RAMAN SPECTRAL AND ELECTROCHEMICAL STUDIES OF SURFACE-FILM FORMATION ON IRON AND ITS ALLOYS WITH CARBON IN NA2CO3 NAHCO3 SOLUTION WITH REFERENCE TO STRESS-CORROSION CRACKING/

In situ and ex situ Raman microspectroscopy and electrochemical measur ements on rotating disc electrodes have been used to identify surface films formed on iron and its alloys with carbon in 0.5 M Na2CO3 + 0.5 M NaHCO3 at 75-degrees-C and potentials where susceptibility and resis tance to SCC occur. A good quantitative correlation between data of in situ Raman spectroscopy, galvanostatic surface film reduction, and SC C tests was found. At active dissolution potentials, where transgranul ar SCC was observed, the iron-carbon alloys' surfaces were blocked by FeCO3 or FeCO3 . H2O salt films. At potentials of the active-passive t ransitions, where intergranular SCC was noticed, in situ Raman spectro scopy revealed the formation of Fe3O4 for both decarbonized iron and i ron-carbon alloys. The magnetite film, formed on iron-0.7% C at potent ials of the maximum SCC susceptibility, in contrast to films formed on decarbonized iron, was found to be strongly heterogeneous and often i ncorporated FeCO3. At more positive potentials, ie - 0.6 V (sce) at wh ich the resistance to SCC was observed, galvanostatic reduction and in situ Raman spectroscopy revealed formation of FeOOH. At this potentia l Raman signals from Fe3O4 were not detectable. It is suggested that i ntergranular SCC is favoured by the electrochemical conditions for whi ch ferric oxides or oxyhydroxides are in equilibrium with Fe3O4 and/or Fe(II) species.