Comparison of aluminum coatings deposited by flame spray and by electric arc spray

Aluminum coatings are widely used as a protection against corrosion on steels substrates. Among the various deposition processes, thermal spray has proved to be a process that is easy to use and offers a long-lasting alternative in maintenance operations. Recent studies [Rodriguez, R.M.H.P. Formação de óxidos nos revestimentos de alumínio depositados por aspersão térmica. Curitiba-PR: ST, UFPR, 2003. Tese (Doutorado) -- Programa Interdisciplinar de Pós -- Graduação em Engenharia -- PIPE, Universidade Federal do Paraná, 2003. 118p; Paredes, R.S.C. Estudo de revestimentos de alumínio depositados por três processos de aspersão térmica para a proteção do aço contra a corrosão marinha. Florianópolis-SC:ST, UFSC, 1998. Tese (Doutorado) - Departameto de Engenharia Mecânica, Universidade Federal de Santa Catarina, 1998. 245p] demonstrated that the source and the gas used for aluminum spraying generate dissimilar corrosion protection behaviors. This paper purports to contribute toward a better understanding of the differences between the flame spray (FS) and electric arc (EA) processes. Initially, the coatings were analyzed by XPS (X-ray Photoelectron Spectroscopy), which revealed differences resulting from the deposition processes. To deepen the investigation into the differences relating to the heat source utilized, the coatings were subjected to a 4000-hour saline mist assay. The corrosion products formed on the surface of the coatings were evaluated based on photographic records, SEM, and X-ray diffraction. The XPS analysis of the aluminum coatings revealed that the aluminum oxide/hydroxide layer persisted at a greater depth in the coating deposited by EA than in that deposited by FS. This difference may also indicate the formation of different aluminum compounds. An analysis of the coatings subjected to the saline mist assay revealed clear differences in the corrosion products formed, which were strongly adherent in the FS coating and only slight adherent in the EA coating, leading to loss in thickness. X-ray diffraction revealed that the principal phase was bayerite in the FS process and boehmite in the coating deposited by EA. It should be noted that the heat source and the deposition material are determining factors in the protection process. In the period during which the coatings of this study were subjected to saline mist, both the FS and EA deposition methods provided satisfactory barrier protection.