Abstract
The cyclic potentiokinetic polarization method (scanning rate 10 mV/min), starting at the free corrosion potential and going up to 100 µA/cm2 (surface area), and microscopic observation of the surface were used to evaluate the susceptibility of commercial stainless steels to localized corrosion for their possible application in solar panels. Aerated and deaerated solutions of 0.6M NaCl and seawater at room temperature were used.
Considering the average value of the pitting potential obtained in numerous tests, the following stability classification was established for the materials studied: 430<430 Nb<304<304L<ELI (C)≤316<ELI (D)
The appearance of morphological differences in pits on the same material and the presence of cavernous pits, detectable after light abrasion of the surface or in a cross section of the sample made their quantification and a determination of any definite relation between surface defects (segregations and/or microflaws) and passive film breakdown susceptibility very difficult.
Crevice corrosion could occur under free corrosion conditions, especially in the presence of air, and it was detectable from potential oscillations.
The crevice potential is not estimated with the same technique used to determine pitting potential because of the strong effect of the critical shield geometry; it is therefore suggested that the redox couple K4Fe(CN)6 − K3Fe(CN)6, in suitable concentration in the system being considered, can give useful indications of this potential. Indeed, the rapid precipitation of Turnbull's blue physically prevents the repassivation process at the electrode potential established by the redox couple.
