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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
Conductives
Solving the Galvanic Corrosion IssueIn EMI Shielding
Harry FuerhaupterPotters Industries Inc.
Research & Development Center280 Cedar Grove Road, Conshohocken, PA 19428
TEL: 610-651-4780 EMAIL: [email protected]
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesOutline
• What is galvanic corrosion• Basic preventative measures• Specific problem with conductive gaskets• Unique behavior of conductive fillers• New conductive filler for low aluminum corrosion
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesTypes of corrosion
1. Uniform 2. Galvanic3. Crevice4. Pitting5. Intergranular6. Selective leaching7. Erosion8. Stress
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesUniform corrosion
• Diagram of zinc in acidZn2+
H+
H+
H+
H2
H+
e-
e-
Zinc
Hydrochloric Acid Solution
Cl- H+
Cl-
Zn Zn2+ + 2 e –
2 H+ + 2 e – H2
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesGalvanic Corrosion
• When two dissimilar metals contact each other in a corrosive environment, the less corrosion-resistant metal experiences more corrosion than when it is not in contact with the other metal.
Steel bolt in steel plate Brass bolt in steel plate
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesGalvanic corrosion
• Diagram of zinc/steel in acidZn2+
H+
H+
H+
H2
H+ Cl-
H+Cl-e-
e-
Hydrochloric Acid SolutionH+
H+
H+H2
e-
e-
Zn2+Zinc
Steel
Zn Zn2+ + 2 e –
2 H+ + 2 e – H2
Fe Fe2+ + 2 e –
- 0.763 V
- 0.440 V
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesGalvanic Corrosion
• The greater the difference in corrosion potential between two metals is, the greater the galvanic corrosion effect is.
GoldGraphiteSilver18-8 Stainless SteelNickelMonel (70 Ni, 30 Cu)Bronze (Cu-Sn)CopperBrass (Cu-Zn)TinLeadTin-lead solderSteel or ironAluminumZincMagnesium
Cathodic(less corrosive)
Anodic(more corrosive)
Galvanic series of some metals in seawater
(Corrosion Engineering, Fontana and Greene, 1978)
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesGeneral prevention
• Eliminate one of the conditions necessary for galvanic corrosion.– Corrosive environment– Dissimilar materials– Contacting surfaces
GoldGraphiteSilver18-8 Stainless SteelNickelMonel (70 Ni, 30 Cu)Bronze (Cu-Sn)CopperBrass (Cu-Zn)TinLeadTin-lead solderSteel or ironAluminumZincMagnesium
Cathodic(less corrosive)
Anodic(more corrosive)
• Coating• Material matching• Size ratio of
anode/cathode• Add sacrificial element
even more corrosive
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesEMI Gaskets and Galvanic Corrosion
• Typically a concern with aluminum frames used in outdoor environment: base stations, aircraft, marine.
GoldGraphiteSilver18-8 Stainless SteelNickelCopperSteel or ironAluminum
Cathodic(less corrosive)
Anodic(more corrosive)
• Cannot avoid contact with aluminum because electrical contact is required.
• The fillers that are most galvanically similar to aluminum tend to oxidize and lose conductivity.
• The most conductive filler materials (silver or silver-coated) are resistant to oxidation, but they have large galvanic difference to Al.
The ideal filler would be galvanically similar to aluminumand resistant to oxidation.
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesUnique Properties of Coated Powders
• A paper presented at IEEE EMC 2004* showed that the powder behaved galvanically more like the core material than the coating.
Open circuit potentials measured in 5% NaCl
PotentialMaterial Volts vs. SCESilver filled silicone elastomer -0.086Silver-nickel filled silicone elastomer -0.100Silver wire -0.106Nickel-graphite filled silicone elastomer -0.157Silver-copper filled silicone elastomer -0.161Silver-glass filled silicone elastomer -0.281Nickel metal -0.367Nickel filled silicone elastomer -0.411Silver-aluminum filled silicone elastomer -0.746Aluminum metal -1.181
Salt Spray Corrosion
* A.R. Pawlowych, “Galvanically compatible elastomericgasketing material for EMI shielding applications”.
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesRepetition of salt spray experiment
• Aluminum corrosion with fluorosilicone gaskets
0
100
200
300
400
500
600
700
0 200 400 600
Hours of Salt Spray
Alum
inum
Wei
ght L
oss
(mg)
Ag/NickelAg/Nickel-graphiteAg/GlassAg/CopperNickel GraphiteAg/Aluminum
Ag/Nickel---
Ag/CopperAg/GlassNi GraphiteAg/Aluminum
Rank byElectrodePotential
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesA new filler for EMI shielding
• Objective – create a non-silver conductive particle with low corrosion properties but galvanically similar to aluminum.
AluminumAluminumoxide layer
• Untreated aluminum power is not conductive due to the natural oxide layer on the surface.
Tungsten carbide
• The tungsten carbide penetrates the oxide layer and provides an electrical path to the aluminum core.
• Since tungsten carbide is galvanically inert, the filler creates virtually no galvanic corrosion on aluminum.
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesGalvan-O-Free Product Details
FIP Grade Molding Grade
Particle Size, µmD10 > 15 > 40D50 35 – 50 55 – 80D90 < 65 < 100
True Density 3.0 – 3.2 3.0 – 3.2
Tap Density, g/cc 1.7 – 1.9 1.7 – 1.9
Powder Resistivity, Ω-cm < 0.8 < 0.8
GF-45 GF-60
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesWC-Aluminum Corrosion Testing
Aluminum Corrosion in Salt Spray
1
10
100
1000
0 200 400 600
Hours of Exposure
Wei
ght L
oss
(mg)
WC-Aluminum
Ag/AluminumNi-Graphite
WC-Aluminum causes significantly less galvanic corrosion on aluminum than nickel/graphite and silver/aluminum
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesWC-Aluminum in Fluorosilicone Gasket
0.1
1
10
100
66 68 70 72 74 76Filler (wt%)
Volu
me
Res
istiv
ity (Ω
-cm
)
60
65
70
75
80
85
90
Har
dnes
s (S
hore
A)
Resistivity Aged Resistivity Hardness
Aging = 48 hours at 200C
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesWC-Aluminum Shielding Effectiveness
Shielding Effectiveness
0
20
40
60
80
100
120
140
30MHz
40 60 80 100 200 400 600 800 1GHz
2 4 6 8 10
Frequency
SE (d
B)
WC-Aluminum has excellent shielding effectiveness between 30MHz and 10 GHz
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesTheory of shielding at 1 Ω-cm
• Porous materials with relatively high volume resistivity have shown shielding capability– Nickel-loaded black rice husk: 80 dB @ 30 MHz to 60 dB @ 1.5 GHz
Journal of Materials Science, 39 (2004), pp. 6209 - 6214– Carbonized wood ceramic: 50 dB from 30 MHz to 1 GHz
Journal of Porous Materials, 4 (1997), pp. 269 – 275
Aluminum
WC
Pore
A C
DB
Pore
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesWC-Aluminum conductive filler applications
• WC-Aluminum filler materials have the right combination of properties to meet the challenges of corrosive environments.
• Some environments where WC-Aluminum could offer improved EMI gasket performance include:– High salt environments - marine– Outdoor enclosures exposed to various weather– High sulfur environments, such as exposure to fuel vapors– Strong chemical exposure, such as bleach for disinfection– Large thermal variations, e.g. aerospace and automotive
• Since WC-Aluminum contains no silver, the cost is closer to nickel-graphite.
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesAdditional considerations• Blending WC-Aluminum and Ag/Aluminum
1
10
100
0 200 400 600
Hours of Salt Spray
Alu
min
um W
eigh
t Los
s (m
g)
Ag/Aluminum60:40 Ag/Al : WC-Al50:50 Ag/Al : WC-AlWC-Aluminum
The amount of aluminum corrosion caused by Ag/Aluminum can be reduced by addition of WC-Aluminum
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesAdditional considerations
1
10
100
1000
0 200 400 600
Hours of Salt Spray
Vol
ume
Resi
stiv
ity (O
hm-c
m)
Ag/Aluminum60:40 Ag/Al : WC-AlWC-Aluminum
• Blending WC-Aluminum and Ag/Aluminum
The gasket resistivity of WC-Aluminum can be significantly decreased by addition of Ag/Aluminum
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Potters Industries Inc.an affiliate of PQ Corporation Potters
ConductivesPotters
ConductivesSummary
Reduction of galvanic corrosion in EMI gaskets can be accomplished by remembering a few key rules:
For finger stock – match materials and keep contact area small
If possible, keep corrosive elements out with secondary seal
For conductive elastomer gaskets in contact with aluminum substrate, Ag/Aluminum and WC-Aluminum have the lowest corrosion potential