Scientific Understanding of Non-Chromate Inhibitors for Coatings Applications

Transcription

1 Overview Scientific Understanding of Non-Chromate Inhibitors for Coatings Applications R.G.Buchheit, Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio USA. An inhibitor screening approach for corrosion resistant coatings applications. Evaluation of lanthanide, transition metal oxoanion, Zn 2+ inhibitors. Assessment of what matters for good corrosion protection from inhibitor pigment additions. The support SERDP and the DOD is gratefully acknowledged.

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3 Paired reduction-oxidation reactions constitute a corrosion reaction, otherwise known as a corrosion cell. Reduction: a reaction that consumes electrons. Oxidation: a reaction that produces electrons. M M n+ + ne ions O 2 + 2H 2 O +4e 4OH OH H M n+ 2 O +O OH 2 solution metal ne Reaction sites sites must must be be in in electronic and and ionic ionic contact: electronic --thru thru metal metal ionic ionic --thru thru solution

4 Chromate-Free Inhibitors in Current Organic Coatings Inhibitor technologies used in state-of-the-art protective coatings Material Characterized Form Active corrosion inhibitor Cytec BR Adhesive bond primer, cured Calcium silicate, zinc film phosphate Deft 02GN083 Paint primer, liquid resin Praseodymium hydroxide, Deft 02GN084 Paint primer, liquid resin Praseodymium hydroxide Deft 44GN098 Paint primer, liquid resin Praseodymium hydroxide Hentzen 16708/709 Paint primer, liquid resin Calcium and magnesium silicate Shervvin Williams Paint primer, liquid resin Zinc oxide, aluminum cm phosphate PPG RW Paint primer, liquid resin Magnesium hydroxide EcoTuff Inhibitive pigment Cercus citrate, zinc molybdate Based on these findings, model (reagent grade) inhibitor systems have been selected for solubility testing 0 Courtesy of Mark Jaworoski, UTRC (2009). U1ltltl Ttcllnt legln Rntarch Ce1ltr 6

5 The use of high capacity, synthetic ion exchange compounds as coating pigments is an approach for inhibitor delivery. Cationic clays (montmorillonites, bentonites) Bohm et al., Werkst. u. Korros., 52, 896 (2001). Anionic clays (hydrotalcites). Buchheit, et al., POC, 47, 174 (2003). silicate layer Al 3+ /Mg 2+ hydroxide layer silicate layer cation layer 1000nm A A ~ Zn 0.65 Al 0.35 (OH) Å ~ Z net = eq/mol Gallery height depends on layer composition. Huang, et al., J. Coll. & Interf. Sci. (2002). Exchangeable cations: Ce 3+ Pr 3+ Y 3+ Tb 3+ La 3+ Zn 2+ Hosts: Li 2 [Al(OH) 3 ] 2 + Ni[Al(OH) 3 ] 2 2+ Zn[Al(OH) 3 ] 2 2+ Mg[Al(OH) 3 ] 2 2+ OH Zn Al Exchangable Anions: CO 3 2 PO 4 3 SiO 3 MoO 4 2 V 10 O 28 6 Approaches Exchange anions: transition metal oxoanions for Cl - or SO 2-4 in solution. Exchange cations: transition metal cations for Na + or K + in solution. Exchange both cations and anions using amphoteric exchangers or mixtures of clays. 4

6 Bentonite preparation. Exchanged cation release is tracked using UV-Visible spectra of 0.5M NaCl solution in contact with the REM bentonite

7 Inhibitor release from exchanged bentonites Cerium 2500 Praseodymium Ce released (ppm) day Pr released (ppm) day Inhibitor release from pigment slurries in a serial washing experiment.

8 Preparation of substrate AA2024-T3 and PVA/PVB coatings. Degreased for 2 minutes Deoxidized for 3 minutes 32.4 g Sodium + Metasilicate Rinsed with Dl water 30g SANCHEM + 72 ml HN ml 01 Water (65 C) 928 ml 01 Water (55 C) (4 grams PVA In 200 ml Dl water)/ (10 wt% PVB in methanol) + Pigment (Exchanged Bentonite) Coated with 4 passes of a roller bar Dried overnight Coated Panel

9 Strontium chromate-pigmented positive control. Z (ohms) 1 day 10 days 42 days Theta (degrees) Cathodic inhibition by chromate adsorption and reduction h SS exposure 8

10 PVB neat resin negative control. Z (ohms) 1 day 10 days 42 days Theta (degrees) Protection by barrier properties h SS exposure 9

11 Scribed 2024-T3 panels after 1000 hours ASTM B117 Salt Spray Exposure. (5 wt.% pigment in PVB). Neat 35ppm Ce 3+ 89ppm Zn 2+ <1ppm PO ppm Zn ppm Zn 2+ <1ppm PO ppm MoO 4 2 Strontium Chromate 53 ppmce ppm PO 3 4 <1ppm PO 3 4 Zn 2+ /PO 3 4 Zn 2+ /PO 3 4

12 Pigment screening using a total impedance metric allows the group of pigments to be broken into groups. EIS carried out on samples subject to salt spray. Z at 0.01Hz (ohm-cm 2 ) 35Ce 3+ /89Zn 2+ /<PO 4 3- SrCrO 4 Zn 2+ /PO 3-4 (1:4) Neat PVB resin 347Zn 2+ 10PO 3-4 Zn 2+ /PO 4-3 (1:1) Group 3 132Zn 2+ /<1PO 3-4 /146MoO Ce 3+ /<1PO 4 3- Group 1 Group 2 Exposure time in salt spray (days)

13 For coatings on Al alloys, high corrosion protection is strongly correlated with inhibition of oxygen reduction reaction Potential (V sce ) Ce 3+ and Zn 2+ are good cathodic inhibitors. Current density (A/cm 2 ) MoO 4 2- and phosphates are not. Potential (V sce ) Current density (A/cm 2 ) 12

14 In immersion experiments, coating corrosion protection scales with Ce or Zn dose. theta (degrees) Z (Ohms) 43 days in aerated 0.5M NaCl solution. B-Ce 24.4 ppm Ce ph 6.22 HT-P/B-Zn (1:1) 4 ppm Zn ph 7.63 HT-P/B-Zn (4:1) 0.2 ppm Zn ph g pigment in 50 ml solution 30 days 13

15 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.

16 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 Cerium Chloride 0 ppm Praseodymium Chloride Praseodimium Chloride 0 ppm E vs. SCE ppm 600 ppm 300 ppm 150 ppm E vs. SCE ppm 50 ppm 150 ppm 300 ppm Log i (Amps/cm 2 ) Log i (Amps/cm 2 )

17 Free corrosion experiment and SEM analysis The decreasing order of corrosion inhibition is Ce, Pr, La and Zn as inferred by inspection. Ce Pr LaZn Al2024-T3 sample is immersed in solution of 100 mm NaCl solution and 300 ppm of the inhibitor (as a chloride salt) for 48 hours SEM, mapping and spot EDAX analysis was done to study the mechanism of inhibition

18 Time-dependent EIS for PVB coatings with 10 wt.% Ce 3+ - and Pr 3+ -exchanged bentonites on 2024-T CeB.PVB_1hr CeB.PVB_3hr CeB.PVB_6hr CeB.PVB_28hr CeB.PVB_50hr CeB.PVB_75hr CeB.PVB_166hr PrB.PVB_1hr PrB.PVB_3hr PrB.PVB_6hr PrB.PVB_28hr PrB.PVB_50hr PrB.PVB_75hr PrB.PVB_166hr Z 10 3 Z Frequency (Hz) Frequency (Hz) CeB.PVB_1hr CeB.PVB_3hr CeB.PVB_6hr CeB.PVB_28hr CeB.PVB_50hr CeB.PVB_75hr CeB.PVB_166hr PrB.PVB_1hr PrB.PVB_3hr PrB.PVB_6hr PrB.PVB_28hr PrB.PVB_50hr PrB.PVB_75hr PrB.PVB_166hr Theta Theta Frequency (Hz) Frequency (Hz)

19 Time-dependent EIS for PVB coatings with 10 wt.% Ce 3+ - and Pr 3+ -exchanged bentonites on 2024-T3 4x10-9 Z at 0.01 Hz in ohm.cm PVB only 10 6 PrB ZnB 10 5 CeB LaB Exposure time (hours) Coating capacitance ( F/cm 2 ) ZnB 2x10-9 LaB CeB PrB 1x10-9 PVB only 5x Exposure time (hours) 10 5 R pore ( ohm.cm 2 ) 10 4 ZnB PrB CeB LaB 10 3 PVB only Exposure time (hours)

20 Some take-away points. Ce 3+ and Pr 3+ inhibit oxygen reduction to a similar extent, and protect to a similar degree in our screening experiments. Zn 2+ is an intriguing, but inconsistent inhibitor for Al alloys in our experiments. None of the lanthanides or Zn 2+ inhibit the ORR as effectively as chromate and corrosion protection in coatings where these inhibitors are present scale accordingly in our experiments.

21 Vanadates are potent inhibitors of oxygen reduction. 50 mm NaCl solution How can we get vanadate inhibitors into coatings?

22 We use a rapid screening test to identify pigments for further development. 3wt.% pigment added to 4g polyvinyl alcohol dissolved in 100mL DI water. Three passes of a #9 roll bar applied to 2024 T3. 96 hours neutral salt spray exposure.

23 Rudimentary primers--epoxy plus IEC pigment give good scribe corrosion resistance hours ASTM B117 salt spray exposure. HT-V primer 25% loading applied by drawdown bar on silane pre-treated 2024-T3 HT-V primer 25% loading applied by drawdown bar on deoxidized 2024-T3

24 Summary--some answers to the core questions. What is the basis for selecting combinations? - Cathodicinhibition specifically inhibition of oxygen reduction. - ph where effective. - Concentration where effective. What inhibitor combinations should be examined? -Focus on cathodic inhibition and account for substrate, environment, and transport. How are inhibitor combinations to be evaluated? - Electrochemical approaches for screening (hard to discriminate exposure results). - Exposure plus electrochemical approaches for evaluation. How are inhibitor combinations delivered? - Ion exchanging pigments and polymers. - Sparingly soluble salts (limited). - Triggered release (ICPs).

25 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

26 Inhibitors and combinations for this discussion. Anion exchangers Phosphate-bearing hydrotalcite Molybdate-bearing hydrotalcite HT-P HT-Mo Cation exchangers Zinc-bearing bentonite Cerium-bearing bentonite B-Zn B-Ce Mixtures Cerium and phosphate Cerium-zinc-phosphate Zinc-phosphate-molybdate B-Ce + HT-P B-Ce + B-Zn + HT-P B-Zn + HT-P + HT-Mo 25

27 Pigment corrosion protection is assessed in high permeability coatings. 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

28 Shift in the (001) peak in the sodium bentoniteindicatescation exchange. (001) Ce-bentonite (001) La-bentonite (001) Na Bentonite 10 (001) A 20 Na Bentonite 30 2 theta (001) Pr bentonite (001) Na Bentonite (001) Zn-bentonite theta (001) 10 2 theta Na Bentonite theta