Post on 21-Feb-2023
ASETSDefense WorkshopDenver, Colorado August 20, 2018
Durable Sol-Gel Surface Treatment to Control Cathodic Current Density
Adam Goff, Rebecca Martin, Charles Sprinkle, and Bryan Koene (Luna)Rob Kelly and Rebecca Skelton (UVa)
Luna gratefully acknowledges that this material is based upon work supported by the United States Navy under Contract No N68335-16-C-0121.
Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the United States Navy.
2
Outline
Program overview Sol-gel coating chemistry Coating application to fasteners Galvanic corrosion testing Electrochemical modeling and down-hole
analysis Conclusions / Future Work
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Problem Statement
Aircraft fasteners and other structural components create galvanic corrosion problems
Aluminum alloy airframe is sacrificial to the more noble alloy (e.g. fasteners)
Instead of focusing on protecting the anode, what about controlling the available cathodic current?
e-
e-e-Anode
CathodeAl → Al3+ + 3e-
O2 + 4e- + 4H2O → 4 OH-
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Luna Solution
A hybrid organic-inorganic sol-gel thin coating that combines: Excellent physical barrier protection against
moisture and corrosive ions Durable and dense electrically insulating
layer between mated components
Cross-linked polymer treatment is impact resistant, flexible, and tough
Environmentally sound formulation that is chrome-free and a water / alcohol based chemistry
Sol-Gel Treated, Cd Plated 4140 Steel
Cd plated 4140 Steel
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Polarization Testing
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1E-10 1E-08 0.000001 0.0001 0.01
E vs
. Ref
. (Ag
/AgC
l), V
Log of Current Density (A/cm2)
316SS PDS
316SS Bare
316SS Sol-gel
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
1E-10 1E-08 0.000001 0.0001 0.01
E vs
. Ref
. (Ag
/AgC
l), V
Log of Current Density (A/cm2)
Ti-6Al-4V PDS
Ti-6Al-4V Bare
Ti-6Al-4V Sol-gel
Sol-gel provides significant reduction in cathodic current density
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Sol-gel Formulation Overview• Combination of hard ceramic component with flexible linkers produce high
toughness for abrasion resistance and durability
Si-(OCH2CH3)4
H2O / H+
- CH3CH2OHSi-(OH)4 SiO2
SiRO
RO OR
ORSi
HO
HO OH
OH
Hydrolysis Condensation SiO
O O
O
SiO O
OSi
O
OSiO
O
O
3D Rigid Network
SiO O
O
SiO
O
-H2O
Flexible component• Luna synthesized• Flexibility, adhesion
Tough, Ceramic• Durability• Abrasion resistance
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Fastener Application Processing Dip-spin application evaluated
by Luna and PPG Initial trials indicated
formulation rheology needed adjusting to enable retention at threads and sharp radii
Formulation adjustments made with various rheology modifiers for improved thread coverage retention Testing showed inadequate
coverage, thereforeabandoned dip application and moved to spray (next slide)
M10 X 40 A4-80 SS Bolts
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Fastener Application Processing (cont)
Spray application pursued with two specialty coaters
Hundreds of 316SS fastener parts coated
Overall, spray application looks very promising and is the primary method moving forward
M10 X 40 A4-80
10-32 x ¾” socket cap
½-20 x 1” 82° countersunk hex
Mated washers for 10-32 x ¾” screws
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Formulation Scale-Up
Similar formulations have been scaled up previously and licensed
All components demonstrated at +5-gallon quantities already and evaluated for shelf life stability
Specific formulation scale-up and final shelf life assessment conducted in follow-on work
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Configuration of Samples
Testing based off of NAVAIR geometry / methodFasteners mated to ¼” thick aluminum alloy plates
Surtec 650V TCP + 44GN072 Primer + 03W127A Topcoat
Ti-6Al-4V fastener locations
316SS fastener locations
0.020” wide scribe, 0.5” extended from center of hole
Testing involves galvanically mating cathodic fasteners to anodic aluminum plates The cathodic fasteners drive
galvanic corrosion of the mated aluminum panel
Simulates common corrosion problems around fasteners on aircraft
Panels are pretreated and coated with desired materials
One row of fastener locations are scribed through the paint down to the base metal before fasteners installed
Panels subjected to whatever environment is of interest; in our case, B117 conditions for 500hrs+
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Galvanic Test Matrix
Panels were tri-chrome conversion coated and primed (both sides) with MIL-PRF-85582 Type I Class C2 (No primer in holes), and top coated with MIL-PRF-32239 polyurethane (unless otherwise noted in table)
Note the Nylon fastener configuration in the table is intended to separate possible aluminum panel crevice corrosion (not related to cathodic fasteners) from galvanic corrosion associated with mating of cathodic fasteners
Galvanic Panel TestingAA2024 Panels AA7075 Panels
ASTM B117 ASTM B117Coating Applicator #1 Fasteners (Luna sol-gel) 2 2Coating Applicator #2 Fasteners (Luna sol-gel) 2 2Bare Fasteners (Control) 2 2Nylon Fasteners (Control) 2 2Everlube 620C Coated Fasteners (316SS hardware only) 1 1Cd Plated Fasteners (HSS hardware only) 1 1Cd Plated Fasteners + Luna sol-gel (HSS hardware only) 1 1
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0000 Hours B117 0504 Hours B117 7075-6
Bare Fasteners, 7075-T6 Plate
Lots of corrosion product from fasteners, discoloration of scribes. Blister initiation at unscribed areas. Deep pitting at scribed areas.
Site of corrosion initiation
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0000 Hours B117 0504 Hours B117 7075-2
Spray Application #1, 7075-T6 Plate
No blisters, very little corrosion product from fastener areas. Reduced pitting at scribed areas. Some loss of fastener coating at
scribe areas
Stain; not corrosion
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0000 Hours B117 0504 Hours B117 7075-3
Spray Application #2, 7075-T6 Plate
No blisters, very little corrosion product from fastener areas. Some slight color change evident. No noticeable loss of
adhesion.
Site of corrosion initiation
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0000 Hours B117 0504 Hours B117 7075-7
Nylon Fasteners, 7075-T6 Plate
No noticeable corrosion product formation. Scribe still bright
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Everlube 620C, 7075-T6 Plate
0000 Hours B117 0504 Hours B117
Large amount of blister formation at scribed fasteners. Product not providing galvanic
protection.
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Cadmium, 7075-T6 Plate
0000 Hours B117 0504 Hours B117
Evidence of corrosion product from fasteners, no panel corrosion or
blistering yet.
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Cadmium + Luna Sol-Gel, 7075-T6 Plate
0000 Hours B117 0504 Hours B117
No noticeable color change of fasteners. No loss of coating adhesion.
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Modeling and Damage Analysis Examples
Light interferometry and microscopy damage analysis
Tota
l Ano
de Cu
rrent
(A/c
m)
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
2.5E-04
3.0E-04
Bare 316 Solgel 316 Bare B7 Solgel B7
Sol-gel reduces galvanic damage by ~98%
Luna provided coupon geometry and e-chem data to Dr. Rob Kelly, along with damaged coupons
Kelly quantified coupon damage and correlated to modeled TAC
Demonstration of a possible prognostic tool to aid in determination of when sol-gel would be needed (or valuable) for specific “hot spots”
Calculated Potential
Distributions
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AA7075-T6 Plate w/ Mated Bare Fasteners After 1,350 hrs in B117 Testing
Note panel contained same primer and topcoat as previous ones but no
conversion coating
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Unscribed hole with Ti-6Al-4V
fastener*Suffered mechanical
damage only
Unscribed hole with SS316
fastener
Unscribed Hole Locations
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Scribed hole with Ti-6Al-4V fastener
Scribed hole with SS316 fastener
Scribed Hole Locations
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Important Take Away Points
Scribed hole with Ti-6Al-4V fastener showed minimal corrosion on surface, but inside hole corrosion damage was comparable to holes with SS316 fasteners
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Important Take Away Points
Different pitting morphologies observed Images from scribed hole with
Ti-6Al-4V fasteners
Thin fissures
Possibly selective grain attack
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Important Take Away Points
All holes with corrosion had similar damage metrics, independent of fastener type and presence of scribe
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Important Take Away Points
Calculated dissolution current using Faraday’s law for alloys
On scribed hole with SS316 fastener Current in hole was more
than current on surface (i.e. the sum of all four scribes)
Current Thrown x10−5 [A]
8.50
7.506.50
5.50
4.50
3.50
2.50
1.50
0.50
0.17
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Conclusions and Future Work
• Luna’s sol-gel coating is being demonstrated to be a valuable tool to mitigate galvanic corrosion around cathodic fasteners• Target use as pre-coated fasteners with unique NAS/stock numbers• Complete galvanic coupon corrosion testing to quantify advantages
of sol-gel application• Down-hole analysis has revealed unexpected corrosion
around titanium fastener holes• Quantify additional coupon damage and compare to UVa modeling
to determine corrosion repeatability and TAC metric• Complete additional torque/tension testing• Work with Navy and other DoD agencies to develop
plans for dem/val on relevant platform (Phase II Option and beyond)