The hardness of water is not corrosive in most cases. Hardness is an indication of Lime or minerals which are neutralizing the PH. The corrosion of the Tank can often be traced to grounding of the electrical system to the plumbing (dielectrolysis) and friction loss from water moving thru the pipes over time. The cutting off of the water supply and the draining of the system to prevent freeze damage is the best method of keeping you from having a surprise when you arrive. The plastic tanks in my opinion have other faults that make them no better than the metal ones. I have replaced them due to cracked fittings, leak in the diaphragm and pressure stress cracks in the assemble.
Art