Sealing of anodic films obtained in oxalic acid baths

The response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acid baths has been compared. Unsealed films obtained in oxalic acid remain virtually unaltered in humid atmospheres, contrary to unsealed films obtained in sulphuric acid which tend to autoseal. The former also appear to take a shorter time to reach the quality thresholds of all the usual control tests on traditional sealing in boiling deionized water. However, after sealing, both types of films age in a qualitatively identical way. Transmission electron microscopy revealed the presence of a complex structure of hexagonal cells, comprising three distinct zones in the films formed in oxalic acid, the pores in which were found to be about five times larger in diameter than those in the films obtained in sulphuric acid. This considerably facilitates microstructural examination and elucidation of the underlying sealing mechanism. Furthermore, the transformation of the film morphology under the electron beam is slower in films obtained in oxalic acid.