The use of b-value and Ib-value of acoustic emission in monitoring hydrogen-assisted cracking of martensitic stainless steel

Hydrogen-assisted cracking phenomena in thiosulphate solution of martensitic stainless steel were investigated by using acoustic emission technique. A time domain approach was adopted in order to analyse the corrosion phenomena by acoustic emission pattern time evolution. An appropriate set of variables was used, some of them directly related to the acoustic event waveform, while others have been derived from them, such as b-value and Ib-value, which are descriptors related to the time domain distribution of the acoustic event energies. Aim of this work is to show how these two descriptors allow to obtain an easy-to-use diagnostic system for damage progression, without a deep knowledge of the acting corrosion phenomena themselves. Tests were carried out on a X12Cr13 martensitic stainless steel, according to NACE TM 0177 standard, using a modified sour simulated environment (5% NaCl, 2.5% acetic acid and 10-2 M Na2S2O3).