Metodologias de avaliação do inibidor volátil de corrosão (VCI) na proteção do aço
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Data
2018-05-18
Autores
Valente Junior, Marco Antonio Gomes
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
Os inibidores voláteis de corrosão (VCI) são compostos que se vaporizam e condensam sobre uma superfície tornando-a menos suscetível à corrosão. Geralmente, são empregados para proteger as peças metálicas durante armazenamento e transporte em longos períodos. Na literatura são relatadas várias condições experimentais para avaliar o VCI. Em vista disso, neste trabalho foram feitos estudos comparativos para avaliar a eficiência do VCI em meios aquosos ácidos ou NaCl, empregando testes de câmara úmida e eletroquímica, quando foram analisados a superfície do metal e do vapor saturado com VCI para compreender o mecanismo de proteção, volatilização e seleção do VCI obtidos no laboratório para estender aos testes em campo. Os compostos de VCI estudados foram aminas, ácidos carboxílicos e seus respectivos sais. Nos estudos em meio aquoso ácido para estimar a eficiência de inibição do VCI foram realizados os testes de câmara úmida em atmosfera contendo vapor de HCl ou testes eletroquímicos em solução aquosa de HCl, que permitiram obter as isotermas de adsorção. Além disso, empregando a teoria do funcional da densidade (DFT) foi feito um estudo sobre a densidade eletrônica do VCI. Com os dados de eficiência de inibição do VCI obtidos no teste de câmara úmida, energias dos orbitais moleculares obtidas por DFT e suas propriedades físico-químicas foram correlacionadas por um modelo de regressão linear múltipla. Nos experimentos em meio aquoso em NaCl sem controle de pH foi estudado a composição do vapor de VCI em função do tempo por micro extração de difusão gasosa (GDME-Gas diffusion micro-extraction) e por análise cromatográfica por HPLC-UV (High Performance Liquid Chromatography). Também foi investigada a influência do vapor de VCI na superfície do metal por espectroscopia Raman in situ através do efeito SERS (Surface-Enhanced Raman Spectroscopy). Além disso, na câmara úmida foram feitos ensaios com cristais de NaCl deliquescentes para simular o eletrólito condensado da fase vapor e, também ensaios de impedância eletroquímica (EIS) em solução aquosa de NaCl com VCI dissolvido. A partir dos resultados obtidos conclui-se que o mecanismo de proteção dos VCI pode ocorrer por adsorção física do VCI saturado na atmosfera e que também pode modificar a composição da camada de óxido primário causada pelas variações de pH resultante da atmosfera do inibidor (〖pH〗_GDME). A seleção do VCI por testes laboratoriais, principalmente os eletroquímicos, podem não ser confiáveis para indicar o VCI mais apropriado para os testes de campo. Neste trabalho sugere que as aminas CHA e DCHA combinada com os sais de VCI podem prolongar a eficiência do VCI, devido à rápida saturação inicial das aminas voláteis e com o tempo de armazenamento das peças, os sais de VCI podem manter a saturação do ambiente em caso de esgotamento das aminas.
The volatile corrosion inhibitors (VCI) are compounds that vaporize and condense on a surface making it less susceptible to corrosion. They are generally used to protect metal parts during long term storage and transportation. Several experiments have been reported in the literature to assess VCI. In this work, comparative studies were performed to evaluate the efficiency of VCI in aqueous or aqueous NaCl using wet chamber and electrochemical tests when the surface of the metal and VCI saturated vapor were analyzed to understand the mechanism of protection, volatilization and VCI obtained in the laboratory to extend the field tests. The VCI compounds studied were amines, carboxylic acids and their salts. In the studies in aqueous acid medium to estimate the efficiency of inhibition of the VCI were carried out the wet chamber tests in atmosphere containing HCl vapor or electrochemical tests in aqueous solution of HCl, that allowed to obtain the isotherms of adsorption. In addition, using the density functional theory (DFT), a study was made on the electronic density of the VCI. With the VCI inhibition efficiency data obtained in the wet chamber test, the energies of the molecular orbitals obtained by the DFT and its physicochemical properties were correlated by a multiple linear regression model. In aqueous NaCl experiments, the VCI vapor composition was studied over time by GDME (Gas diffusion micro-extraction) and by chromatographic analysis HPLC-UV (High Performance Liquid Chromatography). The influence of VCI vapor on the metal surface was also investigated by Raman spectroscopy in situ through the Surface-Enhanced Raman Spectroscopy (SERS) effect. In addition, in the wet chamber tests were performed with NaCl deliquescent crystals to simulate the vapor phase condensed electrolyte and also electrochemical impedance (EIS) tests in aqueous solution of NaCl with dissolved VCI. From the results obtained it is concluded that the protection mechanism of the VCI can occur by physical adsorption of the saturated VCI in the atmosphere and that also can modify the composition of the primary oxide layer caused by the variations of pH resulting from the atmosphere of the inhibitor (〖pH〗_GDME). Selection of VCI by laboratory tests, especially electrochemical, may not be reliable to indicate the most appropriate VCI for field tests. In this work, it is suggested that the amines CHA and DCHA combined with the VCI salts can prolong the efficiency of the VCI due to the rapid initial saturation of the volatile amines and, with the storage time of the pieces, the VCI salts can maintain the saturation of the environment in case of exhaustion of the amines.
The volatile corrosion inhibitors (VCI) are compounds that vaporize and condense on a surface making it less susceptible to corrosion. They are generally used to protect metal parts during long term storage and transportation. Several experiments have been reported in the literature to assess VCI. In this work, comparative studies were performed to evaluate the efficiency of VCI in aqueous or aqueous NaCl using wet chamber and electrochemical tests when the surface of the metal and VCI saturated vapor were analyzed to understand the mechanism of protection, volatilization and VCI obtained in the laboratory to extend the field tests. The VCI compounds studied were amines, carboxylic acids and their salts. In the studies in aqueous acid medium to estimate the efficiency of inhibition of the VCI were carried out the wet chamber tests in atmosphere containing HCl vapor or electrochemical tests in aqueous solution of HCl, that allowed to obtain the isotherms of adsorption. In addition, using the density functional theory (DFT), a study was made on the electronic density of the VCI. With the VCI inhibition efficiency data obtained in the wet chamber test, the energies of the molecular orbitals obtained by the DFT and its physicochemical properties were correlated by a multiple linear regression model. In aqueous NaCl experiments, the VCI vapor composition was studied over time by GDME (Gas diffusion micro-extraction) and by chromatographic analysis HPLC-UV (High Performance Liquid Chromatography). The influence of VCI vapor on the metal surface was also investigated by Raman spectroscopy in situ through the Surface-Enhanced Raman Spectroscopy (SERS) effect. In addition, in the wet chamber tests were performed with NaCl deliquescent crystals to simulate the vapor phase condensed electrolyte and also electrochemical impedance (EIS) tests in aqueous solution of NaCl with dissolved VCI. From the results obtained it is concluded that the protection mechanism of the VCI can occur by physical adsorption of the saturated VCI in the atmosphere and that also can modify the composition of the primary oxide layer caused by the variations of pH resulting from the atmosphere of the inhibitor (〖pH〗_GDME). Selection of VCI by laboratory tests, especially electrochemical, may not be reliable to indicate the most appropriate VCI for field tests. In this work, it is suggested that the amines CHA and DCHA combined with the VCI salts can prolong the efficiency of the VCI due to the rapid initial saturation of the volatile amines and, with the storage time of the pieces, the VCI salts can maintain the saturation of the environment in case of exhaustion of the amines.
Descrição
Palavras-chave
Inibidores voláteis de corrosão, EIS, GDME, DFT, QSAR, Volatile corrosion inhibitors, Perda de massa, Deliquescência, Isotermas de adsorção, Deliquescence, Adsorption isotherms