Partner: Tegan Zurbo
Semester 3 2011
The purpose of this extended experimental investigation is to explore the optimum conditions for restoring rusted iron. By chemically preserving a rusted nail, the methods used aim to simulate the ways in which marine artefacts are restored.
Corrosion occurs very rapidly in marine environments due to the optimum conditions for metals to rust. Both water and oxygen are necessary for rust to form as well salt accelerates rusting, due to this rapid acceleration the ability to reverse the effects of corrosion of metal objects immersed in sea water is a necessity. Variables including temperature, pH and the presence of chloride are involved in corrosion in marine environments and considerable impact on the rate and type of corrosion that occurs. The metal being studied in this investigation, iron, is one of the most challenging of all metals to conserve.
Metal corrosion occurs as a reduction and oxidation (redox) reaction. In this situation, oxidation can be defined as the loss of electrons or the gain of oxygen in going from reactant to product. Reduction can be defined as the gain of electrons or the loss of oxygen in going from reactant to product. Both processes occur simultaneously throughout the reaction.
Corrosion of iron that occurs in a marine environment is a process called electrochemical corrosion the process of rust formation is virtually a galvanic cell. At a weak point on the iron surface iron atoms lose electrons to form Fe2+ ions: Fe -> Fe 2+ (aq) + 2e-
These sites of oxidation are known as anodic sites.
The electrons flow from these sites through the iron to an area of the iron that contains an impurity such as carbon, this impurity may act as a cathode, these electrons then reduce the oxygen that is surrounding the iron in the water. Fe2+ (aq) + 2OH- (aq) Fe (OH) 2 (s)
Sites where this reduction is caused are called cathodic sites. The ions migrate from one location on the irons surface to another through the moisture surrounding the artefact, due to the conduction ability of salt water the rusting process occurs much faster in salt water than in fresh water. This migration of irons preserves electrical neutrality within the galvanic cell by pushing Fe2+ and OH0 together which forms insoluble iron hydroxide: Fe2+ (aq) + 2OH- (aq) Fe (OH) 2 (s)
Iron hydroxide is oxidised to iron forming rust:
4Fe (OH) 2(s) + O2 (g) 2(Fe203H20) (s) + 2(H20)(l)
Figure [ 1 ] Galvanic Corrosion
Archaeological iron is usually covered by a layered structure of corrosion products. The outer layer is a mixture of iron corrosion products (e.g. iron(III) oxyhydroxides, typically goethite) and extraneous material such as small rocks, sand, clay and soil minerals. Below this is another layer of iron corrosion products in a lower oxidation state, usually magnetite, lying on top of any remaining metal. When iron corrodes in a marine environment, it usually becomes covered with concretions, primarily calcium carbonate CaCO3. The Fe2+ ions tend to react and precipitate in the concretion rather than on the surface of the object. The net result of on-going iron corrosion in marine environments is that the cracks, pores, and open spaces within the corrosion layer or beneath concretion become filled with an acidic iron(II) chloride solution, with the Cl- ions concentrated at the metal surface (Turquoise 1993).
The experiment has been based around the National Museum of Australia’s method of conserving iron objects. The artefact collected is conserved by storing the object submerged in an alkaline solution from the point it is taken from the ocean this prevents any further reactions from taking place, the removal of the Cl- minimises the further corrosion of the artefact. In order to restore the corroded object the museum then uses an electrolytic cell, a process that...