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Understanding the electromechanical process of galvanic corrosion

Different Types of Anodes_a
An active metal such as zinc (Zn) (i.e. sacrificial anode) will be the most inclined to decay as it will become the least noble metal in the pool.

By Steve Goodale

Steel pool walls are eroding and ladder handrails are turning black. New heaters are failing after one year and warranties are void because the casing was not bonded. What is going on here? The pool industry has some difficult-to-grasp concepts such as water chemistry; where on the surface it seems easy enough to resolve, but when traditional remedies do not fix the problem, customers are left without answers. There is no better example of this than galvanic corrosion and, more specifically, the damage it can potentially cause to pools, spas, and peripheral components. Understanding how and why galvanic corrosion occurs can help those in the field provide better service to their customers.

What is galvanic corrosion?

Galvanic corrosion is an electromechanical process where a potential difference, measured as voltage, exists between dissimilar metals and alloys when they are placed in electrical contact within an electrolytic solution. For instance, placing two different metals into salt water will make a basic battery. Using a voltmeter, voltage can be measured between the differing metals that are in contact with the water. In essence, a saltwater pool is a large battery that can generate a potential difference across any differing metals introduced to the water.

Stainless steel, galvanized steel, aluminum (Al), brass, copper (Cu), and titanium (Ti) are all commonly found in pools and related equipment, which is one reason why these corrosion-related failures occur.

All pools using chlorine (Cl) can potentially contain enough salt to allow for the water to conduct electricity. In the case of saltwater chlorine pools, the risk and the ensuing damage from galvanic corrosion is greater as salt levels are much higher.

Following this logic, ships that operate in saltwater seas have extensive protection systems in place to mitigate this problem. The salinity of ocean water is roughly 35,000 parts per million (ppm), which is 10 times more than a typical saltwater pool. These extremely harsh conditions have long since exposed the destructive force of galvanic corrosion and the need to control the problems associated with salt water, metal, and electricity in a galvanic couple (the corrosive cell development when two different metals are separated by an electrolyte).

As a frame of reference, the sodium level in municipal tap water might range between 20 and 200 ppm, while a pool maintained with traditional chlorine may contain a sodium level between 300 to 1000 ppm. Sodium levels in saltwater pools typically average 3000 ppm. From zero ppm to the levels of salt found in sea water, the increase in galvanic activity is nearly linear as the sodium level increases.

Two different metals submerged in salt water may have a potential difference between them, allowing an electrical current to pass from the least noble metal (anode) and travel through the electrolyte (salt water) to the more noble, less active metal (cathode). During this process, the electric current results in advanced corrosion of the anode as it dissolves into the electrolyte (i.e. galvanic corrosion), and inhibited corrosion of the cathode (i.e. cathodic protection). The greater the electrolyte’s conductivity in the solution, the greater the damaging effects of the electrochemical process.

That said, pool water is an electrolytic solution. Pure water does not conduct electricity; however, impurities in the water do. Salt, in particular, is a good conductor, and since chlorine—in all forms—is sodium based, all pools using this sanitizer are at risk of galvanic corrosion. This means the damaging effects multiply as the salt levels increase (due to the increased conductivity of the water). Therefore, saltwater pools have an elevated risk for this type of damage. To prevent this destructive process, a fair amount of electrical comprehension is required.

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14 comments on “Understanding the electromechanical process of galvanic corrosion”

  1. The author should be familiar with the galvanic chart as well as fundamental electrical theory and basic chemistry and related terminology…..which is not evident in the content of these articles.

    1. Hi Doug – thank you for your feedback. If you would like to share some sources to inform readers that would be fantastic – we are all just trying to learn. I do have a formal education in electrical engineering and a lifetime working in swimming pools though there is certainly more that I can learn. I think this article goes a long way to increase awareness and understanding of galvanic corrosion but I would agree that there is room for improvement.

  2. Steve, from what I’ve read here (& I’ve also watched all of your videos), am I correct that a lone zinc anode attached to the ladder is not sufficient to protect the pump and heater? The pool tech that just replaced my heater believes one anode attached to the ladder will protect everything. I think I need an inline anode in the pump room to protect my equipment, and that the ladder anode will protect only the ladder. (To my knowledge, the ladder is not connected to a bonding grid – pool built in 1978 by previous homeowner, and a pool tech who knows the installing contractor has told me last year the contractor was infamous for taking shortcuts). 18×36 inground concrete. 3000 ppm salt requirement.

    1. The deck anchors for the ladder should have been included in the bonding grid and the stainless ladder is bonded by the mechanical connection to the wedge anchor. That being said, many builders, especially older ones, float those anchors in freehand into the concrete when finishing the deck. If some part of the bonding grid would be skipped / missed this is it. The ladder anode is better than no anode however an inline anode in the pump room with a direct low resistance connection to the bonding grid would be superior. For the price of a heater vs the price of installing an anode I would not hesitate to get an inline one installed. I personally do not believe that a zinc disc provides any systemic protection for the pool.

      1. Steve, thanks so much for your reply. My ladder slips into *plastic* cups, not metal, and at the bottom of the cup is dirt & gravel. There is no metal in contact with the ladder. The ladder already has brown streaks that look like burns which I believe to be galvanic corrosion since there were no such spots until I switched to salt 6-7 years ago. I will order an inline anode for the pump room as you suggest. Thanks again for your reply.

      2. Hi Steve, I have an 24′ round above ground pool without a heater. Will a zinc disc in the skimmer basket suffice?

  3. Steve – Great article and love your YouTube channel. You’ve been a huge help with the installation of our first pool. I have a 33′ round pool half in the ground. Can you recommend a few good sacrificial anodes?

    1. There are not a lot of options on the market Tiver – basically which ones can you get in your area. Most pool stores will have perhaps one inline anode option…so that is the one you want. If no local stores have any then you can also order them from Amazon. You are already going above and beyond with an above ground pool and a sacrificial anode as most above ground pool owners do not have one installed.

  4. Steve, great article. (From a layman, who is not fully technical) I was perusing the web, after I was in an a minor “accident”, (in a in-ground indoor pool, using “salt water” and a number of other chemicals) at a senior’s residence, where I was visiting a friend. Basically, I used the ladder, not the stairs to exit the pool. As I reached up and put my (right) foot on the second step and raised up my (left) foot to go the the next step, the lower (second) step broke (or collapsed) underneath me. As such, I heavily “whacked” my left elbow, on the metal railing of the step and smashed my right shin on the other side, as well as incurred a number of scrapes, from the falling plastic step. (It was fortunate, that I had a good grip with both of my hands and arms, so I didn’t fall forward to hit my face or mouth (teeth). After taking a few minutes to recover, I dived down to recover the broken plastic step, and I looked at the metal ladder, where the step “should” have been. There was some corrosion on the metal handles and pins and a bit on the plastic step at the side, where the plastic step would have been, if it had not collapsed on me, as I tried to exit the pool. (I took the broken step to the senior’s residence (maintenance) and explained what happened, and suggested that someone needed to check the pool, if there were any plastic fragments or metal pins in the water AND someone needed to check that ladder and the ladder on the other side of the pool, possibly more frequently for security and safety. So, with all that I have written (or said) … Based on parts of your article, Would my “non-tech” understanding be correct to suggest that “Is it possible that the “salt water” in the pool contributed to the “plastic” step collapsing underneath me and as such the senior’s residence needs to do more frequent checks on those two steps to exit the pool, in order to alleviate any additional “accidents”? Thank-You for your input.

    1. Hi dpi Bassett. First, I am sorry to hear about your pool related accident, but very pleased to hear that you managed to avoid even more serious injury during your fall. From a technical perspective, there are a few different things happening with your comment. From experience I can tell you that rusting from the hardware connecting the steps to the rails is pretty common. This is a function of lower quality metals, combined with the close contact of dissimilar metals which causes some galvanic activity between the hardware and the ladder…but this would only result in rust staining. Even in the most extreme cases I have encountered the hardware for the steps is not structurally compromised, but more just cosmetically damaged. In your case, it sounds like the plastic of the step broke, not the metal from the hardware. Now this is something that I have seen many times before. This would be a function of aging plastic steps in an adverse chemical environment. The steps were likely weakened from both the pH of the water, as well as the chlorine content. Stress fractures would be visible before the step failed. I would guess that close inspection of the other would reveal the same. Plastic steps are common in residential pools because they are the most cost effective option. In a commercial pool I would have expected to see a high quality steel tread step. Much more durable and stable. Unless I misunderstand your comments, I do not believe the failure that you have experienced to be related to galvanic corrosion, but more likely regular wear and tear of plastic installed in a chlorinated water environment. A much stronger case could be made that improper chlorine or pH levels possibly contributing to the early step failure (if the water maintenance logs reveal any long withstanding water balance issues), or perhaps a failure to inspect equipment regularly, but I do not see salt being a main factor in either of these cases.

      1. Steven, Thank-you for your (detailed) response. I will pass along your comments, along with my initial comments to the management of the seniors residence in Lasalle (Mtl) Quebec.

  5. After the frustration and work of many days of throwing in shock, phosphate remover, non-chlorine shock, more shock, 25 bucks today, 50 bucks tomorrow and still almost no free chlorine reading, I told my chlorine only pool store that I was too old (73) to keep doing this and I was ready to switch to salt. They immediately told me that besides the 2500 cost to change to salt (pretty high quote from what i’ve seen) these fixtures (ladder and rails) being discussed here would corrode to the point that I wouldn’t be able to get them out. I thought it was just them scaring me to keep my business. But, from reading this, maybe there is something to it. I know it is a personal choice, but is this scary enough to keep one from choosing a salt system? Thanks for any replies.

  6. deck anchors for the ladder should have been included in the bonding grid and the stainless ladder is bonded by the mechanical connection to the wedge anchor.

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