I just installed a 50 gallon Economizer 6 Sears brand hot water heater. The heater came with an Aluminum Anode rod. In reading most of what I have found on the forums, the only difference between a 12 year warranted tank and a 6 year warranted tank is the inclusion of a Magnesium Rod with the 12 year warranted tank.
I installed the new water heater with the aluminum rod and ordered a reliance Magnesium Anode Rod.
I received the new rod about a week after I installed the heater.
My question is: Should I replace the Aluminum rod with the Mag rod right away, or should I leave the Aluminum rod in place for a couple of years?
I guess the real question is whether the steel tank starts to corrode right away with an aluminum rod because it is not as good at preventing corrosion, or do both types work just as well but for longer or shorter periods of time based on the type of rod?
Each metal has a reactivity index. As long as the anode rod is more reactive than the steel, it will be degraded first. If there's enough surface area, in theory anyway, the tank would never rust as long as you maintained enough anode rod volume and surface area. There's lots of info on the web, and it's part of high school chemistry class. http://chemistry.about.com/od/charts...-Of-Metals.htm is one easily found source. Mg and Al are very close to each other, with Mg slightly more reactive, thus potentially a better protector (give similar volume and surface areas of the actual rods). It won't hurt to leave the Al one in there for awhile, then change it. You will likely require an impact wrench to get it out, and it will be harder later, but should come out. An anode won't reverse any existing damage, but it should stop or slow down any from occurring in the future.
Also note, though, that depending on your water chemistry, some anode rods can cause the water to smell, so you may be stuck with one or the other, regardless of how well it might stop internal corrosion.
I would suggest taking the anode rod out now, while it's still pretty easy (hopefully), then reinstalling with Teflon tape and pipe dope. Will make it much easier to remove down the road.
I did this with my new WH and I check the rod every year or two. When you start to see bare wire, replace the rod.
IF there were no steel exposed to the water then there would be no need for ANY anode rod. This was the original concept behind the glass lined tanks, until they discovered that the glass lining was not always perfect, so the anode rod was installed to "coat" any exposed metal surfaces. It was also the concept behind the State Duron tanks which used a plastic liner iinstead of glass, and thus did NOT have an anode rod.