Why Water Tub has rust stains?

Discolored HumidAir Tub Metal Plates

The stains are due to minerals deposited after taped water dried up, please use distilled water (no mineral) only and don't use taped water.

The white residue is likely from non-distilled water. Users should only use distilled water for the humidifier.

Local waters contain dissolved minerals such as salts and carbonates that leave white residue and cause corrosion.

The user guide recommends cleaning with a diluted solution of vinegar. Vinegar slowly dissolves some of the carbonates.

It is evident that the main substance in the water that has caused this corrosion was undoubtedly the high level of chloride from the taped water used in the tubs. On evaporation of this taped water within the tub, residual salts of calcium, magnesium, potassium and sodium chloride (as well as carbonates and sulphate salts) would have formed on the surface. The deposition would have held the corrosive chloride anion close to the metal surface allowing the chloride in the salts to readily attack the stainless steel resulting in both surface and pitting corrosion.