Electrolysis-induced hardness variation for quantitatively assessing the scaling tendency of circulating water

The current global water crisis affects millions, with around 4 billion people experiencing water stress annually (Aliasghar et al. 2022). Effective management of circulating water is of great significance for improving water resource utilization efficiency. Deposition of scale on heat exchanger walls and pipes can lead to numerous serious problems, including reduced heat transfer efficiency, blocked pipes, and under-deposit corrosion (Chhim et al. 2020; Jiang et al. 2022). Usually, calcium carbonate scale is the most easily precipitated scale in the high-temperature environment of a circulating cooling water system (Wei et al. 2008). Therefore, assessing the scaling tendency of water quickly and accurately is of inestimable significance for the solution of the scaling problem (Chen et al. 2023; Guo et al. 2023).

In previous studies on scaling tendency, there are usually two main types of methods for calcium carbonate scaling tendency (Li et al. 2022). One is to make a preliminary qualitative assessment of scaling tendency through the water stability evaluation index which is based on the dissolution equilibrium of calcium carbonate. So far, a variety of water stability evaluation indices have been used to assess the scaling tendency of water, such as the Langelier saturation index (LSI) (Langelier 1946), the Ryznar stability index (RSI) (Ryznar 1944), the Puklius scale index (PSI) (Ravikumar & Somashekar 2012), the Fouling factor (J_water) (Bengao & Xieqing 2008). The water stability evaluation index mentioned above all have their own applicability conditions and limitations. For example, the RSI is often used in intercooled open-cycle cooling water systems and is generally believed to be more accurate at pH 6.5–8. However, LSI is often used in cooling water systems where groundwater is used as supplementary water and is generally believed to be more accurate at pH 7–9 (Li et al. 2022). Since the RSI in steady state is an interval index compared with other single point indices, it is more able to meet the actual industrial water fluctuation situation and it is applied widely. Research shows that different results may be obtained when using different water stability evaluation indices to assess the same water (Kalyani et al. 2017; Song et al. 2020). Therefore, the assessment results of the water stability evaluation index can only be used as a relative reference, rather than an absolute judgment. The other is to make a quantitative assessment of the scaling tendency of water through the key indicators of the experimental methods. Some researchers set up hanging pieces in the water, and measure the mass variation of hanging pieces and the variation of calcium ion solubility in the solution regularly (Dalas & Koutsoukos 1989; Cailleau et al. 2006), so as to calculate the scaling rate. Although this method can directly reflect the scaling tendency of water, the above method is time-consuming and difficult to know the scaling status of water accurately. Therefore, it is difficult to know the scaling rate and scaling condition of water accurately in time. Some researchers proposed to assess the water scaling tendency by obtaining electrochemical parameters through electrochemically induced scaling to solve the time-consuming problem (Haaring et al. 2019). Some researchers have compared the interfacial capacitance obtained by electrochemical impedance spectroscopy (EIS) as a measure of mineral-scale surface coverage, which in turn reflects the scaling tendency of water (Devos et al. 2006; Gao et al. 2020). The scaling tendency of water has also been reflected by the values of open circuit potential (OCP) and self-corrosion current density I_corr (Sebastiani et al. 2017; Zhang et al. 2020). The electrochemical testing method is suitable for the study of the microscopic scaling mechanism, and its experimental conditions are harsher and more influenced by other environmental factors, thus its data lack a certain degree of reliability and reproducibility.

Assessing the scaling tendency of water quality is a long-standing challenge in industrial circulating cooling water systems (Pääkkönen et al. 2012). Although there have been some advances in studies to assess the scaling tendency of circulating water, most of the studies have discussed and studied the water stability evaluation index and the key indicators in the experiment separately. They merely focus on the effect of a single water indicator on the scaling tendency of water (Li et al. 2022; Zhang et al. 2022). Therefore, this paper proposes a method of electrolysis to induce hardness variation to assess the scaling tendency of water quantitatively. The hardness variation is associated with the water stability evaluation index. The circulating water dynamic simulation experiment is carried out to provide support for the hardness variation which clarifies the criteria that can meet the actual water requirements. It realizes the quantitative assessment of the scaling tendency of circulating water.

The circulating water simulating solutions were used as experimental water and configured according to the fluctuating range of circulating water in a chemical plant, mainly 1,000 series, 700 series, and 400 series of simulating solutions. The water quality indicators and formulations are shown in the Table 1.

The fixed size 304 stainless steel round piece was selected as the hanging specimen (the contact surface area with the solution was 11.38 cm²). Its surface was polished with 1,000 mesh silicon carbide sandpaper before immersion, and sonicated in ethanol solution for 10 min to ensure that the surface state of each specimen was consistent (Deng et al. 2022). It was suspended in 250 mL of the above experimental solution and left to stand in a constant temperature water bath at 60 °C for 12 h. The test piece was dried in an oven and weighed. The weight variation was recorded to obtain the scaling rate. The hardness titration using Ethylene Diamine Tetraacetic Acid (EDTA) was performed after the solution was cooled and made up to its initial volume with deionized water to obtain the hardness variation of water. Digital image of specimens was obtained by taking photos with a mobile phone. Three sets of experiments were done in parallel for each water quality.

The electrochemical testing method mainly refers to microscopic testing to obtain electrochemical parameters on the surface of the working electrode. The epoxy-encapsulated stainless steel specimen was selected as the working electrode for testing. The area of the specimen exposed to the solution was 0.64 cm². The surface was polished before use and sonicated in ethanol solution for 10 min to ensure that the surface state of the specimen was consistent each time. The three-electrode system was constituted to place the solution in a water bath at 60 °C for testing, where a platinum sheet was used as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and a stainless steel specimen as the working electrode. Electrochemical testing mainly included potentiostatic polarization and electrochemical impedance spectroscopy (EIS) . To accelerate scaling and avoid the effect of hydrogen precipitation, each group of water was first deposited at a potential of −0.9 V versus SCE for 1 h with potentiostatic polarization. Then electrochemical impedance spectroscopy was tested at a potential of ±10 mV around −0.9 V with a range of frequency of 10 mHz to 10 kHz. The hardness variation is obtained according to the above method.

Macro- and microscopic images of the surface of the working were obtained by taking photos with mobile phones and metallographic microscopes, respectively. Three sets of experiments were done in parallel for each water quality.

The electrolysis-induced method referred to accelerate scaling by electrolysis, and hardness variation was chosen as an evaluation indicator of scaling tendency. The circulating water dynamic simulation experiment was used as its supporting basis to assess the water scaling tendency accurately. The electrode system with an effective positive area of 7 cm × 7 cm was selected for electrolysis. The cathode was a stainless steel flat plate and the anode was a dimension-stable anode flat plate. The distance between them was 2 cm. It was placed in 1 L of the above solution, and electrolysis experiments were carried out with the optimal current density and electrolysis time screened under the water bath conditions at 60 °C. The cathode stainless steel plate was cleaned with acetic acid before use to ensure that its surface state was consistent each time. The hardness variation is obtained according to the above method. It is recorded as H.

The evaluation index of each experimental water and their stability states are shown in Table 2 through the calculation. It can be seen that the RSI has three states in its assessment, while the other three indices have only one or two states, thus its assessment is indeed more detailed compared to the others. In addition, the assessment results of different evaluation indices on the scaling tendency also appear in various situations for the same water quality. Especially the assessment results of each evaluation index are more likely to be different for water with a weak scaling tendency. Therefore, merely using the water stability evaluation index cannot assess the scaling tendency of water accurately and reliably. The result is consistent with literature reports (Li et al. 2022). It is necessary to combine the experimental results to make a more accurate assessment of the scaling tendency of water.

Since the advantages of the RSI in the practical application process, the subsequent research mainly focuses on the RSI and other indices as supplementary research.

The hardness variation of different series of water are examined as shown in Figure 6. The graph clearly shows that the hardness variation presents a better exponential relationship with the RSI and is applicable to a series of water quality widely. Moreover, the fitting relationships between hardness variation and RSI all reached above 0.99. Therefore, it also demonstrates that the hardness variation of water reflects the scaling tendency more accurately.

In summary, the hardness variation of water compared to the key indicators of the above methods is more accurate to reflect the scaling tendency of water. Although the hanging piece method and electrochemical testing method can be used to assess the scaling tendency of water to a certain extent, there are problems with hysteresis and data stability in the actual field application. Therefore, the hardness variation can establish a good correlation with RSI in the above methods, which is chosen as the key indicator to reflect the scaling tendency of water. The electrolysis-induced method is used as an experimental means to assess the scaling tendency of water.

The electrolysis-induced hardness variation is used as the key indicator to assess the scaling tendency of water quantitatively. The functional relationship is established between hardness variation and water stability evaluation index. The circulating water dynamic simulation experiment supports the results of the electrolysis-induced method. It further clarifies the values of hardness variation to meet the actual water requirements for different series of water quality. This work not only realizes the further assessment based on the water stability evaluation index but also provides a more accurate guiding strategy for assessing the scaling tendency of circulating water. However, this study is only applicable to natural water containing calcium ions, and it is also necessary to consider the impact of electrolysis on the reagents for non-natural water (water with added chemicals). Subsequent research can conduct electrolytic induction experiments on various types of circulating water, in order to establish the comprehensive evaluation system for scaling tendency.

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 21978036).

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

The authors declare there is no conflict.