Zirconium resistance

Zirconium resists attack by nitric acid at concentrations up to 70 wt % and up to 250°C. Above concentrations of 70 wt %, zirconium is susceptible to stress-corrosion cracking in welds and points of high sustained tensile stress (29). Otherwise, zirconium is resistant to nitric acid concentrations of 70—98 wt % up to the boiling point. 

For example, when field tests were conducted on zirconium and other materials in acetic acid environments, zirconium was the only metal tested to exhibit excellent corrosion resistance under most conditions [i]. However, severe general corrosion and pitting occurred in acetic acid when the acid contained both acetic anhydride and copper ions. The presence of copper ions was the result of the corrosion of copper specimens, which were tested at the same time. Zirconium resists attack by a wide range of acetic acid and anhydride environments when certain contaminates, such as copper ions, are avoided [2]. 

Zirconium resists attack in H3PO4 at concentrations up to 55% and temperatures exceeding the boiling point. Above 55% H3PO4, the corrosion rate could increase greatly with increasing temperature Figure 22.12. The most useful area for zirconium would be dilute acid at elevated temperatures. Zirconium outperforms common stainless alloys in this area. ... 

Zirconium resists attack in most alkalies, including sodium hydroxide, potassium hydroxide, calcium hydroxide, and ammonium hydroxide. 

Zirconium resists attack in some molten salts. It is very resistant to corrosion by molten sodium hydroxide to temperatures above 1000°C. It is also fairly resistant to potassium hydroxide. The oxidation properties of zirconium in nitrate salts are similar to those in air. 

Zirconium resists some types of molten metals, but the corrosion resistance is affected by trace impurities, such as oxygen, hydrogen, or nitrogen. Zirconium has a corrosion rate of less than 1 mpy in liquid lead to bOO C, lithium to 800°C mercury to 100°C, and sodium to 600°C. The molten metals known to attack zirconium are aluminum, zinc, bismuth, and magnesium. 

Reactor-grade zirconium is essentially free of hafnium. Zircaloy(R) is an important alloy developed specifically for nuclear applications. Zirconium is exceptionally resistant to corrosion by many common acids and alkalis, by sea water, and by other agents. Alloyed with zinc, zirconium becomes magnetic at temperatures below 35oK. 

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines. 

Flame-Retardant Treatments For Wool. Although wool is regarded as a naturally flame-resistant fiber, for certain appHcations, such as use in aircraft, it is necessary to meet more stringent requirements. The Zirpro process, developed for this purpose (122,123), is based on the exhaustion of negatively charged zirconium and titanium complexes on wool fiber under acidic conditions. Specific agents used for this purpose are potassium hexafluoro zirconate [16923-95-8] [16923-95-8] K ZrF, and potassium hexafluoro titanate [16919-27-0], K TiF. Various modifications of this process have been... 

Both zirconium hydride and zirconium metal powders compact to fairly high densities at conventional pressures. During sintering the zirconium hydride decomposes and at the temperature of decomposition, zirconium particles start to bond. Sintered zirconium is ductile and can be worked without difficulty. Pure zirconium is seldom used in reactor engineering, but the powder is used in conjunction with uranium powder to form uranium—zirconium aUoys by soHd-state diffusion. These aUoys are important in reactor design because they change less under irradiation and are more resistant to corrosion. 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

Zirconium is used as a containment material for the uranium oxide fuel pellets in nuclear power reactors (see Nuclearreactors). Zirconium is particularly usehil for this appHcation because of its ready availabiUty, good ductiUty, resistance to radiation damage, low thermal-neutron absorption cross section 18 x 10 ° ra (0.18 bams), and excellent corrosion resistance in pressurized hot water up to 350°C. Zirconium is used as an alloy strengthening agent in aluminum and magnesium, and as the burning component in flash bulbs. It is employed as a corrosion-resistant metal in the chemical process industry, and as pressure-vessel material of constmction in the ASME Boiler and Pressure Vessel Codes. 

Corrosion Resistance. Zirconium is resistant to corrosion by water and steam, mineral acids, strong alkaUes, organic acids, salt solutions, and molten salts (28) (see also Corrosion and corrosion control). This property is attributed to the presence of a dense adherent oxide film which forms at ambient temperatures. Any break in the film reforms instantly and spontaneously in most environments. 

Zirconium is completely resistant to sulfuric acid up to Foiling temperatures, at concentrations up to 70 wt %, except that the heat-affected zones at welds have lower resistance in >55 wt % concentration acid (Fig. 1). Fluoride ions must be excluded from the sulfuric acid. Cupric, ferric, or nitrate ions significantly increase the corrosion rate of zirconium in 65—75 wt % sulfuric acid. 

Zirconium is not attacked by caustics up to boiling temperatures. It is resistant to molten sodium hydroxide to 1000°C, but is less resistant to potassium hydroxide. 

Zirconium is totally resistant to corrosion by organic acids. It has been used in urea-production plants for more than two decades. 

Zirconium is totally resistant to attack of hydrochloric acid in all concentrations to temperatures well above boiling (Fig. 2). Aeration has no effect, but oxidizing agents such as cupric or ferric ions may cause pitting. Zirconium also has excellent corrosion resistance to hydrobromic and hydriodic acid. 

Zirconium oxide is fused with alurnina in electric-arc furnaces to make alumina—zirconia abrasive grains for use in grinding wheels, coated-abrasive disks, and belts (104) (see Abrasives). The addition of zirconia improves the shock resistance of brittle alurnina and toughens the abrasive. Most of the baddeleyite imported is used for this appHcation, as is zirconia produced by burning zirconium carbide nitride. 

Hafnium-free zirconium is particularly weU-suited for these appHcations because of its ductiHty, excellent oxidation resistance in pure water at 300°C, low thermal neutron absorption, and low susceptibiHty to radiation. Nuclear fuel cladding and reactor core stmctural components are the principal uses for zirconium metal. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

---