Salt Water Pools Part #2October 17, 2016
How To Prevent Damage From Salt Water In Pools
So now you understand a little more about how salt water can damage your swimming pool and you can begin to ask the correct questions. Salt water can damage swimming pools but there are a number of things you can do to help prevent this problem (or at least dramatically reduce the amount of damage). Every swimming pool has something called a bonding grid. A bonding grid is different from grounding however this is another area where most pool guys can not explain the difference since they do not fully understand the difference between bonding and grounding for a pool. The bonding grid connects every piece of metal in and around your swimming pool (and equipment) with a heavy gage, low resistance conductor (copper wire). To understand why this is done you need to understand some basic electrical theory:
Voltage (volts) is equal to the current (amps) multiplied by the resistance (ohms)
If you rearrange this equation to solve for current it becomes current is equal to voltage divided by the resistance. This equation is called Ohm’s Law and is the fundamental basis of the relationship between electrical variables. This equation is incredibly important in both the world of electricity as well as in swimming pools as they relate to galvanic corrosion.
When you add salt to the pool what you are doing is changing (reducing) the electrical resistance of the water. Water, at least in a theoretical pure form, does not conduct electricity. It is the impurities in the water that increases the conductivity of the water. As you add impurities such as salt, the ability for the water to conduct electricity increases since the resistance in the water is reduced. This in turn affects the current being generated. As the resistance of the water goes down, the current goes up. Since current and resistance are inversely proportional, when you lower the resistance of the water with salt you are directly increasing the current passing between differential metals within a galvanic couple.
So if you have a bonding grid in your pool (you better!) then all metal components in and around the pool are connected with a low resistance wire. If every piece of metal is connected directly then it is not theoretically possible for a voltage to exist. To understand why this is, you need to understand that another way of saying “voltage” is to say “potential energy difference”. If there is a potential difference between any two points then it can be said that there is a voltage differential. By connecting all metal points within the pool system with a low resistance wire you are forcing them to have the same potential difference. Going back to the Ohm’s law equation and solving for voltage, voltage is equal to the current multiplied by the resistance. If the resistance is equal to zero, then at least theoretically the voltage will be equal to the current multiplied by zero…which is zero. In the real world it is not quite that simple, but you can see where this is going with the bonding grid.
How Bonding Reduces Galvanic Corrosion
So the bonding grid connects all metal components in the pool and prevents current from transferring, problem solved, right? Not so fast. In theory the bonding grid prevents voltage differential which means no current can flow. In theory. In practice, the bonding grid does not have zero resistance…it just has a very low resistance. The difference between zero resistance, and low resistance, is very important. With a low resistance the current is not zero, it is just lower than it would be without a bonding grid. Low levels of current, applied constantly 24/7 still has a cumulative negative effect on the metal components in your pool. To combat this there is another electrical theory that we can utilize.
Metals within a galvanic couple will act in a predictable way. The weaker metal (less noble) of the two will become an anode, and the stronger (more noble) metal will become a cathode. Since galvanic corrosion only occurs on the metal that is the anode in the galvanic couple, you can manually add a metal that is even less noble and this will become the new anode. The cathode metal in a galvanic couple experiences an enhanced protection from corrosion called cathodic protection. A sacrificial anode is a metal that is less noble than any metals currently in the galvanic couple. Most commonly sacrificial anodes are made from zinc or magnesium since these two metals are extremely low on the noble scale of metals. When you add a zinc anode to a pool system the zinc becomes the new anode and is the metal that will receive the brunt of the damage from galvanic corrosion, where the more noble metals of your pool system experience cathodic protection against corrosion.
Is it just that simple – add a sacrificial anode to your pool and no more galvanic corrosion, right? Not exactly. Again when we get deeper into the practical application of this theory we discover that it is not a perfect system. While a sacrificial anode dramatically reduces pool equipment damage from galvanic corrosion, it does not 100% stop the process. There is no silver bullet that will absolutely stop galvanic corrosion since localized anodization can still happen.
One of the factors that impacts the strength of galvanic corrosion is the proximity of the differential metals. The closer the metals are to each other, the stronger the current transfer. If you have a brass screw installed into an aluminum or stainless steel light niche, which is tied into galvanized steel walls (or steel rebar in the base of a concrete pool) then having a sacrificial anode installed 50 feet away in the pump room will not stop galvanic corrosion from happening between these metals in close contact with each other. In this case using bolt on anodes may help. There are sacrificial zinc anodes that you can install directly on lights, as well as ladder anodes and handrail anodes. You can also buy zinc anodes that are designed to sit inside skimmer strainer baskets or pump strainer baskets.