Zirconium alloys, corrosion rates

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

All processes still require use of oxygen for pasava-tion in the synthesis loop. Metallurgical advances have reduced the amount required. Snamprogetti now utilizes a bimetallic zirconium/25-22-2 (Ni, Cr, Mo) tube in its stripper. The corrosion rate for zirconium in urea service is nil. Toyo utilizes a duplex alloy (ferrite-austenite), which requires less ojqjgen. Stamicarbon working with the Swedish steel producer Sandvik, has patented a proprietary material called Saferex, which requires very little oxygen future plants wnll use this new material,... 

Zirconium, hafnium, and their alloys can be either highly reactive or highly corrosion-resistant. In an incompatible environment, corrosion may occur within a short period of time, i.e., a few hours to a few days. On the other hand, in a compatible environment, there may be little change in mass (gain or loss) for years. Consequently, the commonly suggested test duration in hours, which is 50 divided by the corrosion rate in mm/y or 2000 divided by the corrosion rate in mpy, may be excessive and impractical. For example, if the corrosion rate determined from a short-term test is... 

Figure 2-24. Rates of corrosion of zirconium alloys in boiling sulfuric acid.

Fig. 2-27 shows the rates of corrosion of Zr-702, Zr-705, and B-3 alloys in boiling solutions of hydrochloric acid. In contrast with the behavior of zirconium alloys in sulfuric and phosphoric acids (Figs 2-24 and 2-26), in hydrochloric acid the rate of corrosion of Zr-705 is lower than that of Zr-702 alloy. Anodic polarization curves of... 

Exposure to fast neutron fluxes increases the corrosion rate of zirconium-based alloys. Even in the highly oxidizing environment of a BWR coolant, however, the corrosion rate of the principal zirconium alloys is low enough that cladding corrosion is not a limiting factor on the life of the fuel element. 

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 appears to be nontoxic and biocompatible. It has very low corrosion rates in many media. It is ideal for making equipment used in food processing, in the manufacture of fine chemicals, and in pharmaceutical preparations. Zirconium and its alloys are suitable for certain implant applications as well. 

With regard to the zirconium-niobium alloy, it appears that its corrosion rate is not much affected by irradiation, but it is very dependent on the local chemistry and can be accelerated by oxidizing conditions. The corrosion rate is also dependent on heat treatment condition, and good resistance is shown by material aged for 72 hours at 500°C. With such material and with suitable chemical control, it is expected that, as with Zircaloy-2, there will be no problem due to loss of section and the important consideration is that of hydrogen absorption and its consequent effect on mechanical properties. 

Although zirconium and its alloys are costly compared with other common corrosion-resistant materials, their extremely low corrosion rates, resulting in long service life and reduced maintenance and downtime cost, make zirconium and its alloys quite cost effective. Table 8.45, which compares costs between S31600 stainless steel and various corrosion-resistant metals and alloys, shows that although R69702 is more costly than stainless steel. Inconel, and titanium alloys, it costs roughly the same as or less than some of the Hastelloys and considerably less than tantalum. 

Zirconium shows excellent corrosion resistance to hydrochloric acid and is superior to any other engineering metal for this application, with a corrosion rate of less than 0.125 mm y" at all concentrations and temperatures well in excess of the boiling point. Aeration does not affect its corrosion resistance, but the presence of oxidizing impurities such as cupric or ferric chlorides in relatively small amoimts will decrease it. Therefore, either these ions should be avoided, or suitable electrochemical protection should be provided. Zirconium also shows excellent corrosion resistance to nitric acid in all concentrations up to 90% and temperatures up to 200°C, with only platinum being equal to it for this service. Welded zirconium and its alloys retain this high corrosion resistance. In concentrated nitric acid, zirconium may exhibit SCC at nitric acid concentrations above 70%, if under high tensile stress. ... 

As expected, zirconium and zirconium alloys showed no resistance to attack by uranyl fluoride solutions at high temperatures. In fact, other tests have shown that as little as 50 ppm fluoride ions in uranyl sulfate solutions leads to appreciable attack of zirconium [25]. On the other hand, titanium demonstrated high resistance to uranyl fluoride solutions. Tests with more highly concentrated uranyl fluoride solutions have shown the corrosion rate of titanium to be low. The titanium alloys showed higher rates, ranging from 5 to 8 mpy. However, in crevices where oyxgen depletion occurs, titanium and its alloys are severely attacked [26]. 

In aqueous media where hydrochloric and hypochlorous acid and halogens are present in either vapor or liquid phase, the utility of the above materials of construction is severely lin ited and can best be determined by rates of corrosion study during pilot laboratory operation. Tantalum, zirconium, and titanium are usually resistant but expensive. The plastics are of variable resistance and are severely limited by temperature and solvent attack. Stoneware, Karbate, glass, glazed tile, carbon brick, and enameled steel all have utility within rigid limits. The other metals and alloys are usually questionable but may be desirable for replaceable piarts i... 

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