Galvanic corrosion. The zinc is attacked before the steel walls... For me it came with the pool build, so I'll keep in in. I know that I have seen 1" thick anode rods come out of water heaters that are now pencil thin, I have seen the sacrificial blocks on boat outboards and lower units badly worn while the rest of the metal is in good shape, and I have seen what happens when a copper water pipe rests on a black-iron fire sprinkler line. I have to think there is something to it.
Yep.
Except for the fact that, in order for metals to be used in that way, they need to be properly matched to application. When one wants to create a sacrificial galvanic couple, the surface areas need to be matched, in other words, the areas of the cathode (your steel pool wall) and the anode (your chunk of zinc), need to be equivalent. Otherwise you will completely passivate the zinc metal with a zinc oxide/zinc hydroxide layer and it no longer conducts current. The anode becomes "polarized" (which is just a fancy way to say non-conducitve and charged like a capacitor) and it no longer protect anything. At that point, the only thing that keeps the anode going is by mechanically disturbing the surface and hoping that enough passivation flakes of so that it can conduct again and develop an cathodic current.
In water heaters and boats, the placement of the anode is such that the water current is able to remove the passivation a bit and allow for it to act in a sacrificial manner. In most saline marine applications, one uses magnesium bars and blocks to protect outboards because magnesium is a better sacrificial material. In buried steel tanks, one usually bolts or welds many zinc bars onto the tank to help protect it; for critical applications, you actually use an impressed cathodic current protection (ICCP) system. ICCP systems are powered electrical devices that actually supply current to the thing they are protecting to keep oxidation from occurring.
The chunk of zinc sitting in the bottom of a PVC well with pool water flowing above it is barely doing anything at all except slowly rotting away and probably protecting the first few feet of bonding wire that is buried underground. And that's the other thing, not only is a steel wall hundreds of times larger in surface area than the bar, the wire connecting it is usually very far away which limits it's performance as an anode. Sac anodes, like surge protectors, are point-of-use devices - they work most efficiently when they are bolted to the thing they are protecting.
Unfortunately, this is just another one of those pieces of "pool equipment" that are added on to a pool because there some underlying "science" associated with them that is not at all matched to the actual application. Like UV disinfection lamps and ozone generators, these devices do something but the something they do is not necessarily significant. You could completely remove the device tomorrow and, over the years, not ever notice a difference.
