Influência do substrato metálico e da camada de zinco na resistência à corrosão de aços eletrogalvanizados fosfatizados e pintados

• Ano de defesa: 2007
• Autor(a) principal: Evandro de Azevedo Alvarenga
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Substrato metálico; Aços revestidos; Resistência à corrosão; Mecanismo de corrosão; Zinco; Engenharia metalúrgica; Aço Corrosão; Engenharia de minas; Resistência de materiais
• Link de acesso: http://hdl.handle.net/1843/MAPO-7REJWV

The automobile industry has used electrogalvanized steels sheet with zinc layer masses varying from 50 g/m² to 60 g/m² for the manufacture of its products,which is generally not very problematic from the point of view of atmospheric corrosion resistance. However, nowadays some car makers use electrogalvanized steels with zinc layer mass equal to or even less than 40 g/m², which is concerning because a too low zinc layer mass do not satisfactorily protect the steel against corrosion.Thus this study was proposed in order to investigate the influence of the zinclayer mass on the corrosion resistance of phosphatized and painted electrogalvanized steels. Moreover, it also evaluates the technical viability of using the Usigalve/Plus steel, whose metallic substrate is the USI-R-COR-III steel which has characteristics of atmospheric corrosion resistance , with thinner zinc layers, in order to improve theformability of the steel through the reduction of the zinc layer mass.For this study, Usigalve/Plus steels with zinc mass of 20/20 g/m2, 30/30 g/m2 and 40/40 g/m2 were produced in an industrial scale. As a reference for the evaluation of the atmospheric corrosion resistance, a drawing quality carbon steel, a USI-R-CORIII steel and a Usigalve steel were also used, the latter ones with zinc layer masses of 35/35 g/m2 and 60/60 g/m2.The test specimens used in this study were prepared by a car manufacturer.Before the corrosion tests the zinc and phosphate layers and the dry paint film were characterized. Cyclic accelerated corrosion tests (Cycled I and GM 9540P/B), field tests with saline solution spray and non-accelerated corrosion test at the Usiminas atmospheric corrosion stations located in Ipatinga-MG (industrial atmosphere) and Arraial do Cabo-Rio de Janeiro (marine atmosphere) were conducted in order toevaluate the corrosion resistance of the steels.This study shows that both the zinc layer mass and the metallic substrate play an important role in the corrosion resistance of phosphatized and painted electrogalvanized steels.The electrogalvanized steels (Usigalve/Plus) produced with metallic substrate with atmospheric corrosion resistance characteristics (USI-R-COR-III steel) presentedhigher corrosion resistance than the electrogalvanized steels manufactured with carbon steel substrate without atmospheric corrosion resistance.The pure and simple reduction of the zinc mass with the objective of solving the problems associated with formability of the electrogalvanized steels is not a good option because depending on the amount of zinc used, the corrosion resistance of electrogalvanized steels is lower to that of uncoated corrosion resistant steels with atmospheric corrosion resistance characteristics.The benefits of using electrogalvanized steels with atmospheric corrosion resistance characteristics are still higher when one considers that lower zinc layermasses could be used, thus improving the material performance in the forming and welding processes, while still keeping the same corrosion resistance of conventional electrogalvanized steels of the same class.For painting schemes similar to the ones used in this study the curves ofmean corrosion advance versus zinc mass can be used as a reference for the calculation of the zinc layer thickness, otherwise the construction of such curves is necessary.