Abstract
The effects of additives and super-heated water vapor treatment on the composition and structure of boehmite films were studied by measurement of weight changes, X-ray diffractiometry, thermal analysis, infrared spectra, and measurement of impedance. Al specimens of 99.85 and 99.99% in purity were Pretreated and dipped in boiling baths of 0.03-0.15mol triethanol amine, diethyl amine, and monoethyl amine for 0-60min. After the boiling treatment, some specimens were treated in super-heated water vapor at 4kg/cm2 for 30min. The following results were obtained: (1) The effects of amines as additives were remarkable: the weight of coated film was 10-15 times as large as that in case of no amine additives. The super-heated water vapor treatment resulted in the increase in the coated weight of boehmite films. (2) Bayerite was not generally contained in boehmite films having been formed in boiling water and various amine solutions; but some of boehmite badly crystallized was often contained. Such boehmite films produced would be assumed to be pseudo-boehmite, because of containing hydrate with 1.7-1.9 H2O per Al2O3. (3) The crystallization of boehmite films was not developed by the super-heated water vapor treatment. However, the hydration degree decreased to 1.5-1.6 H2O per Al2O3 and the impedance of the films increased. (4) It was considered that boehmite films consisted of thin, but dense inner layer adjacent to the metal and a thicker and more permeable outer layer; the former was mainly composed of monohydrate and the latter of hydroxide. (5) It was also considered that the corrosion resistance of boehmite films depended upon the film thickness and the amount of monohydrate.
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