Not always. There are 3 types of stainless, austenetic, ferritic and martensitic stainless, this is where your grades come into it. The composition of the stainless alloy determines the mechanical properties of the metal. Austenitic is what you are after if you want top corrosion resistance. Martensitic and ferritic stainless does respond to magnesium and you can even non destructively test both types of stainless using magnetic particle testing. Ferritic stainless steel has a much higher proportion of iron in it, and is generally alot cheaper and what the Chinese make many cheaper pots and more than likely this cheap urn out of.