O.R. Mattos

Mass‐Transport Study for the Electrodissolution of Copper in 1M Hydrochloric Acid Solution by Impedance

The dissolution of copper in 1M chloride solution was studied by steady-state (current-voltage curves for various disk rotation rates) and transient [frequency analysis of the electrochemical and electrohydrodynamical (EHD) impedance] measurements. The anodic polarization curves have presented one region of mixed kinetic and another of mass-transport control characterized by a current plateau. The limiting current is a Ω 1/2 function throughout the whole range of rotation rates. From the EHD impedance diagrams observed below the anodic plateau, it was shown that the limitation by mass transport is due to CuCl 2 - . 0n the current plateau, the presence of two time constants on the EHD impedance diagram and the reductibility of the curves have been attributed to the existence of a salt layer covering the surface, the rate of formation of this layer being identical to the rate of dissolution. A layer model has been developed

Reaction Model for Iron Dissolution Studied by Electrode Impedance II . Determination of the Reaction Model

The electrode impedance determined experimentally shows a certain number of time constants as capacitive or inductive features. These impedances arise from various processes occurring at or near the electrode interface, such as charge transfer, adsorption‐desorption of reaction intermediate species, changes of the roughness factor, changes of the number of active sites or of volume concentration in the vicinity of the electrode, etc. The origins of impedances are examined on the basis of experimental results given in Part I . Then, hypotheses describing reaction rates, such as the Tafel law, reaction reversibility, and the adsorption isotherm law, are analyzed in order to translate reasonably the reaction models into mathematical expressions. With hypotheses retained, it is concluded that the experimental results should be interpreted by a model including three adsorbed reaction intermediate species. Forty possible reaction schemes, as the complete set of prospective models, were written and examined according to both steady‐state polarization curves and electrode impedances. The appearance of two current maxima implies at least two dissolution paths in the reaction models and allowed us to eliminate 10 of the 40 reaction schemes. On the other hand, the appearance of inductive impedance was found to constitute a very selective criterion. Only one model, given in Part I , was found to simulate suitably the whole set of experimental results.

Undoped polyaniline anticorrosive properties

The anticorrosive properties of undoped polyaniline (PAni) casted onto mild- and galvanised steels were studied by a classical methodology used for organic coatings. Pure PAni, as well as, PAni plus an epoxy topcoat were tested in total immersion conditions monitored by electrochemical impedance. The influence of the substrate and the electrolyte composition were discussed. PAni performance was evaluated in terms of underfilm corrosion and adhesion loss in comparison with other coating systems. It was shown that undoped PAni did not have good barrier properties and adhesion to the substrates was very poor. Even with an epoxy topcoat, PAni coated samples had the worst performance in comparison with the other coating systems herein used as reference.

Mechanism of anodic dissolution of iron-chromium alloys investigated by electrode impedances—I. Experimental results and reaction model

Abstract Stainless steels are characterized by their ability to passivate spontaneously in a corrosive medium. This ability is dependent on many factors, among them the kinetics of the anodic process has a particular importance. In fact, the addition of chromium to iron decreases drastically the critical current density, that is the current maximum during the alloy dissolution, making the passivation process easier to set up. Steady-state polarization curves and electrode impedances measured on FeCr alloys of various chromium contents show how the chromium addition modifies the kinetics of iron dissolution. On the basis of experimental results a reaction model for the dissolution-passivation process of iron—chromium alloy in acidified sulfate media is proposed. The reaction mechanism of alloy is depicted as that of iron perturbed by the chromium addition.

Application of the impedance model of de Levie for the characterization of porous electrodes

From the de Levies theory for a porous electrode, the impedance can be expressed in function of the cylindrical pore characteristics: the pore length, the pore radius and the number of pores. If the pores have a finite length, these parameters can be regressed from the experimental impedance diagrams. In the case of a semi-infinite pore length, only the product r3/2n can be obtained. In this paper, two practical examples of porous electrodes were presented: the corrosion of cast iron in drinking water and the electrodissolution of copper in 1 M hydrochloric acid solution. In each case, the pore parameters were assessed.

Electrochemical characterization of chromate coatings on galvanized steel

Chromate conversion coatings are still one of the most efficient intermediate surface treatments for galvanized steel. The performance of the complete system depends strongly on the composition and structure of the chromate layer, which is normally characterized by destructive methods. The main purpose of this paper is to develop an indirect non-destructive method, sensitive to changes in the chromate layer, that could be easily applied in industry quality control. Open circuit monitoring, polarization curves and electrochemical impedance are the techniques investigated. Chromate layers with different chromium contents and structures were tested in various electrolytes. It was verified that resistances and capacitances obtained by impedance measurements at low frequency range, can be very sensitive even to little changes in the chromate layer, depending on the composition of the electrolyte.

Reaction model simulating the role of sulphate and chloride in anodic dissolution of iron

Abstract The role of chloride and sulphate anions on anodic dissolution of iron was studied. A gradual transition from sulphate to chloride system was found. Based on a previously proposed model it was possible to take into account this experimental result. The role of anions was explained by a numerical parameter representing the loss of the self-catalytic characteristic of electrochemical reactions.

High‐Rate Copper Dissolution in Hydrochloric Acid Solution

The dissolution of copper in 1 M chloride solution was studied by steady-state (current-potential curves for various disk rotation rates) and transient measurements [frequency analysis of the electrochemical and electrohydrodynamical (EHD) impedance]. The anodic polarization curves present four regions according to the potential : one region of mixed kinetics followed by another of mass-transport control characterized by one plateau, followed by a second mixed kinetics region and a second plateau. The results obtained for the first domain of mixed kinetics and for the first plateau were previously published. In this work, the second mixed kinetics region is considered. From the analysis of the ac and EHD impedance measurements, it was shown that a transition between the presence and the absence of a CuCl salt layer occurs at a critical potential. The value of this critical potential depends on the rotation rate ; the higher the rotation rate, the higher the potential value. A model of kinetic dissolution for this transition is presented, and a coupling between the currents corresponding to the two reactions is demonstrated.

ZnFe anomalous electrodeposition: stationaries and local pH measurements

Abstract The kinetics of ZnFe codeposition was investigated in acid solutions. The effects of solution composition and pH were analyzed. Inhibition of H + reduction and Fe deposition occurs with increasing Zn ++ concentration in sulfate solution. An activation of Zn deposition is also observed. Increasing pH causes Zn deposition activation during ZnFe codeposition. The anomalous codeposition is also favored in chloride medium. When alloy deposition becomes the main process, the interfacial pH is governed by the individual metal deposition that controls the kinetic behavior. The interfacial pH increases during separate Fe deposition, meaning that it occurs with simultaneous consumption of H + . Individual Zn deposition brings about an H + inhibition. A correlation between the codeposition behavior of ZnFe and ZnNi in sulfate and chloride solutions suggests that the prevailing cathodic reaction governs the interfacial pH. Anomalous codeposition process does not seem to be associated with a saturation of any thermodynamic species at the electrode surface. It can only be described by kinetic arguments.

Mechanism of anodic dissolution of iron-chromium alloys investigated by electrode impedances—II. Elaboration of the reaction model

Abstract On the basis of experimental results exhibited in Part I, a reaction model of the dissolution—passivation process of FeCr alloy in acidified sulfate media is proposed. This reaction mechanism of alloy is considered as that of Fe perturbed by the Cr addition. However, the change in the dissolution—passivation process of Fe alone is not sufficient to interpret the whole results. Hence, firstly an interaction between adsorbed species was introduced: a chromium passivating species hinders drastically the rate of the Fe(II) ads → Fe(II) sol dissolution path. Finally, it was also considered that the uniform dissolution of alloy is accounted for by the change of the surface concentration of alloy elements. The proposed model interprets satisfactorily the experimental results.