Why has my water tub corroded?

humidifier | rusty | rusted |corrosion

Corrosion occurs due to high chlorine concentrations or due to salts in the water used.
Whenever you see white residues on the tubs surface, indicates high concentration of minerals in the local water.
Please follow user guide, empty the water each morning (after used) and wash weekly with vinegar to dissolve minerals and wash with soapy water.
Remove any white powder deposit using a solution of one part household vinegar to 10 parts water. 
If the water in the water tub is left to evaporate in the tub, the metal plate will accumulate minerals, causing corrosion.
It is recommended to use distilled water, it will prevent the corrosion.