In industry classes, at trade shows, in articles and in product
manuals, the word “electrolysis” comes up in a wide
range of contexts. Many service technicians know that it’s
the process electrolytic chlorine generators (ECGs) use to make
chlorine from salt water. But others also warn that it can
contribute to certain types of metal corrosion.
What exactly is electrolysis — and how can one process be
involved in such seemingly unconnected chemical reactions?
Experts in pool chemistry have the answers. Here, they share their
explanations of electrolysis, from its effects on pool equipment
all the way down to its contributions on a molecular level.
A thorough grasp of electrolysis is far more than just “book
knowledge” — it enables more accurate diagnoses of many
pool problems that other service people might incorrectly attribute
to water chemistry.
Understanding its nature and effects will also contribute to better
pool maintenance overall — not only for salt pools, but for
any swimming setup.
Proper comprehension of the effects of electrolysis must begin with
an awareness of how the process works at the chemical level.
Professionals throughout the pool industry hold some diverse
opinions about what, precisely, electrolysis is — some say it
involves metals, while others mention water or salt.
According to chemists, electrolysis is simply the process of using
electric current to drive a chemical reaction that wouldn’t
occur spontaneously. This can be intentional or accidental, and it
requires just three types of components: A direct current (DC)
electrical supply, a pair of electrodes to conduct the electrical
current, and an electrolyte — a substance containing mobile
free ions (positively or negatively charged atoms) to carry the
“The electrolysis itself is essentially the transfer of
electrons from one electrode into the solution, and then from
different components in the solution back into the other
electrode,” says Zach Hansen, technical services engineer at
BioLab Inc. in Lawrenceville, Ga.
As in any electrical circuit, electrons — the particles
responsible for negative electrical charge — need a pathway
through which to flow. At one point in this pathway sits the anode,
from which electrons flow into the metallic portion of the circuit.
At the other end of the circuit’s metallic portion sits the
cathode, which receives electrons that have come through the metal
from the anode. Meanwhile, in the electrolyte portion of the
circuit, negatively charged ions (anions) move toward the anode,
while positively charged ions (cations) move away from it and
toward the cathode.
What makes an electrolysis circuit unique is the electrolyte
— the solution that allows ions (and thus, electrical
current) to move around relatively freely. This contrasts with
circuits in which the current is restricted solely to the movement
of electrons through metallic conductors, such as wires. Certain
atoms, molecules or ions in the electrolyte accept the free
electrons that flow in from the cathode, while others have their
electrons pulled away by the positive charge of the anode —
and these changes cause those atoms, molecules or ions to transform
into ions, molecules or atoms of another oxidation state.
For example, when common salt — known chemically as sodium
chloride (NaCl) — dissolves in water, it splits up into two
ions — positively charged sodium (Na+) and
negatively charged chloride (Cl-). As electric current
passes through salt water, electrons are ripped away from the
chloride ions and pulled toward the positive charge of the anode.
The removal of these electrons converts the chloride ions into
neutral chlorine atoms, which are then able to join into molecules
of electrically neutral chlorine gas (Cl2) — one
form of the commonly mentioned “free available
chlorine” that acts as an oxidizer in swimming pools.
Having a clear sense of how the electrolytic process works in
general, at the level of individual atoms, will make it much easier
to understand how it contributes to such a wide variety of effects
up here at the macroscopic level.
The most common purposeful use of electrolysis in the pool industry
is for electrolytic salt chlorination — the creation of free
available chlorine from the chloride ions found in salt water.
Though the terms “electrolysis” and “salt
chlorination” are sometimes used interchangeably, they
aren’t actually synonymous — electrolytic salt
chlorination is just one application of electrolysis.
As the “Chemistry” section above explained, passing
electric current through salt water leads to the conversion of
chloride ions into molecules of chlorine gas (Cl2). As
this happens, the chlorine molecules diffuse away from the anode
and react with water molecules (H2O) to form
hypochlorous acid (HClO) or hydrochloric acid (HCl).
But what happens to the positively charged sodium ions
(Na+) that are left over? They’re drawn toward the
cathode, where they balance the negative charge of the
free-floating hydroxide ions (OH-) which are formed by
reduction of water molecules at the cathode. The resulting solution
near the cathode is sodium hydroxide (NaOH) — a caustic base
commonly known as lye. The sodium hydroxide thus formed will
quickly react with the chlorine formed at the anode — or with
the hydrochloric and hypochlorous acid made by the reaction of
chlorine with water — to make sodium hypochlorite (bleach)
and sodium chloride (salt). It also reacts with the sodium
bicarbonate (“bicarb”) that acts as a pH buffer in the
pool’s water to form carbonate — which soon bonds with
positively charged calcium ions concentrated around the
Because of this series of reactions, scale tends to accumulate on
the cathode, where it often creates problems for the flow of
electric current. Thus, some manufacturers design their chlorine
generators to flip the direction of electrical current back and
forth on a regular basis — turning the anode into the
cathode, and vice versa. This is known as reversing polarity, and
it allows the high-pH areas to be bathed in hypochlorous and
hydrochloric acid, which help dissolve any scale that’s
beginning to accumulate.
Thus, if there’s scale buildup in a chlorine generator
— but not in other areas of the pool — there may be
problems with the ECG’s electrolytic circuit. “Scale is
public enemy number one for the ECG,” says Geoffrey Brown, a
developmental scientist at Pristiva Inc. in Overland Park, Kan. “If you get
scale, you could get heat buildup inside the cell, and you could
ultimately ruin your ECG and have to replace it. And even in the
short term, your ECG will have to work harder to make
Because of this potential for long-term damage, it’s
important to immediately investigate scale treatments such as
antiscalants, acid washes for the plates, or — if applicable
— performing or outsourcing an electrical repair to restore
the generator’s ability to reverse polarity.
As electricity flows through water, its influence isn’t
always limited to intended chemical reactions. One potentially
problematic side effect of electrolysis is galvanic corrosion
— the gradual disintegration of one metal that’s in
direct metallic contact with another metal while both metals are in
contact with the same electrolyte.
The term “dissimilar metals” is commonly brought up as
a cause for galvanic corrosion, but the phrase is often
misunderstood — galvanic corrosion won’t necessarily
happen just because two metals in an electrolyte have different
names. Instead, the term refers to the metals’ relative
positions on a galvanic corrosion chart, which compares the
similarity of metals on an electrochemical basis. Metals toward the
“less noble” (anodic) end of the chart are more easily
oxidized — and thus, more likely to be corroded — than
those toward the “more noble” (cathodic) end. In fact,
the word “noble” in reference to metals just means
“resistant to oxidation.”
But what exactly is oxidation, and what does it have to do with
electrolysis? In general terms, oxidation refers to an interaction
between an oxygen (O2) molecule and any other substance.
If one type of metal in an electrolytic circuit is more easily
oxidized — that is, more prone to react with oxygen —
than another metal in contact with the same electrolyte, the flow
of electricity will accelerate that process of oxidation,
eventually leading to rust or other forms of corrosion.
To put all this in more specific electrochemical terms, oxidation
often involves the loss of one or more electrons by a molecule or
atom. For example (as indicated on the chart), iron that’s
connected to an electrolytic circuit will have a greater tendency
to lose one or more of its electrons than copper will. This
electron loss will leave room for the oxygen in free-floating
hydroxide ions (OH-) to bond with the iron (Fe), forming
iron (ferric) oxides such as FeO, Fe3O4 or
Fe2O3 — in other words, rust.
Though the presence of applied electric current accelerates
oxidation reactions, merely placing two dissimilar metals in an
electrolyte is sometimes enough to generate an electric current, as
electrons from the less noble (anodic) metal naturally drift away
from the electrolyte through the metallic part of the circuit
toward the more noble (cathodic) metal. In fact, this is exactly
how the earliest batteries generated electricity. Though this
process brought some obvious advantages to its users, they also
realized that the life of a battery is inherently limited —
if the electrochemical difference between the metals is strong
enough to generate much electric flow, one of the metals must
oxidize and degrade fairly rapidly.
However, this very principle has been adapted for a more helpful
purpose in home and pool appliances such as water heaters. In some
of these appliances, iron may serve a useful structural purpose,
but must be kept sealed off from a water-filled area that would
allow it to form an electrolytic circuit with a dissimilar metal,
such as a copper pipe nipple. In a case like this, a
“sacrificial anode” made from an even less-noble metal
— such as zinc — might be installed in the water heater
in such a way that it makes direct physical/ electrical contact
with the steel of the water tank. Then, if a crack develops in the
glass lining of the tank — exposing the steel itself to the
water — the sacrificial zinc anode will corrode first,
transferring electrons through the point of metal-to-metal contact
to the iron, making the iron cathodic. Being negatively charged or
cathodic, the steel will not oxidize or corrode until the
sacrificial anode is fully spent and the cathodic protection is
Galvanic corrosion can also happen in less predictable
circumstances, though — for example, it may take hold in pool
environments where dissimilar metals are in contact both with one
another and with salt water, which is more electroconductive than
pure H2O. In a circuit like this, electrolysis and
galvanic corrosion can occur in the absence of any applied
electrical current — the electrochemical difference between
the metals, and the conductivity of the water, are enough to
produce an electric current.
“Galvanic corrosion is an instance where the influence of
electrolysis is so often missed, because people don’t
recognize it for what it is,” says John Puetz, director of
technology at Advantis Technologies Inc. in Alpharetta, Ga.
“They can’t understand why this corrosion is happening,
because there’s nothing wrong with the water chemistry
— but in fact, the chemistry can be perfectly fine and
galvanic corrosion can still occur.”
One helpful tip for diagnosing galvanic corrosion is the fact that
it often — though not always — happens near a weld or
other point of electrical contact between dissimilar metals.
“If you see that corrosion is much more severe near such a
point of contact than it is farther away from that point of
contact, you can be suspicious that galvanic corrosion is involved
there,” says Stan Pickens, a senior research associate at
PPG Industries in Monroeville, Pa.
In cases like these, one way to prevent galvanic corrosion is
simply to break the circuit. “You can greatly slow down the
rate of corrosion — or perhaps even stop it entirely —
simply by installing a ceramic junction between those two
metals,” Puetz says. Other pool professionals recommend PVC
pipe fittings. In either case, interrupting the flow of electric
current will reduce the rate at which electrons are pulled away
from the anodic metal, making it much less likely to oxidize and
corrode. Installing a sacrificial anode, as discussed just above,
is an alternate means of protecting base metals from
The chemistry of salt water can also play a part in galvanic
corrosion: As iron oxidizes, some of the ferric oxides described
above may form a fairly inert coating on the iron, which can help
protect the metal against further corrosion. However, salt water
may counter that protective effect. “Chloride in the water
tends to destabilize — depassivate — an oxide layer, so
that’s one reason that chloride salts tend to make water more
corrosive,” Pickens says.
As these examples demonstrate, metals, water, electric current and
salt can all interact in a variety of ways to produce a wide range
of effects. Thus, a solid grasp of electrolysis makes it much
easier to understand why these reactions can lead to results as
diverse as chlorine generation and metal corrosion. It’s just
one more way in which a little chemical background can vastly
improve the care regimen for a pool and its equipment.