Firstly, there are many kinds of stainless steel, but nearly every stainless steel you’ll find outside of specialized industry is either ferritic or austenitic. If you’ve heard of 440 stainless or sometimes referred to imprecisely as “surgical stainless steel”, that’s a ferritic stainless steel and only features chromium and iron to improve strength and save on costly nickel. It will corrode, albeit slowly, and generally only when exposed to chemicals. 440 is popular for cheesy gift shop novelty knives.

If you’ve ever had cheap stainless steel silverware rust in the sink, this is usually a 400 series ferritic stainless steel.

The costlier, softer, weaker, much more corrosion-resistant austenitic stainless steels, like 304 and 316 stainless contains chromium and iron like ferritic stainless, but adds in some nickel. They will generally only rust when exposed to extremely corrosive salts like lithium chloride, or if it is heated up glowing hot, generally by welding. This heat-affected zone or HAZ, will rust a little bit unless you use a very low carbon alloy, referred to as 304L or 316L. The L suffix denotes a low enough carbon content that the HAZ will not generally rust under any circumstances.

You can see these welds led to ugly corrosion in the neighboring metal.

So 316L or 304L, and even regular 316 or 304 that hasn’t been welded on, will basically never corrode in normal outdoor conditions (not next to the ocean, not exposed to pool chemicals, etc.). I have seen 55 year old stainless equipment dug out from mud and other rusted carbon steel parts that looks brand new.

So, to keep stainless from rusting outside, you need an austenitic alloy and reasonably mild chemical conditions.