Due to weather conditions, aluminium can start to become darker in colour over time, this is called tarnish.
To remove tarnish from aluminium, clean the area with hot soapy water and then create a solution of calcium carbonate, white vinegar and boiled water. Using a very fine brush, dip it into the solution (once cooled a little) and gently rub the tarnished area of the frame in circular motions until the tarnish has lifted.
If you have any stubborn areas, apply white vinegar to the area and leave to sit for 10-15 minutes before rinsing with hot water.