Existence of an electrically insulating layer in amalgam-containing galvanic couples.

Objective: Avoiding the placement of amalgam and noble metal restorations in interproximal contact is recommended due to anticipated galvanic corrosion of the amalgam. There is a similar concern for amalgam/amalgam galvanic couples. It was the objective of this study to determine if an electrically insulating layer forms in the contact area of these galvanic couples. The existence of an electrically insulating layer, which could reduce the galvanic corrosion rate, would be indicated by different corrosion potentials for the two restorations of the couple.

Methods: Using a convenience sample of 158 human subjects, corrosion potentials were measured on each restoration of three types of galvanic couples: amalgam/noble metal (n=8), amalgam/amalgam (n=93) and noble metal/noble metal (n=7). Measurements were made with a Ag/AgCl micro-reference electrode and a high impedance voltmeter. All restorations were at least 6 months old.

Results: Statistical analysis showed that the mean absolute corrosion potential differences and the simultaneous confidence intervals of the couples were, respectively, amalgam/noble metal: 62 (31)mV and (27, 99)mV, amalgam/amalgam: 11 (14)mV and (7, 15)mV and noble metal/noble metal: 7 (10)mV and (0, 19)mV.

Significance: The amalgam/noble metal couples had consistent and mostly large corrosion potential differences between their restorations, which indicated the presence of an electrically insulating layer. An electrically insulating layer was also indicated for the amalgam/amalgam and noble metal/noble metal couples. The layer is probably composed of non-metallic corrosion products, biofilms, and possibly, dental calculus, which could reduce galvanic corrosion rates to small or negligible values.