Corrosion Chemistry

Corrosion Cell


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.

Battery

In a battery, electrons build up in the negative end, also known as the anode.  The positive end, 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. 
Corrosion cell.

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 electrolyte accepts 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 cannot occur.



Anode Reactions


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 corrosion. 

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 Fe(OH)3.

Oxidation of the elemental iron occurs at the anode.  First, 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 the anode. 

Elemental Iron Ferrous iron + Electrons
Fe0 Fe2+ + 2e-

The reaction produces ferrous iron and two electrons.  The electrons are then able to flow through the pipe wall to the cathode.  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 Fe(OH)2 + 2H+

The rust builds up a coating over the anode's surface.  Ferrous hydroxide may then react with more water to produce another form of rust 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 carrying 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 increases. 


Cathode Reactions


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 Hydrogen gas
2H+ + 2e- H2

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 corrosion. 

Dissolved oxygen in the water is able to react with the hydrogen gas surrounding the cathode:

Hydrogen gas + Oxygen Water
2H2 + O2 2H2O

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.

Electrochemical reaction of corrosion.