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 zincfree compounds is dependent on their purity,
solubility, morphology, type of ions, pigmentpolymer 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
0.80
Ag+(aq) + e- Æ Ag (s)
0.34
Cu2+(aq) + 2e- Æ Cu (s)
0.0
2H+(aq) + 2e- Æ H2 (g)
-0.44
Fe2+(aq) + 2e- Æ Fe (s)
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
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 È
Protective mechanisms
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
†
†
†
†
soluble) pigment in 500 ml of 0.5 M
NaCl for a period of 24 hrs
Mixture 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
Cell
Electrolyte
Substrate
Electric
Contact
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
Ecorr
Rp
icorr
(mV vs SCE)
(kΩ)
(µA/cm2)
% Inhibition
Efficiency
Blank
-639
0.21 ± 0.07
76 ± 7
-
P1
-578
1.20 ± 0.17
13 ± 3
83
P2
-545
0.60 ± 0.02
34 ± 1
55
P3
-552
1.23 ± 0.07
21 ± 1
72
P4
-550
0.74 ± 0.09
31 ± 1
59
P5
-503
0.90 ± 0.11
25 ± 3
67
P6
-549
0.95 ± 0.05
18 ± 2
76
P7
-585
3.37 ± 0.42
4±1
95
Ref #
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 bioaccumulative in humans & the environment
Demand for High Performance
Corrosion Inhibitors
Thin Films
Clear Coats
Temporary
Coatings
Green
Technologies
Coil coating
5-10 µm
Waterborne
Lacquers
2-10 µm
Rust
Preventative
5-20 µm
UV
Powder
100% solids
High solid
Epoxy
Acrylic
Urethane
Alkyd
Wash Primer
10-15 µm
Conversion
Coatings
1-3 µm
Zero VOC
Low VOC
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
“Bust the Rust”
Thank You All !!
References
Slide #
Source
12-17
Thierry et al – Progress in Organic Chemistry
(25) 339-355 (1995)
Scantlebury et al - Journal of Electrochemical
Society 148 (8) 293-298 (2001)
Calle et al – Corrosion Technology Lab NASA
Kennedy Space Center
21-23
29