The Efficacy of Zinc Anodes in Non-Saltwater Pools
By: Tom Queen
The efficacy of zinc anodes in saltwater pool systems to retard corrosion and rust of rebar and other metal pool components is well documented. Saltwater pool systems create chlorine from salt through the process of electrolysis. This process splits the sodium chloride molecule (NaCl) into sodium (Na) and two forms of chlorine (Cl) known as sodium hypochlorite and hypochlorous acid. The electrolysis process causes low voltage electrical currents to pass through the water which can cause the corrosion and rusting of rebar and other metal hardware used in pools. This rust particularly if in the rebar, can spread rapidly causing stains on the walls and floor of the pool. Eventually, the entire pool's rebar will rust and weaken its structural integrity. Treating rusted rebar in a pool is a major project that can require scuba gear and special tools. Only a licensed, pool repair contractor (CPC) should be hired to do this. But using a sacrificial anode or zinc anode such as the one above can slow down this corrosion process.
When two dissimilar metals are submerged in a salt generator pool, one metal will be more positive or more noble (cathodic) than the other. The more positive metal will steal electrons from the more negative (anodic) or less noble metal. This process is known as galvanic corrosion.
"Galvanic corrosion can only occur when there exists an electric conducting path connecting the metals and when an electrolyte which can offer a channel for the migration of ions is present." (Galvanic Corrosion - Definition, Mechanism, Examples, Causes & Prevention with Videos (byjus.com)
Salt generator pools contain the three components needed for galvanic corrosion to occur.
Two or more dissimilar metals
The electrochemical cell in most salt cells is made of Titanium. Most pools also have copper pipe and or brass fittings.
The presence of an electrolyte
The electrolysis process creates an electrolyte which as stated above creates the channel for the migration of ions.
A conductive path
The saltwater serves as a conductive path for the electrical current
Metals have a hierarchy of nobility; Meaning the more noble a metal is the more positively charged it is. The less noble a metal is the more it will attract the oxidizing ions to itself.
As you can see from the above diagram Zinc is the least noble metal of the common metals. Therefore, a zinc anode attracts the oxidizing ions much faster than the other metals causing itself to corrode first, thereby serving as a sacrificial anode and providing a cathodic protection to the other more noble metals.
The question we are asking here however is: Does a zinc anode have any efficacy in a non-salt pool?
Many non-salt generator pools also rust and corrode due to a variety of possible reasons such as, acidic water, low total alkalinity, excessive shocking or very high chlorine levels over long periods, too much or too little sun, mulch from a garden, and other phosphates that blow into the water. But these causes don't meet the criteria for galvanic corrosion since there is seemingly no electrical path for ions to travel between dissimilar metals. However, claims have been made about proximity to step down transformers causing an electrical current to pass through pool water.
When this author worked for a pool company in Southern California, we were selling these zinc balls on the premise that being within 100ft of a step-down transformer could cause either a current or bolt of electricity to hit a pool and damage the surface or corrode the pipes. Although the zinc balls that had been installed in these non-salt generator pools clearly had corroded, I was always suspicious of the cause.
While there are over 23 other forms of corrosion such as high temperature, uniform, pitted, crevice, intergranular, stress, etc. There doesn't seem to be much information about zinc anodes being degraded by other chemical factors such as acidic water, or water with a high chlorine content, low PH etc.
There also seems to be a shortage of information about transformers and their connection to something called stray voltage.
Stray voltage occurs when an electrical current moves from flowing within an electrical wiring system to another conductive object, such as a metallized water piping system, a steel structure or a concrete floor, and then through a person or animal who comes into contact with it. (Stray voltage: The silent killer - Electrical Business (ebmag.com)
I found no information linking transformers specifically with stray voltage but, stray voltage could possibly come from other electrical sources near a pool including pool pumps and heaters. This stray voltage would not be high enough to be noticeable to anyone in the pool.
With this possibility in mind, and the degradation of the zinc anodes I witnessed, my conclusion is that if you have rust or corrosion in your non salt generator pool, first try to detect an electrical current in the water. If one is found, then you must locate and repair the source if possible. In this case a zinc anode would be helpful since galvanized corrosion is confirmed. However, if no electricity is found, then the rust is likely being caused by one or more other factors. A zinc anode cannot work properly without an electrical current to enable ions to escape. Therefore, trying a zinc anode will not hurt anything but may not be helpful either.