Will stainless steel insulated cups rust?

Some friends have bought stainless steel insulated cups and found that they have rusted after using them for a period of time. Isn't it said that stainless steel won't rust? Fake goods? Actually, it doesn't mean that stainless steel insulated cups won't rust. In specific environments, it can cause them to rust.
Stainless steel has the ability to resist atmospheric oxidation, that is, stainless steel, and also has the ability to corrode in media containing acid, alkali, and salt, that is, corrosion resistance. But the magnitude of its corrosion resistance varies with the chemical composition, additive state, usage conditions, and environmental medium type of the steel itself. Like 304 steel pipe, it has absolutely excellent corrosion resistance in dry and clean atmosphere, but when moved to coastal areas, it will quickly rust in sea fog containing a large amount of salt; And 316 steel pipes perform well. Therefore, not any type of stainless steel can resist corrosion and rust in any environment.
Stainless steel is a thin, strong, and stable chromium rich oxide film (protective film) formed on its surface, which prevents the further infiltration and oxidation of oxygen atoms and provides the ability to resist corrosion. Once for some reason, this thin film is continuously damaged, oxygen atoms in air or liquid will continuously infiltrate, or iron atoms in metal will continuously separate, forming loose iron oxide, and the metal surface will also be continuously corroded. This kind of surface facial mask is damaged in many ways, and the following types are common in daily life:
1. The surface of stainless steel is covered with dust or foreign metal particles containing other metal elements. In humid air, the condensed water between the attachments and the stainless steel forms a micro battery, triggering an electrochemical reaction that damages the protective film, known as electrochemical corrosion.
2. Organic substances such as vegetables, soup, and phlegm adhere to the surface of stainless steel, forming organic acids in the presence of water and oxygen. Over time, these organic acids can corrode the metal surface.
3. The surface of stainless steel adheres to substances containing acids, alkalis, and salts (such as alkali water and lime water splashing on decorated walls), causing local corrosion.
4. In polluted air (such as atmosphere containing a large amount of sulfides, carbon oxides, and nitrogen oxides), encountering condensed water can form sulfuric acid, nitric acid, and acetic acid liquid points, causing chemical corrosion.
All of the above situations can cause damage to the protective film on the surface of stainless steel, leading to rusting. So, when we don't use the thermos for a long time, we must keep it dry and avoid storing drinks such as yogurt and salt water as much as possible to prevent corrosion of the inner wall of the thermos.