A Novel Stepwise Thermal Protocol for Sessile Drop Wetting Tests of Liquid Lead and Refractory Metals for Nuclear Applications

Abstract

By applying the sessile drop method, a novel procedure for testing the physicochemical interaction between liquid metals and refractory solid metals used in nuclear reactors was developed. It involves two methodological approaches: (1) in situ surface purification of liquid metal drops from its native oxide film directly in a vacuum chamber, by squeezing liquid metal from a capillary placed above the tested substrate, and (2) cyclical and stepped temperature change involving stepwise heating from 400 to 600°C, followed by cooling back to 400°C using two separate protocols (i) a single heating cycle from 400 to 600°C, followed by gradual cooling to 400°C, and (ii) a repetitive cycle involving heating from 400 to 600°C, cooling to 400°C, reheating directly to 600°C, and final cooling to 400°C. Wettability tests of selected refractory metal substrates (Me = Mo, Ta, and Ti) with liquid Pb using the sessile drop method and a new procedure showed that the wetting kinetics and contact angle of Pb/Me couples are significantly affected by temperature, but only during the gradual heating up to a maximum of 600°C. During the subsequent steps to final cool down to 400°C, the contact angle for all tested Pb/Me couples remains unchanged. Measurement discrepancies were observed depending on the capillary materials used (graphite vs. alumina) and surface conditions (polished vs. ground). The results indicate that under the investigated conditions, liquid Pb does not wet the Mo substrate (with increase in temperature, θPb/Mo ~ 115-100°), shows weak wetting on the Ta substrate (θPb/Ta ~ 87-84°), better wetting on polished Ti (θPb/Ti = 30° and θPb/Ti = 67°, with graphite and alumina capillaries, respectively), and good wetting on rough, ground, and unpolished Ti substrate (θPb/Ti_unpolished = 28°).