Why does the inside of a stainless steel water cup turn black after being used for a period of time?

Why do some water bottlesturn black inside after being used for a period of time? Regarding the problem that the interior of stainless steel water cups will turn black after being used for a period of time, this should be discussed from several aspects:

1. Reasons for materials

It is also a stainless steel water cup. Due to the different materials used, the inside will turn black after being used for a period of time. For example, comparing 304 stainless steel with 201 stainless steel, the interior of 201 stainless steel will turn black after being used for a period of time. At the same time, the phenomenon of blackening inside the stainless steel water cup is mostly caused by the material.

2. Different internal processing techniques

They are also stainless steel water cups. Some water cups are finished with electrolytic treatment inside and sandblasting treatment on the internal surface, which is equivalent to forming a protective layer on the internal surface. Some stainless steel water cups are polished on the inside and do not form a protective layer. Similarly, if you drink acidic and alkaline liquids for a long time, the polished interior will turn black.

3. Related to personal living habits

For example, some consumers are often accustomed to drinking carbonated drinks, tea, or using stainless steel water cups to brew coffee. Consumers who often drink these drinks should clean the water cups in time and take good care of the water cups. If carbonated drinks are stored in stainless steel cups for a long time, there will be obvious corrosion. The tea itself will have tea stains. Since coffee has strong vegetable oils, it will also form pigmentation if it is not cleaned in time.

Therefore, there are many reasons for the blackening of the inside of the stainless steel water cup after being used for a period of time, but most of them are caused by substandard materials. If you find that the inside of the stainless steel water cup turns black during daily use, you can first use magnets to absorb the inner wall to determine whether the material is not up to standard. Once the magnet is found to be adsorbed on the inner wall of the stainless steel water cup, it means that the material is not food grade and please do not continue to use it.