Corrosion parameters resultsTable 2 summarizes the key parameters affecting the corrosion and conservation of buried bronze statuettes, including moisture content, pH, chloride concentration, organic matter, and microbial induced corrosion (MIC). It also addresses the effectiveness of various preservation and conservation treatments under different soil conditions. Moisture content is a critical factor influencing the corrosion of buried bronze statuettes. High moisture levels in the soil facilitate electrochemical reactions that accelerate corrosion. Although the experimental results did not explicitly provide moisture content data, understanding its impact is essential for developing effective conservation strategies. Future studies should aim to include precise moisture content measurements. Improved moisture barriers and effective drainage systems can help mitigate moisture-induced corrosion. The pH level of the soil plays a significant role in the corrosion rates and mechanisms of bronze statuettes. Soils with varying levels of acidity or alkalinity can either accelerate or inhibit corrosion. The experimental results indicated varying pH levels, highlighting the importance of monitoring soil pH. Preservation strategies should include treatments that stabilize the soil pH to optimal levels for minimizing corrosion, such as the application of pH-adjusting agents. Chloride ions in the soil are known to accelerate the corrosion process. The experimental findings confirmed that elevated chloride concentrations significantly contribute to the corrosion of bronze statuettes. To combat this, the application of chloride-resistant coatings and barrier layers is recommended. These treatments can protect the metal surface from the aggressive action of chloride ions, extending the lifespan of the statuettes. Organic matter in the soil can have a dual effect on corrosion. High organic matter content was found to inhibit corrosion by limiting oxygen diffusion to the metal surface. However, the decomposition of organic matter can also produce sulfur compounds that may contribute to corrosion. The study identified brochantite and antlerite, linking their presence to sulfur pollutants from the decomposition of organic matter by sulfate-reducing bacteria. Utilizing organic coatings and treatments that protect the metal while allowing limited oxygen diffusion can be an effective conservation strategy. MIC, caused by the activity of microorganisms in the soil, can either accelerate or inhibit corrosion. The identification of specific compounds like brochantite and antlerite suggests a link to sulfur pollutants from microbial activity. This highlights the potential environmental impact of microorganisms on the corrosion process. Implementing biocidal treatments to control microbial activity in the soil can help manage MIC and protect the bronze statuettes from microbial corrosion. The effectiveness of conservation treatments, particularly wax or oil-based coatings, was found to vary with soil conditions. These treatments showed reduced efficacy in soils with high chloride concentrations. This underscores the need for tailored conservation approaches based on specific soil conditions. Conservation strategies should be developed to address the unique challenges posed by different soil types, ensuring optimal protection for the buried bronze statuettes. The impact of soil constituents on the corrosion of buried bronze statuettes is crucial for developing effective preservation strategies. By considering the effects of moisture content, pH, chloride concentration, organic matter, and microbial activity, and tailoring conservation treatments to specific soil conditions, the longevity and integrity of these cultural artifacts can be significantly enhanced.Table 2 Key parameters influencing corrosion and preservation strategies for buried bronze statuettes.Surface features and analysis of corrosion productsThe statuettes were created using the lost wax process as a single unit. They have a thick layer of soil deposits on their surface, which is accompanied by a greenish corrosion product that varies in intensity between shades of pale and dark green. Some of the statuettes have signs of corrosion, while others still have a significant amount of metal at their core. By using a portable digital microscope, it was discovered that certain areas of the statuettes, such as the head, uraeus, crook, and flail, have been eroded and obscured by the layers of corrosion. There are also sections of the crown and feet that are missing, which can be attributed to the severe corrosion that occurred during the burial process.Alloy microstructureLeaded bronze alloys are a type of bronze that contains a small percentage of lead in addition to copper and tin. The addition of lead to the bronze alloy improves its machinability and wear resistance. The microstructure of leaded bronze alloys consists of alpha phase. In a professional scientific illustration of the microstructure of leaded bronze alloy, the alpha phase can be visualized using a combination of optical and electron microscopy. The alpha phase is characterized by a copper-rich matrix with small tin-rich particles. Under optical microscopy, the alpha phase appears darker due to its higher copper content. The small tin-rich particles in the alpha phase can be seen as dark spots distributed throughout the copper-rich matrix55. Under electron microscopy, the microstructure of the leaded bronze alloy can be further explored in greater detail. The fine structure of the alpha phase can be seen, and the distribution of the lead particles can also be visualized. The lead particles can be observed as small dark spots within the copper-rich alpha phase. In summary, a professional scientific illustration of the microstructure of leaded bronze alloy would show the alpha phase, as well as the lead particles distributed within the alpha phase. Optical and electron microscopy can be used to visualize the different phases and provide a detailed understanding of the microstructure of the alloy56.Figure 3 provides a set of microscopic images that offer valuable insights into the corrosion patterns observed on the surface of the bronze statuettes after burial in different soil types. These images vividly display the presence of green and pale green corrosion products that have formed and spread across the statuette surfaces, all of which are incorporated within soil deposits. The varying shades of green observed in these images are indicative of different corrosion products. These products are frequently linked to the interaction between the bronze alloy and environmental elements, such as moisture, oxygen, and certain ions found in the soil. The corrosion of bronze generally results in the creation of copper-based compounds like brochantite and antlerite, which are characterized by their greenish color. The intensity of this green coloration can act as a measure of the extent of corrosion. The presence of pale green areas suggests ongoing or less advanced corrosion, while darker green regions may indicate more advanced and extensive corrosion. The incorporation of soil deposits over the surface of the statuettes is another critical observation from these images. This phenomenon highlights the complex interaction between the bronze surface and the surrounding soil. The deposition of soil particles on the bronze surface can have a multifaceted impact on the corrosion process. On the one hand, it can create a physical barrier that limits exposure to environmental factors, potentially slowing down the corrosion rate. On the other hand, the soil itself can contain corrosive agents, such as chlorides or sulfides, which, when in contact with the bronze surface, may accelerate corrosion. These microscopic images underscore the importance of studying not only the corrosion products themselves but also their distribution patterns and the influence of soil interactions. Understanding the specific compounds formed and their spatial distribution can provide valuable information about the corrosion mechanisms at play. In summary, Fig. 3 reveals the visual evidence of corrosion products with varying degrees of advancement, as well as the intriguing interplay between these products and the soil deposits covering the bronze statuettes. This information contributes to a more comprehensive understanding of the corrosion processes affecting these cultural artifacts in different soil environments.Figure 3Microscopic images show (A–F) green and pale green corrosion incorporated with soil deposits covering the surface.Figure 4 provides metallographic images that are crucial for the examination of the alloy microstructure of the bronze statuettes. This microstructural analysis is particularly important for understanding the material composition and potential changes that occur because of burial in different soil environments. In the images, we observe both un-etched and etched samples of the bronze statuettes. The distinction between un-etched and etched samples is a standard practice in metallography, used to reveal different features of the microstructure. Etching, in this context, involves applying a chemical reagent to the sample, which selectively reacts with certain microstructural constituents, making them more visible under the microscope. (A-B) Un-etched sample and etched sample of the statuette. These images are particularly valuable for comparing the microstructure of the bronze alloy in its natural state (un-etched) with the same alloy after etching. The un-etched sample (A) shows the as-received microstructure, providing insight into the initial condition of the alloy. In this image, we can observe the grain boundaries and any inherent features in the alloy structure. The etched sample (B) highlights certain microstructural details more clearly by darkening certain features, revealing characteristics that may not be as apparent in the un-etched sample. (C-D) Un-etched sample and etched sample of the plume, Similar to (A-B), these images focus on the plume section of the bronze statuette. The un-etched sample (C) displays the natural microstructure of this specific region. Etching the sample (D) enhances the visibility of microstructural details, enabling a more comprehensive analysis of the plume’s alloy composition. These metallographic images are a valuable tool for researchers studying the effects of burial and corrosion on the bronze alloy. By comparing the un-etched and etched samples, scientists can identify changes in the microstructure that may be indicative of corrosion, material degradation, or chemical interactions with the burial environment. Furthermore, these images can aid in assessing the effectiveness of conservation and restoration efforts. If the etched samples show significant alterations in the microstructure, it may suggest the need for specific conservation strategies to address these changes and preserve the statuettes effectively. In summary, Fig. 4’s metallographic images offer a closer look at the alloy microstructure of the bronze statuettes, providing a foundation for understanding how burial and corrosion impact the material composition and structural integrity of these cultural artifacts.Figure 4Presents the metallographic images (A, B) show the alloy microstructure of the statuettes of the un-etched sample and etched sample and (C, D) show the alloy microstructure of the statuettes of the un-etched sample and an etched sample of the plume.Figure 5 provides scanning electron microscopy (SEM) images that offer valuable insights into the alloy microstructure of the bronze statuettes. SEM is a powerful tool for examining surface morphology and microstructural features at high magnification, making it particularly useful for the analysis of materials like bronze. (A) Alloy microstructure of the plume, In Fig. 5A, we observe an SEM image showcasing the alloy microstructure of the plume, a specific component of the bronze statuette. This microstructural analysis is essential for understanding the condition and composition of this region. The SEM image reveals the topography and surface features of the plume, offering a close-up view of its structure. Key information that can be gleaned from this image includes the distribution of phases within the alloy, grain boundaries, and any signs of corrosion or degradation. The plume’s alloy microstructure is a critical aspect to consider when assessing the overall condition of the bronze statuette. Changes in this microstructure could indicate the impact of burial, environmental factors, or conservation efforts. (B) Alloy microstructure of the statuettes, Fig. 5B presents an SEM image of the alloy microstructure of the statuettes, focusing on a broader view of the statuette’s surface. This image provides a comprehensive look at the alloy’s composition and structural characteristics, which are essential for understanding the overall condition of the statuettes. Similar to the plume, this image allows for the examination of features like grain boundaries, crystallographic structure, and any indications of corrosion or surface alterations. It’s a critical component in assessing the impact of burial and environmental factors on the statuettes. These SEM images serve as a foundation for detailed material analysis. Researchers can use them to identify changes in the alloy microstructure caused by corrosion, material degradation, or chemical interactions with the burial environment. They also help in evaluating the effectiveness of conservation and restoration efforts. If there are significant alterations or signs of corrosion in the microstructure, conservation strategies can be tailored to address these specific issues. In summary, Fig. 5’s SEM images provide valuable visual data for understanding the alloy microstructure of both the plume and the statuettes. These images are instrumental in the study of how burial and environmental factors influence the microstructural integrity of these bronze artifacts.Figure 5Presents the SEM image showing the alloy microstructure of (A) the plume and (B) the statuettes.Alloy chemical compositionThe chemical composition of the alloy was analyzed using SEM/EDS, and the results are presented in Table 3 and Fig. 6. The alloy is composed of 73.49% copper, 9.96% tin, and 15.36% lead. The corroded surface was also analyzed using non-destructive in situ pXRF, and the results are presented in Table 4. The analysis revealed that the alloy used in manufacturing the statuette contained copper ranging from 71 to 76%, tin ranging from 2.5 to 4%, and lead ranging from 10.5 to 14%. The soil components on the surface of the statuette were found to be high in silicon, magnesium, and calcium, while phosphorus, aluminum, and iron were present in very low amounts. These soil elements are commonly found in the weathering coatings of ancient bronzes buried in the soil57. The presence of chloride elements may be attributed to dissolved minerals in the surrounding environment. Elemental mapping for copper, tin, and lead in the metal core of the statuette is presented in Fig. 6, which shows that the lead globules are dispersed throughout the alloy structure. EDS and XRF analysis confirmed that the manufacturing alloy of the statuette is leaded bronze, a type of alloy commonly used for making statues in the late period of Egypt58.Table 3 EDS analysis of different spots in the statuettes’ alloy.Figure 6Shows the sites of the SEM image and the corresponding EDS analysis of the statuette alloy.Table 4 XRF results of different spots in the feather, sun disk, and the statuettes (wt%).Table 3 presents the results of Energy-Dispersive X-ray Spectroscopy (EDS) analysis conducted on different spots in the alloy of the statuettes. EDS is a powerful analytical technique used to determine the elemental composition of a material by measuring the characteristic X-rays emitted from the sample when it is bombarded with electrons. In this context, EDS is employed to gain insights into the composition of the bronze alloy at various locations on the statuettes’ surface. Here, Table 3 provides the weight percentages (wt%) of four key elements. Iron (Fe), Copper (Cu), Tin (Sn), and Lead (Pb). These elements are primary constituents of bronze alloys, and their presence and distribution can reveal crucial information about the alloy’s composition. In spot A: Iron (Fe): 1.19%, Copper (Cu): 73.49%, Tin (Sn): 9.96% and Lead (Pb): 15.36%. In spot B: Iron (Fe): 1.26%, Copper (Cu): 28.19%, Tin (Sn): 2.30% and Lead (Pb): 68.25%. In spot C: Copper (Cu): 34.89%, Tin (Sn): 39.83%, and Lead (Pb): 25.28%. In spot D: Iron (Fe): 0.15%, Copper (Cu): 85.51%, Tin (Sn): 12.03% and Lead (Pb): 2.31%. Table 3 reveals a significant variation in the alloy composition at different spots on the statuettes. Notably, the percentages of copper, tin, and lead vary considerably among the analyzed spots. This heterogeneity can provide insights into the alloy’s manufacturing process, regional differences in composition, or localized corrosion effects. The presence of iron in the alloy, even in relatively small amounts, is indicative of impurities in the bronze. Iron is not a typical component of bronze alloys and can affect their properties, including corrosion resistance. The presence of iron in spots A and D is worth further investigation. Spot B exhibits the highest lead content, comprising 68.25% of the alloy composition. Such a high lead content suggests that this particular spot may have been intentionally alloyed or contaminated with lead. The reasons for this localized high lead content should be explored. Copper and tin are the primary components of bronze alloys. In spots C and D, the copper content is significantly higher than tin, indicating variations in the copper-to-tin ratio. These variations could affect the alloy’s properties and corrosion susceptibility. The variations in alloy composition may correlate with the observed corrosion patterns on the statuettes. Differences in composition can influence the formation of corrosion products and, subsequently, the preservation challenges. In summary, EDS analysis, as presented in Table 3, highlights the heterogeneity and impurity levels within the bronze alloy of the statuettes. These variations in composition could have implications for the corrosion patterns and preservation strategies. Further investigations into the sources of these variations and their impact on the statuettes’ condition are warranted.Table 4 provides the results of X-ray Fluorescence (XRF) analysis for various elements (expressed in weight percentages, wt%) in different parts of the Feather, Sun Disk, and the Statuettes. XRF is a non-destructive analytical technique used to determine the elemental composition of materials, and in this context, it helps in understanding the chemical composition of these archaeological artifacts. Let’s discuss the significance of these findings. In the sun disk, the Sun Disk exhibits a high copper (Cu) content of 70.869%. This high Cu content is consistent with the bronze material commonly used in ancient artifacts, as Cu is a primary component of bronze. Lead (Pb) is also significantly present at 13.792%, indicating potential lead alloying or contamination, which could affect the artifact’s properties and corrosion patterns. In feather, the Feather, compared to the Sun Disk, shows a lower Cu content at 75.418%. This difference in copper content may suggest variations in the alloys used for different parts of the artifact. The Feather exhibits higher tin (Sn) content at 4.137% compared to the Sun Disk, which is an interesting variation and may influence corrosion behavior. In statuettes, similar to the Feather, the Statuettes show a substantial Cu content at 74.921%, reinforcing that bronze, rich in copper, is the primary material. The Statuettes have a relatively high chlorine (Cl) content at 1.99%, which may be associated with burial conditions or previous conservation efforts involving chloride-containing compounds. Regarding the common elements Magnesium (Mg), aluminum (Al), silicon (Si), and calcium (Ca) are present in relatively similar amounts in all three parts of the artifact, likely indicating shared compositional elements. By comparing feathers and statuettes, the Feather exhibits a slightly higher tin (Sn) content compared to the Statuettes. This difference in Sn content might suggest variations in the alloy compositions used for different parts of the artifact. All parts of the artifact contain a high copper content, consistent with bronze composition. Copper is a key component of bronze and plays a significant role in determining its properties. The presence of lead (Pb), chlorine (Cl), and arsenic (As) may require further investigation, as these elements can impact the corrosion and degradation of artifacts. In summary, the XRF results in Table 4 offer valuable insights into the elemental composition of different parts of the artifact. Variations in the composition, particularly in the content of copper and other elements, may provide clues about the alloying, manufacturing techniques, or environmental conditions to which the artifact has been exposed. These findings are important for understanding the artifact’s corrosion behavior and for making informed decisions regarding its preservation and conservation. Further research is needed to interpret these findings in the context of the artifact’s history and archaeological significance.Corrosion products analysisFigure 7b and Table 5 exhibit the principal substances resulting from corrosion that were detected through X-ray diffractometry. Meanwhile, Fig. 7a and Table 6 display the mineral composition of the soil. The XRD analysis revealed that the soil sample examined is composed primarily of quartz (91%), with a small amount of albite (9%). As stated above, pXRF analysis of the soil deposits covered the statuettes showing soil components such as Si, Ca, Mg, P, Fe, and Al. The cultivated lands are examined in the Nile valley, and they found that it is a predominated sandy silt soil type59. The gained corrosion products of a particular soil are often attributable to several soil properties that interact to make the soil corrosive to buried copper. If the agricultural usage of the land ceases, the addition of lime may also be stopped, reducing the pH level from 6 to 5 in clay soils, or to about 4–4.5 in sandy soils. At the same time, draining wetlands may cause dramatic acidification of soils owing to the amounts of sulfur compounds that would be released and the resulting sulfide oxidations. The main corrosion products below the sandy encrustation layer consist of cuprite (Cu2O), atacamite (Cu2(OH)3CI), Cerussite PbCO3, and Massicot PbO, as presented in Fig. 7b. The existence of significant quantities of copper oxychlorides in the deterioration results would be expected for bronze statuettes that had been buried in soil with a high chloride content (Megahed et al., 2022). In environments rich in chloride, cuprous chloride (known as nantokite or CuCl) can form at the interface between the external patina and the metal that still exists. When nantokite is exposed to oxygen and water in the environment, it turns into atacamite and other phases, which then react with copper to create new cuprous chloride and water. This is a continuous process where copper, chlorine, oxygen, and water transform into cuprite and atacamite. The presence of copper oxychlorides is particularly hazardous in corrosion as it makes the metal surface unstable. The deterioration of the statuettes showed high levels of lead content, which were indicated by the presence of lead (II) oxide (massicot or PbO) and carbonate (cerussite or PbCO3)60.Figure 7(A) XRD results of the Ehnasya soil sample and (B) XRD analysis of the corrosion products.Table 5 XRD results of Ehnasya soil sample.Table 6 XRD results of the corrosion products.Table 5 presents the X-ray Diffraction (XRD) results of a soil sample from Ehnasya, providing information about the minerals found within the sample. XRD is a powerful analytical technique used to identify the mineralogical composition of geological materials. In this specific soil sample, two minerals have been identified, and their respective semi-quantitative percentages are given. Let’s discuss the significance of these findings. Quartz (SiO2), quartz is the dominant mineral in the Ehnasya soil sample, constituting a significant semi-quantitative percentage of 91%. Quartz is one of the most common minerals in the Earth’s crust and is known for its high resistance to weathering and erosion. Its presence in the soil indicates a source of silica and suggests that the soil may be derived from rocks rich in quartz. Albite (Na(AlSi3O8)), albite is also identified in the soil sample, comprising a semi-quantitative percentage of 9%. Albite is a plagioclase feldspar, a group of rock-forming minerals commonly found in igneous and metamorphic rocks61. Its presence in the soil suggests that the soil may have originated from rocks containing plagioclase minerals. The XRD results for this soil sample provide insights into its mineralogical composition. The dominance of quartz, a highly stable mineral, suggests that the soil may have undergone minimal alteration or weathering processes. The presence of albite indicates the potential influence of rocks rich in feldspar minerals in the soil’s formation. These findings are significant for various reasons. Understanding the mineralogical composition of the soil is crucial for geological and environmental studies. It can provide insights into the soil’s origin, geologic history, and potential sources of sediment. The presence of specific minerals can impact the soil’s properties, including its ability to retain water, nutrient content, and erosion resistance. In the context of archaeological or conservation research, knowledge of the soil composition around buried artifacts can be essential. It can affect the corrosion behavior and preservation of artifacts, as different minerals can have varying effects on the surrounding environment. In summary, the XRD results presented in Table 5 contribute to a better understanding of the mineralogical composition of the Ehnasya soil sample, which has implications for both geological studies and potential implications for archaeological and conservation efforts in the region.Table 6 provides X-ray Diffraction (XRD) results for the corrosion products found on the examined bronze statuettes. Table 6 lists the identified minerals, their chemical formulas, and the semi-quantitative percentages of each mineral. These results are crucial for understanding the nature of the corrosion and the chemical reactions taking place on the statuettes. Let’s discuss the significance of these findings. Cuprite (Cu2O), cuprite is a significant component of the corrosion products, constituting a semi-quantitative percentage of 40.24%. Cuprite is a copper oxide mineral that often forms as a corrosion product on copper and copper alloy artifacts62. It is typically reddish-brown and is a common product of copper corrosion in the presence of oxygen and moisture. Atacamite (Cu2Cl(OH)3), atacamite is another copper-containing mineral identified, making up a substantial portion with a semi-quantitative percentage of 32.19%. Atacamite is a chlorine-rich copper mineral often found in association with cuprite on corroded copper surfaces. Its presence indicates the influence of chloride ions on the corrosion process. Cerussite (PbCO3), cerussite, a lead carbonate mineral, is also identified as part of the corrosion products, with a semi-quantitative percentage of 18.14%. Cerussite can form a corrosion product on lead-based artifacts and is often associated with the presence of carbon dioxide and carbonate ions. Massicot (PbO), massicot, a lead oxide mineral, is present in the corrosion products, although to a lesser extent, with a semi-quantitative percentage of 9.43%. Massicot typically forms as a result of the oxidation of lead and is one of the oxidation products of lead corrosion. The XRD results in Table 6 reveal the composition of the corrosion products on the bronze statuettes. These minerals are typical corrosion products associated with copper and lead-based alloys. Their presence provides important insights into the corrosion processes that have occurred63. The presence of cuprite and atacamite suggests that the corrosion of the bronze statuettes is predominantly copper-based. Cuprite is often formed when copper reacts with oxygen, while atacamite can result from the interaction of copper, chloride ions, and water. The identification of cerussite and massicot indicates lead-based corrosion products. Lead corrosion often results in the formation of lead oxide and carbonate compounds. Understanding the specific corrosion products on the statuettes is valuable for several reasons. Knowing the composition of the corrosion products is essential for developing effective conservation strategies to stabilize and protect the artifacts. The presence of these minerals can provide insights into the environmental conditions and burial history of the statuettes. These findings contribute to the broader field of materials science and corrosion science, aiding in the understanding of metal degradation processes. In summary, the XRD results in Table 6 shed light on the composition of the corrosion products on the bronze statuettes. The identification of cuprite, atacamite, cerussite, and massicot helps researchers and conservators make informed decisions about the preservation and restoration of these valuable cultural artifacts64.Electrochemical behavior of bronze in the soilBronze is an alloy composed of copper and tin, with the addition of other elements such as zinc, lead, and nickel. It has been widely used for various applications, including art, architecture, and coins, due to its unique properties such as durability, ductility, and corrosion resistance. However, when exposed to soil, bronze can undergo various electrochemical reactions that affect its behavior and integrity. In soil, bronze is subjected to various electrochemical reactions due to the presence of different ions, such as chloride, sulfate, and bicarbonate65. These ions can interact with the bronze surface and cause electrochemical corrosion, leading to the formation of a patina layer on the surface. The patina layer is a complex mixture of copper compounds, such as cuprite, tenorite, malachite, and azurite, and provides a protective barrier that prevents further corrosion. Another electrochemical reaction that can occur with bronze in the soil is galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte, and a potential difference is created, leading to the transfer of electrons from the anode to the cathode. This reaction can occur with bronze and other metals, such as iron or steel, resulting in the corrosion of the more active metal. Furthermore, soil composition, moisture content, and pH can affect the electrochemical behavior of bronze. For example, acidic soils can increase the rate of corrosion of bronze, while alkaline soils can cause bronze to form a protective layer more rapidly. In conclusion, the electrochemical behavior of bronze in the soil is complex and depends on various factors such as soil composition, moisture content, and pH. While bronze is generally resistant to corrosion, it can undergo various electrochemical reactions that can affect its integrity and durability in soil66.Weight loss measurementsThe relationship between weight loss measurements and corrosion of leaded bronze in the soil is like that in marine environments. In soil, corrosion of leaded bronze can occur due to factors such as the moisture content, pH level, and chemical composition of the soil. Weight loss measurements can be used to assess the degree of corrosion in leaded bronze in soil by measuring the difference in weight of the material before and after exposure to the soil environment. In soil environments, weight loss measurements can be used to monitor the corrosion of leaded bronze, and to determine the appropriate maintenance and protection measures necessary to prevent further corrosion. For example, if weight loss measurements indicate that the leaded bronze is corroding at a faster rate than expected, it may be an indication that the soil environment is more corrosive than anticipated, or that the corrosion protection measures in place are not adequate67. In some cases, leaded bronze may be buried in soil as a part of the infrastructure, such as pipelines or buried structures. The exposure of leaded bronze to soil can cause localized corrosion, and weight loss measurements can be a useful tool to assess the degree of corrosion and determine appropriate maintenance measures see Fig. 8a,b. In addition, weight loss measurements can be used to compare the corrosion resistance of different materials in soil environments. Overall, weight loss measurements can be used to assess the degree of corrosion of leaded bronze in soil environments and to guide maintenance and protection efforts to prevent further corrosion. These measurements can help to ensure the longevity of leaded bronze infrastructure buried in soil and to help prevent environmental damage and potential safety hazards that can result from corroded infrastructure. Figure 8a represents an increasing tendency for weight reduction but Fig. 8b, represents the lower rate of corrosion of leaded bronze after being buried in soil for a certain period and these results are matched with the obtained equation from Fig. 8b.Figure 8Inhibition efficiency and average corrosion rate of the bronze alloy during exposure in the soil for different concentrations of inhibitor mixture (benzotriazole BTA 3% in alcohol, then final protection was carried out using two layers of paraloid PA 3% in alcohol) (A) inhibition efficiency and (B) average corrosion rate. Electrochemical measurements of bronze alloy different concentration from conservation mixture in corrosive soil (C) open‐circuit potential, (D) Potentiodynamic polarization curves.Open circuit potential and polarization curvesFigure 8c shows the open circuit potential curves of leaded bronze after exposure to different levels of an inhibitive mixture. The corrosion potential shifts towards the positive direction until reaching a concentration of 2%. After this concentration, increasing the mixture concentration causes the corrosion potential to shift back toward the negative direction. This indicates that the optimal concentration for the coating mixture is 2%. In Fig. 8d, the potentiodynamic polarization curves of leaded bronze after exposure to different concentrations of the inhibitive mixture are displayed. The current density and Tafel slope were calculated from fitting curves. There were some differences observed, such as a positive shift in the corrosion potentials and a decrease in the current density with increasing levels of the inhibitive mixture. The corrosion current density can be used to evaluate the corrosion rate, which suggests that the corrosion rates decrease with increasing amounts of the inhibitive mixture. On the anodic branch, bronze exposed to different concentrations showed significant disintegration, while linear Tafel areas were observed on both the anodic and cathodic branches68.Electrochemical impendence spectroscopyElectrochemical impedance spectroscopy (EIS) is a powerful technique for characterizing the electrochemical behavior of materials. When applied to leaded bronze in soil, EIS can provide valuable insights into the corrosion behavior of this material in its environment. Leaded bronze is an alloy of copper, tin, and lead that is commonly used in applications such as statues, monuments, and building facades. When exposed to soil, leaded bronze can undergo corrosion, which can lead to aesthetic and structural damage. EIS can be used to study this corrosion process by measuring the electrical impedance of the material as a function of frequency. In EIS, a small amplitude alternating current is applied to the material, and the resulting voltage response is measured69. The impedance of the material is then calculated as the ratio of the voltage response to the applied current. By varying the frequency of the applied current, EIS can provide information about the capacitive and resistive components of the material’s impedance. When applied to leaded bronze in soil, EIS can provide information about the formation and growth of corrosion products on the surface of the material. For example, as the material corrodes, a layer of copper oxide may form on the surface. This layer can act as a barrier to further corrosion, but it can also increase the capacitive component of the material’s impedance. By analyzing the impedance data as a function of frequency, researchers can gain insights into the formation and growth of this oxide layer, as well as the overall corrosion behavior of the material in the soil70. Overall, EIS is a valuable tool for studying the electrochemical behavior of leaded bronze in soil. By providing information about the impedance of the material as a function of frequency, EIS can provide insights into the corrosion behavior of the material and can be used to develop strategies for mitigating this corrosion. In Fig. 9, EIS plots of leaded bronze in different exposure periods during an immersion test in soil with 20 wt% water content is compared at open circuit potential (OCP). The EIS plots show similar features, with a low-frequency loop indicating a semi-infinite diffusion process and a high-frequency loop with a high-phase shift induced by Cu/CuSO4 and/or the potentiostat in low soil conductivity. The high phase shift is commonly observed in soil corrosion systems and other porous media systems, such as bronze and aged bronze covered with corrosion products. The equivalent electrical circuit in Fig. 9D can be used to characterize the corrosion system71. In electrochemical impedance spectroscopy (EIS), an equivalent circuit is used to model the electrochemical processes occurring at the electrode–electrolyte interface. The elements in this circuit represent different physical phenomena. Here’s an explanation of the common electrical elements that might be proposed in an equivalent circuit, such as the one depicted in Fig. 9D. Rs represents the resistance of the electrolyte solution between the working electrode and the reference electrode. It accounts for the ohmic drop in the solution and is usually depicted as a resistor in the equivalent circuit. Rct represents the resistance to the transfer of electrons across the electrode/electrolyte interface72. A higher Rct indicates a slower rate of electrochemical reactions, which often corresponds to a less corrosive environment or more protective corrosion products. Cdl presents the capacitance of the electrical double layer formed at the interface between the electrode surface and the electrolyte. It accounts for the storage of charge in the double layer and is influenced by factors such as electrode surface area and electrolyte composition. The Q or CPE is used to represent non-ideal capacitive behavior, often due to surface roughness, inhomogeneities, or porosity of the electrode. Instead of a perfect capacitor, the CPE accounts for a more complex, frequency-dependent impedance. In the context of Fig. 9D, Rs (Solution Resistance) could be represented by the initial resistor at the beginning of the circuit. Rct (Charge Transfer Resistance) and Cdl (Double Layer Capacitance) are often in parallel to represent the charge transfer process at the interface and the capacitance of the double layer. Q (Constant Phase Element) might be used instead of a pure capacitor to model the non-ideal capacitive behavior. The combination of these elements helps to simulate the impedance response of the system and interpret the underlying electrochemical processes influencing the corrosion behavior of the statues.Figure 9EIS curves of bronze alloy different concentration from conservation mixture in corrosive soil, (A) Plots of Nyquist diagrams, (B) Plots of Bode plots, (C) Plots of phase fitted results, (D) The obtained equivalent circuit by fitting the results.Table 7 presents the fitting results for leaded bronze after exposure to red soil with different loads of inhibitive mixture, using the reciprocals of Rct to represent the corrosion rate. The corrosion rate of leaded bronze decreases with inhibitive mixture loads, indicating that the formation of an oxide layer could inhibit corrosion, consistent with the results in Fig. 7b. The Rf value of the leaded bronze in the soil increases with exposure time, indicating that new corrosion products are generated and absorbed on the sample surface, enhancing the defensive impact of the oxide layer on bronze73.Table 7 Electrochemical parameters and inhibition efficiency for bronze in soil containing different concentrations of inhibitive mixture.Table 7 presents a comprehensive overview of the electrochemical parameters and inhibition efficiency for bronze corrosion in soil containing varying concentrations of an inhibitive mixture. This data is essential for evaluating the performance of the inhibitor mixture and understanding how its concentration affects corrosion behavior. Let’s discuss the significance of the findings. Inhibitive Mixture Concentrations (Conc., %), Table 7 includes data for different concentrations of the inhibitive mixture, ranging from 1 to 2%, along with a blank (no inhibitor) for reference. Open-Circuit Potential (OCP) vs. CCE, OCP provides insights into the thermodynamic aspects of the corrosion process. More positive (less negative) OCP values indicate a thermodynamically more stable state. The OCP becomes less negative (i.e., more positive) with increasing inhibitor mixture concentration. This shift suggests that the inhibitive mixture has a positive effect on the corrosion potential, making the system more corrosion resistant. Corrosion potential (Ecorr) vs. CCE; Ecorr represents the potential at which the corrosion rate is at its lowest74. A less negative value suggests a more noble state, indicating lower corrosion activity. Ecorr values become less negative with increasing inhibitor mixture concentration. This shift indicates a reduction in corrosion activity, further supporting the inhibitory effect. Anodic (ba) and Cathodic (bc) Tafel Slopes (V dec−1), Tafel slopes provide information about the reaction kinetics. Lower values suggest faster electrode reactions. The anodic and cathodic Tafel slopes decrease with increasing inhibitor mixture concentration, indicating that the inhibitive mixture facilitates both anodic and cathodic reactions. Corrosion Current Density (Icorr) (A cm−2), Icorr represents the rate of corrosion. Lower values indicate reduced corrosion rates. Icorr decreases significantly with increasing inhibitor concentration, highlighting the inhibitor’s effectiveness in reducing the corrosion rate. Corrosion Rate (C.R.) (mpy—mils per year), C.R. is a practical measure of how quickly the material is corroding75. Lower values indicate slower corrosion. The corrosion rate drops substantially with higher inhibitor mixture concentrations, demonstrating the inhibitor’s capacity to protect the bronze from corrosion. Inhibition Efficiency (I.E.) (%), I.E. provides a percentage measure of the inhibition’s effectiveness, with higher percentages indicating better protection. The inhibition efficiency increases as the inhibitor mixture concentration rises, with the highest concentration achieving the greatest inhibition. Polarization resistance (RP) (Ohm cm2), polarization resistance is related to the ease of charge transfer at the metal-electrolyte interface. Higher RP values signify increased resistance to corrosion. RP increases as the inhibitor concentration goes up, suggesting a decrease in the rate of charge transfer and enhanced corrosion resistance76. The data in Table 7 clearly demonstrate the inhibitory effect of the inhibitive mixture on bronze corrosion in soil. As the inhibitor mixture concentration increases, various electrochemical parameters, including OCP, Ecorr, Tafel slopes, Icorr, corrosion rate, inhibition efficiency, and polarization resistance, all exhibit trends that indicate improved corrosion protection. Understanding how the inhibitive mixture’s concentration affects these electrochemical parameters is essential for optimizing corrosion protection strategies for bronze artifacts buried in the soil. The data provides valuable insights for conservators and researchers working on the preservation of archaeological and cultural heritage objects. In summary, the results presented in Table 7 underscore the effectiveness of the inhibitive mixture in mitigating bronze corrosion, with higher concentrations of the inhibitor mixture leading to improved corrosion protection and inhibition efficiency77.Characterization of surface morphologyLeaded bronze is an alloy composed of copper, tin, and lead. When leaded bronze is buried in soil, it can undergo various physical and chemical changes that affect its surface morphology. The characterization of surface morphology can provide insight into the degree of degradation and corrosion that has occurred. One method of characterizing surface morphology is through visual inspection and microscopy78. By examining the surface of the leaded bronze under a microscope, it is possible to identify changes in texture, color, and shape. Scanning electron microscopy (SEM) can provide high-resolution images that show the details of surface features and any corrosion products that may have formed. Another method is X-ray diffraction (XRD) analysis. This technique can be used to identify the mineral phases present on the surface of the leaded bronze. XRD can also reveal the degree of oxidation or corrosion that has occurred. Additionally, atomic force microscopy (AFM) can be used to measure the roughness of the surface of the leaded bronze. This method can provide detailed information on surface topography and roughness, which can be used to track changes in surface morphology over time79. Overall, the characterization of surface morphology can help to understand the mechanisms of degradation and corrosion of leaded bronze in soil environments and guide the development of preservation strategies to mitigate these effects. Figure 10 displays the physical characteristics of how weathering affects bronze surfaces80. The pictures demonstrate the noticeable changes in surface morphology from an untreated surface to a surface that has undergone corrosion, as seen in Fig. 10a. Upon exposing the surface to an inhibitive mixture, a protective layer covers the surface and prevents any signs of corrosion, as mentioned in Fig. 10b. As the treated sample is exposed to topsoil for a specific duration, the surface layers gradually bond to the metal surface, and there are no cracks or pits visible on the surface, as shown in Fig. 10b. Observing the corrosion morphology shows the variation in the corrosion process in soil, from non-uniform corrosion to pitting, and then to chloride and sulfide attack. The number and depth of pits increase over time, with the pits expanding laterally and the corrosion process dominated by the creation of new pits and pit development in the depth direction81.Figure 10Raman spectral analysis of spot No. 1 in the green patina.Micro-Raman spectroscopy of the green patina
Figures 10, 11 and 12 display the Raman spectra of various locations within the patina cross-section, revealing corrosion products that are identified based on the Raman bands specified in Tables 6, 7 and 8. The Raman spectrum analysis in Fig. 10 indicated the presence of brochantite (Cu4(SO4)(OH)6) through three distinct bands; a highly prominent band at 1110 cm−1, a strong peak at 300 cm−1, and a moderate peak at 245 cm−182,83,84. Additionally, Raman spectroscopy detected the existence of two chloride phases in the green patina, comprising botallackite and paratacamite. Botallackite (Cu2(OH)3Cl) is characterized by distinctive bands at 460 and 890 cm−185,86 and Paratacamite (Cu2(OH)3Cl) at 725, 890 cm−1. Figures 11 and 12 presented two distinct sulfate compounds observed in the green patina. The findings indicated the presence of brochantite (Cu4(SO4)(OH)6) with prominent bands identified at (235, 290, 300, 420 cm−1) and several moderate bands at (415, 1070, 1265 cm−1). Additionally, antlerite (Cu3(SO4)(OH)4) was detected with characteristic bands at (290, 900, 420, and 845 cm−1)87. Brochantite is rarely documented in buried archaeological bronzes88. The presence of both brochantite and antlerite could be linked to sulfur pollutants formed due to the decomposition of organic matter by sulfate-reducing bacteria in the soil89.Figure 11Raman spectral analysis of spot No. 2 in the green patina.Figure 12Raman spectral analysis of spot No. 3 in the green patina.Table 8 Raman spectral analysis of spot No. 1.Table 8 presents the results of Raman spectral analysis conducted on spot No. 1, showcasing the composition, and detected Raman bands along with relevant references. This analysis is crucial for identifying the specific corrosion products and minerals present, shedding light on the corrosion processes and environmental conditions. Let’s delve into the discussion of these findings. Brochantite (Cu4(SO4)(OH)6), Raman bands detected at 245, 300, and 1110 cm−1 are consistent with the presence of brochantite. These Raman bands agree with previous studies, confirming the identification of brochantite in spot No. 1. Brochantite is a copper sulfate hydroxide mineral commonly formed in the corrosion process of copper-based materials. Botallackite (Cu2(OH)3Cl), Raman bands detected at 460 and 890 cm−1 indicate the presence of botallackite90. These findings align with previous research, supporting the identification of botallackite in spot No. 1. Botallackite is a copper hydroxy chloride mineral formed as a corrosion product when copper is exposed to chloride-containing environments. Paratacamite (Cu2(OH)3Cl), Raman bands detected at 725 and 890 cm−1 suggest the existence of paratacamite. These findings are consistent with previous studies, confirming the presence of paratacamite in spot No. 191. Paratacamite is another copper hydroxy chloride mineral formed in chloride-rich corrosion environments. The Raman spectral analysis of spot No. 1 reveals the coexistence of brochantite, botallackite, and paratacamite, all of which are corrosion products commonly associated with copper and bronze artifacts exposed to various environmental conditions. The detection of these minerals provides valuable information about the corrosion history of the bronze statuettes and the specific environmental factors at play. The identification of these minerals contributes to a deeper understanding of the corrosion mechanisms affecting the bronze statuettes92. This knowledge is essential for conservation efforts, as it guides the development of appropriate strategies for preserving and protecting these valuable artifacts. Additionally, it aids in assessing the environmental conditions at the burial sites, which can inform future archaeological investigations. In summary, the Raman spectral analysis in Table 8 offers valuable insights into the corrosion products present in spot No. 1 of the bronze statuettes, highlighting the complex interplay between the artifacts and their burial environment.Table 9 presents the results of the Raman spectral analysis conducted on spot No. 2, providing insights into the composition of the corrosion products and minerals detected in this area. Here is the discussion of the findings. Brochantite (Cu4(SO4)(OH)6), Raman bands detected at 235, 290, 415, and 1070 cm−1 are indicative of the presence of brochantite. These Raman bands align with the characteristics associated with brochantite, as reported in prior studies96,97,98. Brochantite is a copper sulfate hydroxide mineral often formed as a corrosion product when copper-based materials are exposed to sulfide-rich environments. Antlerite (Cu3(SO4)(OH)4), Raman bands detected at 290 and 900 cm−1 suggest the existence of antlerite. These findings are consistent with the Raman bands reported for antlerite in previous research99,100,101,102. Antlerite is another copper sulfate hydroxide mineral that can form as a corrosion product in the presence of sulfur compounds. The Raman spectral analysis of spot No. 2 confirms the presence of both brochantite and antlerite. These minerals are commonly associated with copper and bronze corrosion products and are formed in specific environmental conditions, particularly those influenced by sulfur compounds103. The identification of brochantite and antlerite in spot No. 2 contributes to a more comprehensive understanding of the corrosion history of the bronze statuettes. It indicates that these areas were likely exposed to sulfur-containing compounds, which influenced the formation of these specific corrosion products. This knowledge is essential for conservation efforts and helps guide strategies for preserving the artifacts effectively. In summary, Table 9’s Raman spectral analysis provides valuable insights into the composition of corrosion products in spot No. 2 of the bronze statuettes. The presence of brochantite and antlerite underscores the complexity of the corrosion processes and highlights the significance of environmental factors in the degradation of these artifacts104.Table 9 Raman spectral analysis of spot No. 2.Table 10 provides the results of Raman spectral analysis for spot No. 3, shedding light on the composition of corrosion products and minerals detected in this specific location. Here is the discussion of the findings. Brochantite (Cu4(SO4)(OH)6), Raman bands detected at 300, 420, and 1265 cm−1 are indicative of the presence of brochantite. These Raman bands correspond with the characteristic vibrations associated with brochantite, consistent with previous research105,106. Brochantite is a copper sulfate hydroxide mineral commonly formed as a corrosion product when copper-based materials are exposed to sulfide-rich environments. Antlerite (Cu3(SO4)(OH)4), Raman bands detected at 420 and 845 cm−1 suggest the existence of antlerite. These findings align with the Raman bands associated with antlerite, as reported in previous studies107. Antlerite is another copper sulfate hydroxide mineral that forms as a corrosion product, especially in the presence of sulfur compounds. The Raman spectral analysis of spot No. 3 confirms the presence of brochantite and antlerite. These minerals are typical corrosion products associated with copper and bronze artifacts, and their presence suggests exposure to sulfur compounds or sulfide-rich environments, contributing to their formation108. The identification of brochantite and antlerite in spot No. 3 provides insights into the corrosion history of the bronze statuettes. It indicates the role of environmental conditions, particularly those influenced by sulfur-containing compounds, in the formation of these specific corrosion products. This information is valuable for developing conservation strategies tailored to the artifacts’ unique corrosion patterns. In summary, Table 10’s Raman spectral analysis enhances our understanding of the composition of corrosion products in spot No. 3 of the bronze statuettes. The detection of brochantite and antlerite underscores the impact of environmental factors on the degradation of these artifacts and informs conservation efforts.Table 10 Raman spectral analysis of spot No. 3.Conservation treatmentLeaded bronze artifacts buried in soil can undergo corrosion and degradation over time, making conservation techniques essential for their preservation. Here are some different techniques for conserving buried leaded bronze in soil. The first step in conserving buried leaded bronze is to excavate the artifact carefully and remove any soil and debris that may be present on its surface109. This can be done using hand tools, brushes, and water. Chemical treatments can be used to remove corrosion products and stabilize the metal. For example, tannic acid can be used to remove surface corrosion, while ethylenediaminetetraacetic acid (EDTA) can be used to remove deeper corrosion products. Applying a coating to the surface of the bronze can help to protect it from further corrosion. One option is microcrystalline wax, which can be applied using a brush or sprayer. Electrochemical methods can be used to remove corrosion products and reduce the amount of corrosion occurring on the surface of the bronze110. For example, electrolysis can be used to remove corrosion products while also depositing a protective layer of metal ions onto the surface of the bronze. Finally, controlling the environment in which the artifact is stored or displayed can help to prevent further corrosion. This can involve controlling temperature, humidity, and exposure to light, among other factors. Figure 13c, d show the statuettes after undergoing conservation and treatment procedures. To remove soil deposits and powdery corrosion, brushes, needles, and scalpels were used during the cleaning process. A 5% citric acid solution was then applied to eliminate corrosion products, including Cu+ and Cu+2 oxidization results such as Cu2(OH)3Cl, Cu2(OH)2CO3, CuO, and Cu2O. In addition, citric acid can form stable complexes with Cu (II) compounds, according to MacLeod (1987)111. The objects were then rinsed with flowing water followed by distilled water to remove any excess acid and then dried in an oven at 50 °C. After cleaning and rinsing, the statuettes were treated with a 2% benzotriazole solution in ethanol. This solution acts as a physical barrier between CuCl and moisture from the atmosphere, which could activate the CuCl and cause bronze disease, according to Sease (1976)112. Finally, two layers of 2% paraloid in ethanol were applied to provide further protection. The objects were then stored in sealed polyethylene food storage boxes with silica gel to eliminate humidity from the air inside the storage container, following MacLeod’s (1987) instructions89.Figure 13SEM Microscopic morphologies of bronze samples, (A) Before treatment, (B) After treatment and the statuettes after treatment processes, (C) Front side, (D) Back side.
Understanding soil factors in corrosion and conservation of buried bronze statuettes: insights for preservation strategies
