Any idea what metal this is? Welding options?

[Texasdoc]

Since I don't need it to be magnetic, can I just use a 300/316 series stainless? I am still assuming this is 400 series - it is slightly magnetic. It may even just be steel. I'm not sure how to tell.

Edit: I found this on a welding website.

If the tube is martensitic or ferritic, the magnet should stick just as well as it would to carbon steel. If it is only slightly attracted, I would guess you have austenitic tube there, probably 304. 304 is the most susceptable to martensite transformation during cold working. The strain placed on the tube while forming the sheet into a tube probably transformed some of the austenite into martensite which you are picking up with your magnet.

So, I think I probably have a 300 series stainless. Looking at the photo in the first post, the welds around the plugs have rusted a bit but the tank hasn't. There isn't any surface rust either. So I think the tank is stainless. Since a magnet barely sticks, I don't think it would be ferritic or martensitic.

I'm going to go buy a sheet of 300 and go to town. (Someone hold my beer....)