Interior Discoloration

When heating water, the metal discolors. How can I clean this and why does this happen?

The aluminum interior will discolor due to minerals present in water, which is normal and does not affect the coffee brewed nor does it indicate a material defect. To remove or help lighten the discoloration, fill the coffeemaker with water to the level of discoloration (at least to minimum cup level, but not past maximum cup level). Place the stem and empty basket in place. For every 12-15 cups of water in coffeemaker, add 2 teaspoons cream of tartar to the basket. Secure the cover in place, plug in and turn on if model has a switch. Allow to complete perk cycle, then let stand 15-20 minutes or until discoloration is lightened. Unplug, drain and allow to cool before cleaning parts as normal. Do not immerse coffeemaker base in water or other liquid.