Very common, stain"less" steel is not corrosion proof. There are many types of stainless alloys the ones used for making of barrels has to trade off some of the corrosion resistant properties in order to have strength and wear properties.Without getting into the different alloy makeups, 304, 316 etc, etc, thats about as basic as I can put it. You still must clean and treat "oil" stainless steel. Stainless justbuys you more time between cleanings.