DOI: 10.2495/ECOR090161

Conference Paper Corrosion of mild steel and 316L austenitic stainless steel ...

Potentiodynamic polarization curves could be performed with a scan rate of 0.5 mV/s, starting at -0.5 V (versus Eoc) ending at 0.5 V (versus Eoc). If you want to anayze the passivation region, the positive potential could be increased below the electrolytic potential of water.

Dear Dr. Azim Adzlan,

by polarization potentiodynamic tests, you can obtain interesting data as Ecorr/Icorr and Epit, that are related respectively to the phenomenon of generalized corrosion and initial of passivation and the formation and rupture of the passivation film on the metal substrate. Because I can have a film that is formed in a short time, thin but highly protective or vice versa.

For more details, I suggest you to have a look at the following interesting application notes, both by Princeton Applied Research:

-Basics of Corrosion Measurements

Available at: https://www.ameteksi.com/-/media/ameteksi/download_links/documentations/library/princetonappliedresearch/application_note_corr-1.pdf?la=en

-Electrochemistry and Corrosion: Overview and Techniques

Available at: https://www.ameteksi.com/-/media/ameteksi/download_links/documentations/library/princetonappliedresearch/application_note_corr-4.pdf?la=en

Herewith I am sending you, my “standard” procedure that I generally follow for electrochemical tests, in order to study the corrosion behaviour of different metallic materials immersed in several corrosive environments. These measurements were carried out with 250µV/s scanning rate on specimens previously free corroded for 2, 72, and 168 hours. A conventional three-electrode cell was used for electrochemical tests. The counter-electrode was a platinum coil and all potentials related to the saturated calomel electrode (SCE) were recorded. The working electrode was an abraded sample (down to 800 grit by SiC emery paper, suitably prepared) incorporated into an epoxy resin with stable electric contact, in order to have a flat and regular surface. The exact working electrode area for every sample, ranging from 0.57 cm2 to 0.78 cm2, was inserted in the software for electrochemical data normalization. The corrosive solution was an aqueous 3.5 wt.% NaCl solution, pH = 5.8 and D.O. (Dissolved Oxygen) = 6.5 ppm. All the experiments were conducted at room temperature (25 °C), P = 1 atm, in unstirred conditions and all the additional experimental parameters were fixed in order to allow measurements inter-comparison. In order to drawn polarization curves measurement were started from about -100 mV (SCE) with regard to free corrosion potential (open circuit potential), Efc, then the working electrode potential was increased toward anodic direction. This start potential was chosen in order to avoid any possible surface changing from the steady state. Passive film breakdown potential (critical pitting potential, Ep) and pitting protection potential (Epp) were measured. For this purpose, the working electrode potential was increased up to Ep value, indicated by a sharp anodic current increase. When Ep was established, the working electrode potential was decreased until the backward curve crossed the forward curve. The difference between Efc and Ep was named passivity domain (PD), while the difference between Efc and Epp was defined as perfect passivity domain (PPD). In addition, a domain of about +/- 25 mV around Efc, in which log I vs. E shows a linear relationship, was selected to calculate polarization resistance Rp using a suitable software.

My best regards. Pierluigi Traverso