Dental materials are biomaterials used in dental clinics for the treatment of diseases at oral and maxillofacial regions. Since tooth is very poor in its natural healing potential, dental materials are commonly used to reconstruct its function and morphology. However, circumstance of the dental materials are very severe since they will be suffered high and low temperature and pH cycling as wee as occlusal force. In addition, they will be exposed to bacteria. In this paper, present situation and current problems of dental materials will be reviewed based on the several dental treatment steps.
The corrosion of Al (99.99% in mass) in deaerated methanol with and without H2O, NaCl, and HCOOH as contaminants was investigated using immersion corrosion tests, potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The content of H2O in deaerated methanol was changed from 0.1% to 30%, that of NaCl from 0% to 0.1%, and that of HCOOH from 0% to 0.1%. The immersion corrosion tests showed that Al did not suffer from corrosion in deaerated methanol containing 0.1-30% H2O. When 0.1% NaCl was added to methanol containing 0.1-30% H2O, the corrosion occurred only at 0.1% H2O. The EIS measurement endorsed that corrosion resistance of Al was higher in methanol containing 30% H2O than that in methanol containing 0.1% H2O. In case that 0.1% HCOOH was added, the corrosion occurred only at 30% H2O. Potentiodynamic polarization curves obtained in deaerated methanol with 0.01-0.1% NaCl showed that pitting potential increased with increasing H2O content. In this case, the passive region became obvious with increasing H2O content. The passive films composed of Al2O3 and Al(OH)3 were identified by XPS.
We have investigated the effect of sulfate ion on corrosion behavior of SM 400B carbon steel in carbonate/bicarbonate solutions at 343K by using a conventional potentiokinetic method with potential scan rate of 0.1mV·s-1. In the solution containing sulfate less than 0.001kmol·m-3, there are two active anode peaks and passive region. Addition of sulfate more than 1.0kmol·m-3 gave catholic current between the two active peaks and lowered pitting potential. The active peak current density at lower potential increased linearly with increase in equilibrium concentration of bicarbonate. This suggests to result from the formation of iron complex with bicarbonate ion for the active dissolution of iron. Cathodic polarization curves of Pt in the test solution showed that the increase in the concentration of sulfate led to the linear increase in catholic current around the potential between two active peaks. Accordingly, it is concluded that sulfate ion can act as oxidant against the natural corrosion of the steel in this environment. Pitting potential of the steel was lowered with increase in the concentration of sulfate in concentrated region. In the same concentration of sulfate, the pitting potential increase with increase in pH. The trend suggests the retardation of the pitting is due to the solution properties of high pH and high buffer capacity.
Sulfidation properties of the alloys that include Ti of 1.5 to 2.0mass% and Al of 2.0 to 3.0mass% with basic composition of AISI 685, which is used as a rotor material of FCC power recovery turbine, were investigated at 873K in an atmosphere of PS2=10-0.5andPO2=10-18.5Pa. Sulfidation amount was decreased with increasing the Al content in the alloys of including the same level of Ti. The 1.5Ti-3.0Al, 2.0Ti-3.0Al and 1.5Ti-3.5Al alloys (mass%) showed the mass gain and scale thickness less than half of AISI 685. This is due to the diffusion control of metal ions and gases by the Al-rich oxide and/or sulfide in the innermost scale layer.
SCM 435 is a martensitic low alloy steel that is often used as a bolting material when high strength is required in high temperature domains up to approximately 600K. When bolts made of this steel are used, the threads are coated with a lubricant. Bolts used in wet steam (or hot pure water) sometimes suffer damage from stress corrosion cracking (SCC). In such environments, the SCC behavior will be influenced by the gases generated from the lubricant and the decomposition products soluble to water, together with the lubricant itself. This study was conducted to determine the effects of the lubricants and the S content of the steel on the stress corrosion cracking susceptibility of SCM 435, a bolting material, in high temperature deaerated pure water. The results are as follows. (1) No SCC was observed in SCM 435 after the test at 473K regardless of type of lubricants. Pb3O4 induced SCC in the steel at 523K although Ni and graphite did not, suggesting that Pb3O4 is stronger SCC promoter than the other two lubricants. (2) Corrosion pits ware observed at the crack initiation sites, suggesting that SCC process contains the following sub-processes; corrosion pit formation and growth to a critical size, followed by SCC initiation and propagation. The transition from pit to SCC occurred at K1SCC=18MPa·m0.5 in this case. (3) The number of corrosion pits decreased with decreasing S content in the steels. Thus if we assume that the SCC process mentioned above, decrease in S content will be very effective to mitigate SCC in SCM 435.