Wabi Steam Sterilizer heating plate is made of 304 stainless steel and just like all house cookware, it can have some stain or discoloring issues. There are many variations of cause to discolorations and stains, it is a natural reaction to stainless steel and it is considered wear and tear and not covered under your product warranty.
Heat tint or bluing
Our heating plate dry boils the water to create steam. Overheating or boil dry can cause yellow, brown, dark purple, blue or rainbow tints on the stainless steel surface, namely, heat tint or bluing. Although harmless, these tints can spoil the appearance of the heating plate. To remove this discoloration, use a product like Barkeepers Friend® and a non-abrasive cleaning pad. Wash with warm soapy water, rinse and dry.
Mineral Haze or Scale deposits
Mineral haze is any cloud-like white spots across the heating plate surface, and it is most likely from the water you use to steam and excess water from the tap from rinsing the feeding items. It is the same thing white dry spots you see on clear glassware. All you need is some descaling process to remove these stains.
Don’t overuse the citric acid descaling powder
Although citric acid is a powerful cleaning agent, it can cause issues when adding too much or prolong exposure.
Do note that the longer you left the stains on the harder, it will be to remove them and possibly become a permanent mark, again it is harmless to continue with the usage.