Experimental study of hydrogen embrittlement on AISI 304 stainless steels and Rayleigh wave characterization

• Rayleigh wave characterization of cathodic hydrogen-charged stainless steels.
• Rayleigh wave decreases with charging time, longitudinal wave is insensitive.
• SEM confirms that martensite transformation occurs after hydrogen charging.
• Hydrogen induces gradient hardening distribution of specimen’s subsurface region.
• Rayleigh wave velocity change is due to subsurface martensite transformation.

Many failures due to hydrogen embrittlement or hydrogen damage are widely reported in oil and refinery industry. Despite many ultrasonic testing methods have been developed to assess hydrogen embrittlement, they are applied well to serious hydrogen attack instead of earlier degradation. This paper aims to characterize nascent hydrogen embrittlement of AISI 304 austenitic stainless steels under cathodic hydrogenation using Rayleigh wave. After cathodic hydrogen charging of AISI 304 stainless steel, XRD and metallographic examination show that martensite transformation occurs within the subsurface region of the specimens. Microhardness testing indicates that hydrogen leads to hardening of the material. It is found that Rayleigh wave are better to inspect local degradation than bulk waves. Rayleigh wave velocity of 5 MHz and 10 MHz decreases significantly with cathodic charging time, while longitudinal wave velocity changes not. Acoustic velocity change is due to elastic modulus reduction resulting from hydrogen-induced phase transformation in the subsurface region.