Introduction
Base dental alloys contain mainly base metals, which oxidize and dissolve in acids. Their essential elements are cobalt, nickel, iron and titanium. They are saved by a passivating oxide layer on the surface, which limits corrosion and gives acceptable biological tolerance.
General properties
- Strengths: the highest mechanical properties, so restorations can be very thin. Price is about 40 times lower than noble alloys, and thermal and electrical conductivity are low.
- Weaknesses: hard to cast, solder and cold-work. Some components can be carcinogenic, mutagenic or allergenic.
Dental steels
Stainless steels add 13 to 30% chromium, which forms a protective oxide layer. Adding 8 to 11% nickel gives austenitic steel. A typical brand, 1/18/9, has 0.1% carbon, 18% chromium and 9% nickel.
- Austenite is the best structure for dental use: the most corrosion resistant, fine-grained and non-magnetic.
- The problem: it is unstable. Slow cooling or mechanical work turns it into a ferrite-perlite structure with chromium carbides, which is rigid and corrodes easily.
- The cure: austenitizing. Heat to 1000 to 1150 °C for 15 to 20 minutes, then cool quickly in 25% sodium chloride solution.
- Titanium and niobium bind carbon first, so chromium stays free to protect the surface.