Why is the stainless steel thermos cup rusty-one

After seeing this title, the editor guessed that many friends would be surprised. How can stainless steel water cups still rust? Stainless steel? Doesn’t stainless steel not rust? Especially friends who do not use stainless steel thermos cups on a daily basis will be even more surprised. Today I will briefly share with you why stainless steel thermos cups rust?

Stainless steel is a general term for some special alloy steels. It is called stainless steel because the metal material of this alloy will not rust in air, water cups, steam and some weakly acidic liquids. However, different stainless steels will also rust after reaching their own oxidation conditions. Doesn’t this contradict the name? No, the word stainless steel is an expression of the properties and characteristics of metal materials. For example, the real name of 304 stainless steel as we all know is austenitic stainless steel. In addition to austenitic stainless steel, there are also ferrite and martensitic stainless steel. etc. The difference is mainly due to the difference in chromium content and nickel content in the material, as well as the difference in density of the product itself.

Friends who have the habit of careful observation in daily life will find that there is basically no rust on stainless steel materials with particularly smooth surfaces, but some stainless steel products with rough surfaces and pits will rust at the pits. This is mainly because Because the smoother the surface of stainless steel, there will be a layer of water coating on the surface. This water coating isolates the accumulation of moisture. Those damaged water coating layers with pits on the surface will cause moisture in the air to accumulate, causing oxidation and rust. Phenomenon.

The above is a way for stainless steel to rust, but not all stainless steel materials will oxidize and rust under the above circumstances. For example, the 304 stainless steel mentioned just now and the well-known 316 stainless steel rarely have this phenomenon. Stainless steel products also called stainless steel, such as 201 stainless steel and 430 stainless steel, will appear.

Here we will focus on the three materials commonly used in the production of stainless steel water cups on the market: 201 stainless steel, 304 stainless steel and 316 stainless steel. In the previous article, the editor mentioned that currently 201 stainless steel cannot be used as a production material for stainless steel water cups because it cannot meet food-grade requirements and the element content in the material exceeds. This is actually somewhat imprecise. What the editor meant at that time was that 201 stainless steel cannot be used as the material for the inner wall of the stainless steel water cup. Since 201 stainless steel cannot reach food grade, it cannot be in contact with drinking water for a long time.

People who drink water soaked with 201 stainless steel for a long time will experience physical discomfort and affect their health. However, since the inner tank of the stainless steel thermos cup is double-layered, the outer wall will not be exposed to water, so it has been used by many manufacturers as the production material for the outer wall of the stainless steel water cup. However, the anti-oxidation effect of 201 stainless steel is far less than that of 304 stainless steel, and it is resistant to salt spray. The effect is poor, which is why after using the thermos cups used by many friends for a period of time, the inner wall of the inner tank will not rust, but instead the outer wall will rust after the paint peels off, especially the outer wall with dents.