Ferrous artefacts

Metal artefacts immersed in seawater for long periods of time such as over hundreds of years, may become completely corroded and 100% mineralised. In certain instances this is the case but in others, the metals or alloys have only exhibited minimal amounts of corrosion. Even ferrous artefacts recovered from the same site may experience vastly different corrosion rates. 

If there is a conservation laboratory nearby, immerse the artefact straight away in an electrolyte, which passivates any exposed metal. The pH can be ascertained from the relevant E-pH diagram for the metal concerned and examples for ferrous artefacts would be 0.5 M sodium hydroxide, 0.2 M sodium sesquicarbonate or 0.5 M sodium carbonate. 

Finally, thermal methods such as placing the artefact in a hydrogen furnace at 400°C has been successfully employed on ferrous artefacts recovered from the Mary Rose. Any remaining concretion on the surface was present as a fine powder after conservation and was just gently brushed off the surface. An alternative thermal method involved placing the artefact in an inert atmosphere at 1066°C followed by quenching. The procedure is repeated until all the concretion has spalled from the metal. 

Sodium hydroxide is a very common solution used for ferrous artefacts. A concentration of 0.5 M will give a pH of approximately 10.5 that is well in the passivity region for this class of materials. The problem is that if any ferrous or ferric chloride type compounds are present within the corrosion products, these may react with the hydroxide to produce solid sodium chloride within the pores in the rust film (Equation (12)). 

Sodium carbonate is another widely used solution for the conservation of ferrous artefacts. This maintains the pH in the zone of passivity (pH > 9.5) and is also said to act as an anodic inhibitor. A concentration of 3 g dm 3 (0.2 M) was used in the conservation of Holland 1 in Gosport Submarine Museum. It is advisable to use the sesquicarbonate form as it is far easier to dissolve than the simple carbonate. The use of this solution is said to avoid formation of solid sodium chloride in the rust. 

The treatment of small ferrous artefacts in a 0.05 M lithium hydroxide dissolved in methanol or ethanol has its advocates, particularly in France. The chlorides present in the rust layers react with lithium hydroxide to form lithium chloride that dissolves in the alcohol phase. Any of the hydroxide left on the metal surface combines with any carbon dioxide to form a solution with pH above 9.5, which maintains any exposed metal in the passive region. Hence, this solution is claimed to cause no corrosion of the underlying metal. The real disadvantage of this solution is that any lithium chloride left on the surface of the artefact is very hygroscopic. Water will form on the surface at a relative humidity above 15% RH and corrosion of the metal will take place. Humidity levels below 15% RH are very difficult to maintain in display cabinets or in storage and is one of the main reasons why this solution has not been more widely employed. 

The hydrogen reduction conservation process was first employed in Sweden in 1964 for ferrous artefacts recovered from the Swedish warship, the Vasa. The method was further developed at Portsmouth to treat the large number of finds recovered from the Solent and land-based archaeological sites within the Wessex region. The principle of the process is to heat the artefact in an atmosphere of hydrogen in order to sublime off the volatile chlorides and at the same time reduce the oxides, hydroxides, chlorides and eventually to the metallic state. The volume change associated with the reduction of the iron compounds is sufficiently high to enable the release of deeply-buried chlorides particularly at the metal/corrosion product interface. 

The anodes that have been used include stainless steels, mild steel, lead and platinised titanium, while typical electrolytes for ferrous materials have been 0.5 M sodium hydroxide, 0.2 M sodium carbonate, 0.5 M sodium sesquicar-bonate and tap water. For bronze cannons recovered from the Mary Rose, both sodium hydroxide and sodium carbonate electrolytes were employed while pewter artefacts (plates) from the same ship were treated in similar electrolytes or in a 0.5% solution of EDTA as a sodium salt in alkaline solution. 

---