Stability of dental alloys in artificial saliva: an electrochemical and XPS investigation

Alloys used in dentistry should have good mechanical properties and a very high corrosion resistance in order to be considered biocompatible. The oral cavity is a potentially highly corrosive environment thus stainless steels have been used frequently. Due to allergic reactions of about 10% of the population to nickel ions new nickel-free stainless steels have been developed. In this PhD thesis the Ni-free stainless steel DIN 1.4456 has been studied with electrochemical and XPS surface analytical techniques at 25°C and at 37°C. Surface analysis has shown that the alloy in artificial saliva forms a protective passive film at both temperatures. At 25°C the oxy-hydroxide film formed is enriched in oxidized chromium, after long immersion times (7 days) the nominal composition of oxidized manganese (18%) is found. Molybdenum in the film is slightly enriched. Angular resolved XPS performed on samples exposed at 37°C clearly indicate that the outer part of the passive film is composed essentially of iron oxy-hydroxide whereas chromium oxy-hydroxide is located at the inner part. Experiments with argon ion sputtering confirm that the inner part of the film is enriched in oxidized chromium and manganese whereas oxidized iron is strongly depleted. Both angle-resolved XPS and experiments with argon ion sputtering show that the alloy beneath the passive film is depleted in manganese. The kinetics of initial dissolution and film formation are more rapid at 37°C compared to 25°C but seem to follow the same mechanism. At 37°C the initial corrosion rate is much higher but its decrease with time is more rapid. After 24 h the dissolution rate is already lower then 0.2 μm/year, the steady state dissolution rate will be at least one decade lower. This very low dissolution rate has been confirmed by ICP solution analysis where the metal ion concentration was found to be below the detection limit of the technique for all the alloy elements. Finally a model is proposed that might explain the surface films formed after exposure to artificial saliva solution. During the short initial period with a relatively high corrosion rate iron and especially manganese (non-noble elements) dissolve. Chromium is the film-forming element and an inner chromium oxy-hydroxide film is formed. This film limits progressively the dissolution of the alloy: with time an outer iron oxy-hydroxide film is formed. Due to the dissolution of manganese, the manganese content immediately below the film is strongly depleted. On the contrary, molybdenum is present with about 7%. Such a layered structure of the surface is responsible for the high corrosion resistance and biocompatibility of the DIN 1.4456 stainless steel