Boehmite coatings

Boehmite (7-AIOOH) coatings are natural oxide coatings formed by the reaction with water at a temperature of at least 75 °C according to the reaction  

If the temperature is too high, the formation of boehmite will be localised at grain boundaries and may degenerate to intercrystalline corrosion. The purer the metal, the lower the temperature limit. In practice, it is not recommended to exceed the following 

Before growing a boehmite film, the surface should be cleaned by pickling at room temperature for 30-40 s in a bath of the following composition  

The thickest boehmite coatings are obtained in alkaline media these coatings present the best resistance to corrosion. However, they have a milky, white colour, while coatings obtained with water vapour are very clear. 

Boehmite coatings between 1 and 2 pm thick (depending on the alloys) can be obtained after immersion for 8 h in a solution containing 3 g triethanolamine (C2H5)3N. Ammoniac solutions at 80 °C at a concentration of 2g l are also a good medium for growing boehmite coatings. 

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Yang C. Y., Shih W. Y. and Shih W. H., Gelation, consolidation, and rheological properties of boehmite-coated silicon carbide suspensions. J. Am. Ceram. Soc. 83 (2000)pp. 1879-1884. 

Fig. 6.26. Attempt to apply a mesoporous Y-AI2O3 coating on a layer 2 substrate with cracks by capillary colloidal filtration of a boehmite sol without macromolecular additives. In the layer 2 crack regions no boehmite coating develops. The layer 3 coating shows pinholes (SEM picture).

Fig. 6.30. Nucleation and growth of pinholes during drying is suppressed by the addition of polymer thickener to the boehmite coating sol. SEM picture of the surface of a largely defect-free Y-AI2O3 coating (thickness 3 pm) on the substrate shown in Fig. 6.7b (3 layers).

For the preparation of aluminate coatings, the deionized water was preheated with an electric heater under constant stirring to the desired temperature, a zirconia disc was inserted and then the AIN powder added to the water. The pH and temperature were measured versus time using a combined glass-electrode/Pt 1000 thermometer pH meter (Metrohom 827). In addition, some of the prepared boehmite coatings on zirconia surface were thermally treated in the resistance oven in dry air, at 900 °C, for 1 hour, at a heating rate 10 °C/min. 

Figure 3 SEM micrographs of the boehmite coating on the zirconia surface precipitated using AIN powder hydrolysis at 90 °C (a) after deposition (b) after heat treatment at 900°C in air.

Key words chromate conversion coatings, rare earth conversion coatings, sol-gel coatings, boehmite coatings, rare earth deoxidation, rare earth inhibitors. 

The thickness of the boehmite coating varies both with the solution chemistry and the underlying alloy composition/microstructure. The cmrent model of oxide growth in an aqueous environment includes (a) formation of an amorphous oxide,... 

Boehmite coatings have been known for many years, but have not been used as widely as one could have expected. This is mainly because of the long dumtion of the treatment and its cost. Moreover, solutions for growing boehmite coatings need to be regenerated frequently. 

It is well known that increasing temperature leads to an increase in the rate of chemical reactions. In the case of the corrosion of aluminium, this applies to inorganic acids and bases (see Chapters E.4 and E.5), and also to certain organic media such as alcohols, phenols and chlorinated derivatives, especially when the temperature approaches their boiling point. However, in pure, distilled or poorly mineralised water, an increase in temperature will modify the form of corrosion because the natural oxide film can react with water to form a protective boehmite coating (see Section B.5.1). The corrosion resistance of aluminium in water depends on the temperature (see Section D.1.7). 

Boehmite coatings can be formed on aluminium and its alloys in solutions containing 3% triethanolamine (see Section B.5.1). 

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