waste said:
I've seen zinc disks in skimmers that supposedly do the same thing. Is it true? (the disk is in the water which is in contact with some part of the bonding, is that OK?)
No, this doesn't work. The idea of a sacrificial anode is to connect it electrically (i.e. with conductive wire) to the bonding wire such that it puts a negative charge on the wire and all attached metal exposed to water in order to inhibit corrosion since it creates a greater potential to overcome for such corrosion (think of the negative charge as holding the neutral metal atoms more firmly in place preventing them from forming positive ions separating from some of their electrons). Putting a zinc block in the skimmer would just tend to corrode the zinc block adding zinc to the water that could eventually stain. It won't slow down corrosion with other metal in the pool. Chlorine (and dissolved oxygen) would simply oxidize the zinc block, but would continue to oxidize other metal in the pool as well.
In a somewhat real sense it's like a chemical equilibrium where the metal half-reaction is as follows:
Fe(s) --> Fe
2+ + 2e
- ..... E
0 = +0.447V
and where the negative charge produces a surplus of electrons (or an electromotive force of providing more electrons) that makes it more difficult for the above reaction to proceed. The zinc sacrificial anode has the following half-reaction:
Zn(s) --> Zn
2+ + 2e
- ..... E
0 = +0.7618V
so it effectively lowers the iron half-reaction down to as low as -0.342V. The chlorine half-reaction can still overcome this so corrosion can still occur, but it is slower since the negative charge on the metal tends to keep the iron ions nearby increasing their concentration. These may get oxidized to ferric ions and form an iron oxide layer, but this too will tend to stay attracted to the metal being somewhat protective. The same principle holds with copper corrosion. With stainless steel, the corrosion is inhibited by a chromium oxide passivity layer, but that layer formation gets interfered with by higher chloride ion levels (such as happens somewhat more in SWG pools), but the negative charge on the metal allows for the exposed iron to not corrode as quickly.
By the way, one can accomplish the same cathodic protection effect by using a power source instead of a zinc anode. Connecting the negative terminal of the power source to the bonding wire and connecting the positive terminal to a sacrificial wire to ground will also supply protection (this grounding wire and entry into the ground, however, will tend to corrode, just as a zinc anode would).