Corrosion of Gold and Amalgam Placed in Contact with Each Other

¹ Tokyo Medical and Dental University, Tokyo, Japan

Gold and amalgam restorations were brought into contact with each other under various conditions. Then they were stored in air, in artificial saliva, or in the human mouth for various periods of from 1 day to 6 months. The corrosion of the restorations was observed on their proximal surfaces. Vertical sections were observed grossly and microscopically. The flow of galvanic current between the two restorations and the Knoop hardness values of the sectioned surfaces of gold alloys were also investigated. Findings were as follows: (1) Contact of gold inlays with old amalgam fillings in air or saliva did not cause any significant corrosion of the restorations. Contact with fresh amalgam fillings produced silver-colored stains on the contact areas of gold inlays. However, they were of negligible thickness and of no clinical significance. No change in metallographic appearance, corrosion resistance, or hardness was observed in the underlying gold alloy. (2) Contact with fresh amalgam fillings less than 1 hour old in saliva produced silver-colored stains of negligible thickness on the contact areas of gold inlays. In addition, silver-colored circular swelling appeared in a newly produced layer around the contact areas. This layer had a finite thickness, and its bottom was slightly lower than the original level of the gold surface. The depth of such corrosion was, however, less than 10 µ and did not increase after 1 week. Such a reaction was considered to be of no clinical significance, since the corrosion was so slight and the shallow defect was filled with newly produced gold amalgam having fairly high corrosion resistance. No change in the metallographic appearances, corrosion resistance, or hardness was found in the gold alloy underlying this corrosion. (3) On amalgam filling surfaces no significant corrosion was found except a very faint circular roughness around the contact areas of amalgam fillings that had been brought into contact with gold immediately after filling and stored in saliva. This roughness was, however, readily removed by polishing and did not reappear upon further contact. A theory explaining the mechanism of reaction between gold and dental amalgam in saliva was proposed and discussed.