Effect of Heat History on the Corrosion of Magnetic Attachments

This study investigated the effect of heat history on the corrosion of keepers used for dental magnetic attachments. Ferritic stainless steels of SUS 444 and 447J1 were prepared with heat treatments in the temperature range of 550-850t for 1-5 hours. The stainless steels were electrochemically and metallurgically examined by anodic polarization curves in a 0.9% NaC1 solution and by microstructural observation using an electron probe microanalyzer with WDS. Heating both kinds of stainless steel at 650-750°C for two hours or more led to the deterioration of their corrosion resistance. For example, there was evidence of a reduction in the breakdown potentials and an increase in the current densities of the anodic polarization curves. These phenomena were attributed mainly to the precipitation of the o (FeCr) or X (Fe18Cr6Mo5) phase, which sometimes resulted in intergranular corrosion. When dental alloys are cast in investment molds embedding the keepers, the heat time required for thermal expansion of the molds should be kept under one hour.