1. air oxidation: the discoloration and redness of stainless steel cups may be caused by air oxidation. Usually, at the high temperature above 400 degrees, stainless steel may release low-priced manganese metal, making the surface of the water cup appear red. In addition, there is a vacuum between the cup body and the inner container of the thermos cup, so when the cup body is subjected to a large external force, the cup body will be deformed, usually twisted at a small angle. If the cup is twisted, it will collide, resulting in the destruction of the oxide film, thus exposing the metal layer, changing the appearance of the cup and showing a red situation.

2. Brush hard when cleaning: When cleaning the stainless steel cup, if you brush hard on the cup wall, the stainless steel surface on the cup wall may be brushed off, exposing the internal metal layer, and scratches may occur when brushing, resulting in uneven surface of the stainless steel cup and a hill-like shape. This shape will destroy the oxide layer on the surface, thus exposing the metal layer to the air and causing the stainless steel water cup to turn red.

Both of the above reasons may cause the stainless steel cup to turn red. In order to solve this problem, we can try to use mild detergent and soft cloth to clean and avoid using excessive force to brush the cup wall. In addition, if the stainless steel cup turns red due to air oxidation, we can try to clean it with vinegar or lemon juice.