Stainless steel
Stainless steel is an alloy of iron with chrome and nickel, and sometimes other metals.
Stainless steel comes in different grades that have varying levels of corrosion resistance, and the most common grades used in fencing are 304 and 316. Sometimes, 3CR12 is offered as an alternative to stainless steel, but while it is more corrosion resistant to ordinary steel, it's not as durable as proper stainless steel.
If you use stainless steel for your fence, make sure that you don't use different metals on the fence, which might cause galvanic corrosion.