An image differencing technique was used to analyze optical images from an alloy 625 crevice made using an acrylic window. In these experiments, crevices were immersed in ASTM artificial ocean water and held potentiostatically in the passive region while the current was monitored with time. The technique quantified the change in pixels between successive images due to corrosion damage. By calibrating the number of pixels per unit area we calculated the active area and, from this, the crevice current density. It was found that the crevice propagated in the current density range between 3 mA cm−2 at initiation and 100 mA cm−2 at steady state. It is shown that the mass loss as a function of position calculated from the current density accurately reproduces the damage profile inside the crevice. By knowing the location of the active front and the crevice current, we are also able to calculate the wall potential as a function of time and distance. It is shown that the initiation site has a wall potential that is within millivolts of the applied anodic potential and decreases as the active front moved towards the mouth. Finally, we derive an expression for the crevice shape factor which was used to show that the crevice is under diffusion control at later times in the propagation stage.
In this investigation, the role of copper in MnS dissolution was studied in a series of lab-made austenitic stainless steels (SS) with varying Cu content. The base composition of these samples was that of SS 303 and the Cu content was varied between 0.02 and 0.80 wt%. In potentiodynamic polarization experiments, it was found that Cu deposition passivated the MnS inclusions in all except the 0.02 wt% specimen. The critical potential for this passivation, from potentiostatic experiments, was found to be associated with the onset of metastable pitting. The "apparent" pitting potential in the specimen with 0.02 wt% Cu content was approximately 200 mV more negative than the other specimens. This apparent potential was attributed to MnS dissolution, due to a lack of Cu deposition/passivation, and not pitting. With respect to pit repassivation, at concentrations equal to and greater than 0.2 wt% Cu, repassivation potentials were on the order or +0.10 V SCE. In comparison, for the 0.02 wt% Cu specimen, the repassivation potential was less than the OCP (−0.125 V vs. Ag/AgCl) indicating Cu reduction inside the pit plays a role in the measured repassivation potential. Cu release was quantified using a rotating ring disk electrode. In these experiments, oxidation peaks for Cu(I) and Cu(II) were detected. In these experiments Cu was released at low potentials and low, passive, current densities but the resulting near surface concentrations of Cu(I) (0.02 mM) were insufficient to passivate MnS.
The effects of dispersion method for ink preparation and types of catalyst on the catalyst layer’s structure and characteristics were investigated. Catalyst layers prepared by two dispersion methods, i.e., sonication and ball-milling, and two types of catalyst: Pt-HSC (High Surface Area) and Pt-Vulcan XC-72, were fabricated. Viscosity, particle size distribution of the catalyst inks, catalyst layer’s surface properties, and cell performance were measured. Experimental results with the Pt-HSC at ionomer/carbon weight ratio 0.8 show that ink dispersity strongly depends on the mixing method and large agglomerates form in the ink after sonication. The effect of the dispersion method on the ink prepared by Pt-Vulcan XC-72 at similar conditions is not noticeable. The catalyst layer’s mechanical properties, such as hardness and Young’s modulus, were found to vary widely. With an increase of catalyst layer thickness, the number of pin-holes decreased and cracks gradually increased in size. Polarization curves show that the membrane electrode assemblies (MEAs) made with 60% Pt-HSC have a better performance than those with 30% Pt-Vulcan XC-72. The performance and measured electrochemical active surface area of the MEAs made from both catalysts are slightly affected by dispersion method.
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