Anodic coatings hydrothermal sealing

Fe-, Cd-, and Sn-based alloys. Chromate solutions are also used to seal coatings formed by anodizing or hydrothermal treatment. CCCs are primarily used to improve adherence of subsequently applied organic coatings, or to impart corrosion resistance under atmospheric exposure conditions. Certain types of CCCs contain labile hexavalent chromium (Cr(VI)), which can be leached from the coating when it is contacted by an attacking solution. This Cr(VI) can be transported... 

Anodized coatings are typically 2 to 25 pm thick, and consist of a thin nonporous barrier layer next to the metal with a porous outer layer that can be sealed by hydrothermal treatment in steam or hot water for several minutes. The resultant oxide is also nonconductive. This particular property of the anodic oxide is useful in the production of electrolytic capacitors using a special bath of boric and/or tartaric acids. 

The hydrothermal sealing (90 to 100°C) process consists of immersing the anodized parts in a solution of boiling water or other solution such as nickel acetate, wherein the aluminum oxide is hydrated. The hydrated form of the oxide has greater volume than the unhydrated form and thus the pores of the coating are filled or... 

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