Descaling a steel bottle
Where the taste comes from
A bottle that tastes metallic is almost never at fault: it is a chalk deposit that has formed on the wall and holds on to everything you put in before. Tea is the worst offender, coffee next.
The method
Half-fill the bottle with white vinegar, top it up with warm water, leave it an hour with the lid off. Rinse three times. If the deposit holds, a spoon of baking soda added after the vinegar lifts the rest by fizzing.
The lid gasket comes apart and is treated separately. It is the part that holds smells, and the part everyone forgets.
What not to do
No bleach: it attacks the passivation layer of the steel, which is precisely what stops the bottle rusting. No abrasive sponge and no wire brush, for the same reason.
And no dishwasher on a double-walled bottle. The heat of the drying cycle tires the vacuum seal between the two walls; a bottle that no longer keeps things cold has almost always been through a dishwasher.