Indoor corrosivity classification based on lead coupons

Abstract

Air pollution in museums, libraries, churches, and other indoor environments of cultural heritage objects was studied since 1980s. For classification of corrosivity for such type of indoor environments the ISO 11844 series was developed in 2006. In 2020 the revision of this ISO 11844 series had been finished, where lead was included as standard specimen with high sensitivity to vapour organic acids. This paper gives results of exposure of lead standard coupons in museums and other similar indoor environments together with measurement of climatic parameters and air pollution to verify the new approach.

Full text

TECHNICAL NOTE
Koroze a ochrana materiálu 65(4) 7-12 (2021) DOI: 10.2478/kom-2021-0014 7
INTRODUCTION
In both storage and exposition, the processes of
corrosion may threaten the stability of metal artefacts.
Air pollution in museums, libraries, churches, and other
indoor environments of cultural heritage objects was
studied since 1980s [1, 2]. One signicant source was
outdoor air pollution, mainly by SO2 and NOx, which
concentration decreased according to distance of outdoor
source and type of building. Contemporary outdoor air
pollution situation in Europe has no signicant eect on
indoor environments.
But indoor environments are sources of specic
type of pollution themselves. Especially applied mate-
rials for construction of showcases and storage are
potential emission source for variety of volatile organic
compounds [3, 4]. Formaldehyde, formic and acetic
acids belong to the most discussed pollutants in the
museum and similar environments. Their corrosion
impact increases in the following order: formaldehyde
< formic acid < acetic acid. The acetic acid is known
to aect metals (particularly lead), calcareous materials
(shell, limestone, calcium-rich fossils), soda-rich glass,
and cellulose. Examples of deterioration due to acetic
acid include the corrosion of lead-rich organ pipes in
churches [5], discoloration of pigments, and depolyme-
rization of paper [6]. The acetic acid has been shown
to eect paper lifetime signicantly at values above
250 μg m-3 (100 ppb) [7], bronze and zinc are also aec-
ted by organic acids but to a lesser extent [8].
The concept for classication of corrosivity of
indoor atmospheres was prepared in ISO/TC156/WG4
in 1997. The exposure study was performed at 42 test
sites in industrial locations, storage rooms, museums
and churches. For classication of corrosivity of such
type of indoor environments the ISO 11844 Corrosion
of metals and alloys – Classication of low corrosivity of
indoor atmospheres series was published in 2006. This
classication and mainly the methods for determination
and/or estimation of indoor corrosivity had been applied
in many cultural objects [9 - 13]. In 2020 the revision
of this ISO 11844 series had been nished. The main
changes compared to the previous edition are as follows:
● the model that estimates the indoor concentration and
deposition of pollutants originating from outdoors was
added;
● lead was included as standard coupons with high sen-
sitivity to vapour organic acids – Table 1.
Indoor corrosivity classification
based on lead coupons
Kreislova K.1, Fialova P.1, Bohackova T.2, Majtas D.3
1 SVÚOM Ltd., Prague, Czech Republic
2 University of Chemistry and Technology, Department of Metals and Corrosion Engineering,
Prague, Czech Republic
3 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences,
Telč, Czech Republic
E-mail: kreislova@svuom.cz
Air pollution in museums, libraries, churches, and other
indoor environments of cultural heritage objects was studied
since 1980s. For classication of corrosivity for such type of
indoor environments the ISO 11844 series was developed in
2006. In 2020 the revision of this ISO 11844 series had been
nished, where lead was included as standard specimen with
high sensitivity to vapour organic acids. This paper gives results
of exposure of lead standard coupons in museums and other
similar indoor environments together with measurement of cli-
matic parameters and air pollution to verify the new approach.
Tab. 1. Classication of corrosivity of indoor atmospheres
based on corrosion rate of lead standard coupons
Corrosivity category Corrosion rate rcorr
(mg m-2 a-1)(nm a-1)
IC 1 very low indoor rcorr ≤ 40 rcorr ≤ 3.5
IC 2 low indoor 40 < rcorr ≤ 150 3.5 < rcorr ≤ 13
IC 3 medium indoor 150 < rcorr ≤ 400 13 < rcorr ≤ 35
IC 4 high indoor 400 < rcorr ≤ 700 35 < rcorr ≤ 62
IC 5 very high indoor 700 < rcorr ≤ 1 600 62 < rcorr ≤ 140

Indoor corrosivity classification based on lead coupons Kreislova K., Fialova P., Bohackova T., Majtas D.
Koroze a ochrana materiálu 65(4) 7-12 (2021) DOI: 10.2478/kom-2021-0014 8
The atmospheric corrosion of lead is investigated,
with the aim to quantify the eect of volatile organic
acids. The specic sensitivity of lead as material of
cultural heritage objects was mentioned in many papers
[14-20]. The thin layer of corrosion products which
decelerates atmospheric corrosion is formed on lead
surface in atmospheric condition. The native oxide
on lead is predominantly litharge PbO and is 3 to 6
nm thick. Indoor, historic objects gradually develop
a dark grey patina of lead carbonate PbCO3 or lead
hydroxycarbonate/hydrocerussite Pb3(CO3)2(OH)2. The
colour may vary depending on the composition of the
alloy, the object’s past history, and its present storage
conditions. Lead formate and alkaline lead carbonate are
two main degradation products formed on lead objects.
These corrosion products do not have any protection
ability.
In presence of acetic acid vapour, the voluminous
corrosion products are formed and spall from the surface.
At acetic acid levels as low as 100 µg.m-3 lead becomes
darker, and at above 250 µg.m-3 weight gains were
measurable at both 54 and 75% RH. Dissolving into the
aqueous electrolyte, acetic acid ensures a decrease of the
pH value, which results in the dissolution of the native
basic PbO layer explained by
PbO (s) + 2 CH3COOH (aq) ←
→
Pb2+ (aq) +
+ 2 CH3COO- (aq) + H2O
Low formic acid concentrations (300 µg m-3) are
also very corrosive towards lead, but less eective than
acetic acid. The lower solubility of lead formate in water
and the diculty in forming lead complexes explain why
this acid is less aggressive compared to acetic acid.
This paper gives results of exposure of standard
coupons in 15 localities in the Czech Republic: 4 local
museums, 5 libraries in Prague, including exposure insi-
de the showcases, 5 churches in small villages and simi-
lar indoor environments together with measurement of
climatic parameters and air pollution to verify the new
approach.
EXPERIMENTAL
Exposure environments
The exposure program and measurements were car-
ried out in 15 indoor localities in periods 2018/19 and
2019/20. Metals for standard coupons (zinc, copper, sil-
ver, lead) were supplied in the form of sheets (purity min.
99.45 %). The coupons with dimensions 30 × 80 mm
were designed for weight loss identication and the other
coupons with dimensions 10 × 25 mm were analysed by
means of electrolytic cathodic reduction. Only copper,
silver and lead were involved in such coulometric quan-
tication of corrosion loss.
Each exposed metal reacts to specic kinds of pollu-
tants which may be present in indoor environments.
Copper corrosion is often associated with chlorides, sul-
phides, and acidic pollutants such as NO2 and SO2. Silver
reacts with sulphides such as carbonyl sulphide (COS)
and hydrogen sulphide (H2S). Lead reacts with organic
carbonyl pollutants and acidic pollutants. Zinc is also
sensitive to organic acid pollutants and it is metal mostly
reactive in high humidity.
Temperature and relative humidity in exposure loca-
lities were continually measured by T/RH dataloggers.
In selected localities SO2 and NOx were measured by
passive samplers in monthly intervals according to ISO
EN 9225 [21]. In urban indoor localities SO2 yearly
average concentration was ca 4 μg m-3 and NOx yearly
average concentration was ca 10 μg m-3. The passive
samplers were used for measurement of acetic acid va-
pour concentration – Figure 1 [4].
Metal coupons
The metallic coupons were abraded with silicon car-
bide paper to 320 grids, cleaned with de-ionised water,
and degreased with ethanol prior to exposure. The trip-
licate coupons of each metal were exposed vertically
mounded on small plastic racks in selected indoor envi-
ronments for 1-year period – Figure 2. The coupons
designed for electrolytic reduction were polished with
P1200 emery paper, rinsed with de-ionized water and
degreased with ethanol in ultrasound bath for 5 minutes.
The colour and gloss change of lead coupons´ sur-
face was measured by spectrophotometer Spectro Guide
Gloss S (BYK-Gardner) to use CIEL*a*b* regular coordi-
nates. After the exposure, the coupons´ surfaces were eva-
luated by light standard and 3D microscopes Neophot 32
(Zeiss) and VHX-5000 (Keyence), electron scanning mi-
croscope Vega II (Tescan) with EDX analysis and by IR
method with iS5 (Nicolet) to identify corrosion products.
Fig. 1. Passive samplers for acetic acid measurements

Indoor corrosivity classification based on lead coupons Kreislova K., Fialova P., Bohackova T., Majtas D.
Koroze a ochrana materiálu 65(4) 7-12 (2021) DOI: 10.2478/kom-2021-0014 9
Corrosion mass loss of metallic coupons was deter-
mined by interval pickling procedure according to ISO
8407 [22]. For following data treatment, the triplicate
average value was used. Mass loss of some coupons was
galvanostatically reduced using constant current density
0,125 mA cm-2 in deaerated 0.1 mol dm-3 potassium
chloride solution.
RESULTS
Environmental characteristics of exposures
Results of yearly average environmental parame-
ters in selected exposure localities are given in Table 2.
In churches, locality 1 and 2, the high variability of tem-
perature and humidity were measured, but the summer
season was not measured. There are yearly average va-
lues of acetic acid vapour concentration in Table 2, and
the concentration shows very signicant dependence on
temperature – Figure 3. Extremely high concentration of
acetic acid was measured inside the showcases, ca 3.5-fold
higher than in the room where the showcases are placed.
Corrosion losses of exposed coupons
The coupons´ surfaces were evaluated on 3D confo-
cal microscope at dierent magnication. Characteristic
white corrosion products of lead caused by the volatile
organic acids were found on coupons exposed at locality
13, a local museum. The yearly corrosion mass loss was
over 11 g m-2 a-1 (1.01 µm a-1); the highest value of all
exposure localities.
The same layer was identied by SEM method –
Figure 4. EDX analyses of all exposed coupons show the
only elements present on lead coupons´ surfaces were
carbon and oxygen as corrosion products, in traces also
silicon as particles.
Corrosion products from this exposure were ana-
lysed by FTIR method as alkaline lead carbonate
PbCO3Pb(OH)2.
The colour and gloss change of lead coupons´ sur-
face show the most intensive change for L coordinate
represents the colour lightness in white/black scale.
The mass increase data cannot be used due to high
scatter of values. Corrosion loss data (Tab. 3) shows the
corrosivity categories in studied indoor localities are low
for copper and silver, mostly IC2. For zinc the corro-
sivity categories are higher especially in localities with
low temperature and high humidity (localities 1 – 5 –
churches).
2000
3000
0
123456789101
11
2
1000
1500
2500
500
Vapour acetic acid concentration (µg m-3)
Time (month)
Rooms
Showcases
Fig. 2. Example of coupons´ exposure in locality 13
Fig. 3. Temperature dependence of acetic acid vapour con-
centration during year
Tab. 3. Indoor corrosivity categories
Localities Corrosion loss (μm a-1)/corrosivity category
Zinc Copper Silver Lead
1 – 5 0.15/IC4 0.03/IC3 0.03/IC2 0.13/IC5
6 – 10 0.04/IC2 0.02/IC2 0.02/IC2 0.09/IC5
11 – 13 0.08/IC3 0.02/IC2 0.02/IC2 0.59/>IC5
14 – 15 0.04/IC2 0.02/IC2 0.01/IC1 0.18/>IC5
Tab. 2. Environmental parameters of exposure localities
Localities Temperature (°C) Relative humidity (%) CH3COOH concentration
(μg.m-3)
1 – 5 non heated buildings (churches) -3 – 14 33 – 100 240
6 – 10 tempered buildings (depositaries) 14 – 31 33 – 62 217
11 – 13 heated buildings (claddings, furniture) 16 – 29 30 – 40 394
14 – 15 heated buildings (wooden showcases) 15 – 28 26 – 66 1373

Indoor corrosivity classification based on lead coupons Kreislova K., Fialova P., Bohackova T., Majtas D.
Koroze a ochrana materiálu 65(4) 7-12 (2021) DOI: 10.2478/kom-2021-0014 10
DISCUSSION
Formic/acetic acid concentrations have been esti-
mated theoretically from degassing rates as a function
of temperature and acid emission rates from wood,
combined with the measured dimensions of the room
[23]. The performed laboratory tests of dierent types
of wood show the eect of acetic acid vapour emission
on temperature – according the type of wood ca 10× in-
creasing at increasing temperature from 20 °C to 45 °C,
e.g. for oak from 1740 μg m-3 to 19974 μg m-3 [4]. The
direct acetic acid measurement performed in frame of
this project gives the similar results – Figure 5. The sig-
nicant increasing of the emission was estimated at tem-
perature above 20 °C. In exposure localities in the his-
toric buildings the temperature varies during year in
relatively small range – from 15 °C to 30 °C, but it has sig-
nicant eect on change of acetic acid emission – ca 2×
increasing. The maximal values obtained for summer
months inside the showcase were over 3000 μg m-3,
i.e. 3× higher than concentration measured in winter
season. In localities with stable temperature and humi-
dity conditions the monthly average of CH3COOH con-
centration were practically the same in all measured
periods.
The eect of higher humidity on emission rate is
not evident, which may be the reason of lower corrosion
attack of lead in churches in this program. Indeed, from
this experiment it would appear that moderate control of
temperature is more important than the relative humidity.
Although corrosion losses of copper and silver in
indoor environments are very low (max. 0.03 μm a-1) vi-
sually there are signicant surface changes (max. ΔECu
= 25.7 and ΔEAg = 46.1), which may negatively aect
perception of cultural heritage items. Corrosion losses
of zinc have shown the sensitivity of this material to the
humidity of the environment especially in non heated
buildings.
The lead corrosion loss can be classied according
to ISO 11844 into category IC4 and IC5 only for 10 lo-
calities, for other 5 localities the lead corrosion loss ex-
ceeds maximum classication value of 1 600 mg m-2 a-1,
respectively 0.14 µm a-1, so these localities cannot be
considered as low corrosive indoor atmospheres in
respect to lead. The atmospheric corrosion rates for lead
were estimated as 4.5 to 21.5 g m-2 a-1, respectively 0.4
to 1.9 µm a-1 for rural and urban localities [24]. Obtained
data are in agreement with these values for environments
with low formic and acetic acids pollution.
The formation of lead corrosion products is rela-
tively slow procedure – on the most localities (3, 4, 5, 6)
a) locality 1 b) locality 13
Fig. 4. SEM of lead corrosion product layers
800
1200
0
051015 20 25 30
400
600
1000
200
Acetic acid concentration (µg m-3)
Temperature (°C)
y = 38.90 e0.11x
R2 = 0.78
Fig. 5. Dependence of acetic acid vapour concentration on
temperature

Indoor corrosivity classification based on lead coupons Kreislova K., Fialova P., Bohackova T., Majtas D.
Koroze a ochrana materiálu 65(4) 7-12 (2021) DOI: 10.2478/kom-2021-0014 11
only PbO and PbCO3 occurred even the visual changes of
coupons are evident. Obtained results of corrosion mass
loss of lead show relatively good correlation with acetic
acid concentration – Table 3. and Figure 6, but there are
some exceptions aected by another factors.
CONCLUSIONS
Organic acids, mainly acetic and formic, are known
to be signicant pollutants in the museum and similar
indoor environments. The indoor containing wooden
elements (cladding, decoration, cases, exhibition show-
cases, etc.) is still a source of volatile organic acids even
though the wood is an old one (over 300 years). The con-
centration of vapor organic acids strongly depends on
temperature in indoor space, thus yearly measurement
is not representative only for given indoor environment
and the measurements need to cover at least one month
in each season.
Results of determination of corrosivity category in
15 localities of cultural heritage objects show that lead
is the most sensitive metal from others (zinc, copper,
silver). In many exposure localities the yearly corrosion
loss rcorr of lead was higher than maximal value for
corrosivity category IC5 according to revised ISO
11844-1 (1600 mg.m-2.a-1), although for other standard
metals the corrosivity categories were only IC1 to IC3.
It means these localities cannot be characterised as low
corrosivity for this metal. Ones of such localities are
internal spaces in showcases, localities 14 and 15, where
the open rooms´ corrosivity is classied as category IC5.
Acknowledgement
This article was created with the support of NAKI
project No. DG18P02OVV050 Methodology of classi-
cation of indoor environments for exhibition artefacts
from lead from the Ministry of Culture of the Czech
Republic.
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8
12
0
0500 1000 1500
2000
4
6
10
2
Lead corrosion loss (g m-3)
Acetic acid concentration (µg m-3)
y = 5.23-E04x + 1.14E+00
R2 = 7.54E-01
Fig. 6. Dependence of lead corrosion loss on acetic acid
vapour concentration

Indoor corrosivity classification based on lead coupons Kreislova K., Fialova P., Bohackova T., Majtas D.
Koroze a ochrana materiálu 65(4) 7-12 (2021) DOI: 10.2478/kom-2021-0014 12
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