The crevice corrosion sensitivity of titanium is also related to the size of the crevice. The possibility of crevice corrosion in a narrow gap is greater than that in a wide gap. When titanium is in contact with non-metallic materials, the crevice corrosion tendency is much greater than that in the Ti Ti type crevice. In fact, most of the common crevice corrosion in the equipment also occurs on the flange sealing surface in contact with the non-metallic gasket. Crevice corrosion also occurs in titanium tubes in hydrochloric acid, sulfuric acid, oxalic acid, and formic acid solutions. Since various titanium pipes and titanium alloys have unique characteristics in physical, chemical, mechanical, corrosion resistance and process performance, the structure of commonly used equipment made of ferrous metals and other non-ferrous metals cannot be blindly applied in the structural design. According to the performance characteristics of titanium materials, the general provisions for the structural design of titanium equipment are proposed.
1. Because the mechanical properties of titanium and titanium alloy are different from those of steel, the yield strength ratio of titanium material is high, the plastic deformation range is narrow, and the spring back is large during cold stamping and cold bending. Therefore, the structure of titanium equipment is required to be simple. At the same time, the good structure is also convenient for cleaning the surface near the welding joint, and gas shielded welding is used to protect the quality of the front and back welding joints.
2. Titanium can be welded with Cuo, Ni, Dan, and lead metals without brittleness after welding because these metals are very soluble in titanium. However, the mutual fusion characteristics of titanium with steel and other metals are poor, so titanium and other metals cannot be directly welded. Only bonding, brazing, explosive welding, and bolting can be used for the connection.
3. The impact toughness and fracture toughness of titanium are poor, so the continuity of the structure and the smoothness of the welding joint shall be maintained during the design to avoid stress concentration as much as possible.
4. Pure titanium is prone to crevice corrosion in chloride solution, but the crevice corrosion of titanium is closely related to temperature, chloride concentration, pH value, and crevice size.
5. The plastic deformation range of titanium is narrow and there is an obvious work hardening phenomenon. Therefore, the bending and flanging of titanium parts usually adopt a large bending radius, and the pipe expansion rate is small.
In seawater and NaCl brine, titanium will not corrode when the temperature is below 149 ℃. When the temperature exceeds 121 ℃, titanium may corrode in the extremely narrow gap, especially at the non-metallic gasket. However, when the temperature exceeds 149 ℃, titanium may corrode in the wider gap, such as the gap between the tube and the tube sheet. When there are relatively hard chloride deposits on the metal surface, the effective concentration of chloride under the deposits will be equivalent to the chloride solubility at the pipe wall temperature. Moreover, due to the thermal insulation effect of the crud, the temperature can rise greatly. Therefore, the crud is also a field prone to crevice corrosion. The higher the temperature and chloride concentration, the greater the crevice corrosion tendency of titanium
