Analysis of electrochemical noise generated by different corrosion processes

Analizirali smo elektrokemijski šum, ki ga povzročijo korozijski procesi na površini elektrod. Tokovni in napetostni šum, ki skupaj sestavljata elektrokemijski šum, sta bila merjena med tremi vzporedno postavljenimi elektrodami iz nerjavnega jekla AISI 302. Srednja - skupna elektroda, ki je bila z ostalima dvema povezana v tokovni in napetostni vir, je bila obremenjena s konstantno natezno silo. Elektrokemijski šum smo merili v vodnih raztopinah z različnimi koncentracijami NaCl in HCI. Med meritvami smo z računalniškim vizualizacijskim sistemom posneli spremembe na površini elektrod, ter jih primerjali s posameznimi fluktuacijami elektrokemijskega šuma. Za karakterizacijo izmerjenih signalov, predvsem pa, da bi razločili med posameznimi tipi korozije, smo uporabili različne matematične metode v frekvenčnem (analiza spektralne gostote moči) in faznem prostoru (rekonstrukcija atraktorjev, določitev fraktalne dimenzije in maksimalnega Ljapunovega eksponenta). Raziskava je pokazala, da z merjenjem elektrokemijskega šuma in njegovo analizo lahko ločimo med splošno korozijo, ter različnimi tipi lokalne korozije. Pri lokalnih oblikah korozije detektiramo preboje pasivnega filma, njegovo repasivacijo in rast jamic. Odkrijemo lahko tudi začetek napetostno - korozijskega pokanja, ne moremo pa oceniti hitrosti propagacije posameznih razpok, oziroma napovedati pretrga preskušancaIn the present study the characteristics of electrochemical noise generated by different types of corrosion have been examined. The potential and current noise have been measured between three identical electrodes made from the same material (AISI 302) in aqueous solutions with different concentration of chlorides and/or additions of acid at the higher temperature. The common electrode was under tensile load. The optical microscope supported by a computer visualization technique was used simultaneously during the tests and compared with signals of electrochemical noise generated at the same time. In order to characterise the measured signals, and especially to distinguish between the signals of electrochemical noise generated by pitting corrosion, stress-corrosion cracking and uniform corrosion, various parameters known from the fractal theory and theory of chaos were estimated. It is concluded that the electrochemical noise measurements and analysis can distinguish between uniform corrosion and different types of localised corrosion. We can detect breakdown of passive film, repassivation and growth of stable pitts. We can also detect the initiation of cracks, but we still cannot follow to the propagation of crack