The in-line zinc anode is attached to the bonding wire, thereby protecting all metal parts (heaters, lights, rails, etc.) from galvanic corrosion. Zinc anodes will sacrifice themselves and corrode before all other metals in the pool. Zinc anodes need to be replaced approximately every 4-5 years on average. Many factors can affect how quickly it will need to be replaced. We have encountered some pools where the zinc anode had to be replaced every year.
Any time you have different metals (copper, stainless steel, etc.) in a salt water pool, you create a battery. Some amount of current flows between the metals. The electrons that make up the current are supplied by one of the metals, giving up bits of itself in the form of metal ions to the pool water. This is called galvanic corrosion. Galvanic corrosion causes plaster discoloration and metal erosion. The best way to inhibit the effect of galvanic corrosion is to use a zinc anode. Zinc is a metal that gives up its metal ions faster than other metals in the pool. In other words, the zinc anode will erode instead of other metals (pool light, rails, heater, light niche, ladder, etc.). The zinc ions will not discolor the pool plaster. The zinc anode should be replaced after half of it has eroded.
Galvanic corrosion can also occur pools that do not use salt water. Any stray electrical current around the pool area can cause it. Also, many localities are now requiring that the pool water be bonded. The inline zinc anode also performs this function.
This article is courtesy of the Pool Tool Company (www.pooltool.com). Here is a link to their zinc anode brochure (http://pooltool.com/images/catalong/brochurePrint.zip).