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Improving Adhesion:
Examining the
Electrochemistry
of Organic Inhibitors
Benefits of Organics
• Chemisorb onto metallic substrates
• Complex with metal ions at substrate
• Neutralize & absorb the corrodents
• Decrease permeability (barrier enhancement)
• Anodic passivators
• Improve wetting
• Promote adhesion
• Plug pores (defects)
All Coatings Made Here
Periodic Trends
• In general, Electronegativity increases to
the right and increases upwards
• Greater ability to attract electrons to itself
• Close proximity to associated elements
yields a dynamic sharing relationship
among electrons
• Surface treatments allow for metals to
associate with select elements in a sharing
relationship that might otherwise not exist
Relative Electronegativities
CORROSION CELL DIAGRAM
1
2 3
Iron Substrate Anodic Region Cathodic Region
Water Droplet
2 Fe 4 e-
4 H+ O2
2 H2O n H2O
3/2 O2
2 Fe2+ Rust
Fe2O3 . H2O
2. Electrons at the Fe (inactive) cathode reduce O2 to H2O.
3. The Fe2+ migrates through the drop and reacts with H2O to form rust.
1. Oxidation of Fe yields electrons which travel through the metal.
Relative Bond Strengths
Force Type example E (kcal/mol)
Covalent 1° Most Organics 15-175
Metallic 1° Metals 27-83
H bonding 2° Water <12
Dispersion 2° Most Molecules <10
Dipole 2° Polar Organics <5
Induction 2° Non-polar
Organics
<0.5
Force Type example E (kcal/mol)
Paint Film to
Metal
2° DTM Coatings <25
Water to Metal 2° Coating Defect 40-65
Coating/Substrate Adhesion Promoter
Resin Organic Corrosion Inhibitor
Improved adhesion between coating and metal
substrate
Coating/Substrate interface
• Non-Skid Paint; 2K Solvent-borne Epoxy Polyamide With Poor
Adhesion
• Non-Skid Paint needed better corrosion resistance. Needs to withstand 2000 hrs salt spray with minimal corrosion in the defect
sites created by aircraft damage
Adhesion Leads to Corrosion Protection
Primer
w/o
HALOX
Primer
With
HALOX Organic
reduced corrosion creep severe corrosion creep
Adhesion Leads to Corrosion Protection
Mechanism: Hydrophobic Film
Formers
• Film Formers: • Slow both anodic and cathodic reactions
– Organic
•Benzoates
•Amines
•Sulfonates
• Thiols
•Di-acids
O
O-
NH3+
O
O-
O
-O
S-
S
O
O
O-
(1) Polar end: adsorption & strong affinity for M+/ MO leading to
hydrophobic dense film formation. Also the Acid -Base reactivity
site for modification
(2) Electron donor site
(3) Polycyclic aromatic: corrosion inhibition of non-ferrous substrates.
Electron donors (N, S, O)
(4) Non polar end: van der Waals interaction with multiple organic
binders (Compatibility). Also electron donor to substrate
O
O-
O
O-
S
S
N
(1)
(2)
(4)
(3)
Multi-functional Anti-Corrosive
Orientation of Molecule
• Impacts electron sharing capacity
• Dynamic action, changes during film
formation and cure
• Imparts a surface tension gradient
• DTM adhesion
• Intercoat adhesion (pigment affinity)
Multi-functional Anti-Corrosive
• Adsorption of molecule forms a complex which increases the
electrical resistance of the substrate
• Makes the oxidation of steel more difficult
• A form of active corrosion protection
Adhesion Promotion
Binder
Substrate
Electrons adjusting to
lowest energy state
(wetting)
Dipoles formed during
cure (intercoat)
Polymer chains exhibit 1°
bonding (coupling)
16
No Inhibitor 4% BTTSA-amine
0 0 0
34 25
52 36
5 54 42
52 41
52 43
10 54 45
54 44
57 45
15 57 43
56 44
59 45
20 62 43
61 44
62 43
25 62 42
63 43
63 41
30 65 40
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30
TIME (days)
WA
TER
PER
MEA
TIO
N (
%)
No Inhibitor
4% HALOX Organic
Barrier Properties
Reduced Water Permeation in 2K Epoxy System determined by EIS
Coating Galvanized Objects
• Zn2+ migration under film accelerates
adhesion failure in susceptible systems
• Barrier type organic corrosion inhibition
mechanism
ORGANIC HYBRIDS
18
SiOR
RO OR
SiOH
RO OR
+ ROHH2O
1. SILICONE ESTER HYDROLYZES TO FORMS SILANOL
Si
SUBSTRATE
OH OH OH OH
SUBSTRATE
O OSi
OH
RO OR
+
+ ROH
SiRO RO
Si
SUBSTRATE
O O
SiRO RO
Si
SUBSTRATE
O
Si
O
OSi
O
Si
O
OO
2. SILOXANE BOND FORMS
3. GELATION (CROSS-LINKING)
+ H2O + ROH
POLYMER
Sol-Gel Treatment for Improved Adhesion
Substrate:
Hot dipped galvanized
G70 70 U (ACT labs)
Pretreatment
application:
10% Hybrid Sol-Gel
Cure:
Air dry 24 hrs at RT
Coating applied:
Sherwin Williams high
gloss water-based DTM
at 50 µm DFT
Accelerated test shown:
336 hrs salt spray
Ion-Exchange – Notable Synergy
Ion-Exchange Inhibitors
• Significant release of energy by altering
electron environment
• Decreases the densely packed electron structures
of Alkali Metals
• Binds the strongest, most electronegative atoms
and compounds, ex. Cl- and SO42-
• Significant synergy with Sol-Gel over
Aluminum
Adhesion in Coil Coatings After 400h salt spray test ASTM B 117-90
Without Inhibitor Commercial system (SrCrO4)
4% HALOX Organic 18.5% Ion-Exchange
Substrate: hot dip galvanized
steel
Primer : TPA, phenolic resin,
phosphoric acid pre-
treatment
Topcoat: PES Melamine
de-lamination at scribe (mm) 5 2 1 under-rusting at scribe (mm) 0 0 0
After 1500h Salt Spray Test ASTM B 117-90
Without inhibitor Commercial system (SrCrO4)
Substrate: Al/Zn steel
Primer : TPA, phenolic resin,
phosphoric acid pre-
treatment
Topcoat: PVC Plastisol
de-lamination at scribe (mm) total 0 0 de-lamination at cut edge (mm) total 0 1.5 under-rusting at scribe (mm) 0 0 0 under-rusting at cut edge (mm) 3 1 1.5
4% HALOX Organic 18.5% Ion-Exchange
Ion-Exchange
Organic(2%) Ion-Exchange(9%)
CONTROL EXTENDER PIGMENTS
500 hours – Aluminum Filiform Corrosion Test (1.2 mil epoxy primer, 2.0 mil PU topcoat)
Good gloss for Aluminum wheels
The film thickness of 15-50 µm
Filiform corrosion is < 1 mm
3% Sol-Gel + 0.7% Ion-Exchange
Solvent Based Polyester/Acrylic Hybrid coating
Hybrid Synergy with Ion-Exchange
Organic Corrosion Inhibitors
• Anodic passivation (reduce rust & flash
rusting)
• Improve adhesion
• Increase water resistance
• Reduce blistering
• Form protective films (adsorption)
• Increase coating flexibility (plasticize)
• Increase chemical resistance
