buchheit - for posting
TRANSCRIPT
-
8/7/2019 Buchheit - For Posting
1/27
Overview
An inhibitor screening approach for corrosion resistant coatings
applications.
Evaluation of lanthanide, transition metal oxoanion, Zn2+
inhibitors.
Assessment of what matters for good corrosion protection frominhibitor pigment additions.
Scientific Understanding of Non-Chromate
Inhibitors for Coatings ApplicationsR.G.Buchheit, Fontana Corrosion Center, Department of Materials Science and
Engineering, The Ohio State University, Columbus, Ohio USA. [email protected]
The support SERDP and the DOD is gratefully acknowledged.
-
8/7/2019 Buchheit - For Posting
2/27
Paired reduction-oxidation reactions constitute a corrosion
reaction, otherwise known as a corrosion cell.
Reaction sites must be in electronic and ioniccontact:
electronic - thru metalionic - thru solution
Reaction sites must be in electronic and ioniccontact:
electronic - thru metalionic - thru solution
M Mn+ +ne
Oxidation:areactionthat
produceselectrons.
O2+2H2O+4e4OH
Reduction:areactionthat
consumeselectrons.
Mn+H2O+O2
ne
OH
metal
ions
OH solution
-
8/7/2019 Buchheit - For Posting
3/27
CourtesyofMarkJaworoski,UTRC(2009).
-
8/7/2019 Buchheit - For Posting
4/27
-
8/7/2019 Buchheit - For Posting
5/27
Bentonite preparation.
Exchanged cation release is tracked using UV-Visible spectra of 0.5M NaCl solution in
contact with the REM bentonite
-
8/7/2019 Buchheit - For Posting
6/27
Inhibitor release from exchanged bentonites.
400
500
600
700
800
900
1000
0 2 4 6 8 10 12 14
Cereleased(ppm
)
day
500
1000
1500
2000
2500
0 2 4 6 8 10 12 14
Prrelea
sed(ppm)
day
Cerium Praseodymium
Inhibitor release from pigment slurries in a serial washing experiment.
-
8/7/2019 Buchheit - For Posting
7/27
Preparation of substrate AA2024-T3 and PVA/PVB coatings.
-
8/7/2019 Buchheit - For Posting
8/27
1000 h SS exposure
The
ta(degrees
)
1 day10 days42 days
|Z|(ohms)
Strontium chromate-pigmented positive control.
Cathodic inhibition by chromate adsorption andreduction.
8
-
8/7/2019 Buchheit - For Posting
9/27
1000 h SS exposure
|Z|
(ohms)
Theta
(degrees)
1 day10 days42 days
PVB neat resin negative control.
Protection by barrier properties.
9
-
8/7/2019 Buchheit - For Posting
10/27
Pi t i i t t l i d t i ll
-
8/7/2019 Buchheit - For Posting
11/27
53Ce3+/
-
8/7/2019 Buchheit - For Posting
12/27
For coatings on Al alloys, high corrosion protection is strongly
correlated with inhibition of oxygen reduction reaction
Ce3+ and Zn2+ are good
cathodic inhibitors.
MoO42- and phosphates are not.
12Current density (A/cm2)
Potential(Vsce)
Potential
(Vsce)
Current density (A/cm2)
In immersion experiments coating corrosion protection
-
8/7/2019 Buchheit - For Posting
13/27
In immersion experiments, coating corrosion protection
scales with Ce or Zn dose.
13
theta(degrees)
|Z|(Ohms)
B-Ce
24.4 ppm Ce
pH 6.22
HT-P/B-Zn (1:1)
4 ppm ZnpH 7.63
HT-P/B-Zn (4:1)
0.2 ppm Zn
pH 7.94
43daysinaerated0.5MNaClsolution.
1gpigmentin50mLsolution30days
-
8/7/2019 Buchheit - For Posting
14/27
Some take-away points.
The effectiveness of an inhibitor in a coating application
correlates with its ability to inhibit the oxygen reduction
reaction.
Inhibitor effectiveness is concentration dependent.
- critical minimum concentrations needed
- critical concentration ranges may exist.
Inhibitor effectiveness may depend on pH.
h id d i b i
-
8/7/2019 Buchheit - For Posting
15/27
Lanthanides suppress oxygen reduction, but to varying
extents.
Cathodicpolarization on 2024-T3 substrates in aerated 0.1 M NaCl solution with
chloride salts in various concentrations.
-1.20
-1.12
-1.04
-0.96
-0.88
-0.80
-0.72
-0.64
-0.56
10-7
10-6
10-5
Evs.SCE
Log i (Amps/cm
2
)
0 ppm
50 ppm
600 ppm
300 ppm
150 ppm
Cerium Chloride
10-4
-1.20
-1.12
-1.04
-0.96
-0.88
-0.80
-0.72
-0.64
-0.56
10-7
10-6
10-5
10-4
Praseodimium Chloride
0 ppm
300 ppm
50 ppm
600 ppm
150 ppm
Evs.SCE
Log i (Amps/cm2
)
Praseodymium ChlorideCerium Chloride
-
8/7/2019 Buchheit - For Posting
16/27
Free corrosion experiment and SEM analysis
The decreasing order of corrosion inhibition is Ce, Pr, La and Znas inferred by inspection.
Ce Pr LaZn
Al2024-T3 sample is immersed in solution of 100 mM NaClsolution and 300 ppm of the inhibitor (as a chloride salt) for48 hours
SEM, mapping and spot EDAX analysis was done to studythe mechanism of inhibition
-
8/7/2019 Buchheit - For Posting
17/27
Time-dependent EIS for PVB coatings with 10 wt % Ce3+-
-
8/7/2019 Buchheit - For Posting
18/27
105
106
107
0 50 100 150 200
ZnB
LaB
CeB
PrB
PVB only
Exposure time (hours)
|Z|at0.01H
zinohm.cm
2
5x10-10
1x10-9
2x10-9
0 50 100 150 200
ZnB
LaB
PrB
PVB onlyCoatingcap
acitance(F/cm
2)
Exposure time (hours)
4x10-9
CeB
103
104
105
0 50 100 150 200
ZnB
PVB only
CeB
LaB
PrB
Exposure time (hours)
Rpore
(ohm.c
m2)
Time dependent EIS for PVB coatings with 10 wt.% Ce
and Pr3+-exchanged bentonites on 2024-T3
-
8/7/2019 Buchheit - For Posting
19/27
Some take-away points.
Ce3+ and Pr3+ inhibit oxygen reduction to a similar extent,
and protect to a similar degree in our screening
experiments.
Zn2+ is an intriguing, but inconsistent inhibitor for Al alloys
in our experiments.
None of the lanthanides or Zn2+ inhibit the ORR as
effectively as chromate and corrosion protection in coatings
where these inhibitors are present scale accordingly in ourexperiments.
-
8/7/2019 Buchheit - For Posting
20/27
Vanadates are potent inhibitors of oxygen reduction.
How can we get vanadate inhibitors into coatings?
50mM NaCl solution
We use a rapid screening test to identify pigments for further
-
8/7/2019 Buchheit - For Posting
21/27
We use a rapid screening test to identify pigments for further
development.
3wt.%pigmentaddedto4gpolyvinylalcoholdissolvedin100mLDIwater.
Threepassesofa#9rollbarappliedto2024T3.
96hoursneutralsaltsprayexposure.
Rudimentary primers epoxy plus IEC pigment give good
-
8/7/2019 Buchheit - For Posting
22/27
Rudimentary primers--epoxy plus IEC pigment give goodscribe corrosion resistance.
1008 hours ASTM B117 salt spray exposure.
HT-V primer 25% loading
applied by drawdown baron silane pre-treated 2024-T3
HT-V primer 25% loading
applied by drawdown baron deoxidized 2024-T3
S t th ti
-
8/7/2019 Buchheit - For Posting
23/27
Summary--some answers to the core questions.
What is the basis for selecting combinations?
What inhibitor combinations should be examined?
How are inhibitor combinations to be evaluated?
How are inhibitor combinations delivered?
- Ion exchanging pigments and polymers.
- Sparingly soluble salts (limited).
- Triggered release (ICPs).
- Cathodicinhibitionspecifically inhibition of oxygen reduction.
- pH where effective.
- Concentration where effective.
- Electrochemical approaches for screening (hard to discriminate exposure results).
- Exposure plus electrochemical approaches for evaluation.
-Focus on cathodic inhibition and account for substrate,
environment, and transport.
-
8/7/2019 Buchheit - For Posting
24/27
Pigments and pigment combinations
Hydrotalcite pigments were synthesized by co-precipitation.
Bentonite pigments were synthesized by ion exchange.
Synthesis was verified by XRD.
Inhibitor concentrations developed by soaking 1g of pigment or pigment mixture in 50
mL 0.5M NaCl for 2 days
24
I hibi d bi i f hi di i
-
8/7/2019 Buchheit - For Posting
25/27
Inhibitorsandcombinationsforthisdiscussion.
Anion exchangers
Phosphate-bearing hydrotalcite HT-P
Molybdate-bearing hydrotalcite HT-Mo
Cation exchangers
Zinc-bearing bentonite B-Zn Cerium-bearing bentonite B-Ce
Mixtures Cerium and phosphate B-Ce + HT-P
Cerium-zinc-phosphate B-Ce + B-Zn + HT-P
Zinc-phosphate-molybdate B-Zn + HT-P + HT-Mo25
Pigment corrosion protection is assessed in high permeability coatings
-
8/7/2019 Buchheit - For Posting
26/27
Pigmentcorrosionprotectionisassessedinhighpermeabilitycoatings.
Coating Application
5 wt. % pigment loading in polyvinyl butyral (PVB) (15 wt% in ethanol)
on clean AA2024-T3
Drawdown bar application; air cure.
Evaluations
Salt spray exposure per ASTM B117 and periodic removal for EIS in 5%NaCl solution.
Static immersion in aerated 0.5M NaCl solution.
26
Shift in the (001) peak in the sodium
-
8/7/2019 Buchheit - For Posting
27/27
bentoniteindicatescation exchange.
20 30
2 theta
10
Na Bentonite
A
(001)
Ce-bentonite
(001)
20 30
2 theta
(001)
10
Na Bentonite
(001)La-bentonite
20 30
2 theta
(001)
10
Na Bentonite
(001)
Pr bentonite
20 30
2 theta
(001)
10
Na Bentonite
Zn-bentonite(001)