In this research, a mechanistic approach was used to investigate the effect of molybdate ion on the critical pitting temperature (CPT) of 2205 duplex stainless steel. Firstly, the CPT of 2205 DSS was measured using potentiodynamic and potentiostatic polarization. It was found that with addition of 0.0005, 0.005 and 0.05 M Na 2 MoO 4 to 0.5 M NaCl solution, the CPT increases approximately 4, 9, and 14°C, respectively. Using the lead-in pencil electrode technique, the mechanism by which molybdate ion influences the CPT was interpreted using the CPT model proposed by Salinas-Bravo and Newman. The results showed that molybdate has a negligible effect on the pit solution chemistry, resulting in a slight change in the diffusion-limited current density. However, it reduces the rate of alloy dissolution within the simulated pit solution, which was found as a reduced maximum current density.

In the present study, the criteria of pit stabilization and salt formation have been investigated for DSS 2205 using the lead-in-pencil electrode in order to explore the contribution of molybdate in stable pit growth. Results show that the E pit shifts to positive values with increasing molybdate concentration. It is found that the increase in E pit could not be explained by the changes in DC crit and DC sat, since these values are slightly reduced by adding molybdate to the solution. In contrast, the active dissolution rate of the metal assessed by i diss,max is hindered in the presence of molybdate ions. Therefore, it is proposed that the mechanism for the inhibitory influence of molybdate can be attributed to an increment in the intersection point of i diss,max/i diff,crit (which reflects E crit), and i diss,max/i lim (which reflects E sat). Therefore, increasing the E crit and E sat at the pit growth process is responsible for the observed raising of the E pit in presence of molybdate.