The analysis of the potential emission acoustic sources at the metal-liquid interface: pitting corrosion, crystallization, and gas diffusion

The study of pitting corrosion - local form corrosion and also, the analysis of the processes that have been produced during of this phenomenon (diffusion of a gas and the crystallization of some corrosion products) were done by the analysis of acoustic signals emitted by three types of materials: stainless steel 316L, aluminium 6056T6 and copper 99,99%. The stainless steel and aluminium are materials with the same mechanism of corrosion, that emit similar acoustic signals characteristic of the pitting, short volley in impulses and resonance volley. The copper corrosion does not involve gas diffusion, and for this reason does not emit acoustic signals. The study of acoustic emission in presence of a solution which produces crystallization phenomenon and also in the cathode polarization of the metal have demonstrated that diffusion gas phenomenon is the principal responsible of the acoustic signals. Although the crystallization phenomenon is an emissive process, the work conditions (large and numerous crystals) are different from the crystallization conditions of corrosion products inside of pitting. It is necessary to make new experiences with small crystals, located at the level of the sample. (authors)