A NOTE ON THE EFFICACY OF ETHYLENEDIAMINETETRA-ACETIC ACID DISODIUM
SALT AS A STRIPPING AGENT FOR CORROSION PRODUCTS OF COPPER
Khatibul Huda
Summary—The efficacy of ethylenediaminetetra-acetic acid disodium salt (EDTANa2) as a
stripping agent for the removal of corrosion products of copper has been studied by simulation
experiment. Six corrosion products of copper—malachite, nantokite, basic copper (II) chloride,
paratacamite, atacamite and brochan-tite—were synthesized. The interaction between the solid
metal, corrosion products and EDTANa2 was evaluated by estimating the amount of copper
dissolved in the EDTANa2, and calculating the rate of dissolution at various concentrations and
time. As Cu- EDTANa2 water-soluble complex EDTANa2 is a good stripping agent for the removal
of corrosion products of copper, without affecting the surface of the metallic copper. The rate of
dissolution of corrosion products in EDTANa2 depends on the properties and constituents of individual corrosion products and the concentration of EDTANa2. It is concluded from these findings
that the whole process is controlled by forces of diffusion.
Introduction
Corrosion products [1. 2] of copper or copper alloy artifacts can be removed by stripping agents [3].
for example, alkaline Rochelle salt, alkaline glycerol, sodium hexametaphosphate, citric acidthiourea complex, sodium sesquicarbonate and also ethylenediaminetetra-acetic acid disodium salt
(EDTANa2) [4-7]. Farnsworth [8] studied the effect of sodium hexametaphosphate on copper metal.
A study was conducted by Merk [9] on bronze and its corrosion products, using alkaline glycerol,
alkaline Rochelle salt, sodium hexametaphosphate and citric acid, to ascertain the extent of
dissolution of the corrosion products as well as the metal by a simulation experiment. Angelucci
[10] and Matteini [11] worked on copper metal and its oxide using Rochelle salt. Merk [12] also
conducted an experiment using the same reagents as the earlier study [9], with the addition of
benzotriazole. Sharma and Kharbade [13] studied the action of sodium hexametaphosphate and
tripolyphosphate on copper. Removal of nantokite (Cud) by sodium sesquicarbonate and its efficacy
were studied by Organ [14]. The behaviour of alkaline dithionite for the removal of nantokite was
also investigated by MacLeod [15] and Fox [16], who reported differing reactivity of the reagent.
The present study uses a simulation experiment to study the rate, extent and nature of dissolution of
copper from six different corrosion products and metallic copper at different concentrations of
EDTANa2 and exposure times. The efficacy of EDTANa2 as a stripping agent has been evaluated by
determining the rate and percentage of dissolution of different corrosion products of copper artifacts
and metal. The simulation technique has been adopted to avoid any possibility of damage to valuable artifacts during the research trial.
Method
Graduated glass-stoppered cylinders were labelled for different reaction times, corrosion products
and concentrations. Corrosion products and metal (0-4g) were weighed out into the cylinders, using
a Metlar analytical balance, and standardized [17. p. 321] ethylenediaminetetra-acetic acid disodium
salt solution (100ml) was carefully poured in. The vessels were closed with the glass stoppers and
left undisturbed during the entire experimental period. Corrosion products at the bottom of the
vessel started to dissolve and slowly mixed to the entire volume of the solution. After the
predetermined time-frame of the experiment, each vessel was turned upside down and shaken
gently, for homogeneous mixing of the dissolved material, and immediately filtered to remove any
suspended and insoluble matter.
Filtrates were buffered by acetic acid-acetate buffer to maintain pH5 [17. pp. 263-265]. Absorbance
of the buffered dissolved materials was measured in a UV-Vis Hitachi double beam spectrophotometer (Model No. 210), using a blank solution in the reference cell at a wavelength of
745nm. The total quantity of dissolved copper was estimated using a previously prepared
calibration curve. The entire experimental procedure was car-
ried out for different corrosion products and copper metal with (MM, 0-15M. 0-2M and 0-25M
concentrations of EDTANa2 during a time-frame of 30. 60, 120, 360, 720, 1440 and 2880 minutes
and at an ambient temperature of 30°C. The experiment was carried out in duplicate.
The amount of copper "t' (g) actually present in 0-4g of corrosion products and copper metal was
determined according to conventional methods [17. p. 380]. From the experimental and calculated
data, various plots were drawn (Figures 1-4)
Calculation
The rate of dissolution of powdered solid by a liquid reagent may be expressed by the equation
[18]:
where W is the weight of unreacted solid at time T. A is the surface area. C is the reagent
concentration and k is the rate constant. On solving the equation, the rate of dissolution is given by
a plot of W vs. T [19]. W can be calculated as:
where W0 = amount of corrosion product/metal taken, t = amount of copper in corrosion product/metal and Q = total amount of copper complexed with EDTANa2 during time T. The rate and
percentage of copper dissolution are also calculated according to Bond [20]:
Results and discussion
Ethylenediaminetetra-acetic acid disodium salt is a hexadentate complexing ligand and also a
reducing agent. Due to the presence of nitrogen atoms with a loan pair of electrons, it acts as a
strong corrosion inhibitor for metals [21. 22]. The rate of dissolution of corrosion products and
metal, in the form of powder and fine turnings respectively, in solutions of EDTANa2 in distilled
water are shown as plots of W vs. T. The experimental plots of W vs. T of all the experimental
corrosion products in various concentrations of the EDTANa2 solution produce non-linear plots. A
typical curve is shown in Figure 1. According to Begum et al. [19], this type of non-linear curve
indicates that the surface area of
Figure 1 W vs. T plot for atacamite at different EDTANa2 concentrations: (a) 0-1M, (bj 0-15M, (c)
0-20M, (d) 0-25M.
the corrosion products does not remain constant during dissolution. The dissolution may cause
opening of the metallic surface beneath the corrosion layer in an artifact at some point but other
sites may remain partially covered with the corrosion product. This discrepancy can be mitigated by
selecting a suitable reaction time during conservation treatment. Moreover, the metal surface may
not be affected by the EDTANa2 solution, as shown in Figure 2, since the W vs. T curve follows an
almost horizontal straight line for copper metal in different concentrations of EDTANa2, indicating
almost no reaction at any time. When W is plotted against log T. the results show straight lines for
almost all the corrosion products used in the dissolution experiment, in the range of concentrations
used. Typical curves are shown in Figure 3. It may be inferred that the surface area changes
inversely with time of dissolution.
According to North and Pearson [23], release of a quantity (Q) of metal ion is directly proportional
to the square root of the exposure time T, i.e., If the dissolution is governed by diffusion
Figure 2 W vs. T plot for metallic copper at different EDTANa2 concentrations: (a) 0-1M, (b) 015M, (c) 0-20M. (d) 0-25M.
Figure 3 W vs. log T plot for brochantite in different EDTANa2 concentrations: (a) 0-1M, (b) 015M, (c) 0-20M, Id) 0-25M.
processes, plots of Q vs. T½ show a linear character [22]. Q vs. T'½ plots for all the corrosion
products at different concentrations of EDTANa2 show an almost linear character, indicating that the
dissolution phenomenon is driven by the process of diffusion. A typical plot at 0-1M concentration
of EDTANa2 is shown in Figure 4. The rate of dissolution calculated according to Bond [20] also
indicates that an increase in concentration increases the rate of dissolution for the experimental
corrosion products (Table 1).
Conclusion
•
EDTANa2 solution in water is a good stripping agent for the removal of corrosion products of
copper and copper alloys without affecting metallic copper.
Figure 4 Plot of percentage dissolution of copper vs. T½ at 0-lM EDTANa, concentration: (a)
malachite, (b) nantokite, (c) basic copper(II) chloride, (d) paratacamite, (e) atacamite, (f)
brochantite, (g) copper metal.
Increase of concentration accelerates the elimination of corrosion products without affecting metal.
During removal of the corrosion products, surface area change is not homogeneous.
The dissolution process is governed by the force of diffusion.
Change of surface area of corrosion product is inversely proportional to time.
Different corrosion products do not dissolve at the same rate: it depends on the properties and
constituents of the corrosion product.
Appendix: experimental corrosion product, metal and reagents
Malachite (basic copper carbonate) CuCO3-Cu(OH)3, nantokite (copper(I) chloride) CuCl. basic
copper(II) chloride Cu(OH)Cl, paratacamite γ-Cu2(OH),Cl, atacamite S-Cu2(OH),Cl, brochantite
(basic copper sulphate) CuSO4 . 3Cu(OH)2 were prepared according to standard methods described
elsewhere [24—26]. Purity and identification were confirmed by X-ray diffractometry by
comparing the d-spacing value of the prepared corrosion product with the JCPDS card file. X-ray
diffractograms were taken in a JDX-8P Jeol X-ray diffractometer using nickel filtered CuKα
radiation at 30KV and 20mA and at a scanning speed of 2° (26) min~' with a chart speed of
20mm.mirr1. A detector counting rate of 2000cps was employed for good resolution. Copper metal
in the form of turnings was obtained from BDH (England). All other reagents were AnalaR (BDH,
England) or Extra Pure (Merck, Germany).
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Author
KHATIBUL HLDA is
working as a conservation chemist at Bangladesh National Museum. He graduated with Honours and obtained an MSc in chemistry from the University of Rajshahi,
Bangladesh. Later he studied for a PhD in chemistry at Dhaka University, Bangladesh. He was
trained in conservation science at ICCROM. His principal research interest is the application of
complexing ligands for the preservation of metals and alloys. Address: Department of Conservation
Laboratory, Bangladesh National Museum, Shahbag, Dhaka 1000, Bangladesh.
Resume—On a etudie I'efficacite du sel disodique de I'acide ethylenediamine tetracetique
(EDTANa7j en tant qu'agent complexant pour I'elimination des produits de corrosion du cuivre par
des experiences de simulation. Six produits de corrosion du cuivre - malachite, nantokite, chlorure
de cuivre (II), paratacamite, atacamite, et brochantite - ont ete synthetises. L'interaction entre le
metal solide, les produits de corrosion et I'EDTANa, a ete evaluee en estimant la quantite de cuivre
dissoute dans I'EDTA et en calculant le taux de dissolution d diverses concentrations et divers
temps. Comme le cuivre-EDTA est un complexe soluble dans I'eau, I'EDTA s'avere un bon agent
complexant des produits de corrosion du cuivre, sans affecter la surface du cuivre metallique. Le
taux de dissolution de produits de corrosion depend des proprietes et des constituants de chacun
des produits et de la concentration en EDTANa,. On peut conclure d partir de ces resultats que
Vensemble du processus est regi par des forces de diffusion.
Zusammenfassung—Die Effektivitat von Ethyiendiamintetraessigsdure-Dinat riumsal(EDTANa^j als Reinigungsmittel :ur Entfernung von Korrosionsprodukten des Kupfers wurde
durch Simultanexperimente untersucht, Sechs Korrosionsprodukte des Kupfers - Malachit,
Nantokit, basisches Kupfer-fU)-Chlorid, Paratacamit, Atacamit und Brochantit - wurden
synthetisiert. Die Wechselwirkung iwischen dem festen Metall. den Korrosionsprodukten und dem
EDTANay wurden anhand der Menge des von EDTANa^ komplex-ierten und gelosten Kupfers
bestimmt und daraus die Geschwindigkeit der Auflosung bei verschiedenen Kon:entrationen in
Abhangigkeit von der Zeit errechnet. Da Cu-EDTANa, ein wasserloslicher Komplex ist, 1st
EDTANa^ ein gutes Agens :ur Entfernung von Korrosionsprodukten des Kupfers, ohne daft die
Oberflache des Metalls dabei angegriffen wurde. Die Auflosungsrate der Korrosionsprodukte in
EDTANa^ hdngt von den ehemischen Eigenschaften der einielnen Korrosionsprodukte und von der
Konzentration des EDTANa7 ab. Es wurde daher gefolgert, dafi der Pro:efi durch Diffusionskrdfte
kontrolliert wird.
Resumen—La eficacia de la sal disodica del dcido etilendiamino tetraacetico (EDTANaj como un
agente decapante para remover los productos de corrosion del cobre ha sido estudiada con un
experimento de simulation. Se sintetizaron seis productos de corrosion del cobre: malaquita,
nantokita, cloruro bdsico de cobre(II), paratacamita, atacamita y brocatita. La interaction entre el
metal solido, los productos de corrosion y el EDTANa, se evaluaron estimando la cantidad de
cobre disuelto en el EDTANa, y calculando el nivel de disolucion a varias concentraciones y
tiempos. Como en el caso del complejo soluble en agua Cu-EDTANa}, el EDTANa^ es un buen
agente decapante para remover los productos de corrosion del cobre, sin afectar la superficie de
cobre metdlico. El nivel de disolucion de los productos de corrosion en EDTANa, depende de las
propiedades y constituyentes de los productos de corrosion individuales y de la concentration del
EDTANa^. Se concluye de estos resultados que el proceso completo es controlado par fuerzas de
difusion.