Towards a new method for coating heritage lead
© Grayburn et al.; licensee Chemistry Central Ltd. 2014
Received: 24 February 2014
Accepted: 6 June 2014
Published: 17 June 2014
Ethanolic solutions of long-chain carboxylic acids can be applied to lead metal substrates to form a coating of lead carboxylate which provides protection against atmospheric pollutants.
Results and conclusions
In this paper we describe the optimal inhibitor concentration for the coating on lead. Electrochemical impedance data taken before and after immersion in media modelling oak emitted volatile organic compounds (VOCs) polluted atmospheres show that coating effectiveness decreases after exposure, but the effect is lessened if longer chain carboxylates are used.
Heritage metals are often coated as part of a conservation treatment to protect the metal surface from atmospheric deteriogens. At the same time, the coating is expected to both preserve the aesthetic appeal of the artefact and be retreatable. Common protective coatings used in conservation practice are paraffin and microcrystalline waxes . These coatings are applied physically to a surface. Inhibitive coatings on the other hand form a protective layer on a surface after a chemical reaction.
Lead is of particular interest to conservation scientists due to its surface reactivity in air and its occurrence in reactive atmospheric environments such as within pipe organs , the proximity of VOC-emitting oak to lead pipes creating the ideal conditions for corrosion. Another similar example is the storage of small lead artefacts in wooden display cases.
Long-chain carboxylic acids have been studied as possible corrosion inhibiting coatings for lead [3–8] and other heritage metals [9–11]. These coatings are usually formed by the immersion of the lead sample into an aqueous solution of the sodium salt of the carboxylate . For lead samples, a lead dicarboxylate coating forms on the surface, the carboxylic acid having reacted with the uppermost layer of lead atoms. Typically, studies have focussed on chain lengths up to the decanoate because the sodium salts are reasonably water soluble to that point.
Long-chain carboxylic acids are cheap and readily available. The coating precursor solution is also easy to produce. As opposed to conservation waxes, which are derived from crude oil, these coatings are derived from natural fatty acids, thus making these coatings the most ‘environmentally-friendly’ option. However, the solubility of the sodium carboxylates above the decanoate becomes too low for use in aqueous deposition. In addition, decanoic acid has a distinctive odour, which deters application.
In this work we exploit the solubility of some long chain carboxylates in ethanol and use this property to move towards a new method of coating lead metal samples. The longer chains are expected to improve coating effectiveness due to increased stability with chain length . Impedance measurements are used to determine coating effectiveness, whereas time-lapse synchrotron and laboratory X-ray diffraction (XRD) are used to look at drying properties and coating thickness respectively.
Results and discussion
Synchrotron x-ray diffraction of coating formation
Coating mass thickness
The lead metal reflections are smaller for samples coated with lower inhibitor concentrations indicating that the coating of lead tetradecanoate is thicker for low concentrations. A thicker superficial lead compound will absorb more of the X-rays arriving at and reflected from the underlying lead surface leading to a reduction in lead signal.
The extent of absorption by compounds can be measured using the mass attenuation coefficient [13, 14] and this leads directly to a method for estimating average layer thickness using the Beer-Lambert law, described in detail previously (equation 3 ref. ). Figure 4B shows the results for coating mass thickness calculated using the peak areas for the lead signal. The mass thickness is the mass of the lead ditetradecanoate coating per unit area (and the thickness is this divided by the density). In this way we can quantify the observations from Figure 4A. The mass thickness of the coating from 0.01 mol L−1 and 0.05 mol L−1 solutions of the acid is the greatest at 0.045 g cm−2. The mass thickness of the coating formed from higher concentrations of the acid is up to 30% lower. Deposition of mixed coatings of acid and lead carboxylate form for the higher concentrations, and the attenuation of the crystalline tetradecanoic acid will be significantly lower than that for the same thickness of lead tetradecanoate.
Effectiveness against acidic pollutants and a longer chain solution
The lead coupon and coating preparation have been described elsewhere . 0.05 mol L−1 C18 and solutions of 1 mol L−1, 0.5 mol L−1, 0.25 mol L−1, 0.1 mol L−1, 0.05 mol L−1 and 0.01 mol L−1 C14 were similarly prepared. Coupons were immersed in the appropriate solution for 24 hours and left to dry in air.
Time-lapse synchrotron x-ray diffraction
Real-time growth was studied for Pb(C14)2 in two ways. (a) A polished lead coupon 12.5 mm diameter was immersed in 0.05 mol L−1 tetradecanoic acid in ethanol for 10 hours in an environmental cell, the MkIV eCell . During the immersion the surface was analysed in-situ using synchrotron X-ray diffraction on the UK CRG beamline XMaS (BM28)  at the European Synchrotron Radiation Facility (ESRF). Surface powder diffractograms were collected for 2 s every 125 s using a Mar CCD 165 camera (Mar, USA, Inc.). The sample surface was automatically moved to within 125 μm of an X-ray transparent, 6 μm thick, Kapton® window for 10 s for x-ray exposure and then moved deeper into the solution to ensure unrestricted growth. It then remained immersed at all times. After 10 hours, the window was removed and the surface allowed to dry until no further changes occurred in the diffraction patterns. (b) In a different experiment conducted on the Belgian-Dutch CRG beamline DUBBLE (BM26a) at the ESRF, the same solution was flowed in 2 mL droplets across a polished lead sample surface in ambient air. The droplets readily wetted the surface and started evaporating immediately. Using an identical Mar CCD 165 camera a sequence of 10 surface powder diffraction images were recorded with a collection time of 15 seconds and an interval of 30 seconds between scans. During the first four scans 4 × 2 mL aquilots of the solution was flowed over the surface using PTFE piping fixed to the top of the sample. The remaining six scans observed the drying and coating formation, although some evaporation occurred between the first four scans. SR-XRD patterns from (i) and (ii) above were extracted and analysed using the esaProject software  which can produce images such as those shown in Figure 2.
XRD measurements were performed using a ARL X’TRA diffractometer using Cu-Kα X-rays with a wavelength of 1.5405 Å with a scintillation counter detector. A scan from 2 - 67° 2Θ was performed on each sample at a scan rate of 0.8°/minute and a step size of 0.02°. One sample per concentration was measured.
SEM images of 0.05 mol L−1 C14 coated coupons were collected on a ZEISS SUPRA 55-VP microscope at 20 kV acceleration voltage. Optical microscope images were collected on a Nikon SM2800 microscope.
Electrochemical impedance spectroscopy
Impedance measurements were performed using a PGSTAT20 potentiostat with a FRA2 frequency response analysis module. Data were acquired over a frequency range of 100 mHz to 10 kHz (0.1 V signal amplitude). The frequency range 10 kHz – 1 Hz was distributed logarithmically across the first 40 points and from 1 Hz – 100 mHz logarithmically over the last 10 points. Measurements were controlled by the Nova software (version 1.8). For all experiments a three-electrode system in a glass cell was used. The set-up consisted of a saturated calomel electrode (SCE), a carbon counter electrode and the lead coupon mounted in a working electrode holder. The electrolyte used was ASTM D 1384–87, which models a typical atmospheric environment for lead and is a standard electrolyte for corrosion inhibition tests . For the immersion tests, bare and coated coupons were tested for impedance before and after immersion in 0.01 mol L−1 acetic acid solution for 10 minutes. Samples were rinsed with deionised water before and after immersion. All EIS measurements were repeated three times.
Long chain carboxylates are easy to prepare and quick to apply to lead surfaces when an ethanolic solution of the carboxylic acid is used. Evaporation of the solvent leaves behind the Pb(C14)2 coating only, if the correct inhibitor concentration is used. For this lead surface preparation method, a concentration of between 0.01 and 0.05 mol L−1 is optimal. This will afford a coating mass thickness of approximately 0.045 g cm−2.
Impedance tests of the coating showed that the coating is susceptible to electrolyte penetration, especially by acetic acid. Coatings prepared using longer chain carboxylates, are less susceptible to electrolyte penetration, although the compounds become progressively less soluble in ethanol.
Further work will concentrate on longer chain carboxylates as coatings for lead, the coating of corroded lead samples (without prior surface preparation) and the long-term stability of these coatings in less aggressive environments. In addition the coating method will be reviewed. At present the mounting of the sample within the working electrode holder can cause damage to the coating (as seen in the microscopy results). The result of this damage is irreproducibility in the impedance data. Coating bare samples after mounting in the working electrode holder is possible, and demonstrates the ease of application of this coating.
We would like to thank the Physics Department at Warwick University for studentship of R. Grayburn, Tom Plankaert (UGent) for the XRD analyses, and to Lapon Orgelbouw, Diksmuide for the introduction to organ manufacture.
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