Electrochemistry and the Corrosion of Steel
The corrosion of metal, steel is particular, is due to an electrochemical reaction that converts the metal into oxides. The extent of corrosion depends on the nature of the oxides that form. In the case of steel, "rust" is highly porous, so corrosion continues to penetrate the metal. In metals such as aluminium, a tight protective oxide layer quickly forms that blocks further corrosion. Stainless steel alloys form a similarly protective oxide layer.
Electrochemistry
Electrochemistry is the process by which electricity is generated from a chemical reaction. This is the process that takes place inside a battery. A basic battery consists of a single galvanic (also called voltaic) cell.Galvanic Cell Structure
A galvanic cell consists of two electrodes suspended in an electrolyte. An example is a cell with a zinc electrode, a carbon electrode, and an electrolyte of dilute sulfuric acid. The electrodes are connected externally through a wire. Zinc gives up electrons that travel through the wire to the carbon electrode. This is the electric current supplied by the battery.The zinc which supplied the electrons goes into solution as positively charged zinc ions, Zn++. These ions react with negatively charged sulfate ions, SO4++ from the sulfuric acid electrolyte, H2SO4.
At the carbon electrode, hydrogen ions, 2H+, react with the electrons that were supplied by the zinc electrode, forming hydrogen gas, H2. This process continues until either the zinc electrode or the sulfuric acid electrolyte is used up. The overall formula is:
Zn + H2SO4 = ZnSO4 + H2
The process is shown in Galvanic Cell.
Electrical Currents and Corrosion
In most cases, the corrosion reaction is associated with electrical currents that originate from galvanic cells on the surface of the metal. The electrochemical activity of galvanic cells takes place in separate cells called half cells. Each half cell supports the activity of either an anode or a cathode:
- Anode: The positive electrode. Oxidation (loss of electrons) takes place at the anode.
- Cathode: The negative electrode. Reduction (gain of electrons) takes place at the cathode.
The anode and cathode must be connected in such a way that electrons can travel from the anode to the cathode. In a battery, this connection takes place through an external curcuit, such as a wire that connects the battery to some piece of equipment. In the case of the corrosion of a piece of metal, such as steel, the metal inself provides the path that connects the anode with the cathode.
There must also be a path by which ions from the anode can travel to the cathode. This path is supplied by an electrolyte, for example, sea water.
Note: The conductor cannot be pure water, which is a very poor conductor of electricity.
Origin of the Galvanic Cells in Steel
All steels contain a certain amount of carbon, which determines the general properties of the steel. There are also varying amounts of other elements, such as sulphur or phosphorus, that either were not entirely removed in the refining process or were intentionally added to give desireable properties, such as machinability, to the metal. From the electrochemical standpoint, these elements are all impurities that contribute to the formation of galvanic cells, which in turn cause corrosion. Very pure metals show little, if any, corrosion.Rusting of Steel
The reactions that take place during the rusting of steel are shown in the following table:
| Reaction | Description |
|---|---|
| Fe = Fe2+ + 2e- | Iron is oxidized (losses electrons) at the anode. Ferrous ions go into solution. The electrons travel through the metal to the cathode. |
| 2H+ + 2e- = H2 | This reaction occurs if the electrolyte is strongly acidic. Hydrogen ion is reduced (gains electrons) at the cathode, forming hydrogen gas. |
| 2H+ + 2e- + 1/2O2 = H2O 2e- + H2O + 1/2O2 = 2OH- Fe2+ + 2OH- = Fe(OH)2 2Fe(OH)2 + H2O + O2 = Fe2O3 . 4H2O |
This reaction occurs if the electrolyte is neutral or alkaline. Free OH- ions react in solution with the ferrous ions from the anode, forming ferrous hydroxide. The ferrous hydroxide is oxidized by reacting with water and oxygen to form hydrated ferric oxide - rust. |
Cathodic Protection
The galvanic cell shown in Galvanic Cell uses one metal (zinc) electrode and a carbon electrode. Both electrodes in a galvanic cell can be metal, provided the metals used are different. For example, the anode can be zinc while the cathode is iron. That is because zinc is a more active metal than iron: zinc gives up electrons more readily than iron does. In this case, zinc from the anode goes into solution as before. Ferrous chloride (FeCl2) is used as the electrolyte. Ferrous ions (Fe++) in solution acquire electrons at the cathode, so that additional iron plates onto the iron electrode. Zinc ions (Zn++) replace the ferrous ions in solution. Because zinc is used up in the process, zinc is considered to be a sacrificial metal. The iron at the cathode is "protected" by the negative charge it acquired by obtaining electrons from the zinc anode. The general name for this process is cathodic protection.Note: The protection of iron pipes by coating them with zinc, a process called galvanization, utilizes the high activity of zinc to protect the iron from rusting. In moist air the zinc is oxidized to basic zinc carbonate, Zn2CO3(OH)2. The zinc carbonate forms a tough layer on the zinc, protecting the zinc, and by extension the iron underneath the zinc, from further corrosion. If the zinc carbonate and zinc layers are broken, so that the underlying iron is exposed, the zinc will corrode sacrifically, protecting the iron. Although the electrolyte is water, possibly salt water, rather than the ferrous chloride used in the example, this sacrifical protection occurs galvanically according to the principles described.