Using DC Electrochemical Techniques to Assess the Relative Corrosiveness of water-Based Coatings and their Ingredients

1. Using DC Electrochemical Techniques to Assess the Relative Corrosiveness of water-Based Coatings and their Ingredients F. Louis Floyd, Sumeet Tatti and Theodore Provder Eastern Michigan University Coatings Research Institute Ypsilanti, MI

2. Corrosion Protection Mechanisms • Matrix Barrier Properties • Electrochemical Protection • Combination of Above.

3. DC Electrochemical Techniques • Passivation Index • Current densities • Open Circuit Potential

4. Objective of Study • Assess Relative Corrosiveness of Waterborne Coatings (Liquid Paints) and Ingredients – Salts – Pigment Dispersants – Thickeners – Surfactants –Inhibitive Pigments

5. Potentiodynamic Scan of Steel in Various Environments Transpassive Region PI • Open Circuit Potential (OCP), (Ecorr) ; no current flow • Breakdown Potential, (Eb),, pitting potential • Current density is inversely related to inhibition behavior, e.g., A>B>C in terms of current density for OCP< E< Eb

6. Materials and Methods • DC Measuring Equipment & Software – Gamry PC 14/300 Potentiostat/Galvanostat/ZRA, Warminster, PA – CorrView Electrochemical Analysis software, Scribner Associates Inc., Southern Pines, NC used to determine PI • Electrochemical Measurement Conditions – NaCl + Sodium tetraborate at 0.1N – 500<PI<1000 mv – 7<pH<10 – OCP ~ 30 minutes – total scan time ~ 1hour • Accelerated Corrosion of Water-based Paints – ASTM B-117 continuous salt spray testing – GM 9540 cyclic salt spray testing

7. Materials and Methods, Cont’d. • Steel Panels – A single lot of ground steel panels selected from 3 lots for all experiments; uniform w/r to average consistency1 – vapor cleaned in boiling xylene; rinsed in D.I. water • Salts – 0.1N • Raw Materials (Thickners, Surfactants) – 1% active solution or extract of the ingredient used as electrolyte • Pigments – 20 gms slurried in 250 ml DI water for 24 hrs at RT, supernatant used • Liquid paints – diluted to 5wt% with DI water, supernatant used 1. “Characterization of the Variability in Corrosion Resistance of Steel Using Electrochemical Techniques”, R. Groseclose, C. M. Frey, F.L. Floyd, J. Coatings Technology, 56 (714), July 1984, 31-41.

8. Results—Common Salts Passivation Indexes Anodic Scans NaCl Na2SO4 Na2SO3 NaNOP3 NaHCO3 Na2HPO4 Na2B4O7

9. Results—Common Salts sulfate ~ chloride> sulfite> nitrile> tetraborate> biphosphate> bicarbonate Not all materials that reduce corrosion rates also create a passive state on the metal 7. “Salt Spray Performance of water-Based Maintenance Paints:A Multiple Correlation Study”, F.L.Floyd and C. M. Frey, Org. Ctgs. Preprints, 43, ACS, 1980, 586-592 8. “Molybdate in Corrosion Inhibition- A Review”, M.S. Vuskasovich, J.P.G. Farr, Materials Performance, May . 1986, 9-18 9. R. Groseclose, C. M. Frey and F.L.Floyd unpublished extension of work reported in “the Corrosion Inhibition of Metals by Molybdate, Part l: Mild steel” M.A. Stranik, Corrosion, 40 (6) June,1984, 296-302

10. Results—Dispersants Passivation Indexes Anodic Scans Tamol 731A Tamol 165A Tamol 1124 • Tamol 731 & 165 distinctly passivating • Tamol 1124 freely corroding

11. Results—Thickeners, Surfactants Anodic Scans Passivation Behavior Thickeners • No passivation behavior for thickeners • Rank order based on current density: • 2020>>RM7~Polyphobe>> • HEC Acrysol RM-7 Acrysol RM-2020 Polyphobe TR-115 HEC-Cellosize ER15M • Gafac LO-529, distinctly passivating surfactant • 4 other surfactants are non-passivating • Based on current density Triton CF-10 and Monazoline-O somewhat less corrosive than SLS and Aerosol MA-80 Surfactants SLS Aerosol MA-80 Triton CF-10 Monazoline-O Gafac LO-529

12. Results—Surfactants • Based on passivation behavior and current density Surfactant Gafac LO-529, latexes employing passivating surfactants may show improved corrosion resistance relative to those employing non-passivating surfactants • Monazoline-O inhibits both anodic and cathodic half-cell reactions, but does not confer any passivation to the metal Cathodic Scans of Diluted Monazoline-O and 0.1N NaCl

13. Results—Inhibitive Pigments Anodic Scans Cathodic Scans Molywhite MZAP/ED + Heucophos ZCP + ZnO-Zinc Oxide SrCO3 PBCrO3 None of the pigment extracts exhibited passivation behavior in anodic scans Based on current density from anodic scan, least to most corroding; ZCP>Molywhite>SrCO3> PBCrO3 Based on current density from cathodic scan, least to most corroding; ZCP>Molywhite> PBCrO3> SrCO3

14. Results of Model Liquid Paints—Correlation to Corrosion Testing # • Electrochemical (EC) prediction of “best” for each pair was followed in 3 of 4 comparisons • Combination of PI and current density is a useful DC EC technique for screening/selecting ingredients for coatings designed to resist corrosion on steel • EC test requires 1 hr—save considerable time in formulating WB products with improved corrosion control. # B-117 Ratings: sum of 2 replicates and 5 time intervals (4,24,48,72,96 hrs); error is ± 1 unit

15. Passivation Index Results for Liquid Commercial Waterborne DTM Paints and Control Paints

16. Passivation Index Results for Liquid Commercial Waterborne DTM Paints and Control Paints • Fail controls exhibit no passivation behavior • Pass control exhibits strong passivation behavior • 3 out of 4 Commercial DTM products (G,C,E)showed strong passivation behavior with one exception (F) • Paint F could have been a bad production batch. • Only PI information was required to differentiate products, current density was not required • PI information of value in detecting rogue paint behavior • PI technique useful for testing batch-to-batch behavior of products

17. Accelerated Corrosion Results for Liquid Commercial Waterborne DTM Paints and Control Paints—Sample Results for Paint G Passivation • GM9540 (cyclic corrosion testing), 3,6,10,20,30 cycles • Exclude sample F due to flash rusting • Add in sample H, pass control, 2 part solvent epoxy • B-117 (continuous salt fog testing), 50,100, 200, 400, 600 hrs

18. Accelerated Corrosion Results for Liquid Commercial Waterborne DTM Paints and Control Paints—Correlations of PI with B-117 and GM9540 Ratings Note: Actual Ratings were determined by summing all ratings for each coating including replicates for all exposure times or cycles. Larger numbers represent better corrosion resistance results Rank orders were from best (1) to worst.

19. Accelerated Corrosion Results for Liquid Commercial Waterborne DTM Paints and Control Paints—Correlations of PI with B-117 and GM9540P Ratings .

20. Accelerated Corrosion Results for Liquid Commercial Waterborne DTM Paints and Control Paints—Correlations of PI with B-117 and GM9540 Ratings and Conclusions • Paints subjected to continuous exposure exhibited considerably more corrosion than those subjected to cyclic exposure • Fail controls failed and pass controls passed (with one exception) • Continuous salt fog exposure – Broad field failure + significant scribe creep – Paint A best with induction period of 50-100hrs, followed by failure – Paint C 2nd best, barely surviving 50 hrs – Paint fail control D last with massive failure – Paint H (solvent borne epoxy) performed superbly even at 600 hrs – Reversal of lab pass and fail control– to be discussed later .

21. Accelerated Corrosion Results for Liquid Commercial Waterborne DTM Paints and Control Paints—Correlations of PI with B-117 and GM9540 Ratings and Conclusions • Cyclic exposure – Results do not agree with continuous exposure results in extent of failure, scribe creep or in rank order of systems; not surprising – Performance of Paint G and lab pass control B similar with no significant field failure and scribe creep until after 10 cycles – Performance of Paint C was next with failure starting in 3-6 cycles – Commercial fail control D much worse than previous coatings –Except for one paint PI for liquid paints correctly predicted subsequent rank order as films on steel panels – High Correlation of PI to GM9540 is primarily electrochemical • Rank vs Regression Correlation – Rank correlations preferred where a lot of judgement (e.g.,ASTM visual ratings) went into determining ratings, ratings have significant uncertainty and relative differences more important than absolute differences) .

22. Accelerated Corrosion Results for Liquid Commercial Waterborne DTM Paints and Control Paints—Correlations of PI with B-117 and GM9540 , Analysis of Lab Control Exception • Greatest impact is on B117 vs PI correlation, rank correl. 0.088 0.750 • B117 vs GM9540, rank correl. 0.232.0709 • Experimental results on lab control paints were repeated and confirmed previous results • Future work may explain reversals of lab control model paints

23. Conclusions • Water-based coatings appear to fail uniformly during corrosion testing, as one would expect from uncoated metal, but more slowly • Water-based coatings protect metal from corrosion primarily through an electrochemical (EC) mechanism (passivation and or inhibition) rather than a barrier one • PI and relative current density are useful EC parameters for describing the relative corrosiveness of water-based paint ingredients and assessing the degree of success in formulating DTM water-based paints • The DC EC technique can be used to assess batch-to-batch uniformity of DTM water-based products in production • The EC technique using PI and relative current density takes only ~ 1 hr to determine PI Compared to about one month to perform accelerated corrosion tests on panels • DC EC measurements of finished liquid paints correlates well with the results of cyclic corrosion testing though less well with continuous salt fog testing • Cyclic testing does not correlate well with continuous salt fog testing, a well recognized fact in the standards community

24. Conclusions • Corrosion inhibition is not the same as passivation, although they do occur simultaneously. • Some systems signifiicantly reduce corrosion (e.g., inhibitive pigments and imidazoline surfactants) while not contributing any degree of passivation • Passivation appears to be a stronger effect than reducing corrosion current in preventing subsequent corrosion of coated panels • A formulator can use EC techniques (PI and current density) to screen ingredients to find preferable alternatives before engaging in paint making and testing • A formulator can substantially improve the corrosion resistance of any water-based paint • Manufacturers of paint ingredients may find that this effect applies to components of their ingredients, such as surfactant, initiator, and buffer in an emulsion polymerization

25. Future Efforts • Using EC techniques to assess the relative corrosiveness of water-based coatings and their ingredients on aluminum substrates

26. Acknowledgments • This work was supported by  the U.S. Department of the Army, Tank -Automotive and Armaments Command (TACOM), Contract No. DAAE07-03-C-L127 The authors would like to acknowledge the contributions of the following people in completing this study: • I. Carl Handsy of the U.S. Department of the Army, Tank-Automotive and Armaments Command, Warren MI; and John Escarsega, CARC Commodity Manager at the Army Research Lab, Aberdeen Proving Grounds, MD; for very useful technical discussions during the course of this project. • Pauline Smith, Army Research Lab, Aberdeen Proving Grounds, MD, for conducting the accelerated exposure testing and providing visual ratings and photographic results for those tests. • Martin Donbrosky, Jr., Chemir Services, Ypsilanti, MI, for preparing the water-based laboratory coatings, and coating the panels for accelerated exposure testing. • Allayna Lee, undergraduate student in the coatings program at Eastern Michigan University, for some of the potentiodynamic scans. • Dr. Stephen Tait of Pair O Docs Professionals L.L.C. for very useful technical discussions during the course of this project.