ASTM G5: Potentiodynamic anodic polarization measurements

The sample was immersed in 1 N (0.5 mol/L) aqueous sulfuric acid and two Metrohm platinum sheet electrodes were used as the counter electrode. For the reference electrode, an Ag/AgCl 3 mol/L KCl Metrohm reference electrode was chosen. The cell used in this study was the ASTM-compliant 1 L Metrohm Autolab corrosion cell.

To minimize dissolved oxygen, the sulfuric acid solution was deaerated by bubbling nitrogen gas through it for one hour. The Type 430 stainless steel disk was immersed in the solution 30 minutes before the experiment during the N₂ bubbling step. A nitrogen blanket above the solution was maintained throughout the experiment in order to obstruct any atmospheric oxygen diffusion into the solution.

A VIONIC potentiostat/galvanostat was used for the measurement. The procedure and data treatment were performed with the INTELLO software.

The ASTM G5 standard method requires that a voltage scan is initiated from the corrosion potential (E_corr, also known as the open-circuit potential OCP) to 1.60 V vs saturated calomel electrode (SCE) [1]. In this case, the reference electrode system employed was Ag/AgCl 3 mol/L KCl, so the end potential was adjusted to 1.630 V.

The data was plotted in accordance with the guidelines laid out with ASTM Standard Practice G3 [2].

The Active Region (Figure 3, in blue) is characterized by a large increase in the current density, corresponding to oxidation (corrosion) of the stainless-steel sample, until the primary passivation potential (E_pp) is reached. The E_pp also corresponds to a critical current density (j_cc). For potentials higher than the E_pp, the current density drops off, which is usually associated with the formation of a protective (passivating) layer on the surface of the electrode.

After a small peak, the origin of which is likely related to active dissolution of stainless steel from a Cu-enriched layer [3], the current density does not change substantially from its passive current density value (j_p) even as the potential is increased. This is called the Passive Region (Figure 3, in red).

At potentials higher than where j_p occurs, the current density increases again due to breaking the passive layer. This is termed the Transpassive Region
(Figure 3, in green).

Beyond the Transpassive Region, the curve enters another section where the current density does not increase much with the applied potential. This is termed the Secondary Passivity Region (Figure 3, in orange). Further from this, oxygen evolution can occur, which is not shown in Figure 2.