Impact of types of water source on pipeline corrosion: a comprehensive electrochemical and microscopic analysis

With the significant increase in population and increasingly scarce urban water resources, a single water source can no longer guarantee an adequate water supply. Many cities frequently switch water sources and adopt multiple water sources for water supply (Lin et al. 2021). In China, the government is vigorously promoting rural drinking water safety projects, leading to an increasing number of single-village water supply projects. Internationally, governments provide financial and technical support to rural areas, promoting single-village water supply projects to improve local living standards. However, in rural areas, especially mountainous regions, heavy rainfall can degrade the quality of surface water and increase turbidity, threatening water supply safety and sometimes necessitating a switch from surface water to groundwater sources.

Due to the differences in water quality between surface water and groundwater, these switches may cause sudden changes in the water quality within existing pipelines, disrupting the original chemical equilibrium between the water and pipe walls and leading to the establishment of a new equilibrium (Qin et al. 2022). This process can affect the corrosion of raw water pipelines and impact water safety (Ouyang et al. 2020; Saraswat et al. 2020; Singh et al. 2020). Switching is feasible when two water sources have similar water quality, while a gradual switch with appropriate mixing ratios is needed when there are large differences in water quality.

Moreover, due to the complex chemical and biological composition of corrosion products in the pipeline system, water quality issues remain a concern (Hu et al. 2018). The complex relationship between water source switching, pipeline corrosion, and water quality changes highlights the importance of studying the mechanism of pipeline corrosion under water source conversion to ensure the safety of raw water pipelines (Verma et al. 2019). There are three main types of switching in water source transportation: switching between local water sources in the same area (e.g., surface water, groundwater, and reservoir water); water diversion between river basins (e.g., the South-to-North Water Diversion Project); and the development of unconventional water sources to replace conventional water (e.g., seawater desalination projects) (Hasson & Bendrihem 2006; Hilbert 2006; Service 2006; Wang et al. 2023, 2024).

Switching water sources typically introduces corrosive anions such as sulfate and chloride ions, along with changes in pH, alkalinity, and hardness. These factors lead to pipeline corrosion through chemical or electrochemical reactions between the metal and substances in the water (Ansari et al. 2020; Ramkumar et al. 2020). Most research on ductile iron pipeline corrosion focuses on the use of tap water as the electrolyte solution, with comparatively less attention given to raw water (Zhang et al. 2010). Over the past few decades, pipeline corrosion mechanisms have gained global research interest. While corrosion studies on drinking water and reclaimed water pipelines are relatively comprehensive, research on raw water pipelines remains in its early stages (Tang et al. 2006).

Some researchers have examined the effects of water source switching on metal pipeline corrosion and proposed prevention measures. However, there is a lack of electrochemical experiments using electrochemical workstations to study the corrosion process (Hu et al. 2018). Therefore, this study aims to deeply analyze the corrosion mechanism using electrochemical techniques, scanning electron microscopy (SEM), X-ray diffraction (XRD), and other methods, to offer theoretical support for controlling corrosion in raw water pipelines and propose a more suitable raw water transport method.

Due to the complex chemical and biological composition of corrosion products in pipeline systems, maintaining water quality remains challenging. The relationship between water source switching, pipeline corrosion, and water quality changes highlights the need to study corrosion mechanisms during water source transitions. By using electrochemical analysis, such as SEM, XRD, and X-ray photoelectron spectroscopy (XPS), this study seeks to provide a detailed understanding of the corrosion process, offering theoretical support for improving and optimizing rural water supply systems.

The study investigated the impact of water source conversion on pipeline corrosion utilizing advanced biofilm reactors. One set of reactors housed test specimens previously exposed to surface water (Group G1), whereas another set housed those exposed to groundwater (Group G2). The transition of G1 to groundwater and G2 to surface water was simulated within the study. Throughout the 14-day experiment, the reactors were sealed, light-shielded, and maintained at a constant flow rate, with continuous water introduction and daily sampling. Key water quality parameters, including alkalinity, hardness, conductivity, total dissolved solids (TDS), dissolved oxygen (DO), pH, turbidity, and total iron concentration, were meticulously measured. Electrochemical analyses, encompassing continuous polarization curve tests and alternating current impedance spectroscopy, were conducted to monitor corrosion progression. Post-experiment assessments utilized SEM, XRD, and XPS to elucidate morphological, structural, and compositional changes in the test specimens. Additionally, water source conversion strategies were examined by mixing various ratios of surface and groundwater (1/9, 3/7, 5/5, 7/3, and 9/1) and introducing these mixtures into pipelines historically utilized for mixed water transport. A similar 14-day monitoring regimen of routine water quality parameters and electrochemical testing was conducted to evaluate the impact of varying water source proportions on water quality and corrosion.

Semi-corroded test pieces were utilized in this experiment to simulate the material of the original water pipeline. The ductile iron test pieces conformed to Type I standard corrosion specimens, with dimensions of (50.0 ± 1/9) mm × (25.0 ± 1/9) mm × (2.0 ± 1/9) mm (Ansari et al. 2020). Before use, the specimens were sequentially polished with 400#, 800#, and 1,200# water sandpaper. The polished test pieces were then soaked in acetone to remove oil stains, rinsed with anhydrous ethanol, and air-dried before being placed in sealed bags for later use. The cement mortar formula followed the Chinese standard Cement Mortar Coating for Ductile Iron Pipes and Fittings GB/T17457-2019. To simulate the actual pipeline environment as closely as possible, one side of the specimens was coated with epoxy resin. After preparation, the specimens were placed in reactors containing surface water, groundwater, and mixed water for a total of 12 months to simulate the transportation of different water qualities in old pipelines.

The two water sources used in the experiment were divided into two groups. One group represented surface water sources, while the other group represented groundwater sources. The water quality standards were referenced from Groundwater Quality Standards (GB/T14848-2017) and Surface Water Environmental Quality Standards (GB3838-2002). The water quality of the two sources is shown in Supplementary Table S1. The mixed water was prepared according to different proportions.

A three-electrode system consisting of a silver chloride reference electrode, a platinum auxiliary electrode, and a working electrode was employed for open circuit potential measurements, polarization curves, and electrochemical impedance spectroscopy (EIS) using a Gamry workstation and software. SEM was utilized to examine microstructural changes in post-switching samples, following a preparation process that included dehydration, freeze-drying, metal coating, and analysis with an FEI QUANTA FEG250 microscope. XRD analysis was conducted to determine the phase composition of powder samples, involving freeze-drying, scraping, grinding, sieving, and spreading the powder on glass slides for testing with a Thermo K-Alpha diffractometer, using a cobalt target to minimize fluorescence radiation and analyzing results with Jade software and the PDF2004 card database. Additionally, XPS was used to analyze the phase composition of powder samples, following similar preparation steps, and utilizing a Thermo K-Alpha X-ray diffractometer, with calibration using the C1s method and analysis conducted with Avantage software (Supplementary Table S3). The conventional methods for testing water quality indicators described in this article follow the relevant provisions of the Drinking Water Quality Testing Methods (GB/T 5750-2006) and the Standard Methods for the Examination of Water and Wastewater (4th edition). For detailed information, please refer to Supplementary Table S3.

Figure 2(c) and 2(d) displays consistent variations in conductivity and TDS, with conductivity approximately twice that of TDS. Initially, both values increased due to two primary factors: the influx of ions from the new water source and intensified pipeline corrosion, resulting in the release of divalent iron ions and other ions, thereby rapidly elevating conductivity and TDS levels. Substantial changes occurred within the first 5 days, succeeded by stabilization.

Figure 2(e) indicates a downward trend in DO within the initial 5 days post-switch, followed by a subsequent rise from the 6th day onwards. Initially, the alteration in water quality prompted intense pipeline corrosion, resulting in the consumption of DO in the cathodic reaction of the electrochemical process. As corrosion progressed, a stable corrosion scale layer formed, suppressing the anodic process and indirectly weakening the cathodic reaction, consequently reducing DO consumption and causing a gradual increase in DO (Marcus et al. 2008). Figure 2(f) demonstrates a rapid increase in pH value during the initial stage, peaking on the 8th day before gradually declining. The initial rise could be attributed to the electrochemical corrosion of iron, where Fe lost electrons to become Fe²⁺ at the anode and dissolved oxygen gained electrons to form OH⁻ at the cathode. Some Fe²⁺ reacted with OH⁻ to form precipitates, thereby neutralizing it. Bases and other anions, such as and ⁠, reacted with Fe²⁺, producing OH⁻ and increasing pH. Over time, the reduction in dissolved oxygen reaction decreased OH⁻ formation, while Fe²⁺ reacted with OH⁻ to form Fe(OH)₂, resulting in a decline in pH.

Figure 2(g) and 2(h) indicates that turbidity and total iron changes in G1 were more pronounced than in G2. During the initial 8 days post-switch, both G1 and G2 exhibited higher total iron concentrations and turbidity in the effluent, with maximum iron concentrations of 5/5 and 1.5 mg/L, and maximum turbidity of 2.5 and 9 NTU, respectively. Following this period, both measures stabilized at lower levels, signifying that the initial drastic alteration in water quality led to iron release within the pipeline, thereby increasing effluent turbidity and resulting in the ‘yellow water’ phenomenon (Li & Du 2022). Subsequently, the above phenomenon was ultimately eliminated as the persistent water supply and stabilized water quality.

As shown in Figure 4, both G1 and G2 exhibited negative shifts in the polarization curves during the initial stages of water source switching when water quality was unstable. Over time, as the pipelines adapted to the new environment, the polarization curves shifted in both positive and negative directions, indicating a gradual decrease in corrosion tendency (Rios et al. 2013). Figure 4(a) demonstrates that the G1 pipeline, which initially transported surface water but was switched to groundwater, experienced decreased sulfate and chloride ion concentrations and increased alkalinity and hardness (Melo et al. 2022). Initially, the polarization curve showed a negative shift, with a significant positive shift occurring after 5 days. Table 1 reveals that during the first 5 days, the anodic Tafel slope (βa) of the G1 pipeline was lower than the cathodic Tafel slope (βc), indicating predominant anodic dissolution reactions. This suggested rapid iron corrosion and a high anodic corrosion rate, producing a substantial amount of Fe²⁺, which reacted with OH⁻ to form iron hydroxides. After 5 days, the anodic Tafel slope (βa) increased and surpassed the cathodic Tafel slope (βc), indicating a weakening anodic corrosion effect and a shift toward cathodic depolarization. The corrosion current density also decreased over time as a corrosion product film formed on the metal surface, inhibiting further corrosion by preventing Fe²⁺ transfer into the main water. In the later stages, the cathodic Tafel slope (βc) showed a minimal change, indicating a stabilization of corrosion tendency once the pipeline adapted to the new environment. Figure 4(b) illustrates that the G2 pipeline, which historically transported groundwater but switched to surface water, experienced increased sulfate and chloride ion concentrations. During the early stages of switching, the corrosion potential changed significantly over time. As the pipeline adapted to the new water quality, the polarization curve shifted positively, and the corrosion tendency diminished (Rybalka et al. 2021). Table 1 shows that throughout the experiment, the anodic Tafel slope (βa) remained greater than the cathodic Tafel slope (βc), suggesting that for G2, corrosion primarily occurred through cathodic depolarization, with significant hindrance to anodic corrosion reactions. This difference in anodic and cathodic changes was attributed to the distinct corrosion scale layers formed in the original water quality environments of G1 and G2. Table 1 also indicates that after 10 days, the change in the cathodic Tafel slope (βc) was minimal, reinforcing that corrosion tendency stabilized once the pipeline adapted to the new environment.

The Nyquist plot in Figure 8(a) showed a single capacitive loop decreasing in size from the 3rd day, indicating reduced anti-corrosion performance due to water quality changes. By the 5th day, the loop's radius was minimal, reflecting peak corrosion from environmental instability within the pipeline. Shedding of corrosion products and physical and chemical changes decreased impedance and accelerated wall corrosion. As corrosion products accumulated, stabilization began by the 7th day, and the loop radius increased until the 11th, 13th, and 15th days, a second capacitive loop appeared in the high-frequency region as the corrosion product film stabilized.

Table 2 presents EIS data fitting results for the 3rd, 5th, 7th, 9th, and 11th days using the R_s (Q_dlR_ct) circuit model (Supplementary Figure S1). For both G1 and G2, R_s values were low on the 3rd and 5th days due to the drastic aquatic environment changes from the water source conversion, leading to increased ion concentration and activity, reduced solution resistance, and stronger corrosion. After 5 days, as pipelines adapted, ion concentration stabilized, R_s values increased, indicating reduced corrosivity. Q_dl remained stable and low, and the dispersion coefficient (n) ranged between 5/5 and 1, indicating a rough test piece surface. R_ct first decreased then increased, with the lowest value on the 5th day, marking the weakest anti-corrosion ability. By the 11th day, R_ct values peaked, indicating improved anti-corrosion stability.

Table 3 shows fitting results for the 13th and 15th days. Over time, R_s values increased, with G1 having larger R_s than G2 due to better water quality, lower ion concentrations, and weaker ion activity, resulting in larger R_s values and less corrosive action. Both pipelines showed increasing R_f values due to corrosion products covering the test piece surface. R_ct values also increased and remained higher than R_f, indicating charge transfer as the main corrosion control step. Both CPE1 and CPE2 stayed low with minimal variation, and n1 values ranged between 5/54 and 5/55, while n2 increased, suggesting a denser and more protective corrosion scale layer (Moradi et al. 2022). The impact of water source changes on pipeline corrosion behavior was demonstrated by this analysis, with valuable insights into the electrochemical processes and the effectiveness of corrosion products in stabilizing and protecting the pipeline over time being provided by EIS.

The corrosion products of iron primarily consisted of iron oxides and hydroxide oxides, including (α-, β-, γ-) FeOOH, Fe₂O₃, and Fe₃O₄. γ-FeOOH (lepidocrocite) had an unstable structure and was prone to reduction reactions, forming loose, reddish-brown crystals (Yang et al. 2012). Under the microscope, these appeared fibrous, scale-like, honeycombed, and fluffy. Fe₂O₃ (hematite) was also reddish-brown with a sheet-like, block-like, and granular crystal structure, classified into (α-, β-, γ-) Fe₂O₃. Fe₃O₄ was spherical, black, and exhibited good electrical conductivity. As shown in Figure 6, before the water source switch, the G1 pipeline, historically carrying surface water, exhibited SEM images characteristic of surface water corrosion products. These products were typically spherical and block-like with a loose structure, likely composed of FeOOH, Fe₂O₃, and Fe₃O₄. The environmental change from switching the water source enlarged the interstitial spaces between the corrosion products, making them looser and more prone to detachment, thereby increasing the total iron concentration in the water. The corrosion scale layer on the 14th day post-switch, shown in Figure 6(d)–6(f), revealed significant changes in the characteristics of the corrosion products due to altered water quality. Compared with pre-switch corrosion products, those on the 14th day post-switch showed fewer spherical corrosion nodules and a transition to denser, layered, block-like corrosion products. It was speculated that the change in water quality stabilized the previously unstable γ-FeOOH into more stable oxides (Herro & Port 1993; Sarin 2002). Additionally, the higher alkalinity and hardness of the new water source resulted in the formation of calcite on the test piece surface, enhancing the stability of the corrosion scale layer.

Figure 8(a) presents the XRD patterns of G1 before and after the water source switch. In the original environment with surface water, dissolved oxygen oxidized zero-valent iron to ferrous ions, which further oxidized to ferric ions, forming ferric hydroxide products, such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and akaganeite (β-FeOOH). CaCO₃ was also identified on the scale layer, existing in both calcite and aragonite forms (Rafique et al. 2021). After switching to groundwater, the corrosion products on the pipe wall changed. The types of ferric hydroxide products decreased from three to one (α-FeOOH), and Fe₂O₃ was detected. This was due to the greater stability of the new water source, leading some ferric hydroxide products to convert to iron oxides. Combined with earlier SEM results, it was evident that the newly formed Fe₂O₃ covered the surface, forming a dense protective layer. Additionally, the sharper CaCO₃ peak after the switch indicated higher crystal content and crystallinity, suggesting increased stability, strength, and hardness, which more effectively inhibited solution infiltration and ductile iron corrosion. The increase in the CaCO₃ diffraction peak also suggested that carbonate products covered the sample surface post-switch, stabilizing the corrosion scale layer.

Figure 8(b) presents the XRD patterns of G2 before and after the water source switch. In the original environment with surface water, dissolved oxygen oxidized zero-valent iron to ferrous ions, which further oxidized to ferric ions, forming iron oxides and hydroxides, such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), Fe₂O₃, and CaCO₃. After 14 days of operation post-switch, the corrosion products on the pipe wall changed. The types of ferric hydroxide products decreased from three to one (α-FeOOH), and due to the change in water quality, SEM and EIS analyses showed that a new passivation film formed on the test piece surface, changing from Fe₂O₃ to Fe₃O₄. The higher and sharper CaCO₃ diffraction peak indicated increased crystal content and crystallinity after the switch (Withers 2005). Consequently, carbonation products primarily protected the G2 test piece surface, gradually stabilizing the corrosion scale layer (Zhang et al. 2010, 2022). Although the new water source was more corrosive, the previously formed stable scale layer, including CaCO₃ and Fe₂O₃, mitigated the impact of the new water source, enhancing adaptation to the new water quality.

Referring to Figure 11(a), the peak total alkalinity values varied across different ratios. The box plot showed that the peak value ranges were larger for ratios of 1/9 and 9/1 and smaller for ratios of 3/7, 5/5, and 7/3. The order of the ratios from the highest to the lowest maximum alkalinity values was 1/9 > 3/7 > 9/1 > 7/3 > 5/5, while for minimum values, the order was 3/7 > 7/3 > 5/5 > 9/1 > 1/9. After the water quality stabilized, the average alkalinity values from days 8 to 14, in descending order, were 1/9 > 3/7 > 7/3 > 9/1 > 5/5. From Figure 11(b), it was evident that the gap between the maximum and minimum total hardness values was more pronounced at smaller ratios. As the ratio increased, the gap between the extreme values decreased, resulting in more concentrated total hardness measurements (Wasim et al. 2018).

Figure 12(a) illustrates that with a water source mixing ratio of 1/9, the variation between the maximum and minimum observed values was the largest. As the ratio increased, this variation initially decreased, reaching its smallest at a ratio of 5/5, and then increased again. The corrosion potential indicated the tendency of materials to corrode; lower values suggested higher corrosion propensity (Wang et al. 2014). At a ratio of 1/9, the minimum corrosion potential was −5/59 V. As the ratio increased, this minimum value initially rose and then fell, with the lowest value of −0.23 V occurring at a ratio of 5/5, indicating the weakest corrosion tendency. It was suggested that the corrosion tendency of the pipeline was significantly influenced by the mixing ratio, likely due to the similarity between the original transportation environment of the test specimen and the water quality at a ratio of 5/5, leading to better adaptability and reduced electrochemical corrosion due to protective factors like pipeline scale.

EIS tests were conducted to gather data on the corrosion scale layers formed in pipelines after using different ratios of raw water for 14 days. This data helped analyze the structure of the corrosion scale layers under varying water qualities (Ramkumar et al. 2020). Charge transfer, the primary controlling step in corrosion during water source switching, was reflected by R_ct (charge transfer resistance) values. Figure 12(b) presents the R_ct values from EIS fitting for different water source ratios, showing that water quality impacted the structure of the corrosion scale layer differently. The order based on maximum R_ct values was 5/5 > 7/3 > 9/1 > 1/9 > 3/7, and for minimum R_ct values, it was 5/5 > 7/3 > 3/7 > 1/9 > 9/1. It was indicated that at a ratio of 5/5, both maximum and minimum R_ct values were high, suggesting that the corrosion scale layer at this ratio was more adaptable and stable, providing better protection.

Overall, pipeline corrosion was complex and could not be assessed using a single water quality indicator, chemical stability parameter, or electrochemical parameter alone. For instance, relying solely on water quality or stability parameters might have led to selecting a smaller mixing ratio as optimal, ignoring the pipeline's existing corrosion scale layer's adaptability and risking severe corrosion (Zhang et al. 2018). Conversely, focusing only on the scale layer's resistance could have resulted in choosing a larger mixing ratio, jeopardizing water supply quality. Therefore, it was crucial to integrate iron release with water quality indicators, electrochemical parameters, and stability indices, considering both the water quality and the pipeline's adaptability to it, to select a scientifically sound and reasonable ratio for water source supply.

The process of switching water sources involves two stages: an initial rapid corrosion phase due to sudden water quality changes, such as decreased alkalinity, increased pH, and spikes in turbidity and iron concentration, followed by a stabilization phase where the pipe walls adapt, water quality improves, and corrosion slows. This is evidenced by changes in electrochemical parameters and the formation of stable corrosion products like α-FeOOH, Fe₂O₃, and Fe₃O₄. Lower mixing ratios during the switch lead to better system stability, reduced corrosion, and improved water quality.

To mitigate corrosion, water utilities should conduct thorough water quality monitoring, focusing on key parameters such as pH, alkalinity, and TDS. Pre-treatment measures, such as optimizing pH and adding corrosion inhibitors, are crucial before switching. Gradual mixing of water sources at lower ratios may reduce risk. Additionally, controlling turbidity and iron release, alongside an integrated approach to managing water quality and electrochemical parameters, is essential for minimizing corrosion and maintaining system performance. In conclusion, careful preparation and monitoring during water source transitions, combined with targeted mitigation strategies, can effectively reduce corrosion risks and ensure water quality.

This work was financially supported by the Zhejiang Province single-village water supply technology research project (RA + 202308).

Z.L.: methodology, investigation, formal analysis, and writing – original draft; G.S.: analysis and writing – review & editing; S.Z.: methodology and resources; B.J.: validation; resources, and validation; H.X.: validation, investigation, and data curation; Y.G.: writing – review & editing; A.W.: visualization, software, writing – review & editing, and supervision; H.X.: conceptualization, funding acquisition, project administration, formal funding acquisition, supervision, and resources.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.