D.C. mode Hydrogen Plasma Treatment for Archaeological iron.

The aim of this project is to determine if the treatment with D.C. mode hydrogen plasma is suitable as a standard conservation method for highly chlorinated archaeological iron. The project has been running since 1997.
For periods since 1985 research on the use of D.C. plasma treatment has been carried out in Denmark. Connecting the object as cathode has shown to be favourable regarding the efficiency of the process [1]. The principle of the current project is a further development of this work.
Until now a number of different aspects have been investigated, such as process parameters: Chamber temperatures and pressure variations during treatment. Chloride content on iron-core surfaces (SEM EDX) on treated and untreated material. Stability in humidity chamber of treated and untreated material. Changes of electric properties of objects during plasma treatment. Effect of plasma treatment on desalination of objects in aqueous solutions.
The apparatus used in the current project consists of a D.C. powerunit (0-3 A, 0-2000 V), Vacuum unit, gas unit, reaction chamber, meters, valves and safety equipment.
The electrodes consists of one big tubular anode. One smaller tubular cathode, which can be switched off from the electrical system. Inside the tubular cathode is a mesh tray, which is also a cathode. The mesh tray carries the objects to be treated.
Investigations of object potential vs. electrode potential during treatment prove that the object when placed on the cathodic mesh tray gains true cathodic potential after a short time covered in plasma (normally between 3 to 15 minutes). This effect occurs even when the object is covered with a thick corrosion crust, and is also independent of the existence of a metallic core. Hereby this DC plasma treatment differs from typical used RF/AC excited plasma treatments for archaeological iron. Measurements indicate that the reductive effect of the process is dramatically enlarged, when the cathodic potential of the objects is achieved.
The research shows that even though the reduction of the corrosion products seems to be quite fast and efficient, the transport of chloride from the objects during the plasmatreatment is rather slow. Often plasma treated objects show extensive formation of characteristic akagenéite-chrystals after short-term exposure to normal indoor humidity. This indicates that the plasma treatment results in the forming of a mineral structure, where the remaining chloride is very reactive. It is possible that plasma treatment alone results in decreased stability of the objects. At the other hand the treated objects are much more suitable for desalination in aqueous solutions. This is illustrated by the fact that objects which have been desalinated in up to five years by alkaline washing (0.01M NaOH) - and considered stabile - liberate considerable amounts of chloride after plasma treatment.
The next step of the project is to determine the amount of chloride left in the structure after plasma treatment and following alkaline washing. To get a reliable impression of the chloride content, we want to measure the total chloride content in a large number of objects. In order to do this we dissolve the objects in nitric acid.
At the moment we have some difficulties in finding a suitable method to detect small amounts of chloride in extremely acidic solutions. We will be grateful to get some good advise in this matter.


[1] : Studies in Conservation, 36 (1991) 161-171