Corrosion of Copper and Lead by Formaldehyde, Formic and Acetic Acid Vapours
Jean Tétreault, Emilio Cano, Maarten van Bommel, David Scott, Megan Dennis, Marie-Geneviève
Barthés-Labrousse, Léa Minel and Luc Robbiola
The formation of corrosion products and the corrosion sensitivity of copper and lead were studied
under experimental conditions in which formaldehyde (methanal), formic (methanoic) and acetic
(ethanoic) acid concentration, relative humidity (RH) and duration of exposure were varied. Levels
of formic acid above 0.4 parts per million based on volume (ppmv) affect the appearance of copper
at 75% RH, and at levels above 4 ppmv the copper gains weight at both 54 and 75% RH. The main
compound found on copper was cuprite, copper(I) oxide. Lead has a higher sensitivity to formic
acid: at levels as low as 0.04 ppmv lead becomes darker, and at above 0.1 ppmv weight gains were
measurable at both 54 and 75% RH. In the presence of different levels of mixed carbonyl vapours
at 75% RH, copper reacted mainly with formic acid. On the other hand, the reaction of lead was
more complex. Acetic acid tends to form a thick white layer (composed mainly of plumbonacrite
and possibly lead acetate compounds) on the lead surface, while formic acid tends to form a rather
thin and darker layer of lead formate hydroxide. In the presence of formic acid, the action of acetic
acid on lead was inhibited. At levels of formaldehyde up to 3 ppmv, no significant contribution of
formaldehyde to the corrosion process on lead and copper was observed.
INTRODUCTION
For the last two decades, there has been an increasing interest in indoor air quality in museums and
archives. Reliable, sensitive monitoring techniques have been developed for some carbonyls and
better information is available on the typical off-gassing of construction products [1]. However,
some pollutant—object interactions have not yet been fully investigated. Many-data refer to the
effect of carbonyls on metals, but few of them were collected in conditions comparable to museum
and archive environments [2, 3].
The aim of this project was to gain a better understanding of the tendency of copper and lead to
corrode within a few months in carbonyl-rich environments — containing formaldehyde
(methanal), formic (methanoic) and acetic (ethanoic) acids — frequently-found in display or storage
settings [1]. The 'no observable adverse effect level' (NOAEL) of formic acid for these metals, the
rate of film formation above the NOAEL, and the nature of the film were investigated.
The formation of corrosion products and the corrosion sensitivity of metals were studied at different
concentrations of carbonyls, relative humidity (RH) and exposure duration. Humid carbonyl
environments were generated by carbonyl—water—salt mixtures in glass jars. Metal coupons were
suspended above the mixtures for up to 135 days. The corrosion of the metals was investigated by
weight gain measurement, colour measurement, scanning electron microscopy (SEM), X-ray
diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Based on the results, strategies for
controlling the adverse effects of carbonyls on copper and lead in enclosures can be optimized.
EXPERIMENTAL METHODS
Two experiments were conducted to investigate the corrosion behaviour of untarnished copper and
lead coupons in a humid carbonyl environment containing formic acid alone (experiment 1) or
mixtures of formaldehyde, formic and acetic acids (experiment 2). A third experiment was also
conducted on lead in contact with a piece of particleboard to simulate the conditions in a
wooden display case or storage cabinet. The methods used for sample preparation, generation of the
humid carbonyl environment and the analytical method used to characterize the vapours are
explained in Appendices 1, 2 and 3 respectively. The exposure conditions are summarized in Table
1.
Experiment 1: Effect of formic acid vapour concentration
The corrosion of copper and lead was conducted at 54 or 75% RH for 135 days with different
concentrations of formic acid. The exposed samples were characterized every 28 days by weight
and colour measurements. Mineralized corrosion products were also characterized by XRD after
108 days of exposure.
Experiment 2: Effect of different carbonyl pollutants
The atmospheric corrosion behaviour of copper and lead was investigated at 75% RH for up to 135
days using different mixtures of formic acid with formaldehyde and acetic acid. Weight
measurements and XRD were conducted as for experiment 1. Surface investigations after 27 days
of exposure were also performed, using optical microscopy, SEM, XRD and XPS.
Experiment 3: Corrosion of lead in a synthetic carbonyl environment or with particleboard
The formation of corrosion products on untarnished and previously tarnished lead samples was
studied by exposure to a carbonyl mixture or a piece of particleboard for 33 days. The comparison
between exposure to a synthetic atmosphere and the environment generated by particleboard at a
RH of 54, 75 or 100% was investigated by weight measurement and by XRD.
Sample examination
For all the experiments, the corrosion products were not powdery enough to fall off during
handling. The weight gain of the metal samples was obtained with a Mettler balance (model
AE163) with a measurement accuracy of ±0.1 mg. The lightness (L*) of the samples was measured
every month with a Minolta chroma meter (model CR-200) using the CIE L*, a*, b* colour system.
Three colour measurements were made for each sample, but only the lightness parameter (L*) is
presented; the measurements have an accuracy of+0.5 L*. To evaluate the nature of the corrosion
films after 27 days of exposure in experiment 2, the SEM observations of the metal surface were
conducted on a Hitachi (model S-2500). The surface of these samples was also analysed by small
angle X-ray diffraction and by XPS. XRD was conducted with an incidence of 0.5° at room
temperature, using a Philips apparatus with a Co source (Kα = 0.17902 nm), running at 30 mA and
40 kV with a step of 0.035° (90 seconds per step). X-ray photoelectron spectra were recorded at
normal angle with a CLAM 2 apparatus (Thermo VG Scientific) using monochromatic Al Kα
radiation (1486.6 eV); an ellipse of around 0.5 cm2 on the surface
Table 1 Summary of the experiments
(a) Number of samples per vessel.
(b) Parts per million based on volume. FA: formic add, AA: acetic acid, F: formaldehyde.
(c) W: weight measurement; C: colour measurement; OM: optical microscopy; p: periodic
measurement, every 28 days up to 135 days; (27d, 33d, 108d): numbers of days of exposure in the
dark and at 21.3 ± 0.7°C prior to sample analysis.
(d) Each vessel contains a sample of newly sanded lead, lead tarnished for five years, and lead
tarnished for five years then scratched with a knife.
of the sample was irradiated. The photoelectron spectra were recorded using a step of 0.1 eV,
looking at the Is signal for carbon, the Is and Auger KLL signals for oxygen, the 3p, 3s, 2p and
Auger LMM signals for copper, and the 4f, 4d, and 3d signals for lead. The binding energies of the
photoelectron peaks were referenced with respect to the carbon Is level, which was set at 285.0 eV.
For experiments 1 and 2, other metal surfaces were analysed, after exposure for 108 days, by XRD
using a Siemens D5005 with Gobels mirror in detection scan mode, running at 40 mA and 40 kV.
RESULTS
Experiment 1
Figures 1, 2 and 3 illustrate the formation of corrosion layers on copper and lead at various
concentrations of formic acid. Figure 1 shows that an increase in the concentration of formic acid
has a strong effect on the corrosion of copper. A significant weight gain for copper samples is seen
when the formic acid concentration is higher than 2 ppmv. This behaviour is observed after 27 days
exposure at both 54 and 75% RH. The film formation is accompanied by a surface colour change,
based on the lightness (L*) measurement at 75% RH, confirming that at a concentration above 0.4
ppmv the copper begins to react significantly with formic acid. A thin brown/green layer is formed
on the copper surfaces. According to the XRD analysis, this is possibly due to the presence of a
small amount of cuprite, copper(I) oxide; some grey patches were also present. Above 14 ppmv, the
copper samples show whitish surface colours. Corrosion compounds found on copper exposed at 14
and 140 ppmv were identified respectively as copper formate and copper formate dihydrate;
formulae for these corrosion compounds are provided in Table 2.
Figure 1 Weight gain for copper exposed to different formic acid concentrations over a period of
135 days at 54 or 75% RH, and lightness changes measured at 75% RH.
Figure 2 Weight gain and lightness changes for lead exposed to different formic acid
concentrations over a period of 135 days at 54% RH.
Figure 3 Weight gain and lightness changes for lead exposed to different formic acid
concentrations over a period of 135 days at 75% RH.
Figures 2 and 3 show the weight gain and lightness change of lead samples exposed to various
concentrations of formic acid, at 54 and 75% RH respectively. At 54% RH (Figure 2), a significant
weight gain for lead samples occurs when the formic acid concentration exceeds 0.1 ppmv. The rate
of weight increase is greatest over the first 27 days. After that, the rate of formation of compounds
slows down. The colour measurement shows a change of lightness at the lowest concentration
studied (0.035 ppmv). At 1.5 and 8.8 ppmv, lead formate and lead hydroxide formate were
identified by XRD. At 75% RH (Figure 3), lead showed a significant weight gain only above 0.35
ppmv. At 0.35 ppmv, mainly lead formate hydroxide was identified. A maximum weight gain was
achieved after 54 days, with little or no further gain as exposure was prolonged. Above 0.35 ppmv,
most colour change occurred in the first 27 days and the lead surface showed greyish and bluish
colours with a matt appearance. It should be noted that the weight gain due to the corrosion
products does not strictly increase with the concentration of formic acid: a maximum weight gain
occurs at about 1.5 ppmv of formic acid at either 54 or 75% RH.
Experiment 2
Tables 3 and 4 show the weight gain for lead and copper after 135 days exposure to different
mixtures of formaldehyde, formic and acetic acids at 75% RH. The nature of the corrosion products
on some samples, determined by XRD (including small angle), is also included in the tables.
Table 2 Characteristics of copper and lead compounds found in this study
(a) 'i' indicates insoluble and 'vs', very soluble.
Table 3 Weight gain measurements and some XRD analysis on copper samples after 135 days
exposure to carbonyl vapour mixtures at 75% RH
(a) With increasing level of formic acid, the level of acetic acid increases (see Appendix 3).
(b) T indicates trace amounts and P, possible presence of this component.
(c) Analysed by small angle XRD analysis after 27 days of exposure.
Table 3 shows that formaldehyde and acetic acid, alone or present together with formic acid, are not
significantly involved in the corrosion of copper. Copper appears visually untarnished in the
absence of formic acid or when a low concentration (0.2 ppmv) is present, even though a thin film
of cuprite was identified for all the samples by both XRD and XPS after 27 days of exposure.
Furthermore a strong oxygen Is signal at 531.8 eV was observed in the XPS spectra in both cases,
suggesting the presence of some hydroxycopper compounds normally formed in clean indoor air.
This type of compound has been detected by XPS on copper exposed to an unpolluted environment
at relative humidities between 40 and 80% [10]. After 135 days there were some areas of grey
corrosion, identified as tenorite, copper (II) oxide. At 8 ppmv of formic acid, with or without other
carbonyls, the copper samples are covered by a thin layer of a matt, dull green to grey film with
some darker grey patches. Cuprite was the most common corrosion compound identified, as well as
a trace of copper formate dihydrate. Copper acetate dihydrate was identified on a sample exposed to
high levels of carbonyls.
For lead, formaldehyde alone, or mixed with formic and acetic acids, does not contribute significantly to corrosion at 75% RH, as shown in Table 4. This is also confirmed by the lead sample
exposed for 27 days at 0.2 ppmv of formic acid; both XPS and XRD results are similar with and
without formaldehyde present in the vapour phase. SEM examination (Figures 4b with 4c and 4d
with 4e) further confirmed that formaldehyde has no significant effect on the corrosion process. On
the other hand, acetic acid plays a major role in the formation of corrosion compounds on lead.
Acetic acid alone forms more corrosion than formic acid. As shown in Figure 4f, after 27 days of
exposure, localized corrosion is well established in the presence of 0.2 ppmv of acetic acid vapour,
and the crystalline product is mainly plumbonacrite, 6PbCO3.3Pb(OH)2-PbO. As seen in Table 4, in
the presence of formic acid, the capacity of acetic acid to produce corrosion products is highly
inhibited. This can be attributed to a modification of the chemical nature of the corrosion layer,
which gives increasing protective properties that were confirmed by XPS measurements made after
27 days.
Table 4 Weight gain measurements and some XRD analysis on lead samples after 135 days
exposure to carbonyl vapour mixtures at 75% RH
Figure 4 SEM photomicrographs of lead samples after 27 days exposure to various carbonyl
mixtures (FA: formic acid, F: formaldehyde, AA: acetic add).
This effect can also be correlated to a decrease in the electronic conductivity of the surface layer,
manifest in an increase of the charge effect in the course of XPS. This charge effect (positive charge
on the surface) is due to the X-ray induced removal of electrons from the surface during XPS
analysis. In an isolating surface, the electronic conductivity is low and cannot compensate for this
charge. Spectra obtained from these surfaces are displaced towards higher binding energies, i.e.,
lower kinetic energies for the detected electrons. Therefore, a greater displacement of the spectra
indicates a lower electronic conductivity in the layer analysed.
This inhibition effect is well illustrated in Figure 5, which shows the kinetics for the formation of
compounds over 135 days at different concentrations of formic and acetic acids. A uniform thin
layer, composed mainly of lead formate hydroxide, is formed on lead exposed to 0.2 ppmv of
formic acid (Figure 4b). When acetic acid is added to
Figure 5 Weight gain for lead exposed for varying periods of time to different formic and acetic
acid concentrations at 75% RH.
formic acid, the weight gain increases due to the formation of lead acetate compounds, which coalesce into tiny white grains (a few microns in diameter) spread uniformly across the surface.
However, the corrosion is retarded by lead formate compounds as the concentration of formic acid
increases. The visual appearance of the corrosion layer on lead exposed to acetic acid alone (either
0.2 or 11 ppmv) changes from a thick white film to a darker thin film as the formic acid
concentration changes from 0 to 0.2, and to 8 ppmv.
It should be noted that apart from the increase of acetic acid with an increasing level of formic acid
as described in Appendix 3, the measurement of carbonyl levels generated by the three carbonyl—
salt solutions did not show any changes in the levels of formaldehyde or formic acid. No significant
oxidation of formaldehyde into formic acid occurred in the vapour phase in the presence of weak
oxidants such as acetic or formic acids or in the presence of the copper or lead metal surfaces.
Experiment 3
The results of experiment 3 are given in Table 5. In general, the corrosion layer formed on lead is
heavier as the RH increases in a carbonyl environment. There is less corrosion produced if there is
already a corrosion layer present, either a continuous (T) or partial (TS) layer. But this increase
varies with the surface state of the lead. The presence of an initial corrosion film on lead has an
important effect on the evolution of further corrosion. The freshly exposed lead surface (N) has a
reactivity higher than a surface with a naturally formed patina. At 54% RH, no corrosion is
observed on lead with a natural patina. The vapours emitted by the particleboard tend to contribute
to a higher weight gain than observed with the vapours from the carbonyl—salt solution. This trend
was surprising, since the level of carbonyls from the board was similar or less (mainly in acetic
acid) than that from the carbonyl-salt solution. Unfortunately, due to lack of time, the level of
carbonyls was measured for only one jar containing a sample of particleboard and one jar of
carbonyl-salt solution. In addition, the weight gains were measured for only one sample of each
type of lead (untarnished, tarnished and scratched, tarnished) per vessel. These factors may limit the
confidence level for these interpretations.
Table 5 Weight gain (g.m-2) and some XRD analyses of lead samples after 33 days exposure to
particleboard or to a carbonyl environment
XRD1: plumbonacrite, XRD2: lead formate hydroxide, XRD3: lead acetate hydroxide hydrate
(possible) and lead formate (possible), XRD4: lead acetate hydroxide hydrate and lead formate
hydroxide.
(a) FA: formic acid, AA: acetic acid, F: formaldehyde. N: newly sanded lead sample, TS: five-yearold tarnished and scratched, T: five-year-old tarnished.
(b) The carbonyl levels were measured only at 100% and 75% RH respectively.
DISCUSSION
Copper
Formic acid reacted with copper at 54 or 75% RH, forming mainly cuprite, while acetic acid and
formaldehyde did not contribute significantly to the corrosion in the range of vapour concentrations
studied. In general, cupric compounds are always formed on copper exposed to polluted or
unpolluted environments for a long time. The typical structure of the patina formed on copper
comprises an inner layer of cuprite and an outer layer of tenorite and/or hydroxy-copper compound.
The presence of these cupric compounds seems to provide the building blocks for the formation of
acetates or formates [11].
The 'no observable adverse effect level' (NOAEL) is defined as the highest level of a pollutant that
does not produce an adverse effect on a specific chemical or physical characteristic of a material in
a particular experimental set-up [1]. In this research, the NOAEL
of formic acid on copper was found to be 0.4 ppmv at 75% RH based on change of lightness (L*)
and about 3 ppmv based on the weight gain at both the relative humidities studied. For acetic acid,
previous work has established a NOAEL (based on weight gain) above the maximum level studied,
50 ppmv at 54% RH [12]. Independent research has demonstrated that copper samples change
colour when exposed to 5 ppmv of acetic acid or 5 ppmv of formaldehyde at 50% RH [2]. It is
important to note that in the presence of high concentrations of formic acid, there is more corrosion
at the lower RH. A similar behaviour has been observed for other corrosive environments, and has
been attributed to the different protective effects of the corrosion products formed at different
relative humidities [10]. As shown in Table 6, there is a low probability of carbonyls damaging
copper inside enclosures such as display cases or storage cabinets, as long as any coatings formed
by oxidative polymerization,
Table 6 Sources and typical range levels of carbonyls {1}
(a) The levels generated by these products were measured in airtight enclosures.
including oils, oil-based urethanes, alkyds and epoxy esters, are avoided, or as long as the copper is
not exposed to these coatings before an extended drying period [13]. Most copper objects will still
be better preserved in an enclosure containing low levels of carbonyls than if they were left exposed
to an ambient environment containing more aggressive pollutants such as hydrogen sulphide,
carbonyl sulphide or sulphur dioxide (unless there is an unexpected source of a sulphur compound
from a product or an object inside the enclosure [1]).
Lead
The research shows that formic acid has a significant reactivity with lead until a stable film is
formed. The NOAEL of formic acid on lead is roughly 0.1 ppmv at both 54 and 75% RH based on
weight gain, and below 0.04 ppmv based on lightness change. Similar NOAELs for acetic acid on
lead were previously determined at 54% RH [3]. Above the NOAEL of the two acids, the rate of
corrosion of lead with acetic acid is higher than that for formic acid. In contrast to the reaction of
acetic acid with lead, formic acid tends to form a stable film, composed mainly of lead formate and
lead formate hydroxide. From Table 6 it can be seen that most wooden enclosures, coated or not,
can generate levels of acetic acid above the NOAEL (0.1 ppmv). This may result in a significant
risk of corrosion of lead objects displayed or stored in such enclosures. However, if a stable film
already covers the lead, or if formic acid is present, the risk, or the rate of corrosion, can be reduced.
Scratches and cracks caused by handling and transportation can produce new unprotected lead
surfaces vulnerable to corrosion. The high probability of lead corrosion in enclosures reinforces the
need for the proper selection of products with little or no acid emission. Guidelines for the optimum
preservation of lead objects, and the factors influencing the deterioration of lead, are covered in
more detail elsewhere [1, 3].
Formaldehyde showed no significant effect on lead in the experimental conditions covered.
Although, in the museum context, corrosion of lead objects in the presence of formaldehydecontaining wood products has been observed [14], it is hard to reproduce this phenomenon in the
laboratory without extreme environmental conditions. The oxidation of formaldehyde to formic acid
is negligible in the atmosphere in the absence of oxidants such as peroxide [15]. However,
potentially high levels of peroxide can occur inside an enclosure during the curing of a newlyapplied coating formed by oxidative polymerization [16, 17].
Monitoring methods and deterioration
Two methods have been used to quantify the NOAEL for formic acid on copper or lead in this
research. In a well-controlled environment, lightness measurements L* are around 10 times more
sensitive than weight gain measurements. The single measurement of L* in this experiment, in situ
using metal coupons in display cases or on the object itself, will not fully explain the kinetics of the
deterioration; the lightness of the metal may not change over months, but deterioration may still
continue as the corrosion film grows. However, in a museum context, visual comparisons of metal
objects or coupons with a control sample can be a very simple monitoring tool to detect early signs
of deterioration. For metal objects or coupons that must remain shiny or for metals that are plated or
fragile, gloss or colour change observations are an appropriate method. Otherwise, weight gain and
full spectral measurements will provide more comprehensive information related to the effects of
indoor and outdoor pollutants, while XRD and XPS analyses remain important to identify the film
composition.
CONCLUSIONS
In the presence of different levels of mixed carbonyl vapours, copper reacted mainly with formic
acid while the reaction of lead proved to be more complex. On lead, there was a competition
between acetic and formic acids, where acetic acid tended to form a thick white layer compared to
formic acid which tended to form a rather thin and darker layer of lead formate compounds. The
latter film was able to inhibit the action of acetic acid on lead. No significant contribution of
formaldehyde to the corrosion process on lead or copper was observed.
Display, storage or transportation cases made of wood products can cause corrosion of lead and
copper, particularly if the interior of the cases is covered by coatings formed by oxidative
polymerization. The selection of products with little or no acid emission is recommended.
ACKNOWLEDGEMENTS
The authors are grateful to Jean-Pierre Dallas of the Centre d'etudes de chimie métallurgique
(Centre
national de la recherche scientifique), Vitry, France, for providing extra data on small angle XRD
analysis. We also would like to thank Carole Dignard, Lyndsie Sehvyn and R. Scott Williams from
CCI for reviewing this article and offering their comments. David Thickett, formerly of the British
Museum, is thanked for his technical advice. Emilio Cano's internship in Canada was made possible
by funding from the Spanish Ministry of Education and Culture.
APPENDIX 1: MATERIAL PREPARATION
Copper foil 1.0 mm thick (composition, by weight percent: 0.015 Pb, 0.009 Sn, <0.001 Al, <6.002
Sb, <0.001 As, <0.001 Bi, <0.001 Fe, 0.003 Ni, 0.019 P, <0.001 Mn, balance Cu; type Cu-DLP
Standard ISO 1337 [18]) was cut into 2.5 X 5 cm pieces and lead foil 0.2 mm thick (purified grade)
was cut into 2x5 cm strips. The samples were cleaned with propanone (acetone), then sanded. Metal
samples used for experiments 1, 2, and part of 3 were rubbed with grade P500 (based on the scale of
the Federation of European Producers Association) sandpaper to produce a rough surface. Other
metal samples for experiment 2 were sanded and polished with different grades of paper up to
P2500 to provide a very flat and glossy surface suitable for small angle XRD analysis. A hole was
pierced through each sample in order to suspend it in the glass jar. For experiment 3, lead samples
were used in three different states: newly sanded, after tarnishing for five years in indoor air, and
scratched with a sharp knife after tarnishing for five years in indoor air.
APPENDIX 2: GENERATION OF THE HUMID CARBONYL ENVIRONMENT
In order to generate a humid carbonyl environment in glass jars at room temperature (21.3 ± 0.7°C),
a three-component system of carbonyl-water-salt was used. This system was based on different
equilibria: water vapour in equilibrium with a saturated salt mixture and a carbonyl vapour in
equilibrium with its solution. Magnesium nitrate (MgNO,-6H,O) or sodium chloride (NaCl) was
used to generate relative humidities of 54 or 75% respectively [19]. Solutions without a salt were
used to generate a RH of 100%. The relative humidities were checked with a datalogger and were
verified as correct.
For experiment 1, a small plastic container containing 15 g of salt and 3 ml of one of the formic
acid solutions was placed in a 0.45 1 glass jar with a hard
plastic screw-cap lid. The formic acid levels in the vapour phase were calculated using the
relationship between the vapour phase and salt-acid solution for each RH value (see Appendix 3).
To generate the carbonyl levels desired for experiments 2 and 3, the following relationships
between the solution concentration and the vapour phase were used as an estimate. These
relationships were established by experimental work conducted by Thickett for the acids [2], and
preliminary vapour measurements for formaldehyde:
Formic acid, vapour phase: ppmv = 33 X content
of salt solution in % (v/v)
Acetic acid, vapour phase: ppmv = 23 X content
of salt solution in % (v/v)
Formaldehyde, vapour phase: ppmv = 25 X
content of salt solution in % (v/v)
Three carbonyl—water-salt systems (one for each carbonyl) were placed in a 0.90 1 glass jar. It was
observed that the tarnishing layer formed on the metal surfaces was more uniform in this larger jar.
Each carbonyl solution was three times more concentrated than in the experiment using only one
carbonyl—water-salt system, in order to compensate for absorption by the other two solutions. The
carbonyl vapour levels were measured in each jar to check that the appropriate levels had been
achieved, and to identify any possible interactions between the solutions.
For experiments 1 and 2, the carbonyl—water-salt mixtures were replaced every 27 days. During
the replacement of the mixtures, the metal samples were removed from their glass jars for
measurement and put back the following day. For experiment 3, some lead samples were exposed to
carbonyl vapours for 33 days in the same way as experiment 2. The other lead samples were
exposed to vapour released by a 30 g piece of five-year-old particleboard containing a urea
formaldehyde adhesive, in an atmosphere conditioned by a salt solution (54 and 75% RH) or water
alone (100%).
APPENDIX 3: ANALYTICAL METHODS TO DETERMINE CARBONYL LEVELS IN
THE VESSELS
The concentration of carbonyls in the vessels was determined by a previously developed method
using sorbents in diffusion tubes and high performance
liquid chromatography (HPLC) [20, 21]. The sorbent for the acids was potassium hydroxide at a
concentration of 1 mole per litre in water mixed with 10% ethylene glycol dimethyl ether. 40 μl of
this solution were transferred to a stainless steel frit, which was placed in a diffusion tube. For
formaldehyde, dinitrophenylhydrazine (DNPH) was used as the sorbent, at a concentration of 0.137
moles per litre in acetomtnle (ethanenitnle). A solution was made by dissolving 150 mg DNPH in
4.75 ml acetonitrile, 0.5 ml ethylene glycol dimethyl ether and 0.25 ml phosphoric acid (85% v/v).
60 μμl of this solution were added to a paper filter (Whatman, 13 mm). The ethylene glycol
dimethyl ether was used as a wetting agent, and the phosphoric acid was added to decrease the pH,
because the reaction of formaldehyde with DNPH is faster at a lower pH.
Carbonyl vapours were collected during a period of 27 days, by hanging diffusion tubes in the space
above the carbonyl solution. For each vessel, the measurements of formic and acetic acids were
made in triplicate and that of formaldehyde in duplicate. Levels of carbonyls were obtained by the
analysis of two sets of measurements. Overall, the limits of detection were 0.08 ppmv for formic
acid, 0.07 ppmv for acetic acid and 0.01 ppmv for formaldehyde.
In experiment 1, the concentration of formic acid was not measured directly for each vessel.
Instead, the levels were calculated after determining experimentally the relationship between the
vapour phase and the acid content in the salt-acid solution for 54, 75 and 100% RH. The results
showed that the level of formic acid in the vapour phase was directly proportional to its content in
the solution over the range 0.0042-0.422% (v/v) at 23 + 0.1 °C according to the following relationships:
At 54% RH: formic acid, vapour phase: ppmv =
8.3 + 0.7 x acid content of salt acid solution in %
(v/v)
At 75% RH: formic acid, vapour phase: ppmv =
32.6 ± 0.6 X acid content of salt acid solution in %
(v/v)
At 100% RH: formic acid, vapour phase: ppmv =
23.7 ± 0.5 X acid content of salt acid solution in %
(v/v)
In experiment 2, the levels of carbonyls for each vessel were measured. It was observed that the
level of acetic acid increased in the presence of formic acid with or without formaldehyde. The
level of acetic acid in the jar increased from 0.2 ppmv to 0.3 and 0.6 in presence of 0.2 and 8 ppmv
of formic acid respectively. Similarly, with the most concentrated solution of acetic acid, the level
of acetic acid moved from 11 to 13 and 1 6 ppmv with increasing levels of formic acid.
In experiment 3, the levels of carbonyls were measured for only two jars: those containing the
particleboard at 100% RH and the carbonyl mixture at 75% RH.
SUPPLIERS
Acetic acid, reagent grade, 80%; lead foil, purified; magnesium nitrate, certified ACS: sodium
chloride, certified ACS: Fisher Scientific, 112 Colonnade Road, Nepean. Ontario, Canada K2E 716.
Copper: Subero, Fuencarral 25, 28004 Madrid, Spain.
Particleboard: Pilon Ltee, 5 Montclair, Hull, Quebec, Canada J8Y2E3.
REFERENCES
1 Tetreault, J., Airborne Pollutants in Museums, Galleries and Archives: Risk Assessment,
Control Strategies and Preservation Management, Canadian Conservation Institute, Ottawa (2003)
26-28,83,84,99, 141.
2 Thickett, D., 'Relative effects of formaldehyde, formic and acetic acids on lead, copper and
silver', The British Museum, Report 1997/12 (1997).
3 Tetreault, J., SiroisJ., and Stamatopoulou, E., 'Study of lead corrosion in acetic acid
environment'. Studies in Conservation 43 (1998)17-32.
4 Weast. C., The Handbook of Chemistry and Physics. 64th edition, CRC Press, Boca Raton, FL
(1993-1994) B89-B91 and BIOS-BIOS.
5 Scott, D.A., 'Copper compounds in metals and colorants: oxides and hydroxides', Studies in
Conservation 42 (1997) 93-100.
6 Kwestroo, W., and Langereis, C., 'Basic lead acetates', Journal of Inorganic \uclear Chemistry
27 (1965) 2533-2536.
7 Cignini, P.L., and Napoli. A., 'VI/X-ray characterisation and thermal stabilities of lead(II)
complexes with thiodiacetic, oxydiacetic, and immodiacetic acids', Annali di Chimica 66
(1976)333-335.
8 Taylor, P., and Lopata, V.J., 'Stability and solubility relationship between some solids in the
system PbO-CO2-H,O', Canadian Journal of Chemistry 62 (1984) 395-402.
9 Vezzoli, G.C., and Krasner, S., 'Studies of PbCO,, PbSO4 and Bi^O^CO, at high pressure: x-ray
spectra and electric
behaviour in situ'. High Temperatures — High Pressures 15 (1983)41-49.
10 Cano, E., Eopez, M.F., Simancas, J., and Bastidas, J.M., 'X-ray photoelectron spectroscopy study
on the chemical composition of copper tarnish products formed at low humidities', Journal of the
Electrochemical Society 148 (2001) E26-E30.
11 Cano, E., and Bastidas, J.M., 'Effect of relative humidity on copper corrosion by acetic and
formic acid vapours", Canadian Metallurgical Quarterly 41 (2002) 327-336.
12 Tetreault, J., 'Corrosion of zinc and copper by acetic acid vapour at 54% RH', Canadian
Conservation Institute, Ottawa, unpublished results (1992).
13 Tetreault, J., Coatings for Display and Storage in Museums, CCI Technical Bulletin No. 21,
Canadian Conservation Institute. Ottawa (1999) 4-6.
14 Leveque, M.A., 'The problem of formaldehyde - a case study' in Preprints of papers presented at
the fourteenth annual meeting, Chicago, Illinois, 27—25 May 1986, American Institute for
Conservation of Historic and Artistic Works, Washington DC (1986) 56-65.
15 Raychaudhun, M.R., andBrimblecombe, P., 'Formaldehyde oxidation and lead corrosion',
Studies in Conservation 45 (2000) 226-232.
16 Feldnian, L.H., 'Discoloration of black and white photographic prints', Journal of Applied
Photographic Engineering 7 (1981) 1-9.
17 Wexler, H., 'Polymerization of drying oils', Chemical Reviews 64(1964)591-611.
18 Lopez-Delgado, A., Cano, E., Bastidas, J.M., and Lopez, F.A., 'A laboratory study of the effect
of acetic acid vapor on atmospheric copper corrosion', Journal of the Electrochemical Society 145
(1998) 4140-4147.
19 Greenspan, L., 'Humidity fixed points of binary saturated aqueous solutions',_/0wrtttf/ of
Research of the Bureau of Standards. Section A: Physics and Chemistry 81A (1977) 89-96.
20 Gibson, L.T., Cooksey, B.G., Littlejohn, D.. and Tennent, N.H., 'A diffusion tube sampler for the
determination of acetic acid and formic acid vapours in museum cabinets', Analytica Chimica Acta
341 (1997) 11-19.
21 Gibson, L.T., and Brokerhof, A.W., 'A passive tube-type sampler for the determination of
formaldehyde vapours in museum enclosures', Studies in Conservation 46 (2001) 289-303.
AUTHORS
JEAN TETREAULT graduated
from the University of Montreal with a MSc in analytical chemistry. In
1989, he joined the Canadian Conservation Institute. He was president of the Canadian Association
for Conservation of Cultural Property from 1995 to 1997. He serves as an advisor and researcher on
general environmental guidelines, pollutants, display and storage products, and light issues.
Address:
Canadian Conservation Institute, 1030 Innes Road, Ottawa, Ontario, Canada K1A OM5. Email:
jean_tetreault@pch.gc.ca
EMILIO CANO obtained
a PhD in conservation from the Complutense University of Madrid in 2001.
He works as scientific researcher at the Spanish National Centre for Metallurgical Research
(CENIM-CSIC), where he studies the deterioration (corrosion) and conservation of metallic objects
of historic and artistic interest. Address: Departamento de Ingenieria de Matcriales, Degradation y
Durabilidad, Centra National de Investigations Metalurgicas (CENIM), Avda Gregorio delAmo 8,
28040Madrid, Spain. Email: ecano@fenim.csic.es
MAARTEN VAN BOMMEL studied
analytical chemistry at the University of Amsterdam, followed by a
PhD at Leiden University, which involved the coupling of enzyme-amplified biochemical detection
to continuous-flow systems. In 1999, he joined the Netherlands Institute for Cultural Heritage in
Amsterdam. His main interests are in the analysis of dyestuffs and organic pigments by HPLC and
the analysis of volatile organic acids and formaldehyde with solid phase micro extraction combined
with gas chromatography-mass spectrometry. Address: Netherlands Institute for Cultural Heritage,
Conservation Research, POBox 76709, 1070 KA Amsterdam, The Netherlands. Email:
maarien.van.bommel@icn.nl
DAVID SCOTT obtained
a BSc in chemistry from the University of Reading (England) in 1971, a BA
in archaeological conservation in 1978 and a PhD in ancient metallurgy from University College
London in 1982. He lectured in conservation at the Institute of Archaeology, London University,
1981—87, before joining the Getty Conservation Institute as head of the Museum Research
Laboratory in 1987. He is now a professor in art history and archaeology at UCLA and director of
the UCLA/Getty MA program in archaeological and ethnographic conservation. Address: Cotsen
Institute of Archaeology, Room A410, UCLA, Los Angeles, CA 90095, USA. Email:
dascott@uda.edu
MEGAN DENNIS studied
archaeological sciences at the University of Bradford. She has worked
within the technology section of the Centre for Archaeology, English Heritage, and at the Getty
Conservation Institute. Her main research interests are the study of ancient technologies,
specifically metallurgy; the application of science to archaeological collections; and the
incorporation of the scientific
work of museums into education and display. Address: LEA MINEL is an engineer specializing in
surface analysis
Queen's College, Oxford OX1 4AW, UK. Email: at the CNRS. Address: as for BarthesLabrousse. Email:
megan.dennis@archaeology.oxford.ac.uk lea.minel@ght-cnrs.fr
MARIE-GENEVIEVE BARTHES-LABROUSSE, PhD,
is re- Luc ROBBIOLA obtained his MSc in
conservation-search director at the Centre national de la recher- restoration (Universite Paris ISorbonne, 1985) che scientifique (CNRS), France, specializing in and a PhD in chemistry in 1990;
since 1990 he has surface science. Her work mainly concerns metallic been head of the electron
microscopy unit at the surfaces and polymer—metal adhesion mechanisms. National School of
Chemistry, Paris. Address: Serv-Address: Centre d'etudes de chimie metallurgique, ice commun des
microscopies electroniques, Laboratoire C^\RSUPR280, 15 rue Georges Urbain, 94407 Vitry de
metallurgie, Ecole nationals superieure de chimie de Cedex, France. Email: mariegenevieve.barthes@glvt- Paris, 11 rue P et M Curie, 75231 Paris Cedex 5, cnrs.fr France. Email:
robbiola@ext.jussieu.fr
Resume — La formation des produits de corrosion du cuivre et du plomb, ainsi que la sensibilite de
ces deux metaux a se corroder, ont ete etudiees en presence de formaldehyde et d'acides formique
et acetique, et ce, a differents temps d'exposition et differents taux hygrometriques (HR). Dans le
cas du cuivre, V'acide formique affecte son apparence lorsque ce gaz est a des concentrations
superieures a 0,4 ppmv et en presence de 75% HR. D'autre part, le gain en poids fut decelable
lorsque les niveaux d'acide formique etaient superieurs a 4 ppmv a 54 et 75% HR. Le principal
compose trouve sur le cuivre fut la cuprite. Par centre, le plomb a une plus grande sensibilite face a
I'acide formique. A 54% et a 75% HR, avec une concentration d'acide formique aussi basse que
0,04 ppmv, le plomb se noircit, et au-deld de 0,1 ppmv, il y a un gain de poids observable. En
presence de differentes concentrations de vapeur des trois carbonyles sous etude, le cuivre a 75%
HR reagit principalement avec I'acide formique. Par centre, la reactivite du plomb est plus
complexe. L'acide acetique tend a former une epaisse couche blanche (composee principalement de
plumbonacrite et possiblement de composes acetates) sur la surface du plomb. Pour sa part, I'acide
formique tend plutot a former une couche sombre et mince d'hydroxy-formate de plomb. En
presence d'acide formique, I'action de I'acide acetique sur le plomb fut inhibee. Avec des
concentrations allant jusqu'd 3 ppmv, le formaldehyde n'a montre aucune contribution au processus
de corrosion du cuivre et du plomb a 75% HR.
Zusammenfassung — Zur Untersuchung der Korrosionsprodukte und der
Korrosionsempflndlichkeit van Kupfer und Blei wurden Experimente durchgefuhrt, in denen die
Konzentrationen der Schadgase Formaldehyd (Methanal), Ameisensaure IMethansaure) und
Essigsdure (Ethansdure), die Relative Luftfeuchtigkeit (RH) soune die Expositionsdauer variiert
wurden. Bei Volumenanteilen van Ameisensaure uber 0,4 ppmv wurde bei 75% RH das Aussehen
van Kupferoberfldchen verdndert, wahren das Geuncht erst bei Anteilen van tiber 4 ppmv bei 54%
und 75% RH zunahm. Als hauptsdchliches Korrosionsprodukt konnte auf der Oberfldche Kuprit
(Cu 7O) festgestellt werden. Blei hat gegenuber Ameisensaure eine hohere Empfindlichkeit: Bereits
bei Volumenanteilen unter 0,04 ppmv wird die Oberfldche dunkler, und bei 0,1 ppmv nimmt das
Gewicht bei 54% und 75% RH messbar zu. Bei Anwesenheit verschiedener Konzentrationen
unterschiedlicher Carbonylverbindungen reagiert Kupfer bei 75% RH vor allem mil Formaldehyd.
Das Reaktionsverhalten van Blei war deutlich komplexer. Mit Essigsdure bilden sich dicke weifle
Schichten vor allem aus Plumbonacrit und moglichetweise Bleiacetatverbindungen auf der
Oberfldche, wdhrend Ameisensaure zur Bildung recht dunner dunkler Schichten van
Bleiformiathydroxid fuhrt. Bei Anwesenheit von Ameisensaure war das Korrosionspotential van
Essigsdure inhibiert. Formaldehyd — Konzentrationen bis zu 3 ppmv hatten gemdfi den
Untersuchungen keinen signifikanten Einflujl auf das Korrosionsverhalten von Blei und Kupfer.
Resumen — Se ha estudiado la sensibilidad a la corrosion del cobre y del plomo, y los productos de
corrosion formados sobre ambos metales, en distintas condiciones experimentales en las que se han
variado las concentraciones de formaldehido, dcido formica y dcido acetico, la humedad relativa
(HR) y el tiempo de exposition. El aspecto del cobre se altera en presencia de acido formica en
concentraciones iguales a superiores a 0,04 ppmv al 75% de HR, y se produce
incremento de masa a partir de 0.4 ppmv, tanto al 54% como al 75% de HR. El principal producto
de corrosion formado sobre el cobre es cuprita. El plomo muestra una mayor sensibilidad al acido
formica. Desde 0,04 ppmv el plomo se oscurece, y el aumento de masa es apreciable desde 0,1
ppmv en ambus humedades, 54% y 75% RH. En presencia de distintas combinadones de los
vapores organicos estudiados, el cobre reacciona principalmente con el acido formica. Par otro
lado, la reaction del plomo es mas compleja. El acido acetico tiende aformar sobre la superficie del
plomo una gruesa capa blanquecina (compuesta fundamentalmente par plumbonacrita y
probablemente acetato de plomo), mientras que el acido formica produce una capa fina y oscura de
hidroxiformiato de plomo. En presencia de acido formica, la action del acido acetico sobre el
plomo queda inhibida. \'o se ha observado un efecta apreciable del formaldehido en la corrosion
del plomo ni del cobre en concentraciones de hasta 3 ppmv y humedad relativa del 75%.