In a Li/V blanket, it is believed that the interior ofthe wall needs to be coated with insulator ceramics for mitigating the pressure drop caused by magnetohydro­dynamic effects (see also Chapter 4.21, Ceramic Coatings as Electrical Insulators in Fusion Blan­kets). Corrosion of vanadium alloys in liquid Li might not be a concern if the entire inner wall is covered with the insulating ceramic coating. However, since the idea to cover the insulator ceramic coating again with a thin vanadium or vanadium alloy layer was presented for the purpose of preventing liquid lithium from intruding into the cracks in the ceramics coating, the corrosion of vanadium alloys in liquid lithium again attracted attention. It is known that the corrosion of vanadium alloys in liquid lithium is highly dependent on the alloy composition and lithium chemistry. Espe­cially, the N level influences the corrosion in complex manners.28,29 Figure 13 shows a summary of the weight

 

deformation in Li with that in vacuum.25 However, the correlation of creep data is subject to the alloy heat and manufacturing processes as well as test methods and environments. Figure 12 shows the comparison of the NIFS-HEAT-2 creep strain rate versus creep strain data for tests in vacuum and Li environments at 1073 K, for the same batch of NIFS — HEAT-2 creep tubes.25,26 The figure clearly shows reduced strain rate in Li environments. A possible factor could be N pick-up from Li and the resulting

Ti

Cr

Figure 13 The compositions of V-Ti-Cr alloys (wt%) with increase (area I) and decrease (area II) of mass (g cm-2) after holding of samples in Li at 973 K, 500 h. Reproduced from Eliseeva, O. I.; Fedirko, V. N.; Chernov, V. M.; Zavialsky, L. P. J. Nucl. Mater. 2000, 283-287, 1282-1286.

gain and loss in V-xCr-yTi systems in Li.30 High Ti alloys showed a weight increase by forming a TiN layer and high Cr alloys exhibited a weight loss as a result of the dissolution of Cr-N complexes. As the boundary of the two contradictory changes, Ti:Cr^2:1 was observed.

Recently, a corrosion test using monometallic thermal convection Li loop made of V-4Cr-4Ti was conducted at 973 K for 2355 h. Because of the temperature gradient, weight loss and weight gain of V-4Cr-4Ti samples occurred at the hot leg and cold leg, respectively. However, the loss rate corresponded to only < 1 pm year- and the degradation of the mechanical properties were shown to be small.31

V-4Cr-4Ti alloys have been developed mainly for use in Li environments, which are extremely reducing conditions. For the use of vanadium alloys in oxidizing conditions, a different alloy optimization may be necessary. The corrosion of vanadium alloys in oxi­dizing environments is of interest both for the perfor­mance of the pipe exterior out of the breeding blanket and application in non-Li coolant systems such as gas and water systems. Oxidation kinetics of vanadium alloys were studied and showed either parabolic or linear kinetics.32, As the surface oxide layer is not formed or, if formed, not protective to the internal oxidation, alloying with other oxide-formers is neces­sary for improvement. The addition of Si, Al, or Y was shown to significantly suppress the weight gain during exposure to air above 873 K as shown in Figure 14.34

50

40

♦

—1—— ▼—- *—

200 300 400

Hydrogen concentration (wppm)

Figure 15 Total elongation as a function of hydrogen concentration for V-4Cr-4Ti alloys with different O levels. Modified from DiStefano, J. R.; Pint, B. A.; DeVan, J. H.

J. Nucl. Mater. 2000, 283-287, 841-846; Chen, J. M.; Muroga, T.; Qiu, S.; Xu, Y.; Den, Y.; Xu, Z. Y. J. Nucl. Mater. 2004, 325, 79-86.

However, the addition of these elements was not effec­tive in suppressing corrosion in water. Increase in Cr level was shown to be effective, instead.

The effects of oxygen level on hydrogen embrittle­ment have been investigated. Figure 15 compares elon­gation versus hydrogen concentration for V-4Cr-4Ti
alloys with various O levels. The loss of ductility by hydrogen charging was shown to be enhanced

35,36

by impurity oxygen.