Editor's note: this discussion is not about electrical safety or prevention of shock hazards, but instead looks at very low levels of electrical energy that typically pose no threat to people, but do present possible corrosion problems.

Sometimes the best way to truly understand a complex issue is to learn about it by way of extreme example. That was certainly the case when I recently encountered a catastrophic corrosion problem in an over-story stainless-steel pool installed in an upscale residential high-rise building.

My company, Holdenwater (Fullerton, Calif.), was retained as a consultant in litigation resulting from a mysterious and spectacularly horrible set of corrosion problems that were well on their way to destroying an otherwise beautiful 15-by-65 foot vessel, located on the sixth floor of a 30-story condominium complex.

I'll leave the specifics of the lawsuit and the property off the page, but without a doubt, what we learned about corrosion — and more specifically the stray current that caused it — stands as a cautionary tale worth sharing.


My journey into the world of stray current started when my colleague Jeff Freeman and I were called in to try to determine why this particular stainless steel pool started corroding from almost the day it was filled.

Typically when we think of corrosion, most of us imagine a process that unfolds over long periods of time, months if not years. Not so in this case. Within weeks of pool's initial start-up, components such as light rings, handrails and the pool shell itself started showing extreme evidence of corrosion.

They owners had made many alterations to the system and tried to adjust the water chemistry but were quickly dismayed to see the same problems reoccur within the same incredibly fast timeframe. No one in the loop had any idea what was causing the problem, let alone what to do about it.

The situation was tense, to say the least; most everyone involved was aggressively pointing fingers, trying to deflect liability. From the start it was apparent that no one had the slightest clue what was happening, including us — at least not at first.

One of the advantages that we had, however, was the luxury of objectivity. We didn't have a so-called "dog in the fight," and unlike most of the other people involved, our only aim was to find out what could cause such rapid and severe damage.

We've inspected literally hundreds of pools over the years and neither of us had ever seen corrosion this severe happen so fast. Furthermore, none of the possible explanations we were hearing made sense given the accelerated nature of the damage.


We've seen the effects of imbalanced water chemistry or improper sanitation on countless occasions, and in some cases the effects of corrosion due to supposed saltwater chlorination system issues. Nothing we had ever encountered looked anything like the damage impacting this vessel.

Truth be told, both Jeff and I love a challenge and the opportunity to play detective. And to date, this was one of the most dramatic and illusive as we've ever encountered. So we turned our attention away from all the legal banter and defensive posturing and instead focused on what science, careful observation and the pool itself could tell us.

The first thing we did was spend several days carefully inspecting every conceivable aspect of the pool and its operation. We examined every piece of equipment, every plumbing connection, every electrical connection, every bolt, nut, gasket, junction box and piece of conduit and valve, as well as every part of the pool-shell structure we could visually inspect. We even spent hours in the equipment room simply watching and listening to the system's rhythmic heartbeat.

It was only after this extensive inspection that Jeff first suggested it might be stray current. Although I wasn't immediately convinced, it did seem to make sense. We were, after all, looking at a metal structure that contains chlorinated water suffering massive corrosion with no other viable explanation.

In a nutshell, our laymen's knowledge of electricity led us both to suspect that the pool very well could be the "path of least resistance" for electrical current that might be present somewhere in the building.


At that point we decided to jump into fill-tilt research mode to find out if stray current might, indeed, be the problem. We read everything we could find online and called in electrical engineers and corrosion experts for advice and testing. It was our goal to test our hypothesis in every way possible.

We learned that stray current is extremely difficult for people to understand, even those you'd think should know about it, such as the manufacturer of the pool shell or the electrical engineering consultants who worked on the project. We also learned the effects of stray current are hard to prove, as it operates on an infinitesimally small level of electrical energy, and it can happen sporadically. It can change by the minute, the hour, the month the season, all depending on an infinite set of potential variables and sources of electrical current.

As I'll describe below, we did conclusively determine the cause of the damage was, in fact, due to stray current, but we never did firmly identify the cause of the current, or electromagnetic field, a term commonly used by corrosion experts to describe electrical energy moving through a conductive environment.

In brief: There are numerous ways to describe stray current, but as it applies to this situation and pools and spas in general, it refers to the unwanted and uncontrolled flow of electricity through the water and metal components of the pool structure and equipment.

You don't have to be an electrical engineer or corrosion-prevention specialist to understand stray current, but it does help to know a little bit about the basics of how electricity works. (There are numerous sources available for fundamentals on electricity and not being an electrical engineer I'll sidestep that kind of explanation.)

What is important to know is that because pools are bodies of water that have both metal parts and electrical devices, they can be susceptible to stray current. That's true when pools are installed in the ground, and certainly so when they're made entirely of metal installed in buildings, such as the case described here.

It is true the bonding grid, or equipotential bonding grid as it's known to electrical engineers, exists to create equal electrical potential across all the pool's metal conductive components precisely for the purpose of preventing stray current. The problem was that the bonding grid, which was for the most part installed to code, was not preventing the problem.


As is sometimes the case in other technical disciplines, the pool and spa industry is not up to speed with the rest of the world of construction and engineering. Corrosion prevention, often referred to as "cathodic corrosion prevention," is a massive field that touches an incredibly broad spectrum of industries, including maritime shipping, manufacturing, petrochemical, water distribution, food processing, major construction and even dairy farming.

One of the corrosion experts we talked to hadn't had much prior contact with the pool industry. He was both amazed and appalled that an industry that places water, electricity and metals in direct contact rarely if ever uses cathodic protection — and he was especially surprised that was the case with stainless steel pools. The more we explored the topic, the more dumbfounded we became that our industry is so asleep at the wheel on this front.

Corrosion prevention is a multi-billion dollar industry driven by incredibly complex science. It's one of those disciplines in which the leading experts often don't agree on how specific types of materials corrode and for what reasons. In this case, we ran into the same thing. One corrosion expert we encountered said the problem wasn't due to stray current but instead caused by a phenomenon known as "crevice corrosion," which refers to a type of chemical corrosion that can occur when tiny amounts of fluid are confined to very small areas, like crevices, in a metal surface.

However, the evidence didn't point to that conclusion. Because the corrosion was obviously starting with the stainless steel bolts on the light niches and handrails, and not in any place that looked like it had crevices to start with, the conditions appeared to point to corrosion caused by stray current.

Although we suspected stray current, there was really no conclusive way to prove it. For starters, we learned that it can come from almost anywhere and that some level of EMF is present everywhere. The static electricity generated by the wind hitting the side of the building, for example, could be the source of stray current, as could any piece of HVAC equipment relatively proximate to the pool. The electrical utility itself could be a possible source, especially if there was some engineering or installation deficiency of some kind.

In this case, we couldn't help but notice that an electric-powered, mass-transit train system ran just below the building every 18 minutes or so, which just happened to result in spikes in the current detected during testing. Indeed, corrosion experts often cite mass transit systems powered by electrical energy as potential sources for stray current.


We were never able to fully determine the source of the current, and really the only way to know for sure was to see of we could mitigate the corrosion by turning to methods commonly used to prevent damage from stray current.

Convincing the owners and other concerned parties to let us give it a try took some time and effort. Despite everything we discovered and all our well-founded suspicions, many of the other so-called experts resisted the idea. Our argument ultimately carried the day because there was no other viable explanation, and we offered that there was little to lose other than a few thousand dollars if it didn't work and everything to gain if it did. So we set about designing a system to protect the pool from the effects of stray current.

Corrosion from stray current is prevented by two means of cathodic protection that in the simplest terms can be described as active and passive. Active cathodic protection is a complex technology in which current is introduced to negate stray current, in essence fighting fire with fire. The active approach is used mostly in extremely large applications such as large manufacturing or refinery facilities, or in massive metal structures such as rail systems or structural steel in large tunnels.

For as significant as the problem with pool appeared to be, it was clear we were looking at the other primary type of corrosion prevention: passive cathodic protection. A far simpler approach, it involves introducing metal components, usually some form of zinc or magnesium, which is a less noble metal and has greater electrical potential, making it more susceptible to corrosion. In turn, the less noble metal will draw the current to it instead of the metal components you want to protect.

We know it commonly as the "sacrificial anode." In the pool industry, we have products like zinc balls placed in stainless steel filters for the purpose of preventing corrosion.

Given the extreme nature of the problem, our original corrosion specialist suggested taking an aggressive approach using multiple zinc plates on the pool shell and on the major metal components. This approach was too extreme for a public pool environment, so we designed a more user-friendly solution with anodes within the pool shell. Again, we had to look outside the industry to find products that we could modify so they could be mounted inside the pool. In all we used eleven six-inch diameter anodes throughout the pool interior to absorb any stray current. Afterwards, we had our fingers crossed that these would show signs of being productive in protecting the pool.

The pool has a an expensive tile surface and although the anodes might be something of eyesore, we thought the best strategy was to place them in direct contact with the water. We also mounted sacrificial anodes on a number of points in mechanical system and a host of other metallic components.


It worked. In fact, it worked so well that our solution initially looked like a new problem.

Within a few days of installing the anodes, piles of metal precipitate were forming below the anodes. We first heard about it from the site engineers who complained it was becoming a new maintenance nuisance, as the dust had to be vacuumed.

Without question, the stray current was attacking the sacrificial anodes and doing so at a rate you could almost watch in real time. But at the same time, the problem with the corrosion completely stopped! I told the site engineers they should glad the anodes were dissolving because that meant they were protecting the rest of the pool.

As I mentioned above, we never did determine the source of the stray current, although the train would be our prime suspect. And certainly when you talk to corrosion experts, they will say if you can identify the source and stop it somehow, that's the best solution. Many times that is impossible with current technologies.

In a situation like this, however, where the current could be coming from any number of sources and likely more than one, given the resources at hand, our best approach was to give the current somewhere to go where it wouldn't do any damage.

In this case, we estimated the anodes would last about six months, so we made sure we left the property managers with an extra set, which we've heard have already been installed. We've also been told the corrosion issues have disappeared completely. They are now on their third set of anodes.

As an interesting side note: We almost immediately noticed black stains forming below the water on the handrails. Jeff later determined that the stains were caused by the zinc dissolving in the water and being electro-plated on the handrails. That's how strong the stray current was.


As I mentioned at the start of this story, this was learning by way of extreme example. Fact is, most pools are not installed in high-rise buildings, or anywhere completely above grade, and certainly most are not made of stainless steel, although many are.

When it comes to inground pools it's useful to keep in mind that the reinforcing steel in concrete pools, or metal components in package pools, can be susceptible to stray current. That's because a swimming pool is almost a perfect path of least resistance for electrical current. Soil will conduct some current to varying degrees, but by comparison, the pool is just about always going to have greater electrical potential.

Common sense would dictate that concrete or composite inground vessels will not experience the kind of extreme trouble we ran into on the project described above. Nevertheless, anywhere metal, water and electricity come together presents an opportunity for corrosion due to stray current. When the water contains chlorides, we know that its ability to conduct electricity increases.

The upshot is stray current is something to consider if your experiencing excessive corrosion. And the good news is, using passive cathodic protection is both affordable and readily available.

For our part, if and when we're involved with designing a project that includes a stainless steel shell mounted in a building, which is an extremely common application for that type of pool structure, we will always include cathodic protection.

And I'll definitely keep contact information for corrosion prevention professionals on my list of people who know more than I do!

Comments or thoughts on this article? Please e-mail eric@aquamagazine.com.