The Efficiency of
Corrosion Inhibitors
Tony Gichuhi, Ph.D.
R&D Manager/Scientist
HALOX
[email protected]
Abstract
The 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
Topics
Basics of corrosion
Corrosion control methods
Features of corrosion inhibitors
Types of Ions, Solubility & Synergy
Applying Synergy to Solve Corrosion
Problems
Meeting the demands of the future
Concluding remarks & references
Basics of Corrosion
Standard Reduction Potentials
Standard Potential (V) Reduction half reaction
1.23 O2 + 4H+ + 4e- Æ 2H2O
Ag+(aq) + e- Æ Ag (s)
0.80 Cu2+(aq) + 2e- Æ Cu (s)
0.34 2H+(aq) + 2e- Æ H2 (g)
0.0 Fe2+(aq) + 2e- Æ Fe (s)
-0.44
Since E0red (Fe2+) < E0red (O2)
iron can be oxidized by oxygen
Basics of Corrosion
Dissolved oxygen in water usually
causes the oxidation of iron
Fe2+ initially formed can be further
oxidized to Fe3+ which forms rust,
Fe2O3.xH2O
Oxidation occurs at the site with the
greatest concentration of O2
Galvanizing to Prevent Corrosion
Corrosion Control
Methods
Corrosion Control Methods
Protective Coatings (92%)
Organic – Paint, Varnishes, Coal tar
Metallic – Galvanizing, Electroplating
Conversion – 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 È Protective film formed
Solubility È Leaching, blistering, protecting ability
Purity / Modification Protective film, blistering, corrosion
Morphology Dispersion, film formation, water transmission
Pigment Polymer Long-term stability, accelerated cross-linking,
Interaction catalytic effects on cure
Moisture content Accelerated cure, decreased corrosion resistance
PVC of CI Gloss, film formation, blistering
Environment Solubility, efficiency of pigment
(pH, Corrosive) Protective mechanisms
Synergy È
Ionic types: Comparison
Ref # 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 Ability
Leaching 1 g of each (sparingly Electrolyte Counter electrode
soluble) pigment in 500 ml of 0.5 M Cell
NaCl for a period of 24 hrs
Substrate
Mixture is filtered and pH &
conductivity of the extracts is Electric
measured Contact
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
Corrosion Efficiency on CRS
Where:
i0 = Corrosion rate in
absence of corrosion inhibitor
iI = Corrosion rate in
presence of corrosion inhibitor
Corrosion Efficiency on CRS
Ref # Ecorr Rp icorr % Inhibition
Efficiency
Blank (mV vs SCE) (kΩ) (µA/cm2)
P1 -
P2 -639 0.21 ± 0.07 76 ± 7 83
P3 -578 55
P4 -545 1.20 ± 0.17 13 ± 3
P5
P6 -552 0.60 ± 0.02 34 ± 1
P7
-550 1.23 ± 0.07 21 ± 1 72
-503 0.74 ± 0.09 31 ± 1 59
-549 0.90 ± 0.11 25 ± 3 67
-585 0.95 ± 0.05 18 ± 2 76
3.37 ± 0.42 4±1 95
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 more noble
Less current
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, P5
Best Performer = Zinc Chromate
Observations
Phosphate was a better inhibitor of steel
Chromate was a better inhibitor of zinc
The 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 this
Synergy 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 between
Zinc and Mild steel
Artificial Rain Water (pH 4.5)
Results of Galvanic Current
Measurements
Observations
All inhibitive pigments decreased the
galvanic currents more than the blank
Blank: Current dropped from 12 to 9 µA
SrCrO4: Current dropped to 0.2 µA
Other individual pigments were down to
4.5 µA
Synergistic 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
Thin Films Clear Coats Temporary Green
Coatings Technologies
Coil coating Waterborne Rust UV
5-10 µm Lacquers Preventative Powder
2-10 µm 100% solids
Wash Primer 5-20 µm High solid
10-15 µm Epoxy
Zero VOC Acrylic
Conversion Low VOC Urethane
Coatings Alkyd
1-3 µm Corrosion
Preventing
Compounds
The Future
Chromate-free
Heavy metal-free
Sub-micron anticorrosive pigments
Smart coatings (e.g. corrosion sensing)
Nanotechnology
Smart Coatings
Nanotechnology
WATERBORNE ACRYLIC
Galvanized – 336 hrs Salt Spray – 2.0 – 4.0 µm thick
Concluding Remarks
Electrochemical methods can be used to
study the efficiency of corrosion
inhibitors
Many factors influence the behavior and
efficiency of corrosion inhibitors
The future is “Green”
New technologies such as Smart
Coatings and Nanotechnology will soon
emerge