ACOUSTIC EMISSION DURING HYDROGEN ABSORPTION AND DESORPTION IN PALLADIUM R. Ramesh, C. K. Mukhopadhyay, T. Jayakumar and Baldev Raj
Materials and Metallurgy Group
Indira Gandhi Center for Atomic Research, Kalpakkam 603 102, India
Keywords: Acoustic emission, hydrogen in metals, hydrogen embrittlement, hydrogen absorption, palladium ABSTRACT
On-line monitoring of Acoustic Emission (AE) generated during hydrogen charging and discharging in metals and alloys would give rise to valuable information about the mechanisms involved in this process. In the present investigation, the AE generated during absorption and desorptin of hydrogen in palladium has been studied. Specimens of the size (12 mm x 8 mm x 0.3 mm) were electrochemically charged with hydrogen at ambient temperature and at a current density of 5 mA/cm2 in NaOH solution. AE signals generated during hydrogen charging and discharging were recorded using an AE system (model 264B) of M/s. AET Corp., USA. A piezoelectric transducer with resonant frequency at 175 kHz, a preamplifier (60 dB gain) and a compatible filter (125-250 kHz) were used to capture the AE signals. The transducer was coupled to the specimen using a wave guide. The AE Root Mean Square (RMS) voltage and total ringdown counts were recorded continuously.
The plots of AE RMS voltage as a function of time indicate different stages of hydrogen absorption involved during charging process. AE signals observed at the initial stages are attributed to be due to the surface activation and initiation of hydrogen absorption. AE activity increases with time and saturates upon completion of hydrogen absorption. The major source of AE is the release of hydrogen bubbles at the surface of the specimens. The AE activity arising due to bubbles, increases as the specimen absorbs hydrogen and becomes constant when the specimen is saturated with hydrogen. The specimens were allowed to disturb hydrogen under a concentration gradient in the same electrochemical cell and the AE parameters were recorded again. The AE RMS voltage decreases steadily and saturates with time. Continued desorption of the hydrogen from the specimen is indicated by a decrease in AE RMS voltage with time, which ceases when all the hydrogen is disturbed completely. The hydrogen concentration was estimated from the "diffusion current transient" recorded simultaneously along with AE parameters during the discharging cycle. It has been observed from these investigations that the variations in the AE RMS voltage with time could be correlated to the residual hydrogen concentration present in the metal at any instant.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2533 - 2536
Publisher: Ashgate Publishing Company