In a fume hood, immerse the specimen for exactly 10 min in an aqueous solution containing 4 parts concentrated nitric acid (70 % by weight) plus one part water at 25°C (77°F). Occasionally brush the surface with a soft bristle brush.
After 10 min, transfer the beaker containing the aluminum specimen and cleaning solution to an ultrasonic cleaning unit.
Apply power for 1 min. (Warning—Nitric acid is a strong oxidant. Avoid contact with skin, eyes, and clothing. Do not breathe vapor.)