A New Methodology for Accelerated Corrosion Testing
Laurianne Robinet and David Thickett
Accelerated corrosion tests have been used for almost 30 years to exclude corrosive materials from
museum display and storage. The method reported by Green and Thickett in 1995 has been
developed to allow the evaluation of three metals (silver, copper and lead) simultaneously in a
single test. Comparison of a wide range of materials has shown that the differences in results
introduced by this modification are less than the differences encountered with replicate tests using
the same material employing the previous method. Problems encountered with corrosion in lead
control tests have been investigated and the method has been modified to remove this effect. The
cleaning method for the test vessels has been modified to exclude the hazardous chemical, chromic
acid.
INTRODUCTION
Accelerated corrosion tests have been used in conservation since 1972 [1, 2]. Such tests have much
to commend them. They are relatively fast and inexpensive; and when used prior to building or
refurbishment work, they are pro-active and exclude unsuitable materials before damage has
occurred. They 'model' the showcase/storage situation and do not require knowledge of air
exchange rates or the susceptibilities and damage levels of different metals. The tests suffer the
drawbacks of all accelerated testing, with uncertainty over the acceleration factors compared to
ambient environments and the possibility of encouraging reactions that would not occur under
normally encountered conditions. This could lead to materials failing the tests, which if used under
the actual display/storage conditions would not cause problems. However, if the object of the
testing is to exclude potentially damaging materials, then this is not a serious failing. The intention
has always been that the test method should be available to any conservator and hence not require
complex equipment or extensive training.
The test method has developed over the last three decades, with modifications introduced to suit
particular institutions or circumstances. This has led to a situation where many minor variations of
the test are in use. A survey of UK institutions using the test highlighted the many differences in
methods in use [3]. An inter-laboratory comparison between these institutions showed that such
small differences could lead to very different results being obtained from the same test material. In
the field of analytical chemistry it is well recognized that perfect reproducibility between
laboratories does not occur, even with well validated and controlled analytical methods. However,
two tests providing different results for the same material can seriously undermine confidence in
both the tests themselves and the need for materials testing. Research conducted into the various
factors affecting the test method generated a method which balances ease of preparation with
reproducibility. Repeating the inter-laboratory comparison, with each participant using the new
method, gave very significantly improved reproducibility [4].
The major and fundamental limiting factor on reproducibility for this type of testing is almost
certainly the reproducibility of the metal surfaces, which influences their reactivity to the test
environment. The aims of the present work were to investigate different ways of running the tests
with all three metals in one vessel, to optimize a method for preparation speed and utility, whilst
retaining control over those test factors that significantly affect the test results. The test reported
here has been evaluated against the method published by Green and Thicken in 1995 [4], using 53
materials, and the procedure is presented in Appendix 1. At the same time, two separate problems
identified with the 1995 method — tube cleaning, and corrosion observed on the lead control
coupon — were investigated. The results are reported below.
TUBE CLEANING
Cleanliness is a very important part of the test. If the tubes are not cleaned properly, the results of
the next test could be adversely affected. The standard cleaning procedure comprises three separate
steps, one of which uses chromic acid. Chromic acid is of major concern because it has serious
health and safety implications and is damaging to the environment. Two weaker, safer, acid
solutions were tested as replacements for chromic acid solution. Decon 90 'Acid Rinse' detergent
and dilute nitric acid solution (10% by volume) were used to clean tubes which had contained an
unsuitable material, which had corroded lead in a test. Tests with lead, but no material, were then
assembled using the cleaned tubes. Lead was used because of its high sensitivity to low pollutant
gas concentrations [5]. At the end of the test, no corrosion was observed on any of the lead test
coupons. The new cleaning procedure recommended is presented in Appendix 1.
ALTERATION OF THE LEAD CONTROL COUPON
Sometimes, in the standard test, a red oxidation appears on the lead control coupon. Corrosion on
the control is problematic, as the batch of tests must be repeated. The effect was not observed on the
materials tested concurrently, hence a dirty tube is unlikely to be responsible for the corrosion.
To try to determine the cause of the corrosion, tests were undertaken to investigate various factors
such as: different cotton wool; different tissue (used to dry the coupon); different source of lead;
coupon not cleaned in acetone (propanone); coupon air-dried after acetone cleaning; coupon in a
tube without water; coupon in direct contact with water; and a different container, to avoid contact
with the interior surface of the glass. Several of the tests produced white and red corrosion which
was identified by micro-Raman spectroscopy and X-ray diffraction as hydrocerrusite (2PbCO3 •
Pb(OH)2), litharge (tetragonal PbO), massicot (orthorhombic PbO) and plumbonacrite (6PbCO3 3Pb(OH)2- PbO).
The tests set up in a vertical position did not show any corrosion. Some of the previous tests were
repeated with the tubes placed vertically, using an aluminium tray with square compartments. At the
end of the test no corrosion was observed on any of the lead coupons. The contact created between
the coupon and condensation on the glass, by placing the tube at an angle, was responsible for the
corrosion on the control coupon. This effect can be avoided by placing the tube vertically in the
oven, utilizing an aluminium tray.
THREE COUPONS IN ONE TUBE
The standard method involves setting up a separate test for each metal coupon: silver, copper and
lead. In a modified test method [6], the three metal coupons are bent on the edge of a glass beaker,
in direct contact with the glass. Unfortunately this arrangement facilitates contact with water
condensation which can induce corrosion. Also, because of stress induced in the metal coupon,
corrosion appears at the bend, which would not normally be observed.
Placing the three metal coupons in a single tube would greatly simplify' the test, and reduce
preparation time. Such a method must avoid contact between the coupons, and also contact with the
glass of the tube. The use of a soft stopper in which slots are cut to insert the three different metal
coupons (Figure 1) was investigated.
Soft stoppers from different manufacturers were tested for out-gassing, using the standard
accelerated corrosion test. Neoprene stoppers were found unsuitable as they tarnish silver and
copper. Silicone stoppers produced by VWR and Radleys proved to be inert with respect to the
metal coupons used. It was noticed that the part of the silver coupon in the slot of the silicone
stopper tarnished sometimes, but the part exposed in the test was not affected.
In order to verify that the modification gives the same results as the standard test, a trial was run on
different materials using the standard method and the new method, called the '3 in 1' test. Radleys
stoppers were used for all the following experiments.
Figure 1 Metal coupons placed parallel in a soft stopper.
Initially, tests were run on three commonly tested materials: wood and two textiles. Different
weights of material (2, 4 and 6 g) were placed in the tubes, to evaluate the effect of the produced
gases being split between the coupons, and hence the dose of gas for each coupon being lower than
in the standard test. The gases might react preferentially with one or more of the metal coupons.
Each standard test was repeated twice on each material and each '3 in 1' test was repeated three
times. The whole assembly was placed in the oven at 60°C for 28 days. At the end of the test
period, the coupons were assessed according to the standard rating definitions used by the British
Museum [7]:
P suitable for use (no corrosion)
T suitable for temporary use, up to six months (slight corrosion)
U unsuitable for use
Better results were always obtained with 2 g of material. The more material present, the less the
coupon corroded. The more material present in the test tube, the less water remained in the small
tube.
The repeats of the same test on the same materials gave identical results. Even though the coupons
in both tests were affected in the same way and gave the same assessment, their appearance was a
little different. In the '3 in 1' test, the corrosion was always slightly less pronounced and mainly
concentrated at the bottom. This is probably because the coupons are at a different height, with
more distance between the coupon and the fabric than in the standard test; and because they are next
to each other, which restricts the mixing of the air in the tube.
Based on the observations from the first trials, the '3 in 1' test was modified. Another batch of tests
was run on the textile which gave the greatest difference in the appearance of the coupons between
the two tests in the previous trial. Various positions and lengths of coupon were mailed. The
arrangement shown in Figure 1 gave results most comparable with the standard test. The
appearance of the coupon will never be exactly the same as in the standard test, because the
environment is different. Nevertheless, the two tests produce the same ratings. The presence of
more pronounced corrosion in the second trial, compared to the first trial, reveals the importance of
the positioning of the coupons in the tube. The '3 in 1' test has the advantage of improving the
repeatability of the coupon positioning, a previously unsuspected factor in the test.
In order to decide if the '3 in 1' test could safely replace the standard test, comparison tests were run
on 53 different samples commonly encountered in museum use, including textiles, woods,
adhesives, paints and paper. The '3 in 1' test was set up according to the procedure described in
Appendix 1. The lead coupon was placed in the central position (Figure 1) to avoid any risk of
contact with the glass of the tube, which would cause corrosion in the case of water condensation.
Lead is by far the most susceptible of the three metals to this effect. The coupon should be assessed
at the bottom, the part which reacts the most. However, corrosion sometimes occurred at a higher
level on the coupon, in the middle or nearly at the top, generally with lead. The same effect was
observed in the corresponding standard test. The vertical stratigraphy of the corrosion had not
previously been considered. The '3 in 1' test is superior to the standard test because metal is
available near the stopper, unlike the standard test.
With 46 of the materials the results were identical, whatever the materials used or the rating
obtained (P, T or U). With seven of the materials, the two tests did not agree for one of the three
metals (Table 1). Among the disagreements, the standard test is sometimes more sensitive than the
'3 in 1' test and sometimes the opposite is true, so it is not possible to generalize that one test is
more sensitive than the other.
The coupons were assessed according to the standard rating described in the test procedure. The
percentage of identical results was 91% for silver, 96% for copper and 98% for lead. A statistical
comparison between the '3 in 1' test and the standard test method is presented in Appendix 2.
Table 1 Comparison of the level of agreement between the '3 In 7' test and the standard test and for
repeats of the standard test
‫٭‬Root mean square deviation
CONCLUSION
During this research three independent aspects were examined and improved: tube cleaning,
changes in lead control coupons, and developing a way to place the three coupons in one tube.
In the cleaning process, a new procedure has been put in place. Chromic acid is now replaced by
either a solution of dilute nitric acid or Decon 90 'Acid Rinse' detergent, both of which are
significantly less dangerous to health and non-hazardous towards the environment.
The unusual red corrosion on lead control coupons was explained by the contact of the coupon with
the condensed water on the edges of the tube, condensation which was formed during the opening
of the oven. To solve the corrosion problem on the control, the position of the tube in the standard
test was changed from a leaning to a vertical position by using an aluminium tray.
The utility of the accelerated corrosion test method has been significantly improved. The placing of
all three metal coupons in a single tube not only saves time in preparing the tests but also improves
the reproducibility of the positioning of the coupons, a factor not previously suspected in the
amount of corrosion produced. The new test method gives a very high agreement with results from
the previously published method.
ACKNOWLEDGEMENT
The authors would like to thank Susan Bradley from the British Museum for her advice and
comments, and Kristie Short for her help in setting up the tests.
APPENDIX 1: '3 IN 1” METHOD PROCEDURE
Equipment required
2 g sample of the material, 50 ml glass tubes, inert silicone stoppers, small glass tubes (0.8 ml),
metal foil (Ag, Cu, Pb). Cotton wool, distilled water, high purity acetone, three Petri dishes (one for
each metal), watch glass, glass bristle brushes (one for each metal), triangular-end scalpel, tweezers,
pair of scissors. Scalpel, tweezers and scissors must be cleaned with acetone before use.
Cleaning procedure
In the case of rust marks remaining on the tube from a previous test, clean with dilute (20%) HC1
for several days. Place the tubes in a dilute nitric acid solution (10% by volume) or Decon 90 'Acid
Rinse' solution overnight. Wash the tube in water with Centiclean Super Detergent, using a brush.
Put the tube to dry in an oven at 60°C. Pass the tube through a Bunsen burner to eliminate any
organic residue.
Preparation of the tubes
Insert 2 g (± 10%) of the material at the bottom of each tube. Place also, at the bottom of the tube,
the small tube (0.8 ml) filled with distilled water, closed with a small piece of cotton wool.
Preparation of the metal coupons
For each metal foil (Ag, Cu, Pb), abrade the foil surface very roughly on both sides with a glass
brush — the objective is to remove any oxide layer from the metal surface. Cut one coupon of each
metal foil to size (10 x 35 mm). Clean the coupons in acetone by placing them in a Petri dish filled
with Spectrosol grade acetone and cover the Petri dish with a watch glass. Hold the coupons with
tweezers and dry them with a tissue (do not leave the coupon to dry off by itself).
Preparation of the stopper (Figure 1)
Cut three slots, about 5 mm deep and 10 mm across, in the bottom of the silicone stopper with the
scalpel. Insert each metal coupon in a slot, placing the lead coupon in the middle (ensure that the
coupons are not touching one another). An easy way to insert the coupon is to squeeze the stopper,
which will open the slots.
Closing the tube (Figure 2)
Place the coupons in the tube by closing the tube with the stopper which holds the coupons; make
sure the tube is perfectly sealed by pushing the stopper, ensuring coupons are not touching the glass
of the tube. Place the tube upright in an aluminium tray and place the whole in the oven at 60°C for
28 days.
A control tube is made in exactly the same way, omitting any test material. One control tube is
needed for the batch. If any corrosion appears on control coupons, the tests should be repeated.
Notes
In order to avoid any contamination by absorption of harmful gases, such as acetic acid from wood,
the stoppers must be stored in an inert container.
Figure 2 Scheme of the '3 in 1' method.
To avoid any additional cleaning, paint or adhesive materials are applied to an inert sheet of
Melinex (grade 50 or 75 um) and inserted in the test tubes so that only the Melinex is in contact
with the glass of the tube.
APPENDIX 2: STATISTICAL COMPARISON BETWEEN THE '3 IN 1' AND STANDARD
TEST METHODS
The standardization of the test method in 1995 achieved greatly improved reproducibility; however,
100% agreement between laboratories was not attained. A statistical analysis was carried out. The
differences in results for the 53 samples tested between the '3 in 1' test and the standard test were
calculated. This was compared with the variation in results when several laboratories used the
standard test method on the same set of five materials. A simple root mean square deviation
(RMSD) statistic was used as a measure of the overall level of agreement for tests on a specific
metal.
In the 1995 inter-laboratory trial, d is the difference between the result and the mode of the set of
results. However, in the '3 in 1' study, d is the difference between the '3 in 1' result and the standard
test result. n is the number of tests on a given metal used for the inter-laboratory trial (50) or the
number of tests on materials reported above (53), respectively. The lower the RMSD, the better the
agreement between a set of results. Full details for calculating a similar statistic comparing the
laboratories' results with the mode results are given in the 1995 paper. For these calculations the numerical transformations of the categories were altered, with P=0, T=2 and U=4. The
RMSD results are shown in Table 1.
Considering the comparison between the '3 in 1' test and the standard test first, the RMSD is highest
for silver, indicating less agreement for this metal than for copper and lead, and showing the same
trend as the simple percentages reported earlier.
Comparing these RMSDs with those calculated from repeats of the standard test, all the statistics
are lower and in the case of copper and lead very markedly so. This indicates that the differences in
results likely to be introduced by adopting the new method are less than those presently inherent in
the standard test.
HEALTH AND SAFETY
Propanone (acetone) is extremely flammable and can cause serious damage on contact with the
eyes; it may cause dermatitis, and the vapour has an occupational exposure standard of 2400 mgnr3
under the UK COSHH (Control of Substances Hazardous to Health) regulation.
Lead is harmful if ingested and the dust has a maximum exposure limit of 0.15 mgm-3 under the UK
COSHH regulation. Because of the toxicity of the lead dust, the lead should be brushed in a fume
cupboard and the person should wear a dust mask and glasses.
Silver, copper and glass bristle dust are irritants.
SUPPLIERS
Centiclean Super Detergent: Century Oils Ltd, PO Box 2, New Century- Street, Hanley, Stoke-onTrent ST1 5HU, UK.
Acetone (Spectrosol); AnalaR lead foil, 0.13 mm; aluminium incubator trays; 50 universal bottles,
cat. no. 402/0683/00; small glass tubes ('Durham tube'), pack 250,25 X 6 mm, cat. no. 267/0035/01;
silicone rubber stopper no. 21, 21 X 24.5 x 28 mm, ref. 226/0100/ 21: VWR International Ltd,
Hunter Boulevard, Magna Park, Lutterworth LE17 4XN, UK.
Silver metal foil, 0.1 mm, ref. S/0880/90; copper metal foil, 0.1 mm, ref. C/7050/50: Fisher
Chemicals, Bishop Meadow Road, Loughborough LE11 5RG, UK.
Decon Acid Rinse, ref. B 29021: Philip Harris, Novara HS, Excelsior Road, Ashby Park, Ashby de
la Zouch LE65 ING, UK.
Silicone stopper, 21x27x30 mm, ref. V263027: Radleys, Shirehill, Saffron Walden CB11 3AZ, UK.
REFERENCES
1
Werner, A.E.A., 'Conservation and display: environmental control", Museums Journal 72
(1972) 58-60.
2 Oddy, W.A., 'An unsuspected danger in display'. Museum* Journal 73 (1973) 27-28.
3 Green, L.R., and Thickett, D., 'Interlaboratory comparison of the Oddy test' in Conservation
Science in the L'K, ed. N. Tennent. James & James Ltd, London (1993) 111-116.
4 Green, L.R., and Thickett, D., 'Testing materials for the storage and display of artifacts - a
revised methodology', Studies in Conservation 40 (1995) 145-152.
5 Tetreault, J., Sirois, J., and Stamatopoulou. E.. 'Studies of lead corrosion in acetic acid
environments1. Studies in Conservation 43 (1998)17-32.
6 Bramberger, J.A., Howe, E.G., and Wheeler, A., 'A variant Oddy test procedure for evaluating
materials used in storage and display cases', Studies in Conservation 44 (1999) 86-90.
7 Lee, L.R., and Thickett, D., Selection of Materials for the Storage or Display of Museum
Objects, British Museum Occasional Paper 111, British Museum Press, London (1996) 24-26.
AUTHORS
LAURIANNE ROBINET obtained
a degree in chemistry from the Institut Universitaire de Technologic
in Orsay. France, in 1997. She specialized in organic and analytical chemistry and completed a
bachelor degree in 1999, followed by a master degree, DESS instrumentation and physico-chemical
analytical methods, in 2000 at the Universite d'Orsay. She acquired her experience in
conservation through internships at the Laboratoire de Recherche des Musees de France, the
Laboratoire de Dynamique, Interaction et Reactivite (LADIR), and at the Canadian Conservation
Institute. She worked as a conservation scientist in the conservation research department of the
British Museum from 2000 to 2002. She is now working in the conservation and analytical research
department of the National Museums of Scotland where she is responsible for inorganic analysis.
Address: \ational Museums of Scotland, Chambers Street, Edinburgh EH1 1JF, UK. E-mail:
l.robinet@nms.ac.uk
DAVID THICKETT graduated
with a BA in natural sciences (chemistry) from Cambridge University in
1988. He worked for two years in industrial ceramics research before joining the British Museum in
1990. His research interests include preventive conservation, particularly the effects of internally
generated pollutants, metals, ceramics and stone conservation. He has recently joined the
collections care team at English Heritage as conservation scientist. He is undertaking PhD research
investigating the post-excavation changes to archaeological iron and is assistant co-ordinator of the
ICOM-CC Preventive Conservation Group. Address: English Heritage, 23 Sarile Row, London
W1S 2ET, UK. Email: david.thickett@english-heritage.org.uk
Resume — Le test de corrosion accelere a ete utilise durant presque 30 ans pour exclure les
materiaux corrosifs des vitrines d'exposition et des reserves. La methode presentee par Green et
Thickett en 1995 a ete modiftee pour pemtettre le test des trois metaux simultanement au sein d'un
test unique. La comparaison d'une grande variete de materiaux a revele que les differences entre
les resultats, causes par cette modification, sont mains importantes que les differences rencontrees
lors de la replication du test en utilisant le meme materiau avec la methode precedente. Les
problemes rencontres lors de la corrosion du test de reference du plomb ont ete etudies et la
methode a ete changee pour eliminer eel effet. La methode de nettoyage de la verrerie utilisee pour
le test a ete modifiee pour exclure I'acide chromique, un produit chimique dangereux.
Zusammenfassung — Seit nahezu dreifiig Jahren u'erden beschleunigte Korrosionstests dazu
veru>endete, die Verwenduno leicht korrodierender Materialien im Depot und im Museum
auszuschliefien. So berichteten Green und Thicket 1995 van einer Methode, die die simultane
Untersuchung van drei Metallen gleichzeitig erlaubt. Durch Vergleich einer grofien Anzahl van
Materialien konnte gezeigt werden, dass die Unterschiede der Ergebnisse bet dieser modifizierten
Methode geringer sind, als diejenigen, die bei Verwendung des gleichen Materials mit der
Wiederholung der Tests mil der ursprunglichen Methode einhergehen. Die Probleme auf die man
bei Tests zur Kontrolle der Bleikorrosion stofit, wurden untersucht und zur Venneidung dieses
Effekts daraus eine modifizierte Methode entwickelt. Um die gefdhrliche Chemikalie Chromsdure
als Reinigungsmittel der Testgefaffe zu ersetzen, wurde eine weitere Modiftkation der Testmethode
vorgeschlagen.
Resumen — Se han venido utilizando tests de corrosion acelerados durante cast 30 anos con elfin
de excluir ciertos materiales susceptibles de corrosion en elementos empleados en exposition y
almacenaje de museos. El metodo descrito par Green y Thickett en 1995 fue desarrollado para
evaluar simultdneamente, en un unico test, el comportamiento de tres metales. La comparacion de
una amplia variedad de materiales ha mostrado que las diferencias en los resultados obtenidos par
estas modificaciones son menores que las encontradas con tests en los que se use el mismo material
empleando metodos analogos previos. Se han investigado los problemas encontrados con la
corrosion en los test de control del plomo, y el metodo ha sido modiftcado con el fin de subsanar
este efecto. El metodo de limpieza de las cubetas de pmeba se ha modificado para excluir un
producto quimico venenoso: el dcido cromico.