Metal artifacts, corrosion

B. E. Brown and co-workers, eds.. Corrosion and Metal Artifacts, NBS Special PubHcation 479, National Bureau of Standards, Washington, D.C., 1977. 

Mechanisms regulating deterioration processes in the burial state are still rather unknown and little research has been done on the consequences of long-term corrosion on the stability of the objects. Nevertheless, some studies have been performed on metal artifacts, seeking to characterize the surface layer as well as establish a relationship between the composition of the corrosion products and the environment where they formed. 

The third category of metallic artifacts includes collections of a most different provenance—such as scientific instruments, fine arts, historic pieces, ethnographic specimens, etc., which are usually kept in museums. Contrary to the belief that an object is safe once it enters a museum, certain storage or display conditions may lead to corrosive reactions that are different from those found in the natural environment [264, 265]. Some of these dangers come from off-gassing from materials used to build display cases and rooms, as well as air pollution introduced by visitors. 

Electrochemistry has been used for more than a century in the treatment of ancient metal artifacts [281], Ideally, this technique should be able to reverse the corrosion processes that have progressively transformed the metal into an ionic compound. Depending on the conservation state of the artifact, priorities have to be attributed and the treatment will be different if consolidation, stabilization, or cleaning is privileged. 

MacLeod ID (1991) Identification of corrosion products on non-ferrous metal artifacts recovered from shipwrecks. Stud Conserv 36 222-234. 

Dent Wed P (1977) A review of the history and practice of patination, NBSSP 479. Proceedings of a Seminar, Corrosion and Metal Artifacts - A Dialogue Between Conservators and Archaeologists and Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, Maryland, March 17 and 18, 1976, 77-92. 

Pourbaix, M. In Corrosion and Metal Artifacts NBS Special Publication, 1977, 479, 1-16. 

Chemistry played a key role in restoring many items taken from the wreck of the Titanic. Electrolysis was used to clean and stabilize many metal artifacts, and electrophoresis was used to remove corrosion from bank notes, leather, and objects such as these casserole dishes. Chemicals that attract and hold metal atoms or ions were used to remove iron stains from delicate objects made of organic materials such as newspapers, textiles, and letters. The study of objects from the ship may help scientists compile information for long-term storage and containment under seawater. 

The degradation of a metal artifact, defined as loss of functionality over time in relation to the intended use, may be due to corrosion, although corrosiou and degradation are not synonymous. The degradation may be a consequence of several phenomena, which, even related to corrosion only, are in fact quite diversified. 

L. S. Selwyn and P. R. Roberge, Corrosion of metal artifacts displayed in outdoor environments, in ASM Handbook, Vol. 13C, Corrosion Environments and Industries, ASM International, Materials Park, OH, 2006, pp. 289-305. 

Deteriora.tlon. Apart from physical damage that can result from carelessness, abuse, and vandaUsm, the main problem with metal objects Hes in thek vulnerabihty to corrosion (see Corrosion and corrosion control) (127,128). The degree of corrosion depends on the nature and age of the object. Corrosion can range from a light tarnish, which may be aesthetically disfiguring on a poHshed silver or brass artifact, to total mineralization, a condition not uncommon for archaeological material. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

During the long burial period, extended redistribution of material has taken place. While metal went outwards, ions like chloride and impurities from the environment diffused inwards, resulting in a mass of corrosion products that occupies a volume approximately double the initial size. As a consequence, part of the artifact remains... 

In any manner, even though some of the classical corrosion forms can be also found in archaeological finds, fhey are frequently configured in a very complex form—either from a morphological, structural, or chemical viewpoint. Moreover, corrosion products bear a large piece of information about the artifact s life and should not be inadvertently removed, contrarily to modern metals, which would simply be etched. It is a big challenge to conserve such objects. On the one hand, it... 

The few reported cases concerning other metals, like zinc, aluminum, and magnesium, attest their susceptibility to corrosion due to volatile compounds in the museum environment [271]. Iron is naturally vulnerable to atmospheric corrosion whatever the pollutants, and the conservation of ferrous artifacts implicates a precise control of relative humidity, often requiring a surface protection like varnish, wax, or oil [272]. 

A cleaning treatment used to be applied to artifacts with a good metallic structure, whose surface is generally covered with a thin layer consisting in a mixture of corrosion products and grime, sometimes called tarnish. Cleaning aims to remove this undesired superficial layer, without (or with minimal) loss of the metallic substrate. In many cases, such a goal is more easily achieved by electrochemical methods than by mechanical and chemical methods [282]. 

The galvanic or contact method was the precursor to electrochemical treatment [283]. In an electrolytically conducting solution, the artifact is brought into contact with a piece of a less noble metal— usually zinc or aluminium. While the metal corrodes, the electrons supplied to the object allow the reduction of the tarnish layer. Although simple, this method presents some drawbacks, like the progressive contamination of the solution by the corrosion products of the active metal and also limitations with respect to the choice of applicable solution. 

Fig. 6.2 Cross section of a corroded lead artifact before reduction (left) the different regions are (1) the porous corrosion patch, (2) some metal veins, (3) the metal core and (4) the electrolyte. Time-elapsed optical images (a-h) of the same section (right) during the reduction of the corrosion layer (from [303])...

Nordgreen E, Gongalves P, Schindelholz E, Brossia CS and Yunovich M (2007) Corrosion assessement and implementation of techniques to mitigate corrosion of large artifacts from the USS Monitor (1862), Metal 07, Book 3 - Use of Electrchemical Techniques in Metal Conservation, Rijksmuseum Amsterdam 55-61. 

Ethanol is widely acknowledged to be less aggressive toward metals and elastomers than methanol, but little research and development has been devoted to the specific problems posed by ethanol. Ethanol typically has more water in it than methanol (an artifact of production) which may affect solubility of contaminants and corrosion potential. One ethanol contaminant that can arise from production is acetic acid, which is water-soluble and will corrode some automotive fuel system components. For instance, General Motors found that E85 caused more corrosion in fuel pumps than M85, presumably because of a higher level of dissolved contaminants [3.2]. Since much more development has been devoted to compatibility with methanol fuels, the general approach for ethanol has been to use materials developed for methanol, even though they may be over-engineered. ... 

In addition to the analysis of physical structural characteristics of textile fabric pseudomorphs, chemical information has been obtained. On bronze and copper artifacts, the pseudomorphs are composed of malachite, tenorite, and cuprite (I, 2), the formation of which probably requires moist conditions, a corrosive metal, and optimum fiber-metal contact (I). Trace elements in their structure vary from object to object and site to site (1-3), but the relationship of these elements and the fiber, metal, and soil composition is not yet known. 

---