Corrosiveness Assessment of Waterborne Coatings and Components by DC Electrochemical Methods Sarjak

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Corrosiveness Assessment of Waterborne Coatings
and Components by DC Electrochemical Methods
Sarjak H. Amin, F. Louis Floyd, Sumeet Tatti, and
Theodore ProvderEastern Michigan University,
Coating Research Institute, Ypsilanti, MI-48197
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Abstract

• It will be shown that a combination of DC
  Electrochemical techniques can be used to assess
  the relative corrosion (or inhibition) of various
  components in waterborne coatings. These results
  are carry over into the same order of behavior
  when coatings formulated with these components
  were applied to substrates in a corrosive
  environment. This will be demonstrated for
  cold-rolled steel. This approach will be shown to
  be a capable of ranking formulated liquid paints
  for their subsequent potential corrosion
  resistance as dried coatings. Initial results
  will be shown for aluminum substrates.

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Introduction

• One of the more common methods of controlling
  corrosion for metals has been the use of organic
  coatings
• Coatings appear to function by combination of
  barrier and electrochemical protection
• One can use the breadth of the passivation region
  in anodic DC scans to assess the
• variability in corrosion resistance of
  cold-rolled steel panels
• Passivation index (PI)-Difference between the
  open circuit potential and the breakdown
  potential
• This technique is superior in achieving
  correlation to corrosion resistance
• In a later section we present the rankings of
  some inhibitors that would have occurred had they
  utilized the passivation index concept
• It is generally accepted that the contribution of
  corrosion inhibitors in paint films is through
  their water-soluble component
• The final section shows how all raw material
  candidates for use in water-based paints can be
  screened for their inherent contribution to
  either corrosion or inhibition, and how
  fully-formulated liquid water-based paints can
  also be characterized for their net behavior

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Experimental

• Terminology
• Open Circuit Potential (OCP)- When a metal is
  exposed to an electrolyte, it will exhibit a
  characteristic potential that can be measured
  relative to a reference electrode
• Passivation- A metal is said to passivate in a
  given electrolyte if the current flow just above
  the OCP first plateaus with increasing potential
  before resuming a smooth continuous rise
• Breakdown Potential (Eb)- Breakdown of the
  passive film at some higher anodic potential
• Passivation Index (PI)- Difference between the
  open circuit potential and the breakdown
  potential
• Current Density- A qualitative estimate of
  relative current flow above OCP

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Transpassive Region
Breakdown Potential (Eb)
E (mv)
Passivating system
Open Circuit Potential (OCP)
Passivating Index (PI)
Non- passivating system
Log current (nano to milli amps)
Potentiodynamic Scan of Steel in Various
Environments
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Materials and Methods

• Equipment - Gamry (Warminster, PA) PC14/300
  Potentiostat / Galvanostat / ZRA
• Software- CorrView Electrochemical Analysis
  software (Scribner Associates, Inc.,) used to
  determine PI
• Electrochemical Measurement Conditions
• -NaCl Na tetra borate
• -500
• -7
• -OCP Equilibration time, 30 min
• -Total scan time,1 hr
• Panel Cleaning- Vapor cleaned in boiling xylene
  and rinsed in DI water
• Steel panels- A single lot of steel panels
  selected from 3 lots, that was fairly uniform in
  average consistency
• Salts- 0.1 N
• Raw Materials (Thickeners, Surfactants)- 1
  active solution (or extract) of the ingredient is
  used as the electrolyte
• Pigments-20 gms slurried in 250 ml DI water for
  24 hrs at RT, supernatant used
• Liquid Paints- The final paint is diluted (5 by
  wt) and the supernatant scanned as before

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Results and Discussion
Common Salts
Anodic Scans for Common Salts

• Scan showed a range of 5 decades in current
  density
• Wide variation in degree of passivation
• Sulfate and chloride demonstrated free corrosion
  behavior and rest showed some degree of
  passivation

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Rank Order (best to worst) in Inhibition as
Measured by Various Techniques (Sodium salt
unless otherwise specified)

• Not all materials that reduce corrosion rates
  also create a passive state on the metal
• Rank order of relative behavior from most
  corrosive to most passive
• Sulfate chloride sulfite nitrite
  tetraborate biphosphate bicarbonate
• The current density is shown to be useful to
  account relative corrosion behavior

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Passivation Indexes for Common Salts
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Paint Ingredients
Pigment Dispersants
Anodic Scans for Dispersants
Passivation Indexes for Dispersants

• Tamol 165 A was only slightly more passive than
  Tamol 731 A
• Tamol 1124 freely corroding
• Tamol 1124 shows a 2 decade higher current
  density than the two passivating dispersants

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Thickeners
Anodic Scans for Thickeners

• None of the classes provided evidence for
  passivation behavior.
• Rank order based on current density
• 2020 RM-7 Pholyphobe HEC

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Surfactants
Anodic Scans for Surfactants
Passivation Indexes for Surfactants

• Phosphate functional LO-529 passivate distinctly.
  
• SLS, MA-80, CF-10, Monazoline O did not
  passivate.
• LO-529 is lower in current density than worst
  non-passivating surfactants.

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Inhibitive Pigments

• Rank order based on current density, from least
  to most corroding
• ZCP Molywhite SrCrO3 ZnO PbCrO3
• None of the pigment extracts exhibited
  passivation behavior.
• Inhibitive pigments might function primarily via
  cathodic inhibition.

Anodic Scans for Pigment Aqueous Extracts

• Rank order based on current density, from least
  to most corroding
• ZCP Molywhite SrCrO3 PbCrO3 ZnO
• Reversal results for PbCrO3 and ZnO.
• Nacl showed same behavior.

Cathodic Scans of Pigment Extracts and 0.1 N NaCl
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Correlation to Corrosion Testing
Results of Model Liquid Paints

• B117 salt spray testing
• - Panels dried for 7 days at ambient
  cond., then exposed.
• - Readings were taken for at 4, 24, 48,
  72, and 96 hrs.
• Ratings listed above is average for all 5 time
  intervals.
• Higher number, better corrosion.
• Notable component was sodium nitrite as a flash
  rust inhibitor.
• RM-1 and RM-6 results were identical.

• Thus the combination of PI and current density
  is a useful DC electrochemical technique for
  screening and selecting ingredients for coatings
  designed to resist corrosion when applied to
  steel.
• The electrochemical test requires only about an
  hour to conduct, save considerable time in
  formulating WB products with improved corrosion
  control.

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Liquid Paints

• Pass controls C, E, and G showed strong
  passivation
• Paint F exhibits freely corroding behavior
• Paint F could have been a bad production batch

Anodic Scans for Commercial WB DTM Paints

• The fail controls exhibited no passivation.
• Pass control B showed passivation.

Anodic Scans for Lab and Commercial Control Paints
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Results of Anodic Scans on Liquid Commercial WB
DTM Paints and Control Paints

• PI information of value in detecting rouge
  paint behavior
• PI technique useful for testing batch-to-batch
  behavior of products

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Accelerated Corrosion Results

• GM9540 (Cyclic Corrosion testing)3,6,10,20,30
  cycles
• B-117 (Continuous salt fog testing), 50, 100,
  200, 400, 600 hrs
• Panels subjected to continuous exposure exhibits
  more corrosion than cyclic test
• Failures on the cyclic test tended to be more
  localized around the scribe
• Fail controls failed and pass control passed

The first number in the above ratings is for
corrosion creep from scribe. A crosshatch in the
shading and a second number indicates that there
is failure in the unscribed portion of the panel
(BIFblisters in field)
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Ranking of Paint Performance in Three Tests
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• Continuous salt fog exposure
• Paint A showed an induction period on the order
  of 50-100 hrs, after which substantial corrosion
  failure set in
• Paint B barely survived 50 hr period
• Commercial fail control E failed massively
• Paint H (SB Epoxy) performed well even at 600 hrs
• Cyclic exposure
• Results do not agree with continuous exposure
  results
• Performance of Paint G and lab pass control B
  not similar 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
• 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)

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Comparison of Pearson Rank Correlations with
Regression Correlations

• Greatest impact is on B117 vs PI correlation,
  rank correl.0.088?0.750
• B117 vs GM9540, rank correl.0.232?.709
• Experiments results on lab control paints were
  repeated and confirmed previous results
• Future work may explain reversals of lab
  control model paints

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Conclusion

• Water-based coatings appear to fail very
  uniformly during corrosion testing
• The corrosion rate at damage sites (scribe) is
  not significantly different from undamaged sites
  (field), suggests that nothing electrochemical
  interferes with the corrosion process
• Waterborne protect metal from corrosion is
  through an electrochemical mechanism rather than
  a barrier one
• The Passivation Index (PI) and relative current
  density are useful electrochemical parameters for
  describing the relative corrosiveness of
  water-based paint ingredients
• The numerical values for PI and current density
  to be considered relative, rather than absolute
• The PI / current density technique is rapid,
  takes only about 1 hour to determine the PI for a
  material
• DC electrochemical measurements of finished
  liquid paints correlates well with the results of
  cyclic corrosion testing, less with continuous
  salt fog exposures.
• Cyclic testing does not correlate with continuous
  salt fog testing
• Corrosion inhibition is not the same as
  passivation
• Passivation appears to be a stronger effect than
  reducing corrosion current in preventing
  subsequent corrosion of coated panels

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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 in the coatings
  program at Eastern Michigan University, for some
  of the potentiodynamic scans

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