It’s not common to find stainless hex keys. Wiha doesn’t sell stainless hex keys anymore. PB Swiss doesn’t sell stainless hex keys. I actually couldn’t find stainless hex keys last time I looked.I was using "fresh" high quality stainless hex keys as sold by a respected local industrial supply company
Most if them are chrome plated tool steel or black oxide coated tool steel. The chrome plating is very hard and mars button head hex screws. The black oxide doesn’t but wears off and doesn’t look good.