material of stainless steel vacuum flask

All we knows,there is single wall and double wall stainless steel water bottle,what we care about more is that the material is safe enough ?

Normally we use 304 stainless steel(18/8) and 201 stainless steel, 304 S/S is food grade but 201 is not.

For double wall bottle,the inner wall must be 304 S/S,outer wall can be 304S/S or 201S/S,because the inner wall contact food,so it must be 304S/S.

For single wall bottle,it must be 304 S/S,because it contact food too.

Why someone use 201 S/S ?because 201 S/S is cheaper than 304S/S.

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