Application of a Direct Current Anodic Voltammetric Technique to a 55mass%Al-Zn Coated Steel Sheet before and after an Annealing Heat Treatment.

Nogueira, Tania Maria Cavalcanti; Cruz, Matheus; Rios, Paulo Rangel

doi:10.2355/isijinternational.39.295

Cited by 5 publications

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“…O processo de dissolução da liga cromatizada tem sido amplamente estudado utilizando-se a técnica de voltametria cíclica, que produziu resultados que foram concordantes com a caracterização microestrutural por microscopia eletrônica de varredura (WALLINDER et al,1991, NOGUEIRA, 1999, 2000, 2001PINTO, 2003).…”

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Liga 55% Al-Zn: uma alternativa anticorrosiva para o setor industrial

et al. 2021

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Chapas de aço carbono, revestidas com a liga 55% Al-Zn são usadas pela indústria por possuírem elevada resistência à corrosão atmosférica, conferindo maior durabilidade ao aço devido à maior resistência à corrosão por barreira conferida pelo alumínio, combinada à proteção galvânica do zinco. Estudou-se a liga submetida a dois diferentes tratamentos de cromatização, T1 e T2 por meio de técnicas que permitem o estudo em nível macroscópico e molecular. O processo de dissolução da liga das amostras T1q e T2 foi estudado utilizando-se a técnica voltamétrica em meio neutro de KNO3, 0,2 M contendo cloreto. Os processos eletroquímicos foram monitorados pela técnica de microscopia eletrônica de varredura. Os resultados de voltametria mostraram uma maior resistência à dissolução para a liga com o tratamento de cromatização T1. Na presença de cloreto ocorreu também a dissolução da fase dendrítica rica em alumínio. Foi observado o avanço da corrosão em T1 e T2 após a exposição de campo, ao ambiente marinho, por meio de microscopia eletrônica da seção transversal no final de 403 dias. Observou-se que o zinco foi completamente consumido nos espaços interdendríticos, evidenciando um ataque muito maior nestas regiões em relação a amostra T1. Na amostra T2 pode ser verificado também um ataque nas dendritas, o que não é evidenciado na amostra T1. Os resultados dos ensaios acelerados de névoa salina e câmara úmida após 1272 horas mostraram que, no ensaio de câmara úmida, T1 apresentou uma área exposta com corrosão branca localizada enquanto T2 teve uma maior densidade de pontos com corrosão branca. No ensaio de névoa salina observou-se que T1 possui área exposta totalmente tomada por corrosão branca com alguns pontos de corrosão vermelha e T2 além de apresentar área exposta completamente tomada por corrosão branca mostrou uma maior densidade de pontos com corrosão vermelha. Os resultados dos ensaios de campo e dos ensaios acelerados de laboratório estão concordantes com aqueles encontrados na voltametria, onde T1 possui maior resistência à corrosão que T2, provavelmente ocasionado pelo maior conteúdo de cromo total. A camada de 55% Al-Zn sobre o aço carbono produz durabilidade, proteção superficial e resistência à corrosão.

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Liga 55% Al-Zn: uma alternativa anticorrosiva para o setor industrial

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“…A more detailed discussion of the relationship of the Zn dissolution with the voltammograms can be found elsewhere. 10) In view of this when one compares a chromated as received specimen with a heat treated one, two effects are being observed together: the effect of the microstructural change and the effect of the heat treatment on the chromate layer itself. This work is mainly concerned with the latter effect so that one must compare the behavior of chromated and non-chromated samples subjected to the same heat treatment.…”

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“…10)), the original dendritic structure could still be perceived but now one could see a copious precipitation within the formerly precipitation free Al dendrites. 10) The voltammograms, obtained by a DC anodic voltammetric technique, are shown in Figs. 1-3 for the as received and heat treated at 200 and 360°C, respectively.…”

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Electrochemical Behavior of a Chromated 55mass%Al-Zn Coated Steel Sheet before and after a Heat Treatment.

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The final production step of a hot-dip 55mass%Zn-Al coating normally consists in the passivation of the coating. The passivation is important to protect the material against corrosion during transport and storage. A common passivation agent used is the chromic acid leading to a complex chromate film which contains Cr(III) and Cr(VI) compounds. 1) Two main reasons have been proposed to explain protection of a hot-dip galvanized zinc coating by the passivation treatment.2) The first reason would be that the Cr(III) compounds form a barrier that isolates the zinc layer from the environment. The second reason would be related to the Cr(VI) compounds. These compounds would be able to form a film, by incorporating water molecules, spreading over the areas not covered by the Cr(III) compounds. Therefore the effect of Cr(III) and Cr(VI) would complement one another. The presence of Cr(VI) compounds and their importance on the corrosion resistance has been proposed in several papers. [3][4][5] It has been suggested that one can significantly improve the ductility of 55mass%Al-Zn coatings by an annealing heat-treatment. 6,7) However a possible inconvenient of these treatments is that they might decrease the protecting effect of the passivation. This would make the heat-treated material more susceptible to corrosion during transportation and storage.Electrochemical techniques are a convenient way to study the effect of the chromate layer on the dissolution of the substrate. These techniques have the advantages of a comparatively simple and inexpensive experimental apparatus and easy specimen preparation. 8) In addition, relatively "large" areas, of the order of 10 cm 2 , can be analyzed.In recent works 9,10) a direct current (DC) anodic voltammetric technique was applied to the qualitative characterization of the phases present in galvannealed coatings and to investigate the relationship between the dissolution behavior and the microstructure of the 55mass%Al-Zn coating that changes as a consequence of the heat treatment. These works demonstrated that the voltammetric technique produced results which were consistent with microstructural characterization by SEM. In other words, the voltammograms were sensitive to the microstructural changes produced by the heat treatment in these coatings.In this work the electrochemical behavior of hot-dip 55mass%Al-Zn coated steel sheets with and without the chromating treatment before and after an annealing heattreatment is studied by means of a DC anodic voltammetric technique. The aim of the present work is to investigate the effect of the chromating treatment on the dissolution behavior of the 55mass%Al-Zn coating that changes as a consequence of the heat treatment.Commercial 55mass%Al-Zn coated steel sheets with and without the final surface chromating treatment were used. The samples had a nominal total coating weight (computing the coating weight on both sides) of 160 g/m 2 . The substrate chemical analysis was (in mass%): C -0.063; Mn -0.23; P -0.008; S -0.0046; Si -0.037; N -0.00...

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