The most common materials on thermos cups are 304 stainless steel and 316 stainless steel, but there is actually a more common stainless steel grade, but manufacturers generally do not mark it: as 201 stainless steel.
The stainless steel thermos cup has at least two layers, one inner tank, and one outer shell. Regardless of the material of the liner, the outer shell is generally 201 stainless steel. The advantage of it is that it is cheap and sturdy. The disadvantage is that it is easy to be corroded, so there is often a layer of plating or paint on the shell.
If it weren't for the ultra-cheap thermos cups that cost ten or twenty yuan, 201 stainless steel would not be used on the inner liner - this kind of stainless steel does not meet the stainless steel grade specified in the national standard for food contact. In other words, when food comes into contact with 201 stainless steel, harmful substances will be produced.
Therefore, the inner tank is generally made of 304 stainless steel or 316 stainless steel. There are two differences between the two:
1.316 stainless steel has stronger corrosion resistance. If you often make drinks (milk, coffee, etc.) with a cup, it is recommended to choose 316 stainless steel. As for tea leaves, the corrosion to stainless steel is relatively small. So whether it is tea, flower tea or fruit tea, 304 stainless steel is enough.
2.316 stainless steel is more expensive than 304 stainless steel, but its hardness is softer than 304 stainless steel. Therefore, although 316 stainless steel has strong corrosion resistance, it is not recommended as the first choice. Again, there is no need to buy 316 stainless steel water cups without making milk tea and coffee in water cups.
In addition, when looking at the material of the water cup, you may also see 304L or 316L. The "L" on the grade refers to the low carbon content in stainless steel. Its advantage is that it can avoid intergranular corrosion during welding. It is not necessary to know what intergranular corrosion is, but only need to know that this phenomenon will lead to weak welding.
But the so-called "not firm" only appears in industry. The welding line of a water cup is only ten centimeters. If it can encounter intergranular corrosion, you should go and buy a lottery ticket! Therefore, 304L or 316L stainless steel is not recommended.