Listening steels during their corrosion

To improve the reliability of equipment, in situ monitoring of the degradation of metallic materials has led to an important step forward. A variety of technologies have been developed for this purpose, including the use of electrochemical phenomena, electrical conductivity and acoustic emissions. The latter is based on the emission of acoustic signals as a result of active corrosion, the waves of which differ in terms of type (volumetric, surface, Lamb) and frequencies (50 Hz – 1.5 MHz). Thanks to the use of suitable sensors, waveguides and amplifiers, it is possible to non-intrusively and continuously monitor active corrosion.

François Ropital Expert Metallurgy-Corrosion-Electrochemistry francois.ropital@ifpen.fr	Jean Kittel Applied Chemistry and Physical Chemistry Division jean.kittel@ifpen.fr

The main obstacle to the development of this method concerns identification of the specific signals linked to the corrosion phenomena studied. Research work has been conducted at IFPEN, with the collaboration of Total and Insa in Lyon (MATEIS Laboratory), to study the embrittlement of steels by hydrogen in aqueous media containing H₂S. In media of these types, the high level of atomic hydrogen penetration into the steel can cause the formation of internal blisters and cracks, and even premature rupture. A specific study methodology has been developed in the laboratory enabling specific areas of damage to be located using appropriate sensors and the acoustic signature linked to the different possible mechanisms of hydrogen induced embrittlement to be identified. As the figure illustrates, signals specifically linked to Hydrogen Induced Cracking (HIC) have been differentiated from other signals triggered, firstly, by surface corrosion associated with the formation of iron sulfides and, secondly, via the release of hydrogen gas into the liquid medium where this corrosion occurs.

Quantitative monitoring of HIC mode could therefore be performed in a non-destructive, non-intrusive manner. Important information, such as latency time before formation of the first cracks, then estimation of their propagation rate, is also accessible.

Discrimination on the basis of their energy of acoustic emission signals according to the different damage mechanisms for a carbon steel immersed in aqueous medium with 1 bar of H₂S at 20°C.

Publications

• V. Smanio, M. Fregonese, J. Kittel, T. Cassagne, F. Ropital, B. Normand, Wet Hydrogen Sulfide Cracking (HSC) monitoring by Acoustic Emission: discrimination of AE sources, Journal of Materials Science, 45 (2010), 5534-5542. DOI: 10.1007/s10853-010-4613-2

• V. Smanio, M. Fregonese, J. Kittel, T. Cassagne, F. Ropital, B. Normand, Acoustic emission monitoring of H2S cracking of linepipe steels: application to HIC and SOHIC, Corrosion, 67 (2010) 6, 065001-12. DOI: 10.5006/1.3595097

>> See also IFPEN Publications database