Rust=bad. There are better ways to do this, and you don't need the 2nd tank. Check out www.solarroofs.com for advice and kits to use your existing water heater as your solar storage tank.
A friend gave me an 80 gallon galvanized steel tank that I plan on using for a solar water collection tank in an open loop design, meaning the water goes from the solar panels into the 80 gallon tank then into the home water tank. The hot water going into the home hot water tank will keep the elements from kicking on.
Here's the problem. The 80 gallon galvanized tank has rust inside of it but holds 50 PSI and has no leaks. Will this cause a problem for me because of the rust inside of the tank or is there rust in all steel tanks that we just never know about?
I hope replying to an old thread isn't considered a 'no-no' - The information may be helpful to others, even if the original poster isn't around to read it
Rust is bad but the fact that it's an uninsulated tank is even worse, IMO. Whatever you store your solar heated water in, you want to be insulated . . . . very WELL insulated - Otherwise, you'll 'give away' most of the heat you gained during the day overnight and you'll have to start all over from cold again in the morning
For an open loop design, just use a really well insulated electric water heater - If 55 gallons isn't enough for you, daisy chain a couple together . . . . with insulated connecting pipes. My open loop system uses 2 of the 'Sears Best' 55 gallon tanks with 3 inches of foam insulation. When there isn't enough solar (after 2 or 3 cloudy days) I apply electricity (via a time clock) to the top element in the tank which feeds the house. The reason that it takes 2 or 3 cloudy days in a row is because there's 110 gallons of hot water 'stored' in very well insulated tanks