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Electrochemical Corrosion of Iron

The electrochemical oxidation of iron results in the formation of ferrous ions as the initial product.

The electrochemical corrosion of metals has been detailed in a number of sources. In the case of iron, it has been shown that in any electrochemical cell where iron establishes a metallic couple in salt water with a more noble metal, such as copper or silver, or even with another piece of iron or a different part of the same iron object, the anodic and cathodic reactions are the same (see Potter 1956:236-237; Evans 1963:28).

At the surface of the more noble metal (the cathode), the following reaction occurs:

2H2O + 2e >> H2 + 2(OH)-

The hydroxides combine with the sodium ion in the solution to form sodium hydroxide as the cathodic product:

Na+ + OH- >> NaOH

At the anode, the reaction is the production of ferrous ions:

Fe+ - 2e >> F+2

which, in turn combine with chloride in the salt water to form ferrous chloride as the anodic product:

Fe+2 + 2CI- >> FeCl2

On exposure to air, or solutions containing dissolved oxygen, the ferrous chloride oxidizes to ferric chloride and ferric oxide. Ferrous chloride and ferric chloride and are freely soluble and may yield ferrous hydroxide when they combine with the cathodic product sodium hydroxide:

FeCl2 + 2NaOH >> Fe(OH)2 + 2NaCl

In solutions containing dissolved oxygen, a secondary reaction oxidizes the ferrous hydroxide to a ferric state. In the presence of hydroxyl ions in a neutral or slightly alkaline solution, this hydrated ferric hydroxide (any form of ferric oxide with internal water, i.e., common rust) is precipitated on or around the electrodes of the cell. The sequence of reaction at an iron anode in the presence of oxygen as stated by Potter (1956:236) is:

ferrous ion Fe - 2e >> Fe+2
ferrous hydroxide Fe+2+ 2OH- >> Fe(OH)2
hydrated ferric hydroxide (red-brown rust) 4Fe(OH)2 + O2 >> 2H2O + 2Fe2O30H2O

The primary anodic reaction of electrochemical corrosion of iron is the production of ferrous ions. The secondary stage, the oxidation of the ferrous ion compounds to a ferric state, is modified in anaerobic environments. Intermediate oxidation products of ferrous hydroxide, such as hydrated magnetite and black magnetite, are formed (Potter 1956:236-237; Evans 1963:28-29, 75):  

6Fe(OH)2 + O2 >> 4H2O + 2Fe3O40H2O
(green hydrated magnetite)

Fe3OH2O >> H2O + Fe3O4
(black magnetite)

Depending on the environment, the corrosion products can take on a variety states of division and hydration, as well as a variety of physical forms. It is common to find corroded iron from marine sites with an outer layer of hydrated ferric hydroxide (common rust), which has restricted the supply of oxygen to the ferrous hydroxide briefly formed at the surface of the metal. Laminated corrosion layers consisting of an inner layer of black magnetite, a thin layer of hydrated magnetite, and an outer layer of hydrated ferric hydroxide are formed:

 Fe3O4/2Fe3OH2O or 2Fe2OH2O

It is easy to see how two different areas of the same metal object can become anodic and cathodic to form an electrolytic cell. Electrons flow from the anodic area to the cathodic area causing the metal to corrode by forming soluble positive ions at the anode. Millions of these cells over the surface of the metal result in massive oxidation, which continues until an equilibrium state is reached. The corrosion process is halted at the cells when they come into equilibrium but may continue at alternate anodic and cathodic positions on the object until the bulk of the metal is oxidized.


iFile 9: Metals Conservation: Preliminary Steps, Methods of Conserving Archaeological Material from Underwater Sites, Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University, 1999