Surface degradation of amalgams in vitro during static and cyclic loading.

Corrosion during static creep of two gamma 2-containing and one dispersed phase amalgam immersed in artificial saliva has been observed, using SEM, to occur in gamma 1-grain boundary regions where grain boundary sliding and dislocation slip take place. Cracking in the interior of gamma 1-grains has also been found, in particular for dispersed non-gamma 2 amalgam. The combined action of corrosion and stress favored the formation of cracks. Signs of similar creep processes which may promote cracks have also been noted after cyclic loading where the accumulative strains were much lower than in the static creep experiments. Due to these low strains and short immersion times corrosion attacks of the same kind as under static loading can only be assumed. Using nuclear tracer technique quantitative measurements of the corroded material from samples exposed to cyclic loading showed the dispersed non-gamma 2 amalgam to release more Hg, Cu and Zn than the two gamma 2-containing amalgams and that 50-90% copper and mercury were present as particulate matter. These observations can be explained on the basis of different tendencies to cracking in the amalgams. Cyclic loading increases the rate of degradation by one or two orders of magnitude compared with corrosion without loading. Reported studies of marginal degradation in non-gamma 2 amalgams compared with gamma 2-containing amalgams do not agree with the present in vitro experiments.