The higher concentration of salt required by saltwater systems has the potential to damage equipment and fixtures, a point that is often glossed over by people selling such systems to pool owners. The higher salt concentration results in greater conductivity of the water, which accelerates a process called galvanic corrosion — a concern that should be addressed by pool professionals.

In simple terms, a swimming pool is just like a giant primary battery. To make an ordinary primary battery in your high school science lab, you would submerge differential metals within an electrolytic solution. On a larger scale, that is the basic setup of a pool, with the water acting as an electrolytic solution (since it has salt in it) and differential metals (such as galvanized steel, stainless steel, aluminum, titanium, brass and copper) acting as the anode and cathode.

Now the amount of current generated between the metal components of the giant battery of your swimming pool is very small. It is not enough to feel shocks in the water, nor enough to be a concern for electrocution. The electric current generated is tiny. You could only detect this current with sensitive electronic testing equipment.

However, though the generated current is small, it runs 24 hours per day, seven days per week. This constant current can dramatically reduce the service life of a pool's integral metal components through galvanic corrosion, which is the result of a transfer of electrons between the dissimilar metals within the water.

RELATED: Case Study: How Stray Current Caused Extreme Corrosion

The reality is that all swimming pools experience galvanic corrosion if they use chlorine sanitizer. Chlorine is salt based, so by adding chlorine to a pool you are actually adding salt. A saltwater pool, however, has approximately ten times the salt level of a traditional chlorine pool. This is especially significant since there is a linear increase in galvanic activity between 0 ppm salt all the way up to ocean water levels of 25,000 ppm of salt. If a saltwater pool has ten times as much salt as a traditional chlorine pool, it means that the rate of galvanic corrosion also increases ten times.

Figure 1. Galvanic chart

Galvanic corrosion occurs when dissimilar metals are in electrical contact, like the metals in a swimming pool system, which includes the metals in light fixtures and handrails and the metals in components like pumps and heaters. In this situation, the metal in the pool furthest to the right in this chart will become the anode, lose electrons and suffer corrosive attack. Note the position of zinc, varying grades of steel, copper, aluminum and other metals likely to be present in a pool system. The placement of a zinc (because of all the metals potentially present in the pool, zinc is furthest to the right on the galvanic chart) sacrificial anode in the system ensures it will become the anode in a galvanic coupling and bear the brunt of corrosive attack, instead of crucial metals elsewhere in the pool.

Bonding

Pool pros can help prevent galvanic corrosion with proper bonding and the use of sacrificial anodes.

While the vast majority of pools in North America are bonded with an equipotential bonding grid, this will often only be applied to the pool structure itself. Swimming pool equipment pads should be included in the bonding loop since they are, of course, part of the pool system. Each component of a pool system that contains metal should be bonded directly to the casing of the machine.

Filters do not usually require bonding, unless they are ancient and made from stainless steel, but almost all pumps and heaters will have a bonding lug somewhere on the casing.

If you do not bond pool equipment, you run the risk of the equipment itself becoming the anode in a galvanic couple with other metal components in or around the pool. If this happens, the anode metal will degrade through galvanic corrosion.

RELATED: How to Test a Swimming Pool Bonding Grid

Specifically, only the anode metal will corrode, while the other metal will experience enhanced resistance to corrosion from a process known as cathodic protection — a predictable electromechanical process. By not bonding the equipment you allow a potential difference (voltage) to exist between each component. By bonding the equipment you are forcing each piece to have the same electrical potential, which limits the potential for damage from galvanic corrosion.

Figure 2.

 

Adding a Sacrificial Anode

Since galvanic corrosion attacks only the anode metal, a popular strategy is to add a piece of metal to the system that will fill that role. Since the metal device is being placed in the system so it will become the target of galvanic corrosive attack (and thus not other essential components), it is called a sacrificial anode. Its job is to suffer degradation so that other components will remain unharmed.

By adding a sacrificial anode to your pool plumbing installation and connecting it to the bonding grid, you effectively make the rest of your pool equipment the cathode in a galvanic couple. As discussed, this will provide cathodic protection to your pool equipment, which results in increased rust resistance. The sacrificial anode that you add will take the brunt of the corrosion damage, "sacrificing" itself in this process. Simply replace the anode over two to three years, or when the anode metal is reduced in size by half. This is something that all pools should have, and all saltwater pools must have.

Every swimming pool, and especially every saltwater pool, should have a sacrificial anode installed. The addition of this simple and low-cost device will dramatically reduce the damage a pool experiences as a result of galvanic corrosion. While you may still experience localized anodization and oxidation of metals in a pool, especially in situations where two different metals are in direct contact, a sacrificial anode is the bare minimum level of protection that every pool needs. It is absolutely silly to not have one of these — plus they can easily be adapted to any existing system.

RELATED: Dealing with Pool Stains? Identify Before You Treat

To help prevent localized problems around the pool, you can add additional sacrificial anodes, such as standalone zinc anodes that bolt directly to the ladder; you can also get zinc anode discs that can sit inside the skimmer basket.

Given their relatively low cost and the huge preventative protection they offer, there is no question that every pool should use sacrificial anodes. 


Steve Goodale is a renowned writer, humorist and swimming pool expert who lives in Ontario, Canada. You can learn more about Steve, as well as swimming pool construction, maintenance and repair (and have a few laughs) at his website: SwimmingPoolSteve.com.

Steve Goodale is AQUA Contributor of AQUA Magazine.
avatar
Rodney Schluterman Sunday, 09 September 2018
Can the anode be installed before the sand filter?
avatar
Yes absolutely. The anode can go anywhere in the plumbing system before the salt cell or erosion feeder discharge.
avatar
Everybody blames salt as the culprit and yes it does increase the conductivity of the water so will increase galvanic corrosion potential. But remember in the distant past pools were routinely sanitized with liquid chlorine ( Sodium Hypochlorite ) which after it sanitized the water left a residual of sodium chloride (salt) . Some times to levels far exceeding what generators require. These pools usually did not see excessive corrosion issues. Was it that there were better materials then? Or is it the stray voltages produced by the generators themselves?
We have noticed a difference between manufacturers of generators as far as corrosion issues. Equa potential bonding (properly applied) is still very important to help reduce the issues.
I have one old pool (1968) that used an salinator system in the 80's @ 12,000 - 14,000 ppm salt with no bonding. Concrete with marbilite, original rails & ladder, and old Laars heater with cast iron headers that works.
I have had other pools were the new rail goods started corroding in weeks even though marine grade & 3200 - 4000 ppm
avatar
Timothy I'm happy to use use my comment on your blog. Dale L
avatar
Steve It's about time a little common sense has come into view! - Thank you for your insight into salt water pools. The 2 most corrosive elements on the planet are now everyone's choice for a pool. Maybe it's the industries choice to just have another reason to replace equipment and find another way for the consumer to soak up more chlorine. Good Job Steve - Thanks again. Dale Luzzi
avatar
When galvanic corrosion, electrolysis or staining occurs in saltwater pools, a SACRIFICIAL ANODE is merely a placebo. It is not the solution that everyone is seeking.

Marine Applications

On a boat, a sailor does not care if the anode erodes, or if the anode's metals dissolve into the water (ocean or lake). The anode on a boat is a lot cheaper than the wiring, prop or shaft. The annode prevents the corrosion of the precious components, by SACRIFICING itself instead.

Swimming Pool Application

By placing a sacrificial anode in a swimming pool, one is doing the same thing as on a boat. You are merely offering a SACRIFICIAL metal for the galvanic corrosion to attack, instead of the metal in your precious components.

So instead of plating surfaces with copper, or dropping metals out of solution, you will be plating them with zinc or magnesium from the sacrificial anode. The sacrificial anode will save the copper, but it will not stop the staining or plating of the less noble metallic components. The plating & staining will NOW occur with the metal from the anode instead. This is exactly why anodes need to be periodically replaced - they are sacrificed.

Bonding Grid

The equipotential bonding grid does not prevent galvanic corrosion. In fact, if there is stray voltage finding it's way to the pool through the property's CONTINUOUS GROUNDING SYSTEM, the equipotential bonding grid may actually be feeding voltage to the pool's metallic components (because the grounding and bonding are cross connected in numerous places & devices).

For decades, chlorinated pools did not have any of these corrosion issues and did not require sacrificial anodes... why the sudden placebo and interest in sacrificial anodes?

Sacrificial anodes are not the solution to the damage caused by galvanic corrosion or electrolysis... stopping the stray current is !
avatar
The sudden interest in sacrificial anodes is directly proportional to the increased number of pools with 3000 to 5000+ ppm of salt. This makes the damage more pronounced, commonly about ten fold, from a "typical" chlorine maintained pool. Much as you have indicated the anode must be sacrificed in order to provide function in this equation, and the potential for staining is a whole conversation unto itself, but I would agree that staining is a concern. From my perspective I see the anode as a form of protection for integral metal components, such as the steel grid of a concrete pool, or a gas heater. The increased galvanic activity puts these components at increased risk, given the wrong parameters such as pool with poor chemical maintenance, or a failing plaster interior surface ghat readily is leeching salt water which allows the rebar to become a part of the galvanic couple. I would agree that the protection from a sacrificial anode is minimal, and far from a perfect solution, but it is also a very minor component, easily installed, and does work to protect the pool against known destructive processes.

A bonding grid does provide protection against galvanic corrosion in that it mechanically is forcing a common potential. Without a difference in potential, there can be no current transfer. Another way to look at this would be to consider that a pool without a bonding grid would be at increased risk for galvanic corrosion as there would be no process in place to keep a common voltage. This could result in current readily passing via the water to multiple components within the system. From an anecdotal perspective, I started changing liners in 1991 through to 2015, sometimes as much as 100 per season. The number of pools that I encounter now that require extensive rust remediation on the walls is far, far more than the beginning of my career. They are always salt pools. Anodes are proven to provide protection to other metals within a galvanic couple. Their depletion is evidence of their function, and I would argue that it provides evidence enough that it is actually providing some benefit within the context of the situation.

I appreciate your knowledge and interest in this subject Paolo and I am very happy to consider any information you have to offer. I certainly do not have all of the answers, but I do have a keen interest in this subject in particular. Cheers.
avatar
In my 25 years in commercial chlorine generators for recreational pools, our approach has been both to install anodes and to address the bonding issue, which some electricians seem to ignore or terminate the bond wire at the ground or some other location causing the problem to be exasperated We build our own anodes and bond them to the bonding loop as a further precaution.
I liked your take on this issue Steve, the ignorance of "salt caused my pool to corrode" is only topped by a YMCA Facilities Director, who insisted they would have to replace their HVAC system and pool equipment if they used chlorine generation.
Would like to use your article as a Blog if you are cool with that?
avatar
Hi Timothy - thank you for your feedback. Yes please feel free to post this information in a blog post. I also have about a half dozen additional articles on my website that you might like to reference. This is a subject that I have written about extensively. I ask that you credit me as the author and place a link to my website www.SwimmingPoolSteve.com where appropriate. Cheers
avatar
How do you protect commercial pools with large diameter face piping? Do you simply use anodes on the rails, ladders, lights and in skimmers or is there an in line anode for larger piped systems? Years ago they used to be on the metal tanks. Now most tanks are not metal.
avatar
Talk to your commercial equipment rep from your favorite manufacturer to see what they have for you. I have seen anodes added as a branch from the main line. Just plumb in a bypass in 2" and plumb multiple anodes.