Corrosion is an electrochemical reaction involving the movement of
electrons. Let's first consider a more familiar electrochemical
reaction - that which occurs when electricity comes out of a battery.
In a battery, electrons build up in the negative end, also known as the
anode. The positive
known as the cathode, is
attractive to electrons due to its positive charge. If the two
ends of the battery are connected with a conductive object, such as a
metal wire through which electrons can flow, the electrons will flow
from the anode to the cathode as an electric current. The battery
and the wire make up what is known as an electrolytic cell, which is a device
which causes an electric current to flow.
Corrosion in a metal object, such as a pipe, acts in the same
manner. A negative area of metal (the anode) is connected to a
positive area (the cathode) by the pipe wall itself. As a result,
electrons can flow from the anode to the cathode.
In addition to the anode, the cathode, and the connecting conductive
material, the electrochemical reaction requires one more element - the
electrolyte. The electrolyte
is a conducting solution, which in the case of a pipe is the water
within the pipe with its dissolved salts. (In a battery, the
electrolyte is found within the battery - the "battery acid".)
the electrons from the cathode, making the cathode maintain a positive
charge which draws more electrons to it.
So, in summary, any electrochemical reaction requires four elements,
all of which must be in contact - the anode, the cathode, the
conductive material, and the electrolyte. In the battery, the
anode and cathode are the two ends of the battery, the conductive
material is a wire or other object touching both ends, and the
electrolyte is found inside the battery. In the case of corrosion
of a pipe, the anode, cathode, and conductive material are all found in
the pipe wall while the electrolyte is the water within the pipe.
If any of these
four elements, which make up the corrosion
cell, are absent or are not touching each other, then corrosion
In the last section, we discussed the electrical side of the
electrochemical reaction occurring during corrosion. In order for
the flow of electrons to occur, however, chemical reactions must also
be happening. In this section, we will consider the chemical
reactions which occur in an iron pipe as it corrodes. Other types
of pipes will
have different, but homologous, chemical reactions driving their
The main force behind corrosion is the tendency of iron to break
down into its natural state. The iron found in pipe is elemental
iron (Fe0) which is unstable and tends to oxidize, to join with oxygen or
other elements. In nature, this oxidation produces an iron ore
such as hematite (Fe2O3), magnetite (Fe3O4),
iron pyrite (FeS2), or siderite (FeCO3). In
corrosion, the result of this oxidation is rust, Fe(OH)2 or
Oxidation of the elemental iron occurs at the anode.
the elemental iron breaks down as shown below. In this reaction,
elemental iron leaves the pipe, so pits form in the pipe's surface at
Elemental Iron →
iron + Electrons
Fe0 → Fe2+
The reaction produces ferrous iron and two electrons.
electrons are then able to flow through the pipe wall to the
Meanwhile, the ferrous iron reacts with the water (the electrolyte) in
the pipe to produce rust and hydrogen ions.
Ferrous iron + Water ↔
Ferrous hydroxide + Hydrogen ions
Fe2+ + 2H2O ↔
The rust builds up a coating over the anode's surface.
hydroxide may then react with more water to produce another form of
called ferric hydroxide (Fe(OH)3). These layers of
rust are what creates the tubercles we mentioned earlier.
Tubercles can become problematic because they decrease the
capacity of the pipe and can be dislodged during high water flows,
resulting in red water complaints. But in the corrosion process,
the tubercle actually slows the rate of corrosion by cutting the anode
off from the electrolyte. When the tubercle becomes dislodged and
the anode comes in contact with water again, the corrosion rate
The electrons from the breakdown of elemental iron flow through the
pipe wall to the cathode. There, they leave the metal and enter
the water by reacting with hydrogen ions and forming hydrogen gas:
Hydrogen ions + Electrons ↔
2H+ + 2e- ↔
Hydrogen gas will coat the cathode and separate it from the water in a
process called polarization.
Just as the buildup of a tubercle breaks the connection between the
anode and the electrolyte and slows the corrosion process, polarization
breaks the connection between the cathode and the electrolyte and slows
Dissolved oxygen in the water is able to react with the hydrogen gas
surrounding the cathode:
Hydrogen gas + Oxygen ↔
2H2 + O2 ↔
This reaction is called depolarization. Depolarization removes the hydrogen
gas surrounding the cathode and speeds up the corrosion process.
So, you can see why water high in dissolved oxygen is more corrosive.
The Electrochemical Reaction
By combining the electrical and chemical reactions discussed above, we
can see what is
really happening during corrosion of a pipe.