D.R. Salinas

An EIS study of aluminium barrier-type oxide films formed in different media

Abstract Impedance measurements were performed on aluminium in different aggressive (0.5 M NaCl) and non-aggressive (0.5 M Na 2 SO 4 , 0.16 M NH 4 -tartrate) electrolytes. Similar impedance responses of the formed passive film were found. The capacitive behaviour observed at high frequencies is related to the thickness and dielectric properties of the barrier oxide film. The impedance spectra obtained at low frequencies were interpreted in terms of a diffusion-controlled process within the oxide film. The Faradaic impedance given by the metal dissolution at the oxide film/electrolyte interphase is strongly affected by the anion type. This fact and the local pH developed at the interphase passive film/electrolyte determine the chemical stability of the film. The increased impedance observed in chloride solution under rotation conditions is explained by a local change of pH at the film/solution interphase. EIS can also be used to study pitting phenomena. In the stabilized active pitted area an ohmic control given by the presence of a salt film is responsible for the capacitive and low frequency inductive behaviour obtained.

Ag UPD on Au(100) and Au(111)

The underpotential deposition (UPD) of Ag in the systems Au(hkl)/Ag + , ClO - 4 and Au(hkl)/ Ag + , SO 2- 4 with (hkl) = (100), (111) was studied by in situ STM and different electrochemical techniques including charge-coverage measurements with the twin-electrode thin-layer (TTL) technique. Ag UPD was found to occur stepwise and similar in both systems. At relatively high underpotentials expanded and commensurate Ag adlayers Au(100)-c(√2x5√2)R45°Ag and Au(111)-(4x4)Ag are formed. At low underpotentials, condensed commensurate overlayers with Au(hkl)-(1 x 1)Ag structures are formed via a first order phase transition. On Au(111), the condensed Ag phase is preferentially created on monatomic steps, whereas on Au(100) a simultaneous 2D nucleation of Ag on flat terraces is observed. Electrosorption valency measurements in the system Au(hkl)/Ag + , SO 2- 4 show γ = z = 1, indicating that coadsorption or competitive adsorption of anions can be excluded. Electrochemical and in situ STM results indicate similar charge-coverage behaviour in the system Au(hkl)/Ag + , ClO - 4 . Ag UPD on Au(hkl) occurs at positive potentials with respect to the potentials of zero charge of Au(hkl) and Ag(hkl). Therefore, a nearly constant anion coverage in the entire Ag UPD range cannot be excluded.

Influence of alloying elements and microstructure on aluminium sacrificial anode performance: case of Al-Zn

The electrochemical behaviour of Al–x%Zn alloys (1 wt% ≤ x ≤ 80 wt% ) was studied in 0.5 m sodium chloride solution. The experiments focused on the influence of casting conditions on sacrificial anode performance. The influence of casting conditions, solidification structure, polarization behaviour and attack morphology on the anode efficiency and operating potential was analysed. For alloys with low Zn content (1–5 wt%), the interdendritic zones or grain boundaries were the initial sites of attack and self corrosion was the principal cause of efficiency loss. Particularly, for Zn contents below 3 wt% the operating potential was strongly affected by the solidification macrostructure. Casting conditions that produced better alloying element distribution (chill structures) promoted higher anode efficiency. For Zn content higher than 5 wt% the operating potential and the anode efficiency were defined by the α/β phases area relationship.

STM tip-induced local electrochemical dissolution of silver

Local dissolution/deposition processes under in situ scanning tunneling microscopy (STM) imaging conditions are studied in the systems Ag(111)/Ag+, ClO4− and Ag(111)/Ag+, SO42−. The results show that in both systems the local kinetics of these processes strongly depend on the polarization conditions. At STM-tip potentials more positive than the Ag/Ag+ equilibrium potential, a local dissolution of the Ag(111) substrate is observed even at cathodic substrate overpotentials at which the overall substrate current density is cathodic. This tip-induced Ag dissolution is in agreement with results obtained recently in the system Cu(111)/Cu2+. The enhanced local Ag dissolution is explained by a reduced Ag+ concentration underneath the STM tip promoted by both an electrostatic repulsion of Ag+ and a reduction of the mass transport due to the shielding effect of the tip. The possibility for a preparation of negative Ag nanostructures by STM tip-induced electrochemical dissolution is demonstrated.

Effect of the concentration and composition on the size and shape of micelles of sodium oleate–cetyltrimethylammonium bromide mixtures

The dependence of the aggregation number (N(agg)) on composition and concentration of sodium oleate-cetyltrimethylammonium bromide mixed micelles as revealed by cyclic voltammetry experiments shows a complex relationship with the total concentration and the composition of surfactant mixtures. This behavior is related to the evolution with composition of the HLB values and with the composition and the excess free energy of mixed micellization, and is explained by the inclusion of the double bonds of oleate chains in the micellar Stern layers. The increase in size probably leads to a reduction of the micelle surface available for the polar headgroups, causing a reduction in the proportion of double bonds in the hydrocarbon-water interface and a change in the mixed micelle composition. Therefore, the generally held supposition that the composition of mixed micelles does not change with concentration seems rather unrealistic.

Nanocrystallization process of the Fe69.5Cu1Nb3B9Si13.5Cr4 FINEMET-type alloy : an AFM study

The different devitrification stages of Fe 69.5 Cu 1 Nb 3 B 9 Si 13.5 Cr 4 (F-Cr4) amorphous alloy were studied by atomic force microscopy (AFM) on samples annealed at 400, 600 and 670°C for different treatment times. The AFM images showed no topographic changes in the alloys treated at 400°C, indicating that only structural relaxations took place at this annealing temperature. Samples thermally treated at 600°C showed hemispherical features uniformly distributed on the surface, suggesting initiation of the nanocrystallization process. The nanocrystals formed had a size distribution of 15-40 nm, which is slightly affected by the annealing time. At 670°C the crystallization phenomena occurred with the formation of large crystals and an increase in roughness. The corrosion behaviour of the nanocrystalline alloys in 2 M HCl solution was correlated with the AFM studies. The potentiodynamic experiments for these alloys showed a significant decrease of the current density peak at the active-passive transition in relation to the F-Cr4 amorphous alloy. The results indicated that the corrosion resistance and passivating ability of the alloys are improved by the thermal treatment.

Inhibitory effects of NO2− on Ni deposition

Abstract The influence of NO2− on the first stages of Ni deposition onto vitreous carbon was studied by means of voltammetry, potentiostatic steps and AFM techniques in neutral sulphate solutions. At low overpotentials and NO2− concentrations lower than 5×10−4 M, the adsorption of NO2− on forming nuclei increases the energy of nucleation, diminishing the nucleation rate. At more negative potentials and/or in the presence of higher concentrations, the reduction of NO2− increases the local alkalisation to a great extent and Ni nucleates by reduction of the precipitated Ni(OH)2 particles which are adsorbed on the active sites of the substrate. Then, further growth is limited by the transport of Ni2+ through the adjacent precipitated hydroxide layer.

In situ STM study of electrocrystallization of Ag on Ag(111)

The electrocrystallization process was studied in the system Ag(111)/Ag+, SO4= by in situ scanning tunneling microscopy (STM). The results show that Ag deposition occurs preferentially at step edges following a layer-by-layer growth mechanism, but polarization and imaging conditions greatly affect the local kinetics of this process. At STM-tip potentials more positive than the Ag/Ag+ equilibrium potential, a local dissolution of the substrate underneath the tip is observed even at low negative substrate overpotentials, at which the overall substrate current density is cathodic. An in situ STM imaging of Ag deposition was possible at sufficiently high negative substrate overpotentials. An estimation of the local deposition current density, however, indicates that the deposition rate underneath the STM-tip is reduced. These results are explained by the presence of an electric field between the STM-tip and the substrate, which affects the potential distribution directly underneath the tip, producing a large shielding of the diffusive flux of Ag+ ions.