Buchheit - For Posting

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.


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


3/27

CourtesyofMarkJaworoski,UTRC(2009).


4/27


5/27

Bentonite preparation.

Exchanged cation release is tracked using UV-Visible spectra of 0.5M NaCl solution in

contact with the REM bentonite


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.


7/27

Preparation of substrate AA2024-T3 and PVA/PVB coatings.


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


9/27

1000 h SS exposure

|Z|

(ohms)

Theta

(degrees)

1 day10 days42 days

PVB neat resin negative control.

Protection by barrier properties.

9


10/27

Pi t i i t t l i d t i ll


11/27

53Ce3+/


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


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


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


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


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


17/27

Time-dependent EIS for PVB coatings with 10 wt % Ce3+-


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)


4x10-9

CeB

103

104

105

0 50 100 150 200

ZnB

PVB only

CeB

LaB

PrB


Rpore

(ohm.c

m2)

Time dependent EIS for PVB coatings with 10 wt.% Ce

and Pr3+-exchanged bentonites on 2024-T3


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.


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


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


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


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.


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


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


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


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)

Date post:	08-Apr-2018
Category:	Documents
Upload:	dungkaizen
View:	222 times
Download:	0 times

Buchheit - For Posting

Documents