A method for the electrodeposition of aluminum on niobium or niobium alloys, containing at least 40 percent niobium

摘要

A niobium base alloy containing at least 40% Nb is coated with a high temperature oxidation-resistant coating of NbAl3 by electrodepositing aluminium from a molten salt bath comprising cryolite and alumina, e.g. 75-95% Na3AlF6 and 5-25% Al2O3 at 1600-1900 DEG F. The bath may also contain NaF and/or AlF3 with an operating temperature of 1350-1900 DEG F. The anode is preferably a graphite crucible container. The alloy is exposed as a cathode in the bath for 1-90 minutes at a C.D. of 0.1-3 amps./sq.m. Prior of electrodeposition the alloy, degreased with an organic solvent and etched with a solution of 2HNO3: 1HF: 1H2O, may be placed in the bath for at least 2 mins. for fluxing. Residual fluoride may be removed from the coated alloy by placing it in a molten bath of NaCl and LiCl at 1200 DEG F. to replace the fluorides with chlorides which may be removed by brushing or ultrasonic cleaning under water, or the coated alloy may be immersed in a solution of 20 g./l. CrO3 in dilute H3PO4 and subjected to ultrasonic vibrations. The coated alloy may be subjected to diffusion in air at 1600-1800 DEG F. or heated in an inert atmosphere. Examples of molten salt baths for use as electrolyte are as follows:- TABS:1024613/C6-C7/1