S.L. Díaz

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.

Electrodeposition of Zn-Ni alloys in sulfate electrolytes

Abstract The electrodeposition of Zn-Ni in sulfate solutions has been investigated by means of polarization curves, morphological analysis and electrochemical impedance using a rotating disc electrode. The polarization curves showed two parts, the first one related to the Zn-Ni normal codeposition and the second one to the Zn-Ni anomalous codeposition. The mass transport influenced only the Zn-Ni normal codeposition and had no influence on the anomalous codeposition. The temperature showed a great influence on the Zn-Ni alloy deposition and affected both parts of polarization curves. The electrolyte composition had no influence on the Zn-Ni anomalous codeposition, but it affected the transition current density between normal codeposition and anomalous codeposition. The impedance diagrams were obtained in sulfate electrolytes and showed four loops at low frequency domain at the beginning of anomalous codeposition : three capacitive loops and one inductive. For the high polarizations, one of the capacitive loops disappears. These results were discussed comparatively to those previously obtained for chloride electrolyte, in view of establishing a kinetic model for the sulfate electrolyte.

Electrochemical characterization of Ni–P and Ni–Co–P amorphous alloy deposits obtained by electrodeposition

Ni–P and Ni–Co–P amorphous alloy deposits were obtained by electrodeposition at 80 °C on carbon steel substrates. The influence of the electrolyte Co2+ concentration and of applied current density was investigated. The corrosion behaviour of amorphous and crystalline deposits was evaluated by polarization curves and electrochemical impedance spectroscopy in NaCl 0.1 M solution at room temperature. Impedances were measured for samples under total immersion (free potential against time) and for polarized samples in predefined regions of the polarization curves. It was found that the alloy deposit composition is highly affected by the composition of the electrolyte but displays no significant dependence on applied current density. The results showed that the presence of Co on Ni–P amorphous alloys improves the deposit performance in the studied corrosive medium. It was also verified that the amorphous structure provides higher corrosion resistance to both Ni–P and Ni–Co–P alloys.

Evaluation of oilfield corrosion inhibitors in CO2 containing media: A kinetic study

Abstract Film-forming corrosion inhibitors are extensively used to control CO 2 corrosion of steel in oil and gas production plants. The performance of these inhibitors is connected to the persistence and/or regeneration of the protective film. Their active mechanism in the presence of CO 2 is still little understood as far as the kinetics is concerned. In this paper, some of the factors affecting this mechanism have been studied with the aid of polarization curves and electrochemical impedance, using rotating disc and cylinder electrodes. The performance of a commercially available film-forming inhibitor used in PET-ROBRAS, the Brazilian petroleum company, was evaluated. The results show that the efficiency of the inhibitor depends on its residual concentration and on the hydrodynamic flow conditions. The impedance diagrams provided information on growth, stability and deterioration of the inhibitor film.

Hardness analysis and morphological characterization of copper-zinc alloys produced in pyrophosphate-based electrolytes

In this work, copper-zinc alloy coatings on mild steel substrates were obtained in nontoxic pyrophosphate-based electrolytes, at room temperature and under continuous current. The effects of bath composition and current density on the hardness of the coatings, as well as on their morphologies, were evaluated. The results showed that the electrolyte composition, and the use of stress relieving additives strongly influence the hardness of the coatings, while the current density directly affect their morphology. Hence, for a current density of 116 A/m2, copper-zinc alloy deposits with no pores or cracks were produced in a pyrophosphate-based electrolyte, especially when allyl alcohol was added to the solution.

A Kinetic Study on Nickel Electrodeposition from Sulfate Acid Solutions I. Experimental Results and Reaction Path

In this work a systematic study on the kinetics of Ni electrodeposition in acid sulfate media over a wide pH range was carried out. Two distinct electrodeposition behaviors were experimentally detected as a function of pH. Accordingly, it is suggested that Ni electrodeposition takes place through different intermediates: In the lower pH range, Ni(I) ads species predominates at the electrode surface, and its formation is deactivated with increasing pH, whereas in less acid solutions, Ni electrodeposition occurs via two species, [Ni(OH)] ads and [Ni(OH)] + ads . The chemical nature of Ni(I) ads , [Ni(OH)] ads and [Ni(OH)]+ads species is discussed on the basis of the interfacial pH behavior.

A Kinetic Study on Nickel Electrodeposition from Sulfate Acid Solutions II. Reaction Modeling

The electrochemical mechanism of Ni electrodeposition in an acid sulfate medium with pH ranging from 1 to 6 is evaluated. For pH ≤ 3, a reaction model already available in the literature accounts satisfactorily for the experimental results shown in Part I of the present paper. However, this model cannot explain the results obtained in the pH interval of 4-6. A complementary model is then proposed, considering the formation of two species: (i) [Ni(OH)]+ads, the relaxation of which is associated with a capacitive loop at medium frequencies, and (ii) [Ni(OH)] ads , the relaxation of which gives rise to an inductive loop at low frequencies. The validation of this model has been accomplished by the comparison with the experimental polarization curves and impedance measurements. A good agreement is found between the simulated and experimental results. By using the previous model proposed by Epelboin et al. [J. Electroanal. Chem., 119, 61 (1981)] for pH ≤ 3 together with the present complementary model, Ni electrodeposition can now be explained in the whole pH range of 1-6.