The Efficiency of Corrosion Inhibitors

Tony Gichuhi, Ph.D.R&D Manager/ScientistHALOXtgichuhi@halox.com

AbstractThe inhibition efficiency of anticorrosive pigments such as zinc chromate, zinc phosphate, modified zinc phosphate and zinc-free compounds is dependent on their purity, solubility, morphology, type of ions, pigment-polymer interactions, pigment volume concentration, the environment surrounding them and the substrate. The objective of this presentation is to review the knowledge of these pigments and the state-of-the-art in terms of anticorrosive materials; this knowledge can be used to simplify the criteria for selecting anticorrosive pigments for a given application

TopicsBasics of corrosionCorrosion control methods Features of corrosion inhibitors Features of corrosion inhibitors

Types of Ions, Solubility & SynergyApplying Synergy to Solve Corrosion ProblemsMeeting the demands of the futureConcluding remarks & references

Basics of Corrosion

Standard Reduction PotentialsStandard Potential (V) Reduction half reaction

1.23 O2 + 4H+ + 4e- 2H2O

0.80 Ag+(aq) + e- Ag (s)

0.0 2H+(aq) + 2e- H2 (g)

0.34 Cu2+(aq) + 2e- Cu (s)

-0.44 Fe2+(aq) + 2e- Fe (s)

Since E0red (Fe2+) < E0

red (O2) iron can be oxidized by oxygen

Basics of CorrosionDissolved oxygen in water usually causes the oxidation of ironFe2+ initially formed can be further oxidized to Fe3+ which forms rust, Fe2O3.xH2OOxidation occurs at the site with the greatest concentration of O2

Galvanizing to Prevent Corrosion

Corrosion Control Methods

Corrosion Control MethodsProtective Coatings (92%)

Organic – Paint, Varnishes, Coal tarMetallic – Galvanizing, ElectroplatingConversion – Phosphate, Chromate

Corrosion Resistant Materials (6-7%)Alloys, Plastics, Composites, Glass

Corrosion Inhibitor Additives (1-2%)Chemical – Inorganic, Organic, Mixtures

Features of Corrosion Inhibitors

Feature What does it Influence?Types of ions Protective film formed

Solubility Leaching, blistering, protecting ability

Purity / Modification Protective film, blistering, corrosion

Morphology Dispersion, film formation, water transmission

Pigment Polymer Interaction

Long-term stability, accelerated cross-linking, catalytic effects on cure

Moisture content Accelerated cure, decreased corrosion resistance

PVC of CI Gloss, film formation, blistering

Environment (pH, Corrosive)

Solubility, efficiency of pigment

Synergy Protective mechanisms

Ionic types: ComparisonRef # Trade Name Chemistry/Ions

P1 Zinc Chromate Zinc Chromate

P2 Butrol 23 Barium Metaborate

P3 Shieldex Calcium Silica Gel

P4 Cotrol 18-8 Amino Carboxylate

P5 HALOX BW-111 Barium Phosphosilicate

P6 K-White 105 Aluminum Triphosphate

P7 Heucophos ZPZ Modified Zinc Phosphate

Pigment Extracts Chosen to Study Pigments Protective AbilityLeaching 1 g of each (sparingly soluble) pigment in 500 ml of 0.5 M NaCl for a period of 24 hrsMixture is filtered and pH & conductivity of the extracts is measured Substrates was submerged in electrolyte for 16 hrs (steady-state)Polarization experiments conducted using the extract solutions over CRS and zinc substrates Fresh electrolytes (0.5 M NaCl) are used each time for the anodic and cathodic polarization scans

Counter electrode

Electric Contact

Electrolyte

Cell

Substrate

Corrosion Efficiency on CRS

Where:

i0 = Corrosion rate in absence of corrosion inhibitor

iI = Corrosion rate in presence of corrosion inhibitor

Corrosion Efficiency on CRSRef # Ecorr

(mV vs SCE)Rp(kΩ)

icorr(µA/cm2)

% InhibitionEfficiency

Blank -639 0.21 ± 0.07 76 ± 7

13 ± 3

34 ± 1

21 ± 1

31 ± 1

25 ± 3

18 ± 2

4 ± 1

-

83

55

72

59

67

76

95

1.20 ± 0.17

0.60 ± 0.02

1.23 ± 0.07

0.74 ± 0.09

0.90 ± 0.11

0.95 ± 0.05

3.37 ± 0.42

-578

-545

-552

-550

-503

-549

-585

P1

P2

P3

P4

P5

P6

P7

DECREASING CORROSION EFFICIENCY:P7 > P1 > P6 > P3 > P2, P4, P5

Best Performer = Modified Zinc Phosphate

Anodic & Cathodic Polarizations on cold rolled steel (CRS)

more noble

Less current

Less current

more noble

ANODIC CATHODIC

DECREASING CORROSION EFFICIENCY:P7 > P1 > P6 > P3 > P2, P4, P5

Best Performer = Modified Zinc Phosphate

Anodic & Cathodic Polarizations on zinc substrates

ANODIC

CATHODIC

DECREASING CORROSION EFFICIENCY:P1 >> P7 > P3 > P6 >> P2, P4, P5Best Performer = Zinc Chromate

Observations

Phosphate was a better inhibitor of steelChromate was a better inhibitor of zincThe 2 best pigments based on these polarization studies were zinc chromate and modified zinc phosphate

Applying Synergy to Solve a Corrosion Problem

Cut-Edge Corrosion Inhibition

Cut edge corrosion is most common failure mechanism of organic coated galvanized steel (HDG)Strontium chromate is generally used in steel primers to mitigate thisSynergy of non-toxic corrosion inhibitors has been found to perform equal to chromate

Cut-Edge Corrosion Inhibition

Model Cell for Measurement of galvanic corrosion current betweenZinc and Mild steel

Artificial Rain Water (pH 4.5)

Results of Galvanic Current Measurements

ObservationsAll inhibitive pigments decreased the galvanic currents more than the blankBlank: Current dropped from 12 to 9 µASrCrO4: Current dropped to 0.2 µAOther individual pigments were down to 4.5 µASynergistic pigments had better current suppression; down to 1.1 µA

Meeting the Demands of the Future

The Future

The future is “Green” Technology – No heavy metals!OSHA PEL Proposed 5 µg/m3 for Cr6+ in workplaces Feb 27, 2006. OSHA ordered to promulgate new PEL. (aerospace PEL now 20 µg/m3)End-of-Life Vehicle (EU Directive 2000/53/EC): Cr6+, Pb, Cd, Hg banned from vehicles marketed after July 1, 2003 California Air Resources Board (CARB) approved an Airborne Toxic Control Measure (ATCM) for Emissions of Cr6+ and Cd from Motor Vehicle and Mobile Equipment Coatings (Automotive Coatings) September 21, 2001.Registration, Evaluation and Authorization of Chemicals (REACH) – Authorization of chemicals causing cancer, mutations, reproductive problems, or are bio-accumulative in humans & the environment

Demand for High Performance Corrosion Inhibitors

Clear Coats TemporaryCoatings

WaterborneLacquers2-10 µm

Rust Preventative

5-20 µm

Zero VOCLow VOC

CorrosionPreventing

Compounds

Green Technologies

Coil coating5-10 µm

Wash Primer10-15 µm

ConversionCoatings1-3 µm

Thin Films

UVPowder

100% solidsHigh solid

Epoxy Acrylic

Urethane Alkyd

The Future

Chromate-freeHeavy metal-freeSub-micron anticorrosive pigmentsSmart coatings (e.g. corrosion sensing)Nanotechnology

Smart Coatings

Nanotechnology

WATERBORNE ACRYLICGalvanized – 336 hrs Salt Spray – 2.0 – 4.0 µm thick

Concluding Remarks

Electrochemical methods can be used to study the efficiency of corrosion inhibitorsMany factors influence the behavior and efficiency of corrosion inhibitorsThe future is “Green”New technologies such as Smart Coatings and Nanotechnology will soon emerge

“Bust the Rust”

Thank You All !!

References

Slide # Source12-17 Thierry et al – Progress in Organic Chemistry

(25) 339-355 (1995)21-23 Scantlebury et al - Journal of Electrochemical

Society 148 (8) 293-298 (2001)29 Calle et al – Corrosion Technology Lab NASA

Kennedy Space Center