Aço revestido com eletrodepósitos de liga ZnFe é um material de uso comum na indústria de construção civil e automotiva. O revestimento à base de zinco necessita, entretanto, ser protegido de ambientes corrosivos através de um tratamento de superfície que geralmente contém cromo hexavalente (Cr6+), tóxico para o meio ambiente e cancerígeno para o homem. O presente trabalho apresenta um estudo com inibidores de corrosão contendo terras raras (Ce ou La), sobre aço revestido com eletrodepósitos de liga ZnFe. Esses filmes foram avaliados através das Curvas de Polarização, da Espectroscopia de Impedância Eletroquímica (EIE), da Microscopia Eletrônica de Varredura (MEV) e Espectrometria de Energia Dispersiva (EDS). A espectroscopia fotoelétrica de emissão de raios-X (XPS) também foi utilizada para comprovar a presença desses filmes contendo óxidos sobre a superfície da liga ZnFe. Os resultados de EIE e polarização indicaram que as camadas estudadas foram eficientes contra corrosão e que seu desempenho dependia do tipo de cátion presente no sal. A amostra representada por Ce+ZnFe mostrou o melhor desempenho na proteção contra a corrosão, havendo um decréscimo de aproximadamente 10 vezes na densidade de corrente quando a liga foi medida com Ce+ ZnFe um aumento de IZI em uma ordem de grandeza.

Zinc-iron alloy electroplated steel is a material commonly used in the construction industries and automotive. This zinc-based material needs, however, to be protected against corrosive environments through a surface treatment that generally has hexavalent chromium, wich is toxic to the environment and may cause cancer to human being. This paper shows a study with the films of rare earth (Ce or La) on the zinc-iron alloy electroplated steel. These films were evaluated using Polarization Curves, Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM) and EDS. The X-ray photoelectron spectroscopy (XPS) was also used to prove the existence of oxide films on the surface of zinc-iron alloy electroplated steel. The EIS and Polarization results have shown that the systems under study were efficient against corrosion and its performance depended upon the type of salt. The sample ZnFe+Ce proved to have the best inhibitors against corrosion.The cathodic branch of the polarization curve presents a decrease in order of 10 times in the current density when the ZnFe alloy coating were treated with rare earth and gZgincrease in order of one time at the same circumstance.

As características protectoras das superfícies de aços revestidos com zinco (imersão a quente ou electrodeposição) são geralmente intensificadas através da passivação por crómio hexavalente (Cr6+). Porém, por ser este um elemento tóxico, vários estudos visando a substituição do procedimento de cromatização vêm sendo realizados. As resinas de silicone, devido a propriedades como dureza, resistência química e aderência, podem vir a ser um excelente substituinte ao processo de cromatização. Neste trabalho, filmes de silicone obtidos pela hidrólise e subsequente condensação do tetraetoxisilano (TEOS) em presença de Polidimetilsiloxano (PDMS - OH) e de uma resina de silicone comercial foram aplicados em aço galvanizado e aço electrodepositado com ZnFe. A composição estudada foi 45% PDMS - 40% TEOS - 15 % Resina. O revestimento foi então avaliado através de Espectroscopia de Impedância Electroquímica, em solução de NaCl 3%. A evolução do comportamento frente à corrosão do sistema de revestimento foi investigada de forma temporal, verificando-se a influência dos diferentes componentes na resposta electroquímica do filme. Os dados obtidos pela Espectroscopia foram modelados através de um circuito equivalente, permitindo a obtenção dos parâmetros electroquímicos para o sistema avaliado. Os resultados obtidos mostram que o comportamento dos filmes de silicone é dependente do tipo de revestimento metálico que foi aplicado no aço. Maiores valores para |Z| foram obtidos para os filmes aplicados sobre o aço galvanizado, sendo este valor pouco alterado com o tempo de imersão.

Corrosion properties of galvanized steels are generally increased through a passivation processes with Cr6+ resulting in the formation of a passive layer. However, these kinds of chromates are toxic and environmentally legislation prohibits pre-treatments with chromates. As a consequence alternative pre-treatments have been studied along of last decades. Thus, alternative coatings of silicone resins due to their specific properties, such as hardness, chemical resistance and hydrofobicity, can be a viable substitute. In the present work silicon films obtained by hydrolysis and posterior condensation of tetraethoxysilane (TEOS) in presence of Poli(dimetilsiloxane) (PDMS - OH) and a commercial silicone resin, in a sol-gel process conditions, were applied on galvanized steel and on steel electroplated with a ZnFe alloy. The composition studied was 45%PDMS - 40% TEOS - 15% Resin. The film corrosion protection properties were evaluated by Electrochemical Impedance Spectroscopy (EIS) in a NaCl 3% solution at different immersion times. EIS experimental data were fitted to an equivalent circuit from which the electrochemical parameters were obtained. Results show that the overall performance of the coatings system appears to be highly dependent on the metallic coating kind applied to the steel. The better results in terms of impedance |Z|were obtained in coatings applied over galvanized steels, these impedance values showing a stable behaviour with immersion time increasing.

Chromatation pre-treatments have been widely used to improve galvanized steel corrosion resistance. However, due to the high toxicity of chromate ions, chromatation pre-treatments tend to be banned and, in last years, alternative coating systems are under investigation. Recently, polysiloxanes have been developed for application as coatings. Among them, and due to their specific properties, such as hardness, chemical resistance and hydrophobicity, silicone resins may be considered as promising substitutes for chromatation pre-treatments. In this work silicone films, obtained from the hydrolysis of a methoxy functional silicone reactive intermediate, were applied on galvanized steel and on steel electroplated with a ZnFe alloy. Electrochemical techniques were used to characterize the degradation behavior of the samples. These consisted on the monitoring of the open circuit potential (OCP), and on the potentiodynamic polarization of the samples, which was performed in a 3% NaCl aqueous solution. Additionally, electrochemical impedance spectroscopy (EIS) was used as a complementary technique for the evaluation of the corrosion mechanisms of the coating system. SEM and EDS were employed to inspect the surface of the samples before and after the electrochemical tests. EIS data was fitted to an equivalent circuit from which the electrochemical parameters were obtained. Results show the protective character of the resin films, when compared with uncovered specimens. The capacitance of the films increased with the immersion time, in accordance to the behavior expected for an organic film. The overall performance of the coating systems appears to be highly dependent on the type of metallic coating applied to the steel. During the first three days of immersion the coatings applied upon galvanized steel showed larger |Z| values when compared with those applied to the electroplated steel, indicating a superior corrosion resistance of the former. However, after that time, an abrupt drop of |Z| is observed in the film applied on galvanized steel. In comparison, the coating system involving ZnFe alloy evidences a better stability throughout the immersion time.