Application of Electrochemical Noise measurements for Stress Corrosion Cracking studies: influence of current partition and instrumental noise

摘要

Electrochemical Noise (EN) measurements have been increasingly implemented in Stress Corrosion Cracking (SCC) studies because of its non-intrusive characteristics and its performance for (ⅰ) crack initiation detection, (ⅱ) SCC progression monitoring and (ⅲ) quantitative evaluation of the dissolved amount of metal during discontinuous cracking by measuring the current generated by each cracking step and using the Faraday law. However, the influence of both the current partition (in the measuring circuit) and the instrument noise on the obtained results was not entirely clarified in these studies. In that context, this work is focused on the investigation of the influence of both the instrumental noise and the rise in asymmetry between the electrodes (inherent in the SCC tests when only the working electrode is stressed) on the EN data. The EN data were acquired by using two electrodes made from Inconel 600 alloy connected with a zero-resistance ammeter (ZRA) and a Saturated Calomel Electrode (SCE) as a reference electrode. Only one of the Inconel electrodes (Working Electrode - WE) was undergoing SCC in an acidified 10 mM tetrathionate solution under constant load, while the other electrode (Counter Electrode -CE) remained unstressed. The WE and CE presented the same exposed surface to the solution and underwent the same surface preparation. The studied SCC system is known to be prone to intergranular SCC and to generate a low level of EN. The results showed that the measured current did not correspond to the totality of the current released by the anodic process occurring on the stressed specimen. Actually, only a fraction of the current due to anodic dissolution occurring at the WE was recorded because of the higher impedance of the CE. On the other hand, an important contribution to the total recorded current came both from the CE due to the asymmetry of the system and from the instrumental noise of the ZRA (Gamry REF600). The instrumental noise was quantified and its effective frequency bandwidth was compared with that of the EN data related to the corrosion process. The consequences of the asymmetry of the system were evaluated in the frequency domain and it remains hazardous to go back to temporal transients. It was finally demonstrated that the contribution of the instrumentation noise can represent the major part of the EN for very weakly emissive SCC systems. In that case, the quantitative evaluation of dissolution processes from EN data appears to be therefore questionable.