Writing as a Chemist / Chemical Engineer, you need to be aware that there are different qualities of stainless steel and that stainless steel is perhaps more correctly called "corrosion resistant steel".
It can rust.
It can be attacked by a number of common chemicals, particularly table salt (Sodium Chloride), highly concentrated solutions of Sodium Oxychloride bleach (Milton tablets), Potassium Permanganate (that pink mouthwash you drink at dentists), and many others.
Try a dedicated stainless steel cleaner from a supermarket or ironmonger, follow the instructions and then keep it clean - washing it out before storage.
If you still have a problem, then it's low grade stainless steel, and you may want to buy a better quality replacement.
Robert
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