Célia Marina de Alvarenga Freire

Zn–Ni alloy deposits obtained by continuous and pulsed electrodeposition processes

Abstract Electroplated zinc coatings are considered one main way for the corrosion protection of steel sheet. Recently, the interest in zinc alloy coatings such as Zn–Ni, Zn–Fe and Zn–Co has increased owing to their better mechanical and corrosion properties compared with pure zinc coatings. New processes such as pulsed electrodeposition (deposition methods based on current pulses) have been producing deposits with better physical and anti-corrosion characteristics. In the present research a chloride bath was used and Zn–Ni alloy deposits were obtained using continuous current and pulsed current processes. The morphologies obtained for the two deposit types are presented, as well as an analysis of some parameters that influence (such as temperature and on-time) its characteristics. Electrochemical test results are also presented in order to verify the corrosion behavior of deposits.

Characterization of organic-inorganic hybrid coatings for corrosion protection of galvanized steel and electroplated ZnFe steel

Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) / Brasil; Conselho Nacional de desenvolvimento cientifico e Tecnologico (CNPq) / Brasil; Fundacao para a Ciencia e a Tecnologia (FCT) / Portugal

CARACTERIZAÇÃO FÍSICO-QUÍMICA, MICROBIOLÓGICA E SENSORIAL DE POLPA DE CUPUAÇU CONGELADA (Theobroma gradiflorum, Schum)

O presente trabalho visou a caracterização microbiológica, físico-química e a aceitação sensorial de polpa de cupuaçu congelada (Theobroma grandiflorum Schum). Analisaram-se 3 marcas do produto, denominadas A, B e C, fornecidas em saquinhos de polietileno de 100 g. A caracterização microbiológica foi fundamentada na quantificação de mesófilos aeróbios, bolores e leveduras, coliformes e na verificação da presença de Salmonella. Os testes físico-químicos basearam-se na determinação de extrato seco, atividade de água (Aa), cinzas totais, proteínas, lipídios, sólidos solúveis (°Brix), acidez titulável, ácido ascórbico, pH e cálculo do ratio. Os testes sensoriais da polpa foram conduzidos com o suco preparado a partir das amostras comerciais. Atributos de aparência, aroma, sabor e impressão global do suco foram avaliados por meio de testes de escala hedônica e de ordenação-preferência. As contagens médias de mesófilos aeróbios foram equivalentes a 1,6 x 102, 1,3 x 102 e 7,9 x 101 UFC.g–1 de polpa, nas marcas A, B e C, respectivamente. Para bolores e leveduras, obtiveram-se 4,9 x 101, <10 e 1,6 x 101 UFC.g–1. As contagens de coliformes a 30 e 45 °C foram inferiores a 0,3 NMP.g–1 nas três marcas. Constatou-se a ausência de Salmonella em 25 g de polpa, para todas as amostras testadas, em conformidade com a legislação vigente. Os resultados obtidos na caracterização físico-química revelaram desacordo de algumas amostras com a legislação para os parâmetros sólidos solúveis totais, acidez titulável e ácido ascórbico. Os resultados da análise sensorial não evidenciaram diferença entre as três marcas de polpa a 5% de significância para os testes de escala hedônica e ordenação-preferência. Concluiu-se que as três marcas de polpa congelada de cupuaçu apresentaram qualidades microbiológica e sensorial satisfatórias. No entanto, notou-se falta de uniformidade em relação aos padrões de identidade e qualidade, conforme determinado pela legislação nacional vigente.

Corrosion resistance of 2024 aluminum alloy coated with plasma deposited a-C:H:Si:O films

AA 2024 aluminum alloy is widely employed in aeronautic and automobilist industries. Its hardness and low density are attractive properties for such industrial areas. However, since it contains copper, it undergoes severe corrosion in aggressive media as saline or low Earth orbit environments. In this work, it was investigated the properties of films deposited by PECVD on AA 2024 aluminum alloy as well as the corrosion resistance of the film/substrate systems under different corrosive atmospheres. Films were prepared in a plasma atmosphere composed of 50% of oxygen and 50% of hexamethyldisiloxane resulting in a total gas pressure of 4.0 Pa. Plasma ignition was promoted by the application of radiofrequency signal (13.56 MHz) to the sample holder while grounding the topmost electrode. The plasma excitation power, P, was changed from 10 to 80 W in the six different set of experiments. Film thickness, measured by profilometer, increases by 5 times as P was elevated from 10 to 80 W. As demonstrated by the infrared spectra of the samples, films are essentially organosilicons with preservation of functional groups of the precursor molecule and with creation of different ones. The oxide proportion and the structure crosslinking degree are affected by the plasma excitation power. According to the results obtained by sessile drop technique, hydrophilic to moderately hydrophobic films are produced with changing P from 10 to 80 W. The corrosion resistance, evaluated by salt spray and electrochemical impedance spectroscopy, EIS, experiments, in general increases after film deposition. It is demonstrated that film deposition improves, in up to 36 times, the resistance of the alloy to salt spray attack. It is also shown an improvement of about 240 times in the alloy resistance under NaCl solution by the EIS data. Micrographs acquired by Scanning Electron Microscopy after the corrosion tests furnish further information on the importance of the layer physical stability on its barrier properties. Furthermore, films highly protect the alloy against the oxygen attack. Interpretations are proposed based on the modification of the plasma kinetics with P, altering film structure, composition and properties.

Improvement of the Corrosion Resistance of Carbon Steel by Plasma Deposited Thin Films

It is estimated that nearly 3% of the global domestic gross product, corresponding to 2.8 trillion US dollars, is wasted every year with problems related to corrosion (Koch et al., 2002). Oil and gas companies, for instance, spend up to US

The Role of Macrostructural and Microstructural Morphologies on the Corrosion Resistance of Zn and a Zn-4% Al Alloy

80 billion only with the corrosion of devices in marine environments (Muntasser et al., 2002). About 90% of the corrosion costs are associated with iron-based materials. Since carbon steels, which account for about 85% of the annual worldwide steel production, represent the largest class of ironalloys in use, the corrosion of such materials is of paramount importance. Notwithstanding many years of intensive research and development, there is not available an ideal protection method. A convenient method to protect metals is by the use of physical barriers against species such as water, oxygen and hydrogen. In this context, organic coatings have been considered as the most effective protective barriers. In particular, epoxy-based resins are widely applied to protect carbon steel due to easiness of processing and excellent mechanical and chemical resistances (Shin et al., 2010). However, in extended exposures to environment such rigid coatings can fail and once a defect occurs, the corrosive species can reach the metal surface resulting in localized corrosion. Owing to that, frequently the protection with epoxy demands pretreatments or the incorporation of corrosion inhibitors (Radhakrishnan et al., 2009), which may incur in prohibitive extra costs.

Experimental analysis of corrosion resistance on columnar to equiaxed transition region of as cast structures of Al–Cu alloys

Different structural morphologies may develop due to a wide range of the operational conditions that may exist during casting. It is well known that corrosion resistance and mechanical properties depend on solidification structures. The aim of this study is to investigate both the influence of columnar and equiaxed structures of zinc as-cast samples and of dendritic microstructural array of a Zn-Al alloy on the corrosion resistance. In order to obtain columnar and equiaxed structures, both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed using electrochemical impedance spectroscopy and Tafel extrapolation. Corrosion tests were conducted in a 3% (vol.) NaCl solution at room temperature. It was found that coarser macrostructures tend to provide higher corrosion resistance for both columnar and equiaxed morphologies. On the other hand, finer secondary dendrite arm spacing is conducive to a better corrosion behavior of a Zn-4% Al alloy.

Comparative behaviour in terms of wear and corrosion resistance of galvanized and zinc-iron coated steels

Abstract In the present article, in order to investigate the corrosion resistance tendency on both sides of the columnar to equiaxed transition (CET), some samples of Al–Cu alloys (5 and 8 wt-%Cu) were prepared on the columnar and on the equiaxed sides of the casting and subjected to corrosion tests. The corrosion resistance has been analysed by both the electrochemical impedance spectroscopy technique and Tafel extrapolation method conducted in a 3%NaCl solution at room temperature. It was found that both columnar and equiaxed morphologies at the CET region, for each alloy examined, have attained similar experimental electrochemical impedance and polarisation results. The secondary dendrite arm spacings λ 2 on both columnar and equiaxed sides of the CET region are very similar in any case experimentally examined and the resemblance on the corrosion resistance of columnar and equiaxed structures has been attributed to the similarity of such microstructural parameter. The corrosion resistance has decreased with increasing alloy Cu content. The experimental evidence has indicated that this can be associated with smaller dendritic arm spacings.

Silicone resin to improve corrosion resistance of Zn and ZnFe coated steel

Materials degradation due to simultaneous chemical and mechanical effects may occur under a variety of conditions. Galvanized and zinc-iron electrodeposited steels are widely used to protect automobile bodies against corrosion. In such applications, it is important to investigate the electrochemical and tribological behaviour of the coatings, in order to understand the behaviour of those surfaces in a tribological contact exposed to a corrosive environment. In the present paper, the evolution of open circuit potential, for both surfaces, was monitored during reciprocating sliding against an alumina pin in a 3% NaCl solution. The coatings are compared in terms of electrochemical behaviour by polarization tests, open circuit potential and friction coefficient during sliding; and loss of mass after sliding. The surface morphology, after sliding, was examined by SEM. Under the experimental conditions of the present study, the results found in terms of corrosion and wear resistance could be correlated to the differences in morphology, rugosity and composition of the two analysed surfaces.

INTERAÇÃO EMBALAGEM-ALIMENTO SOBRE A ESTABILIDADE DE DOCE DE CUPUAÇU (Theobroma grandiflorum Schum.)

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.