By definition, stainless steel is a steel alloy of iron and at least chromium (10% or more, IIRC). Nickel is very commonly added and there might be some molybdenum or other constituents. The 304 and 316 series are highly corrosion resistant and used widely for tanks and process piping in the dairy and brewing industries. 316 is better, especially if it is going to be welded. Welding can deplete the alloy at the location of the weld with 304 stainless but less so with 316.
Most vessels that bees may come across to use for reactions such as beer or soda kegs are probably going to be 304 or 316 series stainless steel. The information posted above about passivation is right on the money. I've done it many times on tanks that one walks into to inspect. It doesn't take a very strong concentration of nitric acid to provide a durable "stainless" surface on a tank wall.