The element titanium was discovered in 1793. It is a subgroup IV element in the fourth period of the periodic table. Titanium in English is the deified Hercules in Italy. It is represented by Ti. Titanium and its alloys have two crystals: α and β. The former is hexagonal close-packed and the latter is body-centered cubic. The industrial pure titanium commonly used in the pressure vessel industry belongs to alpha lattice titanium at room temperature.
Industrial pure titanium has good corrosion resistance to seawater, marine atmosphere, wet chlorine, chloride, hypochlorous acid, sulfide, sulfate, most oxidizing acids, and organic compounds, so it has a wide range of applications in the chemical industry use.
The corrosion resistance mechanism of titanium is that titanium combines with oxygen at room (low) temperature to form a strong and dense passivation oxide film on the surface, which can prevent the corrosive medium from contacting titanium, thus making titanium corrosion resistant. However, this oxide film is only protective if it is formed at low temperatures. The oxide film formed at high temperatures will be loose, porous, and decomposed. Oxygen atoms will use the oxide film as a conversion layer to enter the metal lattice, further increasing oxidation and thickening the oxide film. , the oxide film at this time has no protective properties. Our welding process is a heating process, so preventing titanium from high-temperature oxidation during the welding process is an important task.
Titanium will produce different colors when heated in the air and react with oxygen at different temperatures. It is silvery white with bright metallic luster below 200℃, light yellow (light straw yellow) at 300℃, and golden yellow (golden yellow) at 400℃. Dark straw yellow), 500℃ is purple, 600~700℃ is dark blue~light blue, 700~800℃ is reddish gray, 800~900℃ is reddish gray, 900~1000℃ is sauce yellow, and above 1000℃ in order It is dark gray to white powder until it peels off, as shown in Figures 1 to 3. Figure 2 is a photo of the actual titanium welding oxidation test. The heat source is at the center of the back.
The thickness of the oxide film on the titanium surface is also different at different temperatures, as shown in Table 1:
| Temperature | 316~538 | 649 | 704 | 760 | 816 | 871 | 927 | 982 | 1038 | 1093 |
| Oxide film thickness | Extremely thin | 0.005 | 0.0076 | <0.025 | <0.026 | <0.035 | <0.051 | <0.051 | <0.102 | <0.356 |
Through the oxidation color of the titanium weld surface, you can quickly determine at what temperature the weld is oxidized and the approximate oxide film thickness. Since the oxide film of titanium after high-temperature oxidation has a great impact on the performance of the weld, we generally require that the oxidation color should be silver white or light straw yellow, and other colors should be removed and not mixed into the weld.
