In most highly corrosive media, zirconium has better corrosion resistance than titanium. This is why zirconium materials are more expensive than titanium materials, but they are still used in many chemical equipment. Since zirconium and hafnium often coexist in zirconium ores, the content of hafnium in zirconium ores is about 2%-3% of the zirconium content. The physical and chemical properties of zirconium and hafnium are similar in all aspects. It is necessary to obtain zirconium ores from which zirconium and hafnium coexist. The cost of producing zirconium with a hafnium content of less than 0.01% is high, and because the physical and chemical properties of zirconium and hafnium are very similar, it is not necessary to reduce the hafnium content in the production of chemical grade zirconium. Generally, the hafnium content in zirconium does not exceed 4.5% will do.
Zirconium is also a passivating metal and can easily combine with oxygen to form a dense passivation film on the surface. This makes zirconium resistant to corrosion by most organic acids, inorganic acids, strong alkalis, molten salts, high-temperature water and liquid metals. It has excellent corrosion resistance in hydrochloric acid of all concentrations below the normal pressure boiling point, but there is a possibility of hydrogenation in hydrochloric acid above 149°C. Zirconium can be used in nitric acid at temperatures below 250°C and with a mass fraction of no more than 70%, but it is prone to ignition in concentrated nitric acid with very little water content. Zirconium is corrosion-resistant in organic acids, but not corrosion-resistant in hydrofluoric acid, concentrated sulfuric acid, concentrated phosphoric acid, aqua regia, bromine water, hydrobromic acid, fluorosilicic acid, calcium hypochlorite, and fluoroboric acid. It is not corrosion-resistant in oxidizing chloride solutions such as copper chloride and ferric chloride, but is corrosion-resistant in reducing chloride solutions.
When zirconium is in the air, it will peel severely at 425°C, generate white zirconium oxide at 540°C, and absorb oxygen and become brittle above 700°C. Zirconium reacts with nitrogen above 400°C and reacts strongly around 800°C. Vacuum annealing cannot remove oxygen and nitrogen from zirconium. Zirconium begins to absorb hydrogen above 300°C, causing hydrogen embrittlement. The hydrogen can be eliminated by vacuum annealing at 1000°C.
