Corrosion Engineering,Science and Technology: The International Journal of Corrosion Processes and Corrosion Control

This paper describes microstructural analyses by X-ray portable diffraction and microdiffraction on intentional patina of the bronze museum objects from antique Egypt and the Roman Empire. They bring evidence of the presence in the true black bronze patinas of metallic gold and/or silver presumably as nanoparticles. Three other Egyptian patinas not belonging to black bronze are characterised. Apparent black patination on a Roman scalpel handle is discussed. The discovery of a new patination procedure on Roman artefacts from the Louvre museum is also related, based on intentional high temperature oxidation to obtain a dark patina on a lead bronze object. A presence of lead carbonate cerussite is an important observation.

The aim of the present study is to determine the role of tin on the corrosion behaviour of bronzes. For this purpose, different nuances of single phase α-Cu(Sn) are immersed in a 10^–2 mol L^–1 deaerated sulphate solution, buffered at pH 6·8. Then, the chemical–physical and electrochemical characterisation of an artificial patina, thin layer of corrosion products electrochemically formed at the surface of a 7 wt-% tin bronze are performed. First of all, the synthesised species are cathodically reduced. The successive steps of the potential–time curves, characteristic of the electrochemical reactions occurring at the electrode surface, are successfully calibrated with reference oxides. In order to complete the previous results, X-ray photoelectron spectroscopy (XPS) depth profiles are carried out by abrading progressively the oxidised surface. The deconvoluted XPS and Auger electron spectroscopy (AES) spectra speak in favour of an ‘alloy/SnO₂/Cu₂O’ layered structure similar to that reported in the literature. Analyses carried out on the oxidised samples by transmission electron microscopy (TEM) confirm this pattern.

Bronze surfaces suffer from corrosion processes if they are exposed outdoors. They are affected by the presence of different aggressive species, the alloy composition and surface preparation. Bronze can be exposed outdoors non-protected or can be pretreated in different ways. Artists use chemical patinations in order to achieve visual effects. In the present study, different finishes were tested on three different patinas: green nitrate, green chloride and an electrochemically prepared patina. The tested finishes were: imidazole and benzotriazole type inhibitors in either ethanol or Paraloid B44, and Carnauba wax as a representative of waxes. The effectiveness of the applied finishes was examined by electrochemical impedance spectroscopy in a solution simulating urban acid rain. Aging experiments were performed in a climatic chamber and a salt spray chamber in order to combine different exposure conditions, the presence of sulphur dioxide, high humidity and a mist of chloride ions. After immersion in an urban acid rain solution, acidified to pH 5, the different patinas were investigated by SEM/EDX and Raman spectroscopy.

Silver artifacts suffer tarnishing when exposed to a sulphur containg atmosphere. Plasma deposited SiO_x thin films are proposed for the protection of silver artefacts, owing to their optical transparency and high barrier properties against vapours. The protective effectiveness of the SiO_x films was assessed by means of electrochemical impedance measurements performed in an Na₂S solution on a set of coated silver based alloy samples, and by submitting another set of samples to a tarnishing test in presence of H₂S vapours. The experimental findings reveal that the SiO_x deposition, performed in RF plasma fed with a tetraetoxysilane/oxygen/argon mixture, produces layers with excellent barrier effects against the aggressive agents. The protective effectiveness increases if the deposition is performed at increasing input powers and at decreasing tetraethoxysilane/oxygen ratios in the feeding gas. The tarnishing evolution onto the coated surface was assessed at the microscopical and macroscopical level by means of field emission scanning electron microscopy and by an easy to use diagnostic tool based on digital photography and image processing.

Although the usual aspect of lead surfaces is light grey, in the gardens of the ‘Palácio Nacional de Queluz’, Portugal, some of the sculptures present an atypical red brown appearance. In order to investigate this phenomenon, lead coupons previously prepared with different surface finishes have been exposed in nearby statues presenting both surface aspects. After 15 months, visible surface transformations have been noticed for certain locations, and Raman microscopy has been used to characterise the surface products. Lead(II) oxide near the metal and salt layer at the outer part has been found for the white patina. For the red patina, a mixture of lead(IV) oxide and an uncommon lead oxycarbonate, probably shannonite (PbCO₃.PbO), has been found. Complementary in situ Raman analyses performed during electrochemical oxidation of lead, lead covered with hydrocerussite and pure hydrocerussite have shown that the presence of both lead and hydrocerussite is required to reproduce the field experiment.

The large use of aluminium alloys for the twentieth century induces that numerous components and objects in aluminium are registered in many collections in museums or are classified as historic buildings. In collaboration with the Air and Space Museum at Le Bourget (France), the objective of the authors’ work was to identify and study the different processes of corrosion of Al alloys occurring in the aircrafts collection from 1930 to now. Chemical and metallographic analyses allowed the identification of various alloys used in different aircrafts. The long-term corrosion products are mainly constituted by amorphous Al(OH)₃. The corrosion behaviour of the complex systems ‘alloys/corrosion layer’ was evaluated by electrochemical measurements. The results show that thick corrosion layers have a poor influence on the corrosion rate of the metallic substrate. On the other hand, the presence of painting residues with corrosion products is beneficial for the conservation because of the presence of mineral inhibiting compounds.

This paper presents a novel non-destructive method to measure the actual corrosion rate of precorroded metal objects, such as historical and archaeological artefacts. The corrosion rate is estimated from the oxygen consumption of the objects, which is measured in a small volume of air encapsulated directly on the surface of the object. An optical method is used for the oxygen measurements, making it possible to measure through transparent materials such as glass. The method is tested on iron and copper samples in different environments using both new uncorroded metal and historical artefacts, which have thick corrosion scales from more than 50 years outdoors. The results show the following: the method has a good reproducibility; there is a good correspondence between oxygen consumption and weight loss; the corrosion rates of precorroded cast iron are significantly lower than the rates found for new steel samples, whereas corrosion rates for precorroded copper are equal to or higher than rates for new copper samples; and corrosion rates as low as 0·1 μm/year can be measured by the method.

Preservation of large outdoor cultural objects containing iron and steel for future generations involves protecting them with a coating. To meet the ethics of the conservation profession and the typical budget, coatings should maintain the original appearance and significance of objects as well as require no maintenance for at least three years. Eighteen coatings, waxes and oils, which met the project's requirements based on peer reviewed research publications, were compared on an equal basis in practice for their potential to inhibit corrosion of large industrial objects. They were applied to test Q panels at equal dry film thicknesses and evaluated by mechanical, atmospheric and accelerated corrosion tests. Uncoated Q panels corroded significantly more rapidly than all coatings, suggesting that any treatment is better than nothing. Interim results suggest that the coatings which clearly inhibit corrosion, as shown by salt spray tests after 504 h, atmospheric corrosion for six months and oxygen consumption were Cosmoloid H80, Dinitrol Car/4941, LPS3, Rustilo 3000, SP400 and VpCI-386.

The anticorrosion performances of new non-toxic inhibitors based on carboxylic acids extracted from vegetable oil have been evaluated for the protection of iron artefacts. Electrochemical measurements and natural aging tests have demonstrated the efficiency of those inhibitors in the context of temporary treatments. Surface analysis coupled with in situ X-ray absorption near edge structure experiments has revealed that their anticorrosion properties are correlated to the precipitation at the metal surface of a protective layer made of iron carboxylate.

New protection systems for cultural heritage artefacts can be tested and their efficiency, compared to traditional systems, assessed using artificial metal coupons. The manufacturing of these coupons requires though a thorough investigation of the artefacts that they are supposed to simulate. Not only their composition, but their surface preparation (through artificial or natural aging) should copy as closely as possible real artefacts. Protection systems tested should be applied according to standardised protocols, while short term testing in humid chamber and long term testing on site should be monitored regularly to detect any possible failure. Only then the use of such artificial metal coupons will be validated. This approach is illustrated in this paper through the EU PROMET and Swiss POINT research projects.

In the present work, the authors tried to establish degradation indices for heritage ferrous artefacts, especially those used in ancient buildings and submitted to indoor atmospheric corrosion. The authors focused on the site of the Amiens Cathedral in the north of France. Samples coming from this reference site were carefully characterised in order to identify the different phases constituting the corrosion scale. The scale consists in a matrix of iron oxyhydroxide goethite embedded with several ferrihydrite marblings. Other phases such as lepidocrocite and akaganeite are scarcely present in the external part of the corrosion scale. Moreover, electrochemical measurements on both references and ancient samples enable to define the reduction reactive phases. From the nature of these phases and their localisation, two degradation indices were defined to evaluate rust reactivity. Finally, a curve that links these two factors is proposed as a first step towards a corrosion diagnosis.

In the context of the in situ conservation and preservation of archaeological artefacts, the long term corrosion mechanisms of iron in anoxic soils are studied. To this purpose, a first step is the characterisation of the corrosion layers formed on archaeological artefacts provided from the archaeological site of Glinet (the sixteenth, Normandy, France). On all the corrosion systems formed on artefacts, the main phases constitutive of the corrosion layer are siderite (FeCO₃), an iron carbonate containing hydroxide groups [probably chukanovite Fe₂(OH)₂CO₃] and magnetite (Fe₃O₄). Furthermore, the arrangement of these phases reveals three corrosion distribution types with corresponding corrosion pattern diagrams.

The presence of Fe and S containing compounds inside rust layers covering iron archaeological nails was suspected but their real nature was not clearly determined. However, this finding suggested that sulphate reducing bacteria (SRB) could be involved in the corrosion processes. A thorough study focused on SRB and FeS compounds potentially present inside the rust layers was achieved on other nails recently excavated. For microbial investigations, the authors used a probe targeting SRB and performed fluorescence in situ hybridisation for cells identification. This procedure revealed that SRB were present in all analysed samples. Analysis of rust revealed that FeCO₃ was the major component, indicating that the nails remained in anaerobic conditions, but FeS compounds were detected on each sample. Iron sulphides were localised on a few spots in the outer part of rust layers. This shows that the presence and activity of SRB had little influence upon the corrosion system.

The prediction of very long term corrosion of iron and low alloy steel in atmospheric conditions or in hydraulic binder media is a crucial issue for the conservation and restoration of heritage artefacts. For both media, the typical iron corrosion product layers (CPL) can be described as a matrix of goethite (α-FeOOH) crossed by marblings of reactive phases: maghemite (γ-Fe₂O₃), ferrihydrite (Fe₅HO₈.4H₂O), feroxyhyte (δ-FeOOH), etc. The aim of the experiments presented here is to bring new insights on the role that the maghemite could potentially play in the mechanisms of corrosion. For that purpose, electrochemical reductions have been coupled with in situ Raman microspectroscopy. These experiments enable the authors to propose a hypothesis of local mechanisms in the specific case of marblings of maghemite connected to the metallic substrate. These local mechanisms could drastically influence the global corrosion rate.

The individual chloride content of 116 archaeological iron nails from Romano British and Medieval sites in Wales is reported. The meaning and value of chloride concentration recorded as weight of chloride in object/object weight is discussed in relation to reporting the effectiveness of washing methods designed to remove chloride from archaeological iron. This is theoretically compared to the concentration value weight of chloride in object/metal surface area of object and the difficulty of quantitatively determining the success of washing methods as stability enhancers is discussed. It is concluded that assessing the impact of residual chloride on post-treatment corrosion of archaeological objects has the potential to offer the most significant guide to treatment success.

Three wrought iron ingots immersed during 2000 years at 12 m deep in Mediterranean Sea were stored after excavation for 2 years without specific protection in air. After that period, two of them were treated by immersion in a NaOH solution, while the third was used to describe the corrosion system resulting from the storage conditions. This characterisation was achieved by a combination of microanalytical techniques. It could be concluded that though ferrous hydroxychloride β-Fe₂(OH)₃Cl was the main Cl containing phase at the time of excavation, akaganeite [β-FeO_1–x(OH)_1+xCl_x] was the only one present in the rust layers after storage. In order to determine the influence of corrosion products nature on dechlorination treatment, the evolution of a corrosion system composed of both Cl containing phases β-FeOOH and β-Fe₂(OH)₃Cl has been followed during in situ NaOH experimental treatment. Specific behaviours of each phase to the dechlorination treatment have been revealed.

Corroding glass forms alkali(ne) surface films, which may lead to special metal corrosion products in the contact zone, for example different sodium copper carbonates or basic sodium lead carbonate. Sodium copper formate acetate was found on objects exposed to long term emissions from wood. A higher pH value may also lead to basic compounds, which normally do not form, e.g. the newly characterised Cu₂(OH)₃ HCOO. The alkaline films also create a reactive environment for the formation of formates from formaldehyde (e.g. emitted from glues) due to the Cannizzaro reaction or possibly from the neglected pollutant carbon monoxide. Further products containing, for example, potassium or sulphate may be expected.

Several studies indicate that the decay of medieval stained glass windows is related to both the glass composition and the characteristics of the environment. The kinetics of the decay processes has been always described through experiences performed in aqueous confined conditions, which are obviously not encountered in real condition. A research programme has been set up in order to assess the kinetics of atmospheric weathering. The long and short term weathering has been studied through respectively ancient glass fragments and glass analogues exposed in the field. This paper presents the preliminary results of the long term. A characterisation of the morphology and chemical modification induced by weathering has been performed. Results will be used in the next step to build up a model of long term atmospheric weathering kinetics.