adding aluminium hydroxied sol to nitric acid

It is one type of of Passivation.
Passivation is the process of making a material "passive" in relation to another material prior to using the materials together.
For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and peroxide alternating with deionized water.
The nitric acid and peroxide oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.