Corrosion zirconium

Alkaline corrosion. Zirconium is resistant to corrosion by almost all alkalies, both in solution and in the fiised condition, up to the boiling point. It resists sodium and potassium hydroxide solutions even under anhydrous conditions and resists molten potassium hydroxide and molten sodium hydroxide, the latter at temperatures greater than lOOO C. It resists calcium and ammonium hydroxides at concentrations up to 28% up to boiling. Because it is resistant to both alkalies and acids, it is the preferred material of construction for processes that cycle between acid and alkaline solutions. 

Aqueous media and marine corrosion. Zirconium has excellent corrosion resistance to seawater, fresh water, brackish water, and other polluted water streams and is a material of choice for heat exchangers, condensers, and other equipment handling these media, where it can replace titanium-palladium alloys. Unlike titanium and its alloys, zirconium is highly resistant to crevice corrosion. With their high corrosion resistance to pressurized water and steam, low neutron absorption (with low hafiiium content), good mechanical strength, and ductility, at nuclear reactor service temperatures, and their ability to remain stable even after extensive radiation, zirconium alloys are used extensively in fuel cladding, fuel channels, and pressure tubes for... 

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. 

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. 

Shipment and Storage. The weight per gallon is 5.6 kg. MSG is shipped and stored in 55-gaHon (0.208 m ) steel dmms or 5-gaHon (0.02 m ) steel pads with a polyethylene liner. Anhydrous MSC is also corrosive toward titanium, titanium—palladium, and zirconium as measured in metal strip tests at 50°C. It is classified as a corrosive Hquid. 

Because of its low neutron absorption, zirconium is an attractive stmctural material and fuel cladding for nuclear power reactors, but it has low strength and highly variable corrosion behavior. However, ZircaHoy-2, with a nominal composition of 1.5 wt % tin, 0.12 wt % iron, 0.05 wt % nickel, 0.10 wt % chromium, and the remainder zirconium, can be used ia all nuclear power reactors that employ pressurized water as coolant and moderator (see... 

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 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. 

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. 

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. 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

Zirconium, too, is produced commercially by the Kroll process, but the van Arkel-de Boer process is also useful when it is especially important to remove all oxygen and nitrogen. In this latter method the crude zirconium is heated in an evacuated vessel with a little iodine, to a temperature of about 200° C when Zrl4 volatilizes. A tungsten or zirconium filament is simultaneously electrically heated to about 1300°C. This decomposes the Zrl4 and pure zirconium is deposited on the filament. As the deposit grows the current is steadily increased so as to maintain the temperatures. The method is applicable to many metals by judicious adjustment of the temperatures. Zirconium has a high corrosion resistance and in certain chemical plants is preferred to alternatives such as stainless... 

Gillman, V. A. and Kolotyrkin, Ya. M., The Mechanism of Zirconium Pitting in Halide Solutions , Dokl. Akad. Nauk. SSSR, 143, 640 (1962) C.A., 57, 3184a Videm, K., Pitting Corrosion of Aluminium in Contact with Stainless Steel , Kjeller Rept. KR-18, 16(1962) C./l., 57, 8315a... 

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