Today, more people are installing Clark Synthesis Underwater Speakers in saltwater pools. While customers are very pleased with the underwater music, a few witness a disturbing side effect. It looks like the speaker is rusting and the rust is running down the side of the pool. Yuck!
Let’s see if we can figure out what is going on…
In the 1700s scientists were developing the first rudimentary batteries. The first reliable battery was created around 1799 by Alessandro Volta. He placed 2 different metals, copper and zinc, in a solution of salt water. It worked.
What was good news for Volta is not good news for owners of saltwater swimming pools
When you add salt to swimming pool water, and you have metals in the pool, like niches, ladders, lights, etc, you create a battery. Before you think that freshwater pools don’t have salt, you might want to take another look. Freshwater pools almost always have some amount of dissolved salt. As water evaporates and is replaced by fresh water, the amount of salt increases. If you do this often enough, and for long enough, you can create your own backyard version of the Great Salt Lake in Utah, or the Dead Sea in Israel – both created with fresh water, evaporation, and time.
This swimming pool battery can be problematic. Galvanic corrosion can occur and quickly destroy metal parts. There are two things that are commonly done to prevent Galvanic corrosion. The first is to bond all the metal parts together. The theory is that the bonding will neutralize the battery by equalizing the electrical charge across the pool. Bonding is typically a building code requirement. While bonding reduces much of the electrical charge, enough may be left for the corrosion to continue.
Since this isn’t always a complete solution, a second system has been introduced – a system of sacrificial anodes. To prevent galvanic corrosion from destroying metal parts, the sacrificial anodes give up their metal so that other metal parts in the pool are not harmed.
Is galvanic corrosion causing the speakers to rust? It sure looks like it. But that is not what is happening. Galvanic corrosion involves the transfer of material from one metal to another metal. When we see discoloration around the speaker, we don’t see metal corroding. We do see materials deposited on the face ring and screen, but not corrosion under the deposits. The speaker body is plastic, and will be heavily covered with deposits. These deposits are primarily iron.
So, what is happening?
The speaker body has a very, very powerful magnet, and the face ring and screen are slightly magnetic. In a saltwater pool, the magnetic field attracts the dissolved solids from the water. If there are enough dissolved solids, including iron, the solids can accumulate around the speaker body until they start dripping and running out the bottom of the niche. It looks like rust because of the accumulated iron particles.
That’s when our phone rings. The customer is sure that the speaker is rusting and discoloring their brand-new swimming pool. That’s when we ask, “Does your pool have a sacrificial anode?” The customer typically answers, “I have no idea, but I’ll find out.” Most of the time, the installer never installed a sacrificial anode in the saltwater pool. Oops! After one or more sacrificial anodes are installed, the problem disappears.