Zirconium alloys are highly resistant to corrosion in most organic acids, inorganic acids, salt solutions, strong alkalis, and molten salts. But zirconium alloy has better corrosion resistance than titanium in hydrochloric acid system. Zirconium alloys are expensive -- four times as expensive as graphite and two times as expensive as titanium -- but in corrosive environments, graphite can withstand high temperatures, but not high pressures. With zirconium alloy, the pressure gauge buffer tube extends the service life of the equipment, reduces maintenance costs, reduces downtime, and eliminates product contamination caused by corrosion products. A great deal of experience shows that zirconium alloys show economic superiority in high temperature and corrosive environment.
Two zirconium base alloys, Zr702 and Zr705, are commonly used in chemical industry. Its chemical composition is shown in table 1. Zr702 and Zr705 contain 99.2% zirconium and 95.5% hafnium respectively, while the larger content of hafnium is 4.5%. Zr705 is a zirconium - niobium alloy.
Zr702 can be used in harsh corrosive environments, such as sulfuric acid above 55% concentration; Zr705 can be used for less corrosive occasions, its price is cheaper than Zr702. Zirconium has a very high affinity for oxygen, in the air, room temperature can form a protective oxidation film, which has a strong corrosion resistance. Zr702 is a densely arranged hexagonal lattice structure (alpha) at a temperature less than 865℃, which will be converted to a body-centered cubic lattice structure (beta) at a higher temperature. When the temperature is less than 920℃, Zr705 is a two-phase system composed of the above two lattice structures. When the adjustable shrinkage cavity is higher than this temperature, it is converted into the body-centered cubic lattice structure (beta).
Because of this property, usually rolled zirconium alloys are anisotropic, that is, some physical and mechanical properties of zirconium alloys vary with the direction of the material. Therefore, the metallurgical products of zirconium alloy should be supplied in annealed state. Pipe wall thickness should be determined according to calculation. Where necessary, pipes of the same diameter may have different wall thicknesses in the same grade for economic reasons.
For threaded connections required for blow-out and purge, the hood type vent shall have a smaller wall thickness as specified in the standard. A threaded platform and nipple can be used, and it is unnecessary to increase the wall thickness of all pipelines.
Due to the strict processing requirements of zirconium alloy materials, the principle of equal wall thickness should be applied in butt welding joints. As far as possible to avoid welding processing due to different wall thickness caused by additional processing.
