ELECTROCHEMICAL STUDY OF SOIL CORROSION

10ο ΠΑΝΕΛΛΗΝΙΟ ΕΠΙΣΤΗΜΟΝΙΚΟ ΣΥΝΕΔΡΙΟ ΧΗΜΙΚΗΣ ΜΗΧΑΝΙΚΗΣ, ΠΑΤΡΑ, 4-6 ΙΟΥΝΙΟΥ, 2015.

ELECTROCHEMICAL STUDY OF SOIL CORROSION ON AN ANCIENT-LIKE TERNARY Cu-PbSn ALLOY

O. Papadopoulou, D. Daskalaki-Mountanou and P. Vassiliou Laboratory of Physical Chemistry, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou str., 15780, Athens

ABSTRACT This study focuses on the investigation of the electrochemical behaviour of an ancient –like Cu-Pb-Sn alloy in a series of electrolytes, simulating soil environment enriched in fertilizers. Two soil samples –with and without recent addition of fertilizers– collected from arable fields in Thessaly during autumn and spring respectively, two different types of fertilizers - F1 (30%N- 4%P weight) and F2 (19%N-14%P)-filtrates have been prepared in the laboratory. Fertilizer solutions of both types at various concentrations have also been employed for the study of the corrosion processes. Leaded bronze round coupons were submitted to potentiodynamic sweeps in all electrolytic solutions. A correlation between the chemical composition of the two fertilizers and the corrosion and passivation behavior has been attempted. In particular, the fertilizers with higher nitrogen content enhance passivation behaviour of the alloy, produce lower corrosion rates and favour the accumulation of corrosion products. INTRODUCTION Understanding the complex and heterogeneous corrosion processes involved in the degradation metal cultural heritage objects – both electrochemical and chemical – is fundamental for the conception of more focused and effective conservation protocols. The damage of outdoor exposed bronzes is strictly related to the nature of the pollutants and to their concentration on the metal surface. It is reported that a surface water film in contact with a heavily polluted atmosphere is a highly concentrated electrolyte. Also the soil can be considered as a concentrated electrolyte and, therefore, a highly corrosive environment, The unexcavated metal cultural heritage objects are endangered by active soil corrosion, especially since human interventions into nature from industrialisation and agriculture led to severe local changes in sites where ancient civilizations thrived and where the soil environment has been seriously affected by recent extensive agricultural activities. The addition of some fertilizers supposedly accelerates the metal corrosion processes and many studies have proved the correlation between particular corrosion products and the soil enrichment with modern fertilizers. So, as it is reported by Wagner, even if archaeological finds have rested for centuries in ‘soil archives’ in nearly stable condition (characterized by a slow but steady corrosive decay), today many cultural assets are endangered. Among the major causes of this dramatic change of burial conditions there are the acidification of soil and the intensive addition of phosphorus, sulphur and nitrogen fertilizers [1]. Nitrogen, phosphorus and potassium constitute the three primary nutrients in NPK chemical fertilizers, which have been widely used by farmers for decades. Chemical fertilizers and amendments bring NPK nutrients and other species to soils via soluble salts such as ammonium or potassium sulphate, calcium or ammonium phosphate, ammonium nitrates, phosphoric acid etc [2]. Copper phosphates are very rarely encountered corrosion products, primarily in association with buried bone and in arid climates. Copper nitrates are also relatively rare corrosion products, since the usual nitrate salts are all water soluble [3]. Very few case studies are available in literature concerning the impact of agricultural fertilizers on bronze corrosion [1, 2, 4, 5]. Lead sulphates and lead phosphates were among the uncommon corrosion products of a buried leaded bronze reported in [2]. Lead minerals are the least soluble of the metal-containing minerals. Standard Gibbs free energies of formation demonstrate that lead phosphate minerals are thermodynamically more stable than copper phosphates or calcium oxy-phosphate salts [6]. The principles of immobilisation of heavy metals in polluted soils are based on this property, where the transformation of toxic forms of lead into insoluble hydroxypyromorphite takes place [2]. This work investigates some aspects of the electrochemical corrosion of an ancient-like cast bronze in two different NPK fertilizers solutions of various concentrations and in fertilizer-enriched soil filtrates. The interaction of phosphate and nitrate compounds with soil constituents, their relative abundance in the solution and the role of alloying elements and dendritic segregation to the corrosion evolution are being discussed. The metal dissolution processes and the nature of the corrosion products are being described along with the electrolyte properties (concentration, pH and electrical conductivity).


EXPERIMENTAL Ancient Greek type leaded bronze cast [7], as the ancient craftsmen produced, was employed for this study. The alloy (LB), has a composition of 88% Cu, 8% Pb and 4% Sn and a microstructure similar to the one of ancient alloys. Two soil samples S1 and S2 –with and without recent addition of fertilizers respectively– collected from arable fields in Thessaly during autumn and spring respectively, and two different types of fertilizers - F1 (30%N- 4%P weight) and F2 (19%N-14%P)- have been employed for the production of several electrolytes in the laboratory. Four soil filtrates and four aqueous fertilizer solutions of each type (F1 and F2) at the same concentrations(0.2, 1, 5 and 10% w/v) were employed for the electrochemical tests. The soils were dried until they reached constant weight, then were grinded with a pestle in a ceramic mortar and sieved using a DIN4188 sieve of 0.8mm mesh.Two small soil samples were submitted to a quick test with the addition of 10% w/v HCl solution in order to identify their nature. No reaction was observed and can thus be categirized as acidic with aluminosilicate content. Another S1 soil sample was used for X ray diffraction(XRD) analysis by a BRUKER D8 ADVANCE diffractometer with a CuKa source. The produced soil filtrates were the following:  100g S1 added in 400mL of deionized water, heated at 60ο -70ο C for 30minutes and stirred, filtrated and diluted to a final volume of 500mL (S1)  100g S2 in a final filtrate of 500 mL (20% w/v soil), prepared following the same procedure (S2)  100g S2 and 5g F1 added in 400mL of deionized water, heated at 60ο - 70ο C for 30minutes and stirred, filtrated and diluted to a final volume of 500mL (S2F1).  100g S2 and 5g F2 in a final filtrate of 500 mL (20% w/v soil, 1%w/v fertilizer), prepared following the same procedure (S2F2) The pH and the conductivity of all solutions were measured. A GAMRY CMS 100 potentiostat and software were employed for the electrochemical sweeps. The setup consists of a three-electrode cell - with the alloy specimen surface as a working electrode(WE), an Ag/AgCl electrode as reference and a Pt wire as counter electrode. For each electrochemical test, the electrode surface was grinded from 500 up to 1500 gritt. The WE remained for 1000 seconds at open circuit (OC) conditions and then a Tafel curve was acquired for the potential range of open circuit potential (OCP) ± 300 mV, at a scan rate of 1 mV/s. Subsequently, the alloy was anodically polarized from OCP up to +600mV/OCP. The electrolyte volume was 500mL and all the potentiodynamic sweeps were conducted at scan rate of 1mV/s. The electrode was prepared for metallographic observations by grinding with silicon carbide abrasive papers ranging from 400 up to 2000 gritt and polishing with diamond pastes and alcohol-based lubricant, until a mirror like surface was achieved. Then, the polished surface was cleaned in ethanol in an ultrasonic bath for few minutes. Klemm’ s II reagent was produced for colour etching [8]. Optical Microscopy(OM) observations (in bright field and under polarized light) were carried out using a Leitz Aristomet metallographic microscope in order to study the metallurgical features and the corroded surface of the electrode after each test. The chemical analysis of the two fertilizer samples, as well as the of collected corrosion products, after the electrochemical tests, were performed by a FEI QUANTA 200 Scanning Electron Microscope (SEM) equipped with a tungsten filament and solid state back scattered electron detector. The instrument is coupled with Energy Dispersive Spectroscopy (EDS) for elemental analysis. The accelerating voltage of the incident electron beam was set to 25 kV. RESULTS AND DISCUSSION The metallographic study of the alloy testifies that the alloy has remained in the as cast condition exhibiting extensive dendritic micro-segregation, without subsequent metal working and annealing processes. The tint etchant reveals the coring of the dendrite branches, wich are enriched in copper and appear orange to yellow depending on the Cu content. The interdedritic areas appear grey and are rich in tin. Pb is immicible in the Cu-Sn phases and can be ssen in the form of dispersed black globules (Figure 1b). In Figure 1c, the α+δ eutectoid phase is visible locally, with a colour variation from green to pink around it. The presence of this phase indicates that in some areas the chemical composition of the Cu-Sn-Pb bronze exceeds 5% Sn due to the inhomogeneous chemical composition (macro-segregation) of the casting [9]. According to LB nominal chemical composition the tin content is 4% and therefore a single α phase is expected. The etched surface of the bronze electrode can be seen in Figure 1a. Large grain clusters are clearly distinguished, as well as the basaltic zone in the periphery form the central solidification zone. The XRD spectrum of the fertilized soil sample (S1) identfied peaks at 21.2, 22.5 and 27.1 degrees (2 Theta) attributed to aluminosilicate crystalline phases and several other peaks, characteristic of phosphate salts, ammonium sulphates and nitrates.


(b) (c) (a) Figure 2. (a) The LB electrode surface after etching (b) OM image (x100) depicting coring of the dendritic structure of the Cu-Sn-Pb alloy under polarized light (c) As a first indication of the corrosivity of the four studied soil filtrates, the corrosion rates were calculated from the initial Tafel curves of the alloy (Figure 2) and were compared to the electrolytes with the same fertilizer concentration (1%). The non-fertilized soil (S2) induces the highest corrosion rate and Icorr, while the lowest values are observed in case of S2F2. S1 and S2F1 Tafel curves seem to be a closer match - in terms of the contained fertilizer type. The filtrated fertilizer-soil mixtures have higher conductivity and lower pH than S1 fertilized soil, so it can be assumed that the fertilizer concentration of this collected sample is below 1%w/v. However, these findings can not be considered as indicative of the overall corrosion behaviour of the metal and a detailed investigation of the anodic behavior of the alloy was necessary in order to extract trustworthy conclusions. The anodic polarization curves are presented in Figure 3 and the calculated dissolutiion slopes along with pH and conductivity measurement of the fsoil filtrates are given in Table 1. As expected, the S1 filtrate (fertilized soil) is responsible for a more steep metal dissolution slope compared to that observed in S2 electrolyte. At higher potentials, though, S1 reaches earlier a passivation state. S2 produces a passivation plateau above +0.43 Volts vs OCP and corresponds to the lowest passivation current density among all . Between the two S2 soil-fertilizer filtrates (S2F1 and S2F2), S2F2 presents a tendency for severe metal dissolution very close to the OCP and the alloy passivates above +0.45V/OCP. On the other hand, S2F1 presents a more prolonged Tafel region, before the evolution of the metal dissolution process which starts approximately at +0.38 V/ OCP. The recorded dissolution slope is decreased but still very steep due to the high concentration of fertilizer compared to the collected fertilized soil. S2F1 curve does not exhibit a passivation behavior within the examined potential range. There is no obvious correlation with the measured pH and conductivities since these properties are very similar for S2F1 and S2F2. It is more likely that the relative mobilities of the dissolved ionic species (depending on the NPK fertilizer chemical composition) and the initial formation of insoluble phosphate corrosion products on the electrode surface determine the corrosion processes. S1

S2

S2F1

S2F2

F1 1%

F2 1%

-4,0

-3,0

0,300

Potential (V) vs Ag/AgCl

0,200 0,100 0,000 -0,100 -0,200 -0,300 -0,400 -0,500 -0,600 -9,0

-8,0

-7,0

-6,0

-5,0

-2,0

-1,0

Log Current Density (A/cm2)

Figure 2.Tafel curves of the different soil filtrates presented in a common graph with the 1% w/v fertilizer solutions


S1

S2

S2F1

S2F2

F1 1%

F2 1%

0,600

Potential (V) vs OCP

0,500

0,400

0,300

0,200

0,100

0,000

-0,100 -8,0

-7,0

-6,0

-5,0

-4,0

-3,0

-2,0

-1,0


Figure 3.Anodic polarization curves of the different soil filtrates ploted in a common graph with the 1% w/v fertilizer solutions versus OCP The properties of the fertilizer electrolytes – pH and conductivity values- and the electrochemical parameters calculated for each potentiodynamic test conducted on the leaded bronze are given in Table 2. Above 1%w/v a saturation effect can be assumed, since all the measured parameters have very small diffferences. The highest conductivity values correspond to low concntrations (0.2%). From the plots of Figure 4, it can be deduced that both fertilizers have almost identical conductivuties at all concentrations, F1 has higher pH at all examined concentrations but after a critical point F1 tends to decrease and F2 tends to increase. It is quite interesting that F2 produces higher corrosion rates at all concentrations except for 1%w/v where F1 and F2 provoke the same corrosion rates. The anodic polarization curves of each fertilizer type are depicted in Figures 5 and 6. With increase of electrolyte concentration above 1% the F1 curves exhibit a progressive peak evolution and a subsequent passive region following the Tafel region. At F1 5% and 10% a second passivation plateau is reached above +0.50V/OCP. The same trend can be observed in the case of the F2 solutions. Once again, the first passivation current densities increase with the fertilizer concentration. The rather unusual finding is the passivation effect observed for 0.2% F2 at +0.55V/OCP. It must be noted that 0.2% F1 and F2 curves have the same shape and characteristic regions with S2F1 and S2F2 soil filtrate curves respectively, where the fertilizer concentration is 1%. This testifies that the interaction of soil dissolved ions with the fertilizer ionic species hinders the passivating processes. Table 1. Properties of soil filtrates employed as electrolytes and calculated electrochemical parameters after potentiodynamic tests on LB alloy Soil Filtrates pH Conductivity (mS/cm) OCP (mV) Corrosion Rate (mm/year)

S1 7.0 0.58 -292.3 0.008

S2 7.0 0.48 -356.3 0.039

S2F1 5.5 9.22 -347.3 0.013

S2F2 5.3 7.78 -14.2 0.004

Ecorr (mV)

-251.7

-281.5

-283.4

-106.5

0.624

3.190

1.099

0.330

0.060

0.419

0.091

0.072

0.205

0.237

-

-

-6

2

Icorr (10 A/cm ) 1 anodic dissolution slope (V/decade) 2nd anodic dissolution slope (V/decade) st


(a)

(b)

(c) Figure 4. Plots of (a)pH , (b)electrical conductivity and (c) corrosion rate as a function of the % fertilizer concentration for the two fertilizer types The corrosion products formed on the bronze WE surface were observed after the potentiodynamic tests in 10% w/v F1 and F2 (Figures 7a,b,c). The F1 induced corrosion products are more dense and cover the entire metal surface (Figure 7a). The F2 produces less intense chemical attack. The dendrites were preferentially corroded as is shown in Figure 7b and locally, at the electrode edges, the corrosion patterns indicate severe attack (Figure 7c). The whitish crystalline compouns are probably a mixture Sn amorphous oxides and Pb phosphate salts, while the light blue-green compound are basic Cu phosphates. In thecase of F1, a blue green precipitate was collected for analysis by SEM-EDS. F2 induced corrosion products were detached with a carbon tape from the metal surface and were also analysed. The EDS results are presented in Table 3 together with the elemental chemical composition of the the two fertilizer powders. Due to the alloy dissolution involved in this electrochemical sweeps no matches with the stoichiometry of well crystallized copper and lead phosphate minerals reported in [2,3] were found. F2 patina has higher P content. Nitrogen corrosion compounds have not been detected due to their high solubility. Table 2. Properties of fertilzer solutions at various contrations and calculated electrochemical parameters after potentiodynamic tests on LB alloy Conductivity Corrosion Rate OCP Ecorr Icorr % w/w pH (mS/cm) (mm/year) (mV) (mV) (10-6A/cm2) Concentration of Fertilizer F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 Solution 6.3 5.9 254.00 240.00 0.007 0.018 -230.7 -9.2 -171.6 -287.2 0.582 1.468 0.2 6.0 5.1 10.70 10.10 0.010 0.011 -29.6 -1.8 -68.6 -77.4 0.821 0.875 1 6.0 5.0 41.20 41.50 0.016 0.059 -22.4 -9.8 -65.6 -57.2 1.282 4.835 5 5.5 5.2 74.00 72.90 0.018 0.086 -36.3 -28.0 -92.7 -57.4 1.461 7.062 10


F1 0.2%

F1 1%

-6,0

-5,0

F1 5%

F1 10%

S2F1

0,600


0,500 0,400 0,300 0,200 0,100 0,000 -0,100 -8,0

-7,0

-4,0

-3,0

-2,0

-1,0

0,0


Figure 5.Anodic polarization curves of the F1 electrolyte solutions at concentrations of 0.2, 1, 5 and 10% w/v ploted versus OCP F2 0.2%

F2 1%

F2 5%

F2 10%

S2F2

0,600


0,500

0,400

0,300

0,200

0,100

0,000

-0,100 -8,0

-7,0

-6,0

-5,0

-4,0

-3,0

-2,0

-1,0


Figure 6. Anodic polarization curves of the F2 electrolyte solutions at concentrations of 0.2, 1, 5 and 10% w/v ploted versus OCP


(a)

(b)

(d)

(c)

Figure 7. (a) OM image(x100 under polarized light) of the electrode surface corrosion products after the potentiodynamic sweeps in F1 10% electrolyte (b) and (c) OM images(x100 and x200 under polarized light) of the electrode surface corrosion products after the potentiodynamic sweeps in F2 10% electrolyte (d) the corroded LB surface after the tests in sweeps in F2 10% electrolyte

Table 2. % Atomic Chemical Composition of the two fertilizer types (F1 and F2) and of the bronze corrosion products after the potentiodynamic tests in F1 10% and F2 10% - EDS quantification results. Element

F1

F2

NK OK FK NaK MgK SiK PK SK KK CaK AlK CuK PbL SnL Total N/P ratio Cu/P ratio

38.0 52.5 0.9 0.1 0.3 0.2 2.4 2.8 0.1 2.7 100 15.6 -

25.2 59.2 1.2 0.2 0.5 0.4 8.5 4.3 0.1 0.5 100 3.0 -

F1 10% solution precipitate 73.5 0.4 1.2 0.2 0.6 0.5 0.6 21.4 0.9 100 109.3

F2 10% detached patina 83.4 0.2 8.5 0.1 0.9 6.5 0.2 100 0.8


CONCLUSIONS The simulation of electrochemical processes involved in the corrosion of bronze cultural heritage during burial in sites with heavily fertilized soils was attempted The ternary alloy, which was employed as WE for the conducted potentiodynamic tests – Tafel and anodic polarization- is a cast leaded bronze, without further working or thermal treatment. The extensive macro- and micro-segregation phenomena are revealed by metallographic observations after colour etching. Among the four different aqueous soil filtrates which were used as electrolytes, S2F2 can be ranked as the least corrosive medium, while S2 (the soil filtrate without fertilizers) induces the highest corrosion rate on the bronze electrode. The most crucial parameters for the alloy corrosion behaviour, at a certain concentration, appear to be the relative mobilities of the phosphate and nitrate anionic species rather than pH and conductivity, which do not present significant differences regarding the two fertilizers F1 and F2. The ternary bronze has a similar electrochemical behaviour in S1 and S2F1electrolytes, as can be observed by Tafel curves. Thus, the S1 soil could probably have been fertilized by a product with similar chemical composition to the F1(rich in nitrates) and its concentration can be estimated below 1 % w/v. The influence of the N/P ratio of the two fertilizer types was investigated by the comparison of F1 and F2 fertilizer aqueous solutions at four different concentrations (0.2, 1, 5 and 10% w/v). The F1 fertilizer – with increased nitrate content - enhances the alloy passivation behaviour and favours the accumulation of insoluble phosphate corrosion products while nitrate ions remain cannot precipitate as corrosion products due to their high solubility. For both types, the pH, conductivities and corrosion rate measurements imply that above 1% w/v the electrolytes become saturated in the dissolved ionic species and under those conditions chemical attack is favored against charge transfer. Non of the rarely reported nitrate and phosphate corrosion products found in literature were identified among the analyzed corrosion products of the potentiodynamic sweeps, because the products of a severe alloy dissolution process could not have maintained a fixed stoichiometry and in many cases are amorphous or poorly crystallized. However, the evolution of the precipitated compounds is based on epitaxial growth on the dendritic structures of the alloy substrate producing some characteristic corrosion patterns.

ACKNOWLEDGEMENTS The authors would like to thank Dr. Vassilis Stergiou for his motivation of this work and for the soil samples and fertilizers that he brought for the experimental setup.

REFERENCES [1]. Tommesani L., Brunoro G., Garagnani G. L., Montanari R. and Volterri R., Revue d’ Archeometrie, 21:131 (1997). [2]. Remazeilles C. and Conforto E., Stud. Conserv., 53:110 (2008). [3]. Scott D. A., Copper and Bronze in Art/ Corrosion-Colorants- Conservation, Getty Conservation Institute, Los Angeles (2002), pp. 241-251. [4]. Romanoff, M., Underground Corrosion, Washington, D.C, National Bureau of Standards, 1957, circ. 479. [5]. A.G.Nord, E. Mattsson and K. Tronner, (2005), Factors Influencing the Long-term Corrosion of Bronze Artefacts in Soil. [6]. La Iglesia A., Estudios Geol., 65:109 (2009). [7]. M.P. Casaletto, T. De Caro, G.M. Ingo, C. Riccucci, Appl. Phys. A: Mater. Sci. Process., 83:617 (2006). [8]. Vander Voort G. F., ASM Metals Handbook, Volume 9, pp.229-233. [9]. Scott D. A., Metallography and Microstructure of Ancient and Historic Metals, Getty Conservation Institute in association with Archetype Books, Singapore (1991).