Which electrode is used to weld stainless steel to stainless steel?

It depends on the type of stainless steel. Stainless steels come in various compositions and are identified by numbers and letters. Electrodes should be chosen that match these. For example when welding 316 stainless use type 316 electrodes. In a lot of situations you can safely use the next higher number. 304 stainless can be welded with 308 electrodes, 308 stainless with 310 electrodes. 309 electrodes are generally used to weld stainless to carbon steel.