Salt, Corrosion, and Water Chemistry

If you are considering a salt chlorine generator system for your swimming pool there are many important things to consider and be aware of. When you are in the process of looking to purchase a salt generator system many people will try and tell you as part of their sales pitch that salt is not corrosive and will not cause any harm to your pool or equipment. This is only somewhat true. We are here to help clear up some of the myths and misconceptions out there about whether or not salt is corrosive, its effects on metal, and what to know before deciding what is best for your pool.

Let’s start by covering how a salt chlorine generator works in your pool. A salt chlorine generator is hooked up as part of your swimming pools filtration system. The portion of the system commonly referred to as the “cell” turns regular table salt into chlorine, in a process called electrolysis as your swimming pool water is pumped through the cell by the pool pump. The salt in the water is turned into Hypochlorous acid; the same thing that is produced when any chlorine is introduced to water, no matter what form it is introduced in. As the water enters the swimming pool, it will introduce the newly produced chlorine which prevents algae, bacteria and micro-organisms. This creates a clean and safe swimming environment.

Salt Chlorine Generators are a great sanitation option for swimming pools as long as the owner is aware of everything that comes with them. Corrosion can be defined as the degradation of a material due to a reaction within its environment. Based on that definition corrosion is usually inevitable at some point, in any environment.

This means that whether you use a salt chlorine generator or not, corrosion will likely occur at some point during your pools life. However, when in water, salt acts as an electrolyte, which allows an electric current to occur. This electric current produced in salt water speeds up corrosion through oxidation reduction reactions.  Because corrosion is likely inevitable at some point, people will argue that salt does not cause corrosion which is somewhat true, but it will certainly speed up the process causing corrosion to occur much faster.

When it comes to metals all are susceptible to corrosion at some level, but there are varying degrees of susceptibility depending on their place on the Electromotive or Galvanic Series of Metals.  The following is the galvanic series for stagnant (that is, low oxygen content) in seawater / saltwater, from the top- metals that are least susceptible to corrosion, at the bottom the most susceptible. The order may change in different environments.

• Graphite

• Palladium

• Platinum

• Gold

• Silver

• Titanium

• Stainless steel 316 (passive)

• Stainless Steel 304 (passive)

• Silicon bronze

• Stainless Steel 316 (active)

• Monel 400

• Phosphor bronze

• Admiralty brass

• Cupronickel

• Molybdenum

• Red brass

• Brass plating

• Yellow brass

• Naval brass 464

• Uranium 8% Mo

• Niobium 1% Zr

• Tungsten

• Stainless Steel 304 (active)

• Tantalum

• Chromium plating

• Nickel (passive)

• Copper

• Nickel (active)

• Cast iron

• Steel

• Lead

• Tin

• Indium

• Aluminum

• Uranium (pure)

• Cadmium

• Beryllium

• Zinc plating (see galvanization)

• Magnesium

If there are two metals near each other in salt water, the one that is more reactive or most prone to corrosion will corrode first actually protecting the other metal from corrosion. For example, ship builders often put chunks of zinc next the propeller, without the zinc the steel hull would corrode quickly as it is more reactive than the bronze propeller. Since the zinc is more reactive than the steel it will do all the corroding and the hull will be protected, leaving only for the zinc chunks to need to be replaced periodically.

Similar to what the ship builders do with the zinc near the propeller of a ship, a Pool Sacrificial Zinc Anode can do the same thing for your pool.  Zinc Anodes help to protect things such as handrails, lights, equipment components etc. on your swimming pool from metal corrosion due to electrolysis. Being that zinc is very low on the list it will corrode before other metals higher on the list, this may also be something to consider if you have a copper heat exchanger or copper pipes because copper is also low on the list but with zinc being lower it should help prevent corrosion on the copper elements of your pool.

Now that the basics of salt and corrosion are covered, you are probably wondering if a salt system is right for you and your pool. The truth is they are a great option as long as you are familiar with both the ups and downs of the product and are prepared to properly care for it to avoid problems. Don’t be fooled by sales pitches that tell you that your swimming pool will be maintenance free with the addition of a salt system. A salt generator system will cut down on chemical costs of bromine or chlorine and eliminate having to add them regularly but all other water elements must still be tested and maintained on a regular basis just as they would need to be on any other system.

Acidic water caused by unbalanced pH is another common cause for sped up corrosion. When you have a salt system the combination of the salt and unbalanced pH could quickly wreak havoc on your equipment as it would provide a very fertile environment for corrosion. A common thought that cannot really be explained is that salt generators do cause pH to rise, so when using a salt chlorine generator unbalanced pH could be something you commonly have to deal with.

With the use of a chlorine generator, it is still crucial that pH be maintained at 7.4-7.6 ppm.  Many factors can affect your swimming pools pH level. This includes airborne contaminants, rain, fill water and bathers. Some chemical products can also affect the pH like sanitizers, shock treatments, chlorine salt generators, and balancing products. Since the PH changes frequently due to these and other outside factors it is recommended to test the PH level at least twice a week.

% Active Hypochlorous	pH	% Less Active
Acid (HOCL)	(ppm)	Hypochlorous Ion
97	6	3
91	6.5	9
76	7	24
66	7.2	34
50	7.5	50
33	7.8	67
24	8	76
9	8.5	91
Active Chlorine vs. pH at 86 Degrees F.

 

When chlorine meets water it splits into two parts, Hypochlorous Acid referred to as HOCL and Hypochlorite Ion, known as OCL. The HOCL is the fast, strong, oxidizing disinfectant. OCL is a slow and weak disinfectant. The above chart shows how pH levels affect the ratios of these two parts in your water. This is why it is so crucial to maintain proper pH to keep your sanitizer effective and your water clean. Keeping your pH properly balanced will also cut down on the likelihood of corrosion occurring due to acidic water.

The truth is no matter what system you choose for your pool there are many factors that contribute to corrosion and the best way to avoid problems is by properly maintaining your pool, its equipment, and ensuring you maintain proper water chemistry.