I know, it's my mistake. I used common non-filtered water in the humidifier. And now it has limescale. How to remove it carefully and don't break humidifier?
Limescale can be dissolved by acids.
Vinegar essence is a popular cleaning agent because it doesn't leave residues, but it smells quite pungent.
Citric acid works just as well and doesn't smell. Do use diluted pure citric acid rather than lemon juice because juice leaves residues. It can be bought in most drug stores or pharmacies and even as a cooking/baking ingredient in well-sorted supermarkets.
Be responsible and safe! Wash your hands after handling acids and take care not to get any into your eyes. Keep out of the reach of children.
Plain old sparkling water contains carbonic acid in a low concentration. It's probably the safest detergent of all the listed ones but also the weakest and may take longer or multiple applications to completely clean the humidifier. The more the water fizzes, the better it can dissolve limescale. Do not let the humidifier sit in sparkling water for a whole day, because as soon as it stops fizzing, the carbonic acid is gone and the mineralized water itself builds even more limescale.
Do not use any sugary beverages like lemonade or coke unless you know you can clean sticky sugar from the humidifier sufficiently.
WD-40 contains petroleum distillates which will remain in the water and are somewhat toxic when ingested, It is a water displacing lubricant not a descaler. Lime scale is dissolved minerals in the water left after the water evaporates. Rinsing with hot distilled water (demineralized water) will clean the humidifier.
I don't if you ever have heard the word of "WD-40". As a solution I agree with Elmy but if you want cheap and trustable solution you may want to try WD-40. Here is a page for some examples that you can use WD-40
But you may find it so strong, I can offer use the products of Durgol. ı trust this brand for descaling my coffee machine.
I hope these helps.