THE USE OF ELECTROCHEMICAL HYDROGEN PERMEATION TECHNIQUES TO DETECT HYDRIDE PHASE-SEPARATION IN AMORPHOUS METALLIC ALLOYS

Samples of amorphous Fe40Ni38Mo4B18 and Ni81P19 alloys were submitted to electrochemical hydrogen permeation tests, under cathodic charging conditions, which resulted in elevated hydrogen fugacity. Two differen t permeation techniques were used: the double-potentiostatic and the p otentiostatic step methods. The main difference between these techniqu es is the form of the resulting hydrogen concentration profile across the sample thickness. The hydrogen permeation curves for the Ni81P19 s amples, for both techniques, had shapes identical to the theoretical c urves. The experimental curves for the Fe40Ni38Mo4B18 samples, however , deviated from the theoretical curves. These deviations are attribute d to the formation of a hydride phase due to the high hydrogen fugacit y imposed during cathodic charging. The potential use of these electro chemical hydrogen permeation techniques is demonstrated in the study o f phase separation through the analysis of the growth of a new phase. The diffusivity of hydrogen in the hydride formed in the Fe40Ni38Mo4B1 8 amorphous metallic alloy was found to be 5.7 +/- 0.3 x 10(-15) m(2) s(-1) and the thickness of the hydride layer, formed during the double -potentiostatic test, for a cathodic charging level of -2000 mV/SCE sa turated calomel electrode (SCE) was found to be 18.9 +/- 0.5 mu m. (C) 1998 Published by Elsevier Science B.V. All rights reserved.