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CD Proposal New Markets for Conductives Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Solving the Galvanic Corrosion Issue In EMI Shielding Harry Fuerhaupter Potters Industries Inc. Research & Development Center 280 Cedar Grove Road, Conshohocken, PA 19428 TEL: 610-651-4780 EMAIL: [email protected] www.pottersbeads.com Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Outline • What is galvanic corrosion • Basic preventative measures • Specific problem with conductive gaskets • Unique behavior of conductive fillers • New conductive filler for low aluminum corrosion Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Types of corrosion 1. Uniform 2. Galvanic 3. Crevice 4. Pitting 5. Intergranular 6. Selective leaching 7. Erosion 8. Stress Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Uniform corrosion • Diagram of zinc in acid Zn2+ Zinc e- Hydrochloric Acid Solution - - + e H+ Cl H H2 H+ + H H+ Cl- Zn Zn2+ + 2 e – + – 2 H + 2 e H2 Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Galvanic 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 Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Galvanic corrosion • Diagram of zinc/steel in acid Zn2+ Zinc Zn2+ Hydrochloric Acid Solution e- H+ - Cl- H+ e H+ H2 H+ H+ Cl- - H+ e Zn Zn2+ + 2 e – - 0.763 V - e H+ + – H2 2 H + 2 e H2 H+ Steel Fe Fe2+ + 2 e – - 0.440 V Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Galvanic Corrosion • The greater the difference in corrosion potential between two metals is, the greater the galvanic corrosion effect is. Cathodic Gold (less corrosive) Graphite Silver 18-8 Stainless Steel Nickel Galvanic series of Monel (70 Ni, 30 Cu) some metals in Bronze (Cu-Sn) seawater Copper Brass (Cu-Zn) (Corrosion Engineering, Tin Fontana and Greene, 1978) Lead Tin-lead solder Steel or iron Aluminum Anodic Zinc (more corrosive) Magnesium Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives General prevention • Eliminate one of the conditions necessary for galvanic corrosion. – Corrosive environment Cathodic Gold – Dissimilar materials (less corrosive) Graphite Silver – Contacting surfaces 18-8 Stainless Steel Nickel • Coating Monel (70 Ni, 30 Cu) • Material matching Bronze (Cu-Sn) Copper • Size ratio of Brass (Cu-Zn) Tin anode/cathode Lead Tin-lead solder • Add sacrificial element Steel or iron even more corrosive Aluminum Anodic Zinc (more corrosive) Magnesium Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives EMI Gaskets and Galvanic Corrosion • Typically a concern with aluminum frames used in outdoor environment: base stations, aircraft, marine. • Cannot avoid contact with Cathodic Gold aluminum because electrical (less corrosive) Graphite contact is required. Silver 18-8 Stainless Steel • The fillers that are most Nickel Copper galvanically similar to Anodic aluminum tend to oxidize and Steel or iron (more corrosive) Aluminum 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 aluminum and resistant to oxidation. Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Unique 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 Potential Salt Spray Corrosion Material Volts vs. SCE Silver filled silicone elastomer -0.086 Silver-nickel filled silicone elastomer -0.100 Silver wire -0.106 Nickel-graphite filled silicone elastomer -0.157 Silver-copper filled silicone elastomer -0.161 Silver-glass filled silicone elastomer -0.281 Nickel metal -0.367 Nickel filled silicone elastomer -0.411 Silver-aluminum filled silicone elastomer -0.746 Aluminum metal -1.181 * A.R. Pawlowych, “Galvanically compatible elastomeric gasketing material for EMI shielding applications”. Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Repetition of salt spray experiment • Aluminum corrosion with fluorosilicone gaskets 700 Rank by Electrode g) 600 Potential m ( s 500 Ag/Nickel Ag/Nickel Ag/Nickel-graphite Los --- 400 Ag/Glass ght Ag/Copper i e 300 Ag/Copper Ag/Glass W Nickel Graphite Ni Graphite num 200 i Ag/Aluminum Ag/Aluminum um l 100 A 0 0 200 400 600 Hours of Salt Spray Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives A new filler for EMI shielding • Objective – create a non-silver conductive particle with low corrosion properties but galvanically similar to aluminum. • Untreated aluminum power is not conductive due to the natural oxide layer on the surface. • 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. Tungsten Aluminum carbide Aluminum oxide layer Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Galvan-O-Free Product Details GF-45 GF-60 FIP Grade Molding Grade Particle Size, µm D10 > 15 > 40 D50 35 – 50 55 – 80 D90 < 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 Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives WC-Aluminum Corrosion Testing Aluminum Corrosion in Salt Spray 1000 g) 100 m ( WC-Aluminum s Ag/Aluminum Los Ni-Graphite ht 10 g i e W 1 0 200 400 600 Hours of Exposure WC-Aluminum causes significantly less galvanic corrosion on aluminum than nickel/graphite and silver/aluminum Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives WC-Aluminum in Fluorosilicone Gasket 100 90 ) 85 m ) -c Ω e A ( 10 80 ity hor iv S t s i 75 s e R dness ( e 1 70 r a m H lu o V 65 0.1 60 66 68 70 72 74 76 Filler (wt%) Resistivity Aged Resistivity Hardness Aging = 48 hours at 200C Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives WC-Aluminum Shielding Effectiveness Shielding Effectiveness 140 120 100 ) B 80 d 60 SE ( 40 20 0 30 40 60 80 100 200 400 600 800 1 246810 MHz GHz Frequency WC-Aluminum has excellent shielding effectiveness between 30MHz and 10 GHz Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Theory 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 Pore A C Pore WC B D Aluminum Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives WC-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. Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Additional considerations • Blending WC-Aluminum and Ag/Aluminum 100 g) m ( s Ag/Aluminum Los ht 60:40 Ag/Al : WC-Al g i 10 e 50:50 Ag/Al : WC-Al W WC-Aluminum num i um l A 1 0 200 400 600 Hours of Salt Spray The amount of aluminum corrosion caused by Ag/Aluminum can be reduced by addition of WC-Aluminum Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Additional considerations • Blending WC-Aluminum and Ag/Aluminum 1000 ) m c - m h O 100 ( y Ag/Aluminum it iv t 60:40 Ag/Al : WC-Al s i s WC-Aluminum e R 10 me lu Vo 1 0 200 400 600 Hours of Salt Spray The gasket resistivity of WC-Aluminum can be significantly decreased by addition of Ag/Aluminum Potters Industries Inc. an affiliate of PQ Corporation PottersPotters ConductivesConductives Summary 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.
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