Burnham AL40A Indirect Water Heater - Brown water :(

Water just started looking brown. I have only been in the house a couple years (moved in to the girlfriend's house) and recently discovered the calcite tank was essentially empty. Checked pH and it was around 5.5-6 so I put in 2 - 5 gal buckets of calcite and ph seems to be up around 6.5-6.8 now. Yesterday I saw the brown water and started googling, and I am thinking the years of acidic water has damaged the tank. Is there any value in my draining and flushing the tank or if its at that point is it damaged beyond any futher use?

If so, then should I look at the newer Burnham indirects or should I look at a different brand?

Thanks
Rob