| Abstract: | SUMMARIES. The composition and nature of the different sulfides which can occur on underground iron are reviewed with reference to the literature, together with the various corrosion reactions. The type of pyrites on fossils and minerals is also included. The experimental work involved goethites and magnetites reacting with damp hydrogen sulfide gas to give mackinawite, pyrite and/or greigite. These can react exothermically and may be pyrophoric. Methods to increase stability and reduce the danger of pyrophoric oxidation are discussed.
CONCLUSION. It has been shown in this work that goethite and also magnetite can react with moist hydrogen sul¬fide to form mackinawite according to the reac¬tions: On aging, this mackinawite can react with sulfur to give greigite or pyrite: 3FeS + S = Fe3 S4 and FeS + S = FeS2 All these sulfides can react in air in a pyrophoric manner to give different iron oxides. From this work it has been shown that it is possible to pre¬vent pyrophoric oxidation by: 1 Exposing the iron sulfides to an atmosphere of nitrogen containing 2-4 vol% oxygen; this process gives deaclivation. 2 Storing the sullides in a nitrogen atmosphere, which prevents further reaction. 3 Spraying with copious supplies of water; this will also give deactivation and is suitable for large objects. If immersion or spraying is undertaken, which often happens with newly recovered artifacts, it is recommended that an alkali such as lithium or sodium hydroxide is added to the water to reduce the effect of any sulfuric acid which may be pre¬sent. The alkali also reduces the tendencies for cor¬rosion according to the Pourbaix diagram. A further precaution is that a corrosion inhibitor is used in the water. This should be a safe, non-toxic chemical, such as tannin, and the appropriate con¬centration is needed: the used sprayed water can be recycled if required. Alternatively the method suggested by Costagliola et al. [70] of using Huorinated resins as a coating, or else a layer of polyacrylic/polypyrrole composite as suggested by Sarac el al. [72] may be employed. It should be noted that virtually all the work reported in this paper relates to powdered iron oxides or sulfides. From an archaeological aspect, however, iron artifacts normally consist of an iron core and a coating of sulfide. This sulfide may be pyrophoric but the heat produced during oxidation gives a much lower temperature rise than obtained from the isolated powder. The actual temperature rise depends upon the amount and nature of the iron sulfide and the heat capacity of the iron artifact. It may therefore be concluded that only in cir¬cumstances where there is a large volume of sulfide present and little or no remaining iron is there a danger of pyrophoric action. The potential danger, however, should be considered during excavation of iron artifacts and precautions taken if necessary. |
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