Silver galvanic corrosion

Potters Industries, Ine. developed a new aluminum compatible particles which can be used in gaskets in contact with aluminum. If silver in the gasket were to come in contact with the aluminum of the enclosure, galvanic corrosion may result. The aluminum compatible grade was found to pass 3000 hours in a salt spray chamber without loss of shielding effectiveness. 

Special considerations in conductive applications, special conductive blacks must be employed in silver containing gaskets, galvanic corrosion is a problem, attention should be given to material with which diield is connected (potential difference), with zinc or aluminum casing nickel filled materials are preferred... 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

Technical metals that contain impurities of other metab Examples are technical metals such as zinc and tin, which contain silver or iron as impurities. The standard electrode potentials of silver and iron are more electropositive than those of zinc or tin, causing zinc and tin to corrode through a form of galvanic corrosion, while the impurities remain intact. Because of their more positive electrode potentials, silver and iron are cathodicaUy protected by zinc or tin. 

Fig. 17. Schematic depicting the effect of galvanic corrosion of a nonnoble metal pad on electrical conduction of a silver-filled ICA. (a) Good electrical conduction before corrosion, (b) Poor electrical conduction due to the formation of a metal hydroxide or oxide formation as a result of galvanic corrosion.

Schematic illustration of flexible metal mirror design. Sol-gel coatings serve several protective roles the planarization layer prevents galvanic corrosion of the Fe-Ag couple in humid environments the protective film prevents abrasion or corrosion of the underlying silver reflective surface. From Ashley and Reed (71,72).

Aluminum Foil. Studies of various foods wrapped in aluminum foil show that food products to which aluminum offers only fair resistance cause little or no corrosion when the foil is in contact with a nonmetallic object (glass, plastic, ceramic, etc.) The reactions, when found, are essentially chemical, and the effect on the foil is insignificant. However, when the same foods are wrapped or covered with foil that is in contact with another metallic object (steel, tinplate, silver, etc.), an electrochemical or galvanic reaction occurs with aluminum acting as the sacrificial anode. In such cases, there is pitting corrosion of the foil, and the severity of the attack depends primarily on the food composition and the exposure time and temperature. Results obtained with various foods cov-... 

Zinc (Zn) Zinc is a bluish silver metal. Medical applications of zinc compounds include calamine lotion and zinc oxide (sun block). Galvanized steel is resistant to corrosion and is prepared by coating iron with a thin layer of zinc. 

Galvanized steel plates showed some visual differences between cities, as seen in Figure 8. Some differences were also observed within the same city (e.g. Figure 9). These plates were cleaned using an ammonia wash followed by a chromic acid and silver nitrate dip (ASTM G1). Mils per year penetration rates are shown in Table III. Decreasing corrosivity follows the order ... 

Incompatibilities and Reactivities Strong oxidizers, acids, halogens, salts of silver zinc [Note Corrosive to copper galvanized surfaces.] ... 

---