Corp., 100% RTU) at room temperature (immersion coating). While the actual Alodine TM T5900 composition is proprietary, it contains potassium hexafluorozirconate (K 2 ZrF 6 ), sulfate (SO -2 3 ), fluoroboric acid (HBF 4 ), and a chrome (III) salt. The pH of this solution was 3.85. The coated sample was again rinsed with tap water for 2 min., then with ultrapure water for 30 s, and allowed to air dry. For reasons that remain unclear, the tap water rinses are critical for proper and reproducible TCP film formation on this alloy. It could be that the more basic pH of the tap water (~6.8), as compared to the ultrapure water (~5.8), aids in the formation of the hydrated zirconia coating (see below). In most cases, the coated samples were dried overnight at room temperature in a covered storage dish prior to any electrochemical or structural characterization. Alodine TM 5200 (Henkel Corp.) was also coated (immersion) on some polished AA2024 samples, for comparison. This is a hexafluorozirconate coating similar to the T5900, but it contains no added chromium salt. Samples receiving this coating were prepared by degreasing and deoxidation, as described above. The 5200 solution was diluted with ultrapure water to yield a 3% (v/v) solution. The pH of this solution was 2.74. The alloy sample was then immersed for 2 min. at room temperature (21-25 o C) to form the coating. The coated sample was then removed, rinsed with tap water for 30 s and dried overnight at room temperature in a covered dish before use. In other experiments, AA2024-T3 samples were coated using two procedures. Process I consisted of: (i) etching for 15 min at 55 o C in 15% (v/v) Turco 6849 TM , (ii) desmutting for 5 min at room temperature in 20% (v/v) Liquid Turco Smut-go NC TM , (iii) coating with TCP in Alodine TM 5900S bath for 5 min at room temperature. These solutions are products of Henkel Corp., Madison Heights, MI. Turco 6849 contains nonylphenoxypoly ethanol, sodium xylene sulfonate, ethanolamine and sodium tripolyphosphate and has a pH of 11.7~12.2. Smut-go NC solution contains ferric sulfate, nitric acid and sodium bifluoride and has a pH ~ 0. Alodine TM 5900S is a fluorozirconate based bath. Deionized water rinsing was performed between each step. Samples were given a final rinse and blown dry with compressed air at room temperature (RT) at least 1 h prior to use if not mentioned otherwise. As a comparison, other samples were treated using the second treatment procedure, Process II, consisting of: (i) etching at 65 o C for 2 min in a solution of 32.4 g/l Na 2 SiO 3 + 48 g/l Na 2 CO 3 with pH 13.4, (ii) desmutting at 55 o C for 3 min in a solution of 72 ml/l HNO 3 + 30 g/l Sanchem 1000 (sodium bromate based, Sanchem Inc., Chicago, IL) with pH 0.25 and (iii) conversion coating at RT for 5 min in Alodine TM 5900S bath. Samples were rinsed with deionized (DI) water after each step and blown dry with compressed air at RT at the end. The control samples in polarization experiments were prepared by ultrasonic cleaning in ethyl alcohol to eliminate organic contamination. This condition is termed “as received” hereafter. 15
Page 1 and 2: FINAL REPORT Scientific Understandi
Page 3 and 4: This report was prepared under cont
Page 5 and 6: List of Acronyms AES Atomic Emissio
Page 7 and 8: List of Figures Figure 1.1. Figure
Page 9 and 10: Figure 1.21. Figure 1.22. Figure 1.
Page 15 and 16: Figure 3.11. Cathodic polarization
Page 19 and 20: Figure 4.12. Average Roughness of R
Page 21 and 22: Figure 5.4. Figures 5.5. Apparatus
Page 23 and 24: Figure 5.36. Figure 5.37. R paramet
Page 25 and 26: Figure 6.23. Two commercial samples
Page 27 and 28: Figure 7.1. a) instrumental configu
Page 29 and 30: List of Tables Table 1.1. Table 1.2
Page 31 and 32: Table 6.4. Processing parameters fo
Page 33 and 34: Keywords 1. Al alloys 2. Non-chroma
Page 35 and 36: 1. Executive Summary This report su
Page 37 and 38: the role of electrophoresis in inhi
Page 39 and 40: occurring at low pHs and release at
Page 41 and 42: were obtained by a post treatment w
Page 43 and 44: 4. G.O. Ilevbare, J.R. Scully, J. Y
Page 45 and 46: 3. Objective The primary objective
Page 47: 5. Results and Accomplishments In t
Page 51 and 52: TCP-coated AA2024-T3. A Kratos AXIS
Page 53 and 54: X-ray Photoelectron Spectroscopy (X
Page 55 and 56: Potential (V vs. Ag/AgCl) -0.2 -0.3
Page 57 and 58: Signal Intensity (counts/s) assessi
Page 59 and 60: Al (including both Al 0 and Al 3+ c
Page 61 and 62: TEM or during sample preparation, s
Page 63 and 64: Potential (V vs. Ag/AgCl) and uncoa
Page 65 and 66: 5.1.4.8 Effects of Aging on the Fil
Page 67 and 68: Intensity (counts/s) Concentration
Page 69 and 70: |Z| 0.01Hz (ohm cm 2 ) |Z| 0.01Hz (
Page 71 and 72: The artificial scratch cell was als
Page 73 and 74: Raman intensity, a.u. 4000 880 3500
Page 75 and 76: Figure 1.24. Raman spectra for the
Page 77 and 78: peak at 520 cm -1 and an intense Cr
Page 79 and 80: Figure 1.28. Plots of the intensity
Page 81 and 82: Figure 1.30. (A) Video micrograph o
Page 83 and 84: The Zr and O signals arise from the
Page 85 and 86: Figure 1.35. (A) Corrosion potentia
Page 87 and 88: Table 1.1. Corrosion current (i cor
Page 89 and 90: additional pitting. The TCP coating
Page 91 and 92: Figure 1.41. Cr(VI) peak intensity
Page 93 and 94: Work is ongoing to understand how t
Page 95 and 96: 9. L. Li, D-Y. Kim and G. M. Swain,
Page 97 and 98: carefully rinsed with DI water, air
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decrease to a net anodic current in
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Potential (mV vs. SCE) Potential (m
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i (A/cm 2 ) Immediately after silic
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Potential (mV vs. SCE) Rp (ohm.cm 2
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2.4.4 In situ AFM Scratching Result
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Figure 2.31. In situ AFM scratching
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(2.3) Figure 2.35. Specie diagram f
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concentration and the silicon to ca
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analysis coupled with XPS revealed
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5.2.4.4 In Situ AFM Scratching Befo
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esults in the formation of silicate
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26. P. Schmutz and G.S. Frankel, J.
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Pr 3+ , Y 3+ showed [12] an order o
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0.1M NaCl solution with varying con
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8.7 through pH 12.6 due to the lowe
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(b) (c) 99
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immersed in Ce 3+ remained bright w
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S phase and dissolution of Al matri
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Corrosion rate, i corr (A/cm 2 ) 6x
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I L (microA) 600 500 400 300 NC (-8
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than that of chromate[35]. Historic
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coupon was immersed with the polish
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5.3.2.4 Results 5.3.2.4.1 Speciatio
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(c) Figure 3.8. Speciation diagram
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E vs. SCE (V) -0.70 -0.75 -0.80 -0.
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E vs. SCE (V) 1000 rpm E vs. SCE (V
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Chemical quantification showed a me
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(a) (b) Figure 3.15. (a) (top) SEM
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E vs. SCE (V) E vs. SCE (V) E vs. S
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-Z" (ohm) -Z" (ohm) -Z" (ohm) -Z" (
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(Figure 3.8b). However, no effect o
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CPS CPS CPS x 2 14 10 12 10 8 6 4 2
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CPS CPS 100 95 90 85 80 75 70 x 10
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I k L 0. 2 1 2 AnFC B D 2 3 O 1
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i -800 (A/cm 2 ) 10 -4 NaCl NaCl+5
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Corrosion rate ( 1/R p ) ohm -1 cm
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particles compared to zinc-free sol
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and Zn bentonite compounds have red
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2) Deoxidizing for 3 minutes in an
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Scintag (now ThermoARL) Pad-V TM an
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Table 3.5. Characterization of bent
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Zn 2+ cation release (meq/100g) Pr
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(a) (b) (c) Figure 3.33. (a) An exp
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|Z| (ohm) Phase angle (degrees) 5.3
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|Z| at 0.01 Hz (ohm.cm 2 ) Pore Res
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|Z| at 0.01 Hz (ohm.cm 2 ) |Z| at 0
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pigmented coatings exhibited partia
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5.3.3.6 Conclusions 1. Exchange ben
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39. M. Mahdavian and M. M. Attar, P
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100. F.Liebau, Structural Chemistry
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of the coated sample, which is tigh
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Lubricant Blue (Struers) was used f
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good grip on the dolly, the equipme
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Pressure (kPa) r (mm) Pressure x Ra
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Pressure x Radius (kN/m) plateau bu
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Pressure x Radius (kN/m) Pxr (KN/m)
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Average Roughness (m) In contrast,
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Adhesion Strength (J/m 2 ) PVB PVB
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Pressure (kPa) Constant Infusion up
Page 221 and 222:
(a) (b) (c) (d) Figure 4.19. Coatin
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Min. Potential (V vs SHE) Min. Pote
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Pressure (kPa) exposure in hot wate
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Engineering Stress (MPa) slope of t
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Adhesion Strength (J/m 2 ) ASTM D33
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Pressure (kPa) Pressure (kPa) Radiu
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Adhesion Strength (J/m 2 ) Pressure
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Table 4.7. Adhesion Strength values
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Non-Cl./Deox. No CC Cleaned/Deox. N
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5.4.5 Conclusions and Implications
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5.5 Task 5: Inhibitor Activation an
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solution for equilibration at contr
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Table 5.2. Techniques applied to sp
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The capacitance was determined by E
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combination with zinc molybdate, a
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Concentration, mmol / L Final pH te
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Alkaline affects praseodymium solub
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MoO 4 2- ) inhibition to the system
Page 257 and 258:
Zinc molybdate is reported to be si
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The presence of a salt that has no
Page 261 and 262:
This approach yields two figures of
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Inhibitor mapping of the Hentzen 16
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The molybdate loss in the strontium
Page 267 and 268:
The capacitance dC is given by 0 (
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2 C( r, t) C( r )[1 r 2 exp( D nt
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-thea (degree) logIZI (Ohm-Cm 2 ) P
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Capacitance (F) respectively. The c
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Capacitance (F) Weight gain (g) For
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Normalized Capacitance Change Norma
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Potential V (Ag/AgCl) Corrosion rat
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5.5.4.7 Nanopore structure characte
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Normaalized Transport Rate (g/m^2_d
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S-Parameter S-Parameter Penetration
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S-Parameter S-Parameter Penetration
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R-parameter R-parameter Penetration
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5. T. M. Letcher, “Thermodynamics
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pigment of CaSiO 3 with initial pH
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allowed a comparison of the differe
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The solution resistance (R sol ) va
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Theta [degrees] theta |Z| [ohm] 10
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C [F/cm2] Rcorr [ohm*cm2] 7075-AHN
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C [F/cm 2 ] Rcorr [ohm*cm 2 ] Rcorr
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Visual TTF [hours] Table 6.2. Param
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of Metallast pre-treatment, which h
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Prediction of the TTF obtained by A
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5.6.2 Electrochemical Impedance Spe
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5.6.2.2.1 Coating system descriptio
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Table 6.4. Processing parameters fo
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population of total TTF vectors th
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components of the impedance and the
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Figure 6.17. EIS spectra for sample
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Figure 6.18. Kaplan-Meier survival
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Maximum c i range Maximum ci range
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The type and number of input neuron
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Predicted TTF The BFM method, intro
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The most significant variables to i
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5.6.3 Characterization of Pigment D
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of sputtering gold and carbon paint
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Figure 6.26. Cross-sectional view o
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Figure 6.28. EDX maps of primer cro
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Figure 6.31. EDX maps of primer cro
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In Figure 6.34, Ca and Si existed a
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Table 6.8. Average percentage resid
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strictly limited due to the barrier
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Figure 6.37. Coating systems in det
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Each corrosion volume of scribed ar
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Corrosion Area (mm 2 ) 1200 1000 80
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Corrosion Area (mm 2 ) Corrosion Ra
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Corrosion Rate (mm/yr) Corrosion Vo
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Corrosion Area (mm 2 ) corrosion ra
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corrosion rate (mm/yr) Corrosion Vo
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Corrosion Volume (mm/yr) Corrosion
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ate, PPG CA7233 coated samples had
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Appendix 6B Table 6B.1. Corrosion v
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Table 6B.3. Corrosion rate of sampl
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Table 6B.5. Corrosion area of sampl
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5.7. Task 7: Characterization of Lo
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at 65C. After using DI water to rin
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Transmission Mode Conduct EIS measu
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Cdl(F/cm 2 ) Rp(.cm 2 ) relatively
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pit bottom area (cm 2 ) pit bottom
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2days 2days 1mm 1mm 7days 7days Pit
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(a) (b) Si Si O Al Na Ca K Si Al-Cu
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Pit 1 and pit 2 as shown in Figure
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In these experiments, Deft primers
Page 391 and 392:
For the case of as-deposited thin f
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3. L. Balazs, Physica E, 54, 1183-1
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and subsequently re-oxidized in two
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not represent true adhesion strengt
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• The pit morphology in 0.5 M NaC
Page 401 and 402:
Appendix 2. List of Scientific/Tech
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“The Secret Life of Chromate Free
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