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Titanium and its alloys are preferred for dental implants because they have much better corrosion resistance and mechanical properties than stainless steel. However, its corrosion resistance, which goes down in acidic environments (acidic pH), requires a breakthrough. Indonesian people like to consume foods and drinks that contain a lot of acidic substances (with low pH), such as Pempek, fried foods, chili sauce, soft drinks and carbonated drinks. This is the background of research to determine the corrosion behavior and corrosion rate of titanium and its alloys in an acidic environment. The results expectedly would help dentists select materials with higher corrosion resistance for orthodontic applications. We selected Ti-6Al-4V Extra Low Interstitial (ELI) as the object of this research. The specimens were immersed in Kokubo's Simulated Body Fluid (SBF) solution, a solution modified to resemble human saliva (pH 5.0) in four variations of time (t): 1 week, 2 weeks, 3 weeks, and 4 weeks. The corrosion rate was calculated by the weight loss method. To improve corrosion resistance in this strong acid environment, the specimens were coated with commercial nano-sized hydroxyapatite (HA). To examine the microstructure of corrosion, an optical microscope and a Scanning Electron Microscope (SEM) were used. The results showed that HA coating on Ti-6Al-4V ELI could reduce the corrosion rate. In the fourth week, the corrosion rate of Ti-6Al-4V ELI without HA coating was 0.07575 mm/year. Meanwhile, at the same immersion duration, the corrosion rate of Ti-6Al-4V ELI coated with HA was 0.00841 mm/year. HA can protect the TiO2 passive layer on the Ti-6Al-4V ELI so that the implant material is not easily eroded when installed in the body. Implant materials that are more corrosion resistant are good for implants for their good osseointegration. [ABSTRACT FROM AUTHOR]

The study is intended to investigate the heat loss phenomenon of oil flow in a riser. This heat loss happens due to the difference between the oil temperature in a riser and the surrounding sea water temperature. It causes the formation of wax that may disturb the flow. Heat loss can be reduced by setting up an insulator in a riser or by selecting appropriate pipeline specifications. It is necessary to determine the possible locations and specifications of insulator and pipeline. A mathematical model is formulated by considering the oil temperature and its flow velocity. Assuming that the density variation is small, the fluid behaves as an incompressible fluid. Furthermore, numerical solutions with finite difference methods are presented with some hypothetical data to give an overview of how the system works. Two surrounding conditions are taken into account, i.e. with and without sea current. From the simulation, the location of wax formation can be predicted. At a certain depth region of sea, where the sea current is present, a greater heat loss take place in which wax may be formed immediately. To overcome the formation of wax, we can control the parameters such as conductivity and wall thickness of pipe. [ABSTRACT FROM AUTHOR]