Okay, I'm not getting my points across. Yes salty water, only that containing sodium chloride (salt), is corrosive to steel. Chlorides are also corrosive to steel.
Average water heaters don't contain stainless steel. They do contain mild carbon steel and it corrodes when dry simply by being in a humid environment such as a living room; let alone in a basement or garage.
All steel tank water heaters have a glass lining. If the lining is intact, sea water (usually stated as water with 35,000 ppm TDS) is not going to damage the glass let alone the steel of the tank. TDS is total dissolved solids.
Ion exchange softening does not add chlorides to the water. It does add sodium to the water, that is not "salt".
That addition of sodium may slightly, it depends on how much ion exchange is being done, increase the TDS of the water. Softening does not reduce TDS.
That slight increase will slightly increase the (electrical) conductivity of the water. So, slightly more electrical current would be conducted in the softened water than in the non-softened hard water but...
Remove the TDS (deionize the water) and water can no longer conduct electricity. DI water can get the conductivity down to 18 meg ohms.... (that's 18 million ohms of resistance to electrical current flow). So the TDS of the water is responsible for electrical conduction, not the salt the water contains.
The school science experiment water didn't contain sufficient TDS so we added salt to increase the TDS, thereby the conductivity of the water, so the small current could flow for the project so the light bulb would illuminate.
That's the same as adding salt to the water in a certain type of air humidifier. I have to do that with each refill of water because my city water here has a TDS of less than 100 ppm BUT, I also have a softener for the 4-5 gpg of hardness in my city water and use salt to regenerate it.
The question is/was, is softened water containing salt capable of damaging a water heater. IMO you have to try real hard to get sodium chloride (salt) to stay in the resin tank so as to be able to get it out of the softener into the water heater. Then the glass lining of the heater has to be damaged to allow that sodium chloride to rust the tank. No?
Mike... here are some links concerning septic systems and softened water sodium content.
BTW, IF you use potassium chloride, the formula to find how much added potassium is: 13.4 mg/l per grain of ion exchange. And too much potassium is a serious health concern (I do not know how much is too much but a tablespoon is too much because it has been said that that much can kill you).
I hope it was here that someone mentioned 'salt' substitute and french fries...
Read any 'salt' substitute container lable. You'll find most are potassium chloride. I had it in my spice rack next to the stove for years. And it was used on french fries and all other food in my house that needed 'salt', but we don't use it anymore.
http://www.awqinc.com/sodium_softening.html
http://www.epa.gov/ord/NRMRL/pubs/625r00008/html/fs3.htm
http://wqa.org/sitelogic.cfm?ID=212