ZIRCONIUM ALLOYS

26.2 ZIRCONIUM ALLOYS

Because of good corrosion resistance in both acids and bases, zirconium alloys are widely used in chemical plants. Commercial zirconium, as used primarily for corrosion resistance in the chemical industry [4] , contains up to 4.5% hafnium, which is diffi cult to separate because of the similar chemical properties of zirco- nium and hafnium. The presence or absence of hafnium has no effect on the corrosion resistance, which is controlled by a very stable oxide. At ambient tem- perature, this passive oxide is 2 – 5 nm thick [4] . The pure metal low in hafnium (0.02% max) has a low thermal neutron capture, making it useful for nuclear -

power applications. The outstanding corrosion property of zirconium is its resistance to alkalies at all concentrations up to the boiling point. It also resists fused sodium hydrox- ide. In this respect, it is distinguished from tantalum and, to a lesser extent, tita- nium, which are attacked by hot alkalies. Zirconium is resistant to hydrochloric

and nitric acids at all concentrations and to < 70% H 2 SO 4 up to boiling tempera- tures. In HCl and similar media, the metal must be low in carbon ( < 0.06%) for optimum resistance. In boiling 20% HCl, a transition or “ breakaway ” point is

BEHAVIOR IN HOT WATER AND STEAM

fi nal rate, which is higher than the initial rate, is usually less than 0.11 mm/y (0.0045 ipy) [5] . Zirconium is not resistant to oxidizing metal chlorides (e.g., FeCl 3 ; pitting occurs), nor is it resistant to HF or fl uosilicic acid. Critical pitting potentials of 0.38 V (S.H.E.) in 1 N NaCl and 0.45 V in 0.1 N NaCl [6] indicate that the metal is vulnerable to pitting in seawater. It undergoes intergranular S.C.C. in anhydrous methyl or ethyl alcohol containing HCl, but not when a small amount of water is added [7] . This behavior, similar to that of commercial titanium, suggests that stress may not be necessary and that the failure is perhaps better described as intergranular corrosion.

Although zirconium alloys do have good resistance to S.C.C., they are sus- ceptible in many environments. In general, weldments should be heat - treated to reduce residual stresses. Unalloyed zirconium has been reported to be resistant to S.C.C. because of its low yield strength, whereas the higher - strength alloys are more susceptible. Cracking can be intergranular, transgranular, or mixed inter- granular/transgranular [8] . In Zr – 2.5% Nb, not only S.C.C., but also a delayed hydride cracking process, can occur at highly stressed locations [8 – 11] .

In the nuclear industry, zirconium alloys are used as nuclear fuel cladding and structural fuel assembly components (see Section 8.5 ). The intense radiation within the reactor core accelerates degradation by increasing the rates of corro- sion and hydriding. Zircaloy - 2 (Zr, 1.5% Sn, 0.1% Fe, 0.1% Cr, 0.05% Ni), an alloy used in nuclear reactors, is subject to S.C.C. in chlorides at 25 ° C at potentials noble to the breakdown potential of the air - formed oxide fi lm [0.34 V (S.H.E.)

in 5% NaCl] [12] . Stress - corrosion cracking may also occur in FeCl 3 and CuCl 2 solutions [13] . In slow strain rate tests, commercially pure zirconium, Zircaloy - 2, and Zircaloy - 4 (Zr, 1.5% Sn, 0.2% Fe, 0.1% Cr) undergo S.C.C. in > 20% HNO 3

at 25 ° C with the maximum cracking rate in 70 – 90% HNO 3 . Unlike the situation for titanium, the presence of NO 2 does not have a signifi cantly damaging effect [14] . Constant strain (U - bend, C - ring) tests of commercial zirconium and some zirconium alloys in 70% HNO 3 up to the boiling point showed high resistance to SCC, but not necessarily immunity [15] . Both zirconium and Zircaloy - 2 are subject to SCC in iodine vapor (a major fi ssion product of uranium) at 300 – 350 ° C

[16, 17] . Cold work and irradiation hardening have been reported to increase susceptibility. To help improve the corrosion resistance of Zircaloy, several new zirconium alloys have been developed, such as Zirlo (Zr – 1.0% Nb – 1.0% Sn – 0.1% Fe). Notwithstanding the progress so far, materials reliability does have a signifi cant effect on the economics of nuclear power plants, and there is considerable incen- tive to develop a full understanding of the mechanisms of corrosion of zirconium alloys in reactors and to develop alloys that are resistant to both irradiation and corrosion in reactors [18] .