Surface and corrosion properties of modified passive layer on 304 stainless steel as bipolar plates for PEMFCs

In this study, the relation between surface chemistry and corrosion properties of modified 304 stainless steels (304SS) was investigated. 304SS samples were submitted to plasma nitriding performed at two different temperatures: 420 \ub0C (low) and 520 \ub0C (high). Then, a thermo reactive deposition (TRD) was used in a mixture of ferro niobium, alumina and ammonium chloride. Finally, a pickling treatment was performed to access high corrosion resistant surfaces. Surface treated samples, both before and after pickling, were investigated by using Scanning Electron Microscopy (SEM) and Potentiodynamic (PD) techniques. X-ray Photoelectron Spectroscopy (XPS) was performed for the analysis of the surface layers of the samples after pickling. The surface layers were mostly comprised of iron and chromium oxides and hydroxides. XPS results proved the presence of a surface layer mostly constituted by iron oxides and oxyhydroxides for both the treated samples that resulted more homogenous for the sample nitrided at low temperature. Chromium was detected by XPS in the predominant form of oxide on the surface of the sample nitrided at low temperature

Effect of major degradation products of ethylene glycol aqueous solutions on steel corrosion

The effect of major degradation acid products of ethylene glycol (EG) aqueous solutions, namely glycolic, formic, acetic, and oxalic acids, on the corrosion behavior of low carbon steel was investigated under stirring conditions at 80\ub0C by means of well-established techniques for electrochemical, physico-chemical, and surface analyses. The electrochemical behavior of steel under polarization conditions is dominated by active dissolution of iron with Fe2+ production leading to oxide products and H+ reduction as the cathodic counterpart. No correlation was found between the corrosion current density estimated by Tafel extrapolation method and that determined by the rate of production of Fe2+ under free corrosion conditions. The latter experiments revealed that the nature and the relative proportion of carboxylic acids influence the corrosion behavior of steel. The rate of production of soluble corrosion products increases with the stability and complexation ability of the organic anion towards Fe3+, being more significant in the case of glycolic acid in excess with high chelation propensity. Conversely, formation of Fe2+ and Fe3+ oxalates on iron surface is promoted in the presence of oxalic acid due to a catalytic action on magnetite dissolution. The extent of above processes is compromised if hydrogen bonding interactions between different carboxylic acids are privileged

Relationship between structure and CO oxidation activity of ceria-supported gold catalysts.

Gold catalysts supported on cerium oxide were prepared by solvated metal atom dispersion (SMAD), by deposition-precipitation (DP), and by coprecipitation (CP) methods and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic activity was tested in the CO oxidation reaction. The structural and surface analyses evidenced the presence of a modified ceria phase in the case of the DP sample and the presence of pure ceria and gold metal crystallites in the case of the SMAD and CP samples. The DP sample, after a mild treatment in air at 393 K, exhibited only ionic gold, and it was very active below 273 K. By comparing the activities of the different catalysts, it is suggested that the presence of small gold particles, as obtained by the SMAD technique, is not the main requisite for the achievement of the highest CO conversion. The strong interaction between ionic gold and ceria, by enhancing the ceria surface oxygen reducibility, may determine the particularly high activity

Ion photon-stimulated desorption as a tool to monitor the physisorption to chemisorption transition of benzene on Si(111) 7 × 7

We investigated the use of ion photodesorption as a tool to monitor the transition from the physisorbed to the chemisorbed state on a surface. The adsorption of benzene on Si(111) 7 × 7 in the temperature range 40-300 K is chosen as a prototype. The D+ ion photodesorption yield was monitored as a function of temperature at various benzene exposures. Comparative measurements of the C 1 s photoelectron yield in the same temperature range enable the physisorbed to chemisorbed state transition to be distinguished from that of the multilayer to the chemisorbed state. We find the onset at 110 K in the first case, and at 130-140 K in the second case. These results demonstrate that ion photodesorption is a potentially interesting method to identify physisorption to chemisorption transitions of adsorbed molecules on surfaces