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The Truth about Salt, Corrosion, and Water Chemistry

When it comes to salt there are many myths and misconceptions out there about whether or not it is corrosive and its effects on metal, in this article we hope to clear up a few of them particularly where your hot tub is concerned, and especially if you currently use or are thinking about using a chlorine or bromine generator system.

There are many people out there, a lot of who are trying to sell you a salt generator system or items like a soft water system for your home, who claim that salt is not corrosive. This is only somewhat true.

Corrosion can be defined as the degradation of a material due to a reaction with its environment.

This means that corrosion is often inevitable in some form, in any environment. 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. So, it can be argued that the salt itself is not the root cause of the corrosion but it is definitely rapidly speeding up the process. If you have ever lived near the ocean you are aware of the corrosion that can be very rapidly increased due to salt. When using salt in your hot tub, levels are much lower than in the ocean but the presence of salt can still speed up corrosion.

When it comes to metals, all of them 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.

Because corrosion is a spontaneous process that converts metals to their ionic form, as long as conditions are right, it is bound to happen. Often people will try to prevent this by coating the metal material with something that resists corrosion or by having a sacrificial metal, like the zinc used on the ships.

Corrosion caused (or sped up) by salt can occur at any level of salt exposure but due to the chemical process in which the salt speeds corrosion it can be assumed that more salt equals more rapid corrosion. Therefore, the saltier the water the faster it will corrode the metals in contact with it.

When it comes to your hot tub, probably the most common issue with corrosion is on the heating element. Things such as low pH, or over chlorination can cause water to become acidic which is often attributed to causing corrosion of hot tub heating elements, with salt present this can speed up the process as previously explained, putting your heating element at even greater risk. As a heater element becomes corroded it will start pitting, etching, and have surface breakdown eventually causing it not to work properly.

To help prevent corrosion of your hot tubs heater element, there are two options. The best is to purchase a titanium heater element. In the above chart of which metals are most susceptible to corrosion you will see that titanium is very close to the top, meaning it is not going to corrode nearly as easily as those lower on the chart. The second option, though not quite as preventative, is a coated heater element. This means that the element is coated in a substance made to resist corrosion also allowing you to get a longer life from your element and holding off corrosion.

Another common question about corrosion is regarding Stainless Steel—why isn’t stainless steel as susceptible to corrosion but can still corrode? While stainless steel does not corrode, rust or stain with water in the same way that ordinary steel does—don’t let its name fool you as it is not fully stain proof. Based on the environment that the stainless steel is made to endure there are different grades, each grade has its own susceptibility to corrosion. 304 is the most common grade of stainless steel, used in everything from cars to cookware and buildings.

There is magnetic (400) and non-magnetic (300) grade stainless steel, the magnetic or 400 series is not as durable or corrosion resistant. If you want to test durability in your stainless steel products you can do a simple test with a magnet. If the magnet sticks, to the stainless steel it is not as durable and corrosion resistant as it is claiming to be—also some manufacturers will mix materials so be sure and conduct the magnet test all over the product.

Now we have covered metals but what about plastic, in hot tubs particularly jets and plumbing. Typically plastics are very resistant to salt water corrosion, although after very long periods of time they may become brittle but in general plastic is resistant. When it comes to your hot tub things like bad water chemistry will wreak havoc on your plastic parts far more than salt.

Now that we have covered all the basics about salt and corrosion, you are probably wondering whether a salt chlorine or bromine generator is really right for you. The truth is they are a great option as long as you know both the ups and downs of the product. Just don’t be fooled by the sales pitch that it will make your hot tub maintenance-free. Though a salt chlorine generator or bromine generator system will cut down on chemical costs of bromine or chlorine, all the other water elements must still be checked and balanced regularly, just as they would on any other system.

As previously mentioned, unbalanced pH can cause acidic water which when combined with salt will result in rapid corrosion. It is thought, but not explained as to why, that chlorine and bromine salt generators can cause pH to rise. So if you are using a chlorine generator be prepared that high pH may be something that you are commonly dealing 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 hot tubs PH level. This includes airborne contaminants, rain, fill water and bathers. Some chemical products can also affect the PH like sanitizers, shock treatments 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.

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.

All in all, there are many things that factor into good water chemistry and keeping your water clean and balanced is the best way to avoid corrosion and maintenance problems whether you are using a salt generator system or not.

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