The Efficiency of Corrosion Inhibitors - Flash Rust Inhibitor

The Efficiency of
Corrosion Inhibitors

Tony Gichuhi, Ph.D.
R&D Manager/Scientist
HALOX
tgichuhi@halox.com

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