Durable Sol-Gel Surface Treatment to Control Cathodic ...

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

Program Overview

4Content on this slide subject to markings on cover page.

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-


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

http://portal/NEW%20Luna%20Logos/LUNA_Logo4c.jpg


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

Sol-Gel Coating Chemistry


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

Coating Application to Fasteners


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


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


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

Galvanic Corrosion Testing


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+


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


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


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


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


0000 Hours B117 0504 Hours B117 7075-7

Nylon Fasteners, 7075-T6 Plate

No noticeable corrosion product formation. Scribe still bright


Everlube 620C, 7075-T6 Plate

0000 Hours B117 0504 Hours B117

Large amount of blister formation at scribed fasteners. Product not providing galvanic

protection.


Cadmium, 7075-T6 Plate

0000 Hours B117 0504 Hours B117

Evidence of corrosion product from fasteners, no panel corrosion or

blistering yet.


Cadmium + Luna Sol-Gel, 7075-T6 Plate

0000 Hours B117 0504 Hours B117

No noticeable color change of fasteners. No loss of coating adhesion.

Electrochemical Modeling and Down-Hole Analysis


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


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


Unscribed hole with Ti-6Al-4V

fastener*Suffered mechanical

damage only

Unscribed hole with SS316

fastener

Unscribed Hole Locations


Scribed hole with Ti-6Al-4V fastener

Scribed hole with SS316 fastener

Scribed Hole Locations


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



Different pitting morphologies observed Images from scribed hole with

Ti-6Al-4V fasteners

Thin fissures

Possibly selective grain attack



All holes with corrosion had similar damage metrics, independent of fastener type and presence of scribe



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


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)

Durable Sol-Gel Surface Treatment to Control Cathodic ...

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