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Ultrasonic modification of carbonate scale electrochemically deposited in tap water
Accepted Manuscript Ultrasonic modification of carbonate scale electrochemically deposited in tap water G. Vasyliev, S. Vasylieva, A. Novosad, Y. Gerasymenko PII: DOI: Reference:
S1350-4177(18)30091-9 https://doi.org/10.1016/j.ultsonch.2018.05.026 ULTSON 4184
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
22 January 2018 17 May 2018 18 May 2018
Please cite this article as: G. Vasyliev, S. Vasylieva, A. Novosad, Y. Gerasymenko, Ultrasonic modification of carbonate scale electrochemically deposited in tap water, Ultrasonics Sonochemistry (2018), doi: https://doi.org/ 10.1016/j.ultsonch.2018.05.026
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Ultrasonic modification of carbonate scale electrochemically deposited in tap water Vasyliev G., Vasylieva S., Novosad A., Gerasymenko Y. National technical university of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056, Kyiv-056, Ukraine
Corresponding author Georgii S. Vasyliev Tel.: +380969249888 Fax.: +380442369774 E-mail address: [email protected] Postal address: National technical university of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056, Kyiv-056, Ukraine
Keywords: Chronoamperometry, Calcium carbonate, Nucleation-growth process, Ultrasonic cavitation
Abstract Influence of the ultrasound intensity (28 kHz, 1.17.5 W/cm2) on CaCO3 nucleation-growth on the surface of a cylinder mild steel electrode rotating at 500 rpm was studied in tap water. The deposition kinetics was analyzed by chronoamperometry; the calcareous layer was characterized by gravimmetry, scanning electron microscopy and XRD. Application of ultrasound to calcium carbonate crystallization affects nucleation sites density, masstransport rate and cavitation erosion of the deposits. Lower intensity ultrasound reduces scale porosity and area density by increasing nucleation site density and accelerating the mass transport. Higher intensity ultrasound promotes cavitation erosion of the formed layer, thus cleaning the surface from the scale. A scale layer with the highest blocking properties formed under applied ultrasound intensity of 1.9 W/cm2. The ultrasound doubled crystallization rate, reduced the scale porosity 5 times and halved its area density comparing to non-sonicated conditions. Ultrasound of controllable intensity can solve both scale and corrosion problems of industrial heatexchange equipment by forming a protective scale layer and removing excessive deposits.
Introduction Calcium carbonate precipitation is a major concern of energy production in industry. The scaling phenomenon causes technical problems such as reduction of heat transfer efficiency and obstruction of pipes. The non-productive expenses related to scaling were estimated at 1.5 billion Euros per year in France, about 0.8 billion $US in Great Britain, 3 billion $US in Japan and 9 billion $ in the USA [1,2]. Another problem of industrial heat-exchanging equipment is internal corrosion. Corrosion causes deterioration of materials and leads to economic losses since repairs and equipment replacements are required more often [3]. Three crystalline forms of CaCO3 exist: calcite, which has a cubic shape; vaterite, with a sperulite, hemispherical flowers, or lenses morphology; and aragonite, which is recognised as needles.[4] The type of deposited phases was found to depend on the substrate nature (copper or stainless steel) [5] and the initial state of the substrate surface (clean or damaged) [6]. The presence of a passivating layer on non-noble metals reduces the density of nucleation sites and thus promotes formation of vaterite nuclei [4]. The calcium2+ concentration
and the nucleation rate also influence the allotropic form of calcium carbonate [7]. Calcite predominantly forms in conditions which favore a fast nucleation rate whereas the vaterite form is systematically obtained under experimental conditions leading to a slow nucleation rate [4]. The protection properties of carbonate scale depend on both the size of crystals and their type. Superior protection was achieved through formation of calcium carbonates of the aragonite phase [4,8]. The scaling process and blocking abilities of the carbonate layer appeared to be highly dependent on solution composition [4,9]. In the solution enriched with sulfate ions the deposition of CaCO 3 was almost inhibited [10]. The influence of additives on the carbonate crystallization process has been widely investigated to develop a scale control technique, based on the dosage of chemical reagents to suppress precipitation [2,11,12]. Ultrasound (US) is widely applied to control crystallization in industry [13,14]. It influences nucleation and crystal growth processes, as well as crystal size distribution, agglomeration and even breakage. By tuning all these parameters, crystals with desirable characteristics, i.e. size, shape and morphology, can be obtained. The influence on nucleation is attributed to the cavitation bubbles formation and collapse. The pressure in the collapsing cavitation bubble reaches 1 GPa and the temperature changes from very high during the collapse to very low just after the collapse. Pressure and temperature variations are the main factors that govern crystal nucleation in the presence of ultrasound. This process is utilized in seedless crystallization, especially for sterile systems in pharmaceutical industry. At the same time, application of ultrasound cavitation in seeded crystallization also increases crystallization rate [15] by enlarging the surface area available for crystallization [16]. The collapse of cavitation bubbles forms microstreamings and intense solution stirring. This enhances mass transfer at the solution/crystal interface, leads to faster replacement of the adjacent solution layer and, as a result, increases crystal growth rate. US cavitation of higher power may also lead to deagglomeration and crystal breakage [17]. Surface damage of the crystals also generates secondary nuclei and reduces an average particle size. Ultrasound cavitation is used to mitigate scale formation and remove the deposits in industrial equipment. The formation and collapse of cavitation bubbles of high intensity creates pressure shock waves, which cause erosion of the material and its removal from the solid surface [18]. The destruction of the scale surface layer was found to depend on the cavitation intensity. The power applied to sonotrode and roughness of the scaled surface control the intensity of cavitation erosion [19,20]. However, a complete removal of calcium carbonate from the heat-exchange surface can bring about corrosion problems. Carbonates are known to act as a barrier layer on the metal surface to prevent oxygen ingress [21,22]. The controllable growth of the calcium carbonate layer on the metal surface with high blocking abilities and low thickness can potentially solve both scale and corrosion problems in heat-exchanging equipment. Application of ultrasound to electrochemical deposition processes is known as sonoelectrocrystallization. This technique is used to form various types of nanoparticles. The pulse electrolysis is coupled to ultrasonic vibration of working electrode so that the material deposited during a single electric pulse is shaken off the surface and accumulated at the bottom of the vessel [23,24]. In contrary, the idea of the present work is not to remove calcite crystals from the surface, but to form a thin calcite layer with high blocking abilities, which can act as a barrier for oxygen ingress thus reducing the corrosion rate. In our previous work, it was shown that application of lower intensity ultrasound (2.2 W/cm2 and lower) accelerates crystallization of CaCO3 due to a
higher oxygen supply and the surface ‘pre-treatment’ effect – a strong increase of the number of active sites and cleaning the surface from iron oxides. Higher ultrasound intensities enhance calcite crystallization at the initial stages that lowers calcium concentration in the pre-electrode layer. This promotes formation of vaterite crystals in the growing calcite layer increasing its porosity and overall scaling time. The aim of the present work was to investigate the scale thickness and porosity regulation with ultrasound, find the ultrasound intensities favorable for protective scale deposition and excess scale removal. These results are of high interest to develop a simple physical method for both scale and corrosion protection of heatexchange surfaces in heat-power industry, as well as in chemical, food processing and other industries.
Experimental Electrochemical measurements The tap water from the Kyiv city distribution system was used in all experiments. Its composition is presented in Table 1. The measurements were conducted in a 1L glass beaker filled with tap water and thermostated at 250.5 C (Fig. 1). A mild steel cylinder with a diameter of 6 mm and 30 mm long was rotated at 500 rpm and served as a working electrode. Its surface was prepared by turning, degreasing, rinsing with distilled water and drying in hot air. The counter electrode was a platinum grid and a saturated silver chloride electrode (saturated AgCl) was used as a reference. The working electrode was polarized at −1.1 V (saturated AgCl) for 240 min to induce precipitation of calcium carbonate on its surface. The current-time dependence was registered concurrently. Each test was run for three times to ensure reproducibility. An ultrasonic device working at the frequency of 27.8 kHz was placed under the bottom of the beaker and radiated the solution. Electric power supplied to the ultrasonic transducer ranged between 33 and 234 W, the transducer diameter was 60 mm and calculated US intensity varied in the range 1.17.5 W/cm2 or 0.033–0.234 W/cm3.
Table 1. Water chemical composition. Parameter рН Dissolved oxygen Total Hardness Total Alkalinity Calcium Magnesium Chloride Sulfate Hydrocarbonate Total dissolved solids
Units DO TH TA Сa2+ Mg2+ Cl– SO42– HCO3– TDS
mg/l mmol/l mmol/l mmol/l mmol/l mg/l mg/l mmol/l mg/l
Value 7.8–8.0 6.0 3.9–4.2 3.9–4.1 3.0–3.1 0.9–1.1 18–25 30–35 3.2–3.5 240–260
Fig. 1. Polarization apparatus for carbonate electrodeposition: (1) Pyrex vessel, (2) working electrode (mild steel cylinder), (3) counter electrode (platinum grit), (4) reference electrode (saturated silver chloride), (5) Luggin capillary, (6) salt bridge, (7) ultrasound bath, (8) ultrasound horn, (9) regulable ultrasound generator, (10) direct current source, (11) potentiostat, (12) signal converter, (13) computer, (14) electric motor, (15) regulated water heater
Scaling investigation The electrochemical approach to investigate the scale formation and removal from the metal surface is based on the measurement of oxygen reduction current during nucleation and formation of calcium carbonate. The calcium carbonate crystals reduce the electrochemically active surface area of the electrode [25,26]. Comparison of the modified and bare electrodes in both the electrochemical and surface parameters allows blocking properties of the deposits to be evaluated. During the scale formation the oxygen reduction current, I, is decreased proportionally to the reduction of an electrochemically active surface area. When the surface is free of any deposits (shortly after polarization started) the current value reaches its highest value, I0. After the scaling is finished the lowest oxygen reduction current value, Imin, is achieved. In practice, it was measured at the end of the It curve (250 min of polarization). When scaling is finished oxygen reduction can occur on uncovered areas only or through the pores of the scale. Hence, scale porosity SP can be calculated from the values of I0 and Imin as: SP
Imin I
1
(1)
The rate of scaling or scaling time, ts, was found by linear extrapolation of the It dependence to zero current. The crystallization kinetics of CaCO3 under the ultrasound action was characterised by the values of I0, SP and ts. Due to the US influence on the initial current value I0, the current values at different US intensities were normalized by I0. To determine the mass of the deposited carbonates the electrodes were weighted before and after electrochemical measurements with 0.0001 g accuracy. The mass of the deposits was used to analyze the results as the area density, mg/cm2. The morphology of the surface was studied with use of a SEM-106I Selmi microscope (Ukraine) operated at 20 kV. The X-ray diffraction phase analysis (XRD) of deposits was performed using a Rigaku Ultima-IV powder X-ray diffractometer with Cu K radiation (30 kV amd 30 mA). Full X-ray diffraction patterns were recorded for the scan angles (2) from 10 to 80 with a step size of 0.04. Using the ICDD-PDF database, individual crystalline phases were identified from their observed XRD patterns.
Results and Discussion The experimental chronoamperometric curves with the US intensity ranging from 0 to 7.5 W/cm2 are presented in Fig. 2. The rise of US intensity causes the reduction of crystallization time and above 3.2 W/cm2 all the curves coincide. All the parameters influenced by US are combined in Fig. 3. The initial current I0 values demonstrate an overall double growth along with a 7-fold increase of the ultrasound intensity (Fig. 3a). I0 is a current of the oxygen reduction with the limiting stage of the process being oxygen diffusion to the electrode surface. This process has a constant speed in the absence of ultrasound. The application of US gives rise to I 0 meaning that the rotation of the electrode is much less efficient to cavitation stirring. The US applied to water produces acoustic vibrations and cavitation [27]. The rise of I0 is caused by the agitating action of cavitation bubbles near the electrode surface [28] and by thinning of the diffusional layer under conditions of insonation [27]. However, 2 regions with different slopes are clearly distinguished in the I0 – US intensity plot. A higher rise of I0 is observed in the intensity range 0–1.9 W/cm2. The consequent increase of intensity (1.9–7.5 W/cm2) changes I0 only slightly, from 0.77 to 0.92 mA/cm2. This observation clearly demonstrates that the masstransport intensification induced by ultrasound has a limiting behavior.
Fig. 3. Normalized chronoamperometric curves with the ultrasound intensity as a parameter (lines – a guide for an eye). Linear sections of the experimental curves were extrapolated to zero current to obtain t s values. Scaling time ts (Fig. 3c) combines the influence of both the nucleation and the growth rate. The number of nuclei, initially formed on the surface, as well as their growth rate, determines the rate of surface coverage and, as a consequence, the current reduction rate. ts appears to strongly correlate with I0 (Fig. 3a), in the intensity range 1.9–7.5 W/cm2 ts remains nearly unchanged similarly to the I0 behaviour. In contrary, at the lowest tested US intensity 1.1 W/cm2 ts reaches its highest value of 200 min. By comparing the shapes of the I–t curves at 0 and 1.1 W/cm2 with the curves obtained by Belarbi [29], one can conclude that the number of nucleation sites becomes larger with the application of ultrasound cavitation. However, the supply of Ca2+ and CO32– ions is insufficient to support the stable growth of enlarged number of crystal seeds. Thus, ultrasound promotes the rise of nuclei even at the lowest tested intensity, however the stirring action is not strong enough to support a stable growth of the formed crystals. So, the scaling proceeds slower what leads to higher t s (Fig. 3c). The rise of ultrasound intensity increases the intensity of cavitation. The appearance and collapse of cavitation bubbles creates efficient stirring in the bulk solution. Near the electrode surface the effect is several orders of magnitude stronger due to the asymmetrical collapse of the cavitation bubble. Collapsing bubble forms microstreamings that can reach a speed up to 100 m/s. Such an intense stirring eliminates any diffusion barriers and crystallization occurs at the maximum rate for a given Ca2+ concentration. More nuclei are formed with an increase of the US intensity in conditions of stable supply of Ca2+ and CO32– ions. Further rise of the intensity does not cause any changes and scaling time ts reaches a limiting value of 50 min.
Fig. 4. Influence of the ultrasound intensity on scale crystallization parameters: (a) initial current, (b) scale porosity, (c) scaling time, (d) deposited mass.
While I0 and ts dependences showed a platue behaviour, there is a minimum in the SP–US intensity plot (Fig. 3b). SP expresses the blocking ability of the crystal layer at the end of crystallization. The lower the SP value the better blocking abilities and resulting corrosion resistance. A rise of US intensity to 1.9 W/cm2 caused a SP decrease to the minimum value of 1%, which is 5 times lower than the porosity of the scale deposited in the absence of US. The lowest SP value is provided by the US increase of the nucleation site density and mass transport. Further rise of the US intensity above 1.9 W/cm2 makes SP increase. At the highest tested US intensity of 7.5 W/cm2 the SP values amounts to 12%. At the same time, the values of I0 and ts remain unchanged with rise of the US intensity. So, the increase of SP cannot be attributed to the changes in the crystal nucleation or crystal growth processes. A probable reason of the blocking abilities reduction can be a destructive effect of cavitation. US cavitation of higher intensities causes crystal breakage, deagglomeration, and can ruin scale crystals and even clean the surface [18]. SEM analysis after crystallization was performed to determine the crystal morphology. The SEM images were obtained under two magnifications: 250 (Fig. 4) and 2500 (Fig. 5) to study distribution of the crystals on the surface and their morphology, respectively. The application of US clearly demonstrates several significant changes in the crystal layer. First, the single crystal size is reduced from 10–20 m (Fig. 5a) to 6–8 m (Fig. 5b). The crystal size distribution under US is much narrower. In non-sonicated conditions, large crystals (10–20 m) can be found along with smaller ones (2 m and less). Application of US makes the
crystals size practically uniform (6–8 m). Lowering of the average crystal size agrees with the rise of the nuclei number, previously detected in Fig. 3a. Although it is not possible to determine the exact porosity value from SEM images, it is obvious that the crystal layer is the most uniform and the number of uncovered regions (white spots between grey crystals) is the lowest in the figures 4b-d, which correspond to the intensity range 1.13.2 W/cm2. SP has the lowest values in the same intensity range (Fig. 3b).
Fig. 4. SEM images of CaCO3 deposited on the electrode surface in tap water (t = 25 C; electrode rotation rate: 5
rpm, applied potential E 2
−1.1 V/saturated AgCl). US intensity: (a) 0; (b) 1.1; (c) 1.9; (d) 3.2; (e) 5.3; (f)
7.5 W/cm . Magnification 250.
Fig. 5. SEM images of CaCO3 deposited on the electrode surface in tap water (t = 25 C; electrode rotation rate: 5
rpm, applied potential E
−1.1 V/saturated AgCl). US intensity: (a) 0; (b) 1.1; (c) 1.9; (d) 3.2; (e) 5.3; (f)
2
7.5 W/cm . Magnification 2500.
Crystals, obtained at the highest US intensities, are ruined (fig. 5d, e). The single crystals have smooth edges, while at lower intensities the edges are sharp. At 7.5 W/cm2 it is not even possible to determine single crystals due to ruination. Lower magnification shows that at high US intensities the surface coverage also reduces, confirming the SP rise. The stripes of the scale between uncovered patches can be clearly seen in Fig. 4f.
The areal density vs US intensity dependence combines the trends of I 0, ts and SP (Fig. 3d). The area density value halves when intensity reaches 1.9 W/cm2 comparing to non-sonicated conditions. This means that application of US halves the amount of calcite needed to form a protective layer on a steel surface. The further rise of US intensity reduces the deposited mass due to the surface cleaning with high-energy cavitation. This part of the curve agrees well with the corresponding part of the SP – US intensity plot (Fig. 3b).
Fig. 6. XRD patterns of surface deposits, formed on mild steel in tap water during cathodic polarization at different ultrasound intensities. – calcite; □ – iron.
Fig. 6 shows the XRD patterns of surface deposits for applied ultrasound intensities. In all patterns the peaks at 44.6° and 64.7° correspond to the steel substrate due to low thickness of deposits. All other peaks were attributed to the calcite phase of calcium carbonate according to the JCPDS PDF2 standard card (01-089-1304) (ICDD (PDF-2/Release 2011 RDB). These data indicate that the calcite phase was predominantly formed during crystallization [15]. US was found not to change the crystal phase of calcium carbonate. At the same time the intensity of peaks in the XRD patterns was observed to decrease with rise of ultrasound intensity from 3.2 to 7.5 W/cm2. At the highest applied US intensity of 7.5 W/cm2 calcite peaks are hardly distinguishable, in agreement with the SEM surface observation (Fig. 4f) and deposited mass measurements (Fig. 3d) due to cavitation ruination of the crystals. To summarize, three major factors determine properties of the scaling under US application: the rise of nucleation site density, mass transport acceleration and crystal ruination with cavitation. A rise of nucleation site density occurs even at the lowest tested US intensity 1.1 W/cm2 and is evident from the SEM image (Fig. 4b). At the same time, Ca2+ supply is insufficient and scale deposition requires longer time. Higher US intensities
enhance Ca2+ and O2 supply and the scale layer is built faster with low porosity and thickness (1–3 %). A further rise of US intensity leads to stronger cavitation so that crystals ruination prevails over their deposition. Strong cavitation causes destruction of single crystals and eventually at 7.5 W/cm2 clears the surface. Two practical points can be gained from the obtained results. US intensity of 1.9 W/cm2 applied to the mild steel equipment exposed to tap water may help to build a highly protective scale layer on the surface. If the full surface clean-up is required, the US intensity as high as 7.5 W/cm2 is to be applied. Thus, the application of US cavitation can simultaneously solve both scale and corrosion problems in heat-exchanging equipment also being an environmental friendly and easy-accessible method. Further investigations are to be aimed on ultrasound frequency influence and ultrasound field distribution around the horn to achieve a simple and efficient technique to control both scaling and corrosion in heat-exchangers.
Conclusions The influence of ultrasound intensity on both CaCO3 deposition kinetics and scale properties on the steel rotating cylinder electrode was investigated. The following conclusions can be drawn. 1.
The application of US to the surface crystallization process accelerates the mass transport and increases nucleation site density. This leads to the rise of the initial current density and reduces scaling time. The mass transport acceleration results from the efficient solution agitation with acoustic waves and cavitation.
2.
The ultrasound intensity of 1.9 W/cm2 was found to promote formation of a scale layer with porosity of less than 1 %, what is five times lower comparing to the corresponding value obtained in the absence of ultrasound. The layer crystallization time and area density halved. Further increase of ultrasound intensity does not change the crystallization kinetics significantly.
3.
The increase of ultrasound intensity above 1.9 W/cm2 enhances a cavitation power. The strong cavitation at ultrasound intensity of 7.5 W/cm2 ruins the scale crystal, thus raising the porosity and eventually cleaning the surface.
4.
The application of ultrasound simultaneously solves scale and corrosion problems in heat-exchanging equipment. This environmental friendly and easy-accessible technique can provide the foundation for the development a simple and efficient approach for scale control in industry.
Acknowledgements This work was supported by the Ministry of education and science of Ukraine [grant number 2044, 2017].
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Highlights -
Influence of the ultrasound (28 kHz, 1.17.5 W/cm2) on CaCO3 nucleation-growth was studied;
-
Ultrasound affects nucleation sites density, mass-transport rate and cavitation erosion;
-
The highest blocking properties were obtained under applied intensity of 1.9 W/cm2;
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Ultrasound intensity of 7.5 W/cm2 ruins the scale crystal, thus cleaning the surface.
S1350-4177(18)30091-9 https://doi.org/10.1016/j.ultsonch.2018.05.026 ULTSON 4184
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
22 January 2018 17 May 2018 18 May 2018
Please cite this article as: G. Vasyliev, S. Vasylieva, A. Novosad, Y. Gerasymenko, Ultrasonic modification of carbonate scale electrochemically deposited in tap water, Ultrasonics Sonochemistry (2018), doi: https://doi.org/ 10.1016/j.ultsonch.2018.05.026
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Ultrasonic modification of carbonate scale electrochemically deposited in tap water Vasyliev G., Vasylieva S., Novosad A., Gerasymenko Y. National technical university of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056, Kyiv-056, Ukraine
Corresponding author Georgii S. Vasyliev Tel.: +380969249888 Fax.: +380442369774 E-mail address: [email protected] Postal address: National technical university of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056, Kyiv-056, Ukraine
Keywords: Chronoamperometry, Calcium carbonate, Nucleation-growth process, Ultrasonic cavitation
Abstract Influence of the ultrasound intensity (28 kHz, 1.17.5 W/cm2) on CaCO3 nucleation-growth on the surface of a cylinder mild steel electrode rotating at 500 rpm was studied in tap water. The deposition kinetics was analyzed by chronoamperometry; the calcareous layer was characterized by gravimmetry, scanning electron microscopy and XRD. Application of ultrasound to calcium carbonate crystallization affects nucleation sites density, masstransport rate and cavitation erosion of the deposits. Lower intensity ultrasound reduces scale porosity and area density by increasing nucleation site density and accelerating the mass transport. Higher intensity ultrasound promotes cavitation erosion of the formed layer, thus cleaning the surface from the scale. A scale layer with the highest blocking properties formed under applied ultrasound intensity of 1.9 W/cm2. The ultrasound doubled crystallization rate, reduced the scale porosity 5 times and halved its area density comparing to non-sonicated conditions. Ultrasound of controllable intensity can solve both scale and corrosion problems of industrial heatexchange equipment by forming a protective scale layer and removing excessive deposits.
Introduction Calcium carbonate precipitation is a major concern of energy production in industry. The scaling phenomenon causes technical problems such as reduction of heat transfer efficiency and obstruction of pipes. The non-productive expenses related to scaling were estimated at 1.5 billion Euros per year in France, about 0.8 billion $US in Great Britain, 3 billion $US in Japan and 9 billion $ in the USA [1,2]. Another problem of industrial heat-exchanging equipment is internal corrosion. Corrosion causes deterioration of materials and leads to economic losses since repairs and equipment replacements are required more often [3]. Three crystalline forms of CaCO3 exist: calcite, which has a cubic shape; vaterite, with a sperulite, hemispherical flowers, or lenses morphology; and aragonite, which is recognised as needles.[4] The type of deposited phases was found to depend on the substrate nature (copper or stainless steel) [5] and the initial state of the substrate surface (clean or damaged) [6]. The presence of a passivating layer on non-noble metals reduces the density of nucleation sites and thus promotes formation of vaterite nuclei [4]. The calcium2+ concentration
and the nucleation rate also influence the allotropic form of calcium carbonate [7]. Calcite predominantly forms in conditions which favore a fast nucleation rate whereas the vaterite form is systematically obtained under experimental conditions leading to a slow nucleation rate [4]. The protection properties of carbonate scale depend on both the size of crystals and their type. Superior protection was achieved through formation of calcium carbonates of the aragonite phase [4,8]. The scaling process and blocking abilities of the carbonate layer appeared to be highly dependent on solution composition [4,9]. In the solution enriched with sulfate ions the deposition of CaCO 3 was almost inhibited [10]. The influence of additives on the carbonate crystallization process has been widely investigated to develop a scale control technique, based on the dosage of chemical reagents to suppress precipitation [2,11,12]. Ultrasound (US) is widely applied to control crystallization in industry [13,14]. It influences nucleation and crystal growth processes, as well as crystal size distribution, agglomeration and even breakage. By tuning all these parameters, crystals with desirable characteristics, i.e. size, shape and morphology, can be obtained. The influence on nucleation is attributed to the cavitation bubbles formation and collapse. The pressure in the collapsing cavitation bubble reaches 1 GPa and the temperature changes from very high during the collapse to very low just after the collapse. Pressure and temperature variations are the main factors that govern crystal nucleation in the presence of ultrasound. This process is utilized in seedless crystallization, especially for sterile systems in pharmaceutical industry. At the same time, application of ultrasound cavitation in seeded crystallization also increases crystallization rate [15] by enlarging the surface area available for crystallization [16]. The collapse of cavitation bubbles forms microstreamings and intense solution stirring. This enhances mass transfer at the solution/crystal interface, leads to faster replacement of the adjacent solution layer and, as a result, increases crystal growth rate. US cavitation of higher power may also lead to deagglomeration and crystal breakage [17]. Surface damage of the crystals also generates secondary nuclei and reduces an average particle size. Ultrasound cavitation is used to mitigate scale formation and remove the deposits in industrial equipment. The formation and collapse of cavitation bubbles of high intensity creates pressure shock waves, which cause erosion of the material and its removal from the solid surface [18]. The destruction of the scale surface layer was found to depend on the cavitation intensity. The power applied to sonotrode and roughness of the scaled surface control the intensity of cavitation erosion [19,20]. However, a complete removal of calcium carbonate from the heat-exchange surface can bring about corrosion problems. Carbonates are known to act as a barrier layer on the metal surface to prevent oxygen ingress [21,22]. The controllable growth of the calcium carbonate layer on the metal surface with high blocking abilities and low thickness can potentially solve both scale and corrosion problems in heat-exchanging equipment. Application of ultrasound to electrochemical deposition processes is known as sonoelectrocrystallization. This technique is used to form various types of nanoparticles. The pulse electrolysis is coupled to ultrasonic vibration of working electrode so that the material deposited during a single electric pulse is shaken off the surface and accumulated at the bottom of the vessel [23,24]. In contrary, the idea of the present work is not to remove calcite crystals from the surface, but to form a thin calcite layer with high blocking abilities, which can act as a barrier for oxygen ingress thus reducing the corrosion rate. In our previous work, it was shown that application of lower intensity ultrasound (2.2 W/cm2 and lower) accelerates crystallization of CaCO3 due to a
higher oxygen supply and the surface ‘pre-treatment’ effect – a strong increase of the number of active sites and cleaning the surface from iron oxides. Higher ultrasound intensities enhance calcite crystallization at the initial stages that lowers calcium concentration in the pre-electrode layer. This promotes formation of vaterite crystals in the growing calcite layer increasing its porosity and overall scaling time. The aim of the present work was to investigate the scale thickness and porosity regulation with ultrasound, find the ultrasound intensities favorable for protective scale deposition and excess scale removal. These results are of high interest to develop a simple physical method for both scale and corrosion protection of heatexchange surfaces in heat-power industry, as well as in chemical, food processing and other industries.
Experimental Electrochemical measurements The tap water from the Kyiv city distribution system was used in all experiments. Its composition is presented in Table 1. The measurements were conducted in a 1L glass beaker filled with tap water and thermostated at 250.5 C (Fig. 1). A mild steel cylinder with a diameter of 6 mm and 30 mm long was rotated at 500 rpm and served as a working electrode. Its surface was prepared by turning, degreasing, rinsing with distilled water and drying in hot air. The counter electrode was a platinum grid and a saturated silver chloride electrode (saturated AgCl) was used as a reference. The working electrode was polarized at −1.1 V (saturated AgCl) for 240 min to induce precipitation of calcium carbonate on its surface. The current-time dependence was registered concurrently. Each test was run for three times to ensure reproducibility. An ultrasonic device working at the frequency of 27.8 kHz was placed under the bottom of the beaker and radiated the solution. Electric power supplied to the ultrasonic transducer ranged between 33 and 234 W, the transducer diameter was 60 mm and calculated US intensity varied in the range 1.17.5 W/cm2 or 0.033–0.234 W/cm3.
Table 1. Water chemical composition. Parameter рН Dissolved oxygen Total Hardness Total Alkalinity Calcium Magnesium Chloride Sulfate Hydrocarbonate Total dissolved solids
Units DO TH TA Сa2+ Mg2+ Cl– SO42– HCO3– TDS
mg/l mmol/l mmol/l mmol/l mmol/l mg/l mg/l mmol/l mg/l
Value 7.8–8.0 6.0 3.9–4.2 3.9–4.1 3.0–3.1 0.9–1.1 18–25 30–35 3.2–3.5 240–260
Fig. 1. Polarization apparatus for carbonate electrodeposition: (1) Pyrex vessel, (2) working electrode (mild steel cylinder), (3) counter electrode (platinum grit), (4) reference electrode (saturated silver chloride), (5) Luggin capillary, (6) salt bridge, (7) ultrasound bath, (8) ultrasound horn, (9) regulable ultrasound generator, (10) direct current source, (11) potentiostat, (12) signal converter, (13) computer, (14) electric motor, (15) regulated water heater
Scaling investigation The electrochemical approach to investigate the scale formation and removal from the metal surface is based on the measurement of oxygen reduction current during nucleation and formation of calcium carbonate. The calcium carbonate crystals reduce the electrochemically active surface area of the electrode [25,26]. Comparison of the modified and bare electrodes in both the electrochemical and surface parameters allows blocking properties of the deposits to be evaluated. During the scale formation the oxygen reduction current, I, is decreased proportionally to the reduction of an electrochemically active surface area. When the surface is free of any deposits (shortly after polarization started) the current value reaches its highest value, I0. After the scaling is finished the lowest oxygen reduction current value, Imin, is achieved. In practice, it was measured at the end of the It curve (250 min of polarization). When scaling is finished oxygen reduction can occur on uncovered areas only or through the pores of the scale. Hence, scale porosity SP can be calculated from the values of I0 and Imin as: SP
Imin I
1
(1)
The rate of scaling or scaling time, ts, was found by linear extrapolation of the It dependence to zero current. The crystallization kinetics of CaCO3 under the ultrasound action was characterised by the values of I0, SP and ts. Due to the US influence on the initial current value I0, the current values at different US intensities were normalized by I0. To determine the mass of the deposited carbonates the electrodes were weighted before and after electrochemical measurements with 0.0001 g accuracy. The mass of the deposits was used to analyze the results as the area density, mg/cm2. The morphology of the surface was studied with use of a SEM-106I Selmi microscope (Ukraine) operated at 20 kV. The X-ray diffraction phase analysis (XRD) of deposits was performed using a Rigaku Ultima-IV powder X-ray diffractometer with Cu K radiation (30 kV amd 30 mA). Full X-ray diffraction patterns were recorded for the scan angles (2) from 10 to 80 with a step size of 0.04. Using the ICDD-PDF database, individual crystalline phases were identified from their observed XRD patterns.
Results and Discussion The experimental chronoamperometric curves with the US intensity ranging from 0 to 7.5 W/cm2 are presented in Fig. 2. The rise of US intensity causes the reduction of crystallization time and above 3.2 W/cm2 all the curves coincide. All the parameters influenced by US are combined in Fig. 3. The initial current I0 values demonstrate an overall double growth along with a 7-fold increase of the ultrasound intensity (Fig. 3a). I0 is a current of the oxygen reduction with the limiting stage of the process being oxygen diffusion to the electrode surface. This process has a constant speed in the absence of ultrasound. The application of US gives rise to I 0 meaning that the rotation of the electrode is much less efficient to cavitation stirring. The US applied to water produces acoustic vibrations and cavitation [27]. The rise of I0 is caused by the agitating action of cavitation bubbles near the electrode surface [28] and by thinning of the diffusional layer under conditions of insonation [27]. However, 2 regions with different slopes are clearly distinguished in the I0 – US intensity plot. A higher rise of I0 is observed in the intensity range 0–1.9 W/cm2. The consequent increase of intensity (1.9–7.5 W/cm2) changes I0 only slightly, from 0.77 to 0.92 mA/cm2. This observation clearly demonstrates that the masstransport intensification induced by ultrasound has a limiting behavior.
Fig. 3. Normalized chronoamperometric curves with the ultrasound intensity as a parameter (lines – a guide for an eye). Linear sections of the experimental curves were extrapolated to zero current to obtain t s values. Scaling time ts (Fig. 3c) combines the influence of both the nucleation and the growth rate. The number of nuclei, initially formed on the surface, as well as their growth rate, determines the rate of surface coverage and, as a consequence, the current reduction rate. ts appears to strongly correlate with I0 (Fig. 3a), in the intensity range 1.9–7.5 W/cm2 ts remains nearly unchanged similarly to the I0 behaviour. In contrary, at the lowest tested US intensity 1.1 W/cm2 ts reaches its highest value of 200 min. By comparing the shapes of the I–t curves at 0 and 1.1 W/cm2 with the curves obtained by Belarbi [29], one can conclude that the number of nucleation sites becomes larger with the application of ultrasound cavitation. However, the supply of Ca2+ and CO32– ions is insufficient to support the stable growth of enlarged number of crystal seeds. Thus, ultrasound promotes the rise of nuclei even at the lowest tested intensity, however the stirring action is not strong enough to support a stable growth of the formed crystals. So, the scaling proceeds slower what leads to higher t s (Fig. 3c). The rise of ultrasound intensity increases the intensity of cavitation. The appearance and collapse of cavitation bubbles creates efficient stirring in the bulk solution. Near the electrode surface the effect is several orders of magnitude stronger due to the asymmetrical collapse of the cavitation bubble. Collapsing bubble forms microstreamings that can reach a speed up to 100 m/s. Such an intense stirring eliminates any diffusion barriers and crystallization occurs at the maximum rate for a given Ca2+ concentration. More nuclei are formed with an increase of the US intensity in conditions of stable supply of Ca2+ and CO32– ions. Further rise of the intensity does not cause any changes and scaling time ts reaches a limiting value of 50 min.
Fig. 4. Influence of the ultrasound intensity on scale crystallization parameters: (a) initial current, (b) scale porosity, (c) scaling time, (d) deposited mass.
While I0 and ts dependences showed a platue behaviour, there is a minimum in the SP–US intensity plot (Fig. 3b). SP expresses the blocking ability of the crystal layer at the end of crystallization. The lower the SP value the better blocking abilities and resulting corrosion resistance. A rise of US intensity to 1.9 W/cm2 caused a SP decrease to the minimum value of 1%, which is 5 times lower than the porosity of the scale deposited in the absence of US. The lowest SP value is provided by the US increase of the nucleation site density and mass transport. Further rise of the US intensity above 1.9 W/cm2 makes SP increase. At the highest tested US intensity of 7.5 W/cm2 the SP values amounts to 12%. At the same time, the values of I0 and ts remain unchanged with rise of the US intensity. So, the increase of SP cannot be attributed to the changes in the crystal nucleation or crystal growth processes. A probable reason of the blocking abilities reduction can be a destructive effect of cavitation. US cavitation of higher intensities causes crystal breakage, deagglomeration, and can ruin scale crystals and even clean the surface [18]. SEM analysis after crystallization was performed to determine the crystal morphology. The SEM images were obtained under two magnifications: 250 (Fig. 4) and 2500 (Fig. 5) to study distribution of the crystals on the surface and their morphology, respectively. The application of US clearly demonstrates several significant changes in the crystal layer. First, the single crystal size is reduced from 10–20 m (Fig. 5a) to 6–8 m (Fig. 5b). The crystal size distribution under US is much narrower. In non-sonicated conditions, large crystals (10–20 m) can be found along with smaller ones (2 m and less). Application of US makes the
crystals size practically uniform (6–8 m). Lowering of the average crystal size agrees with the rise of the nuclei number, previously detected in Fig. 3a. Although it is not possible to determine the exact porosity value from SEM images, it is obvious that the crystal layer is the most uniform and the number of uncovered regions (white spots between grey crystals) is the lowest in the figures 4b-d, which correspond to the intensity range 1.13.2 W/cm2. SP has the lowest values in the same intensity range (Fig. 3b).
Fig. 4. SEM images of CaCO3 deposited on the electrode surface in tap water (t = 25 C; electrode rotation rate: 5
rpm, applied potential E 2
−1.1 V/saturated AgCl). US intensity: (a) 0; (b) 1.1; (c) 1.9; (d) 3.2; (e) 5.3; (f)
7.5 W/cm . Magnification 250.
Fig. 5. SEM images of CaCO3 deposited on the electrode surface in tap water (t = 25 C; electrode rotation rate: 5
rpm, applied potential E
−1.1 V/saturated AgCl). US intensity: (a) 0; (b) 1.1; (c) 1.9; (d) 3.2; (e) 5.3; (f)
2
7.5 W/cm . Magnification 2500.
Crystals, obtained at the highest US intensities, are ruined (fig. 5d, e). The single crystals have smooth edges, while at lower intensities the edges are sharp. At 7.5 W/cm2 it is not even possible to determine single crystals due to ruination. Lower magnification shows that at high US intensities the surface coverage also reduces, confirming the SP rise. The stripes of the scale between uncovered patches can be clearly seen in Fig. 4f.
The areal density vs US intensity dependence combines the trends of I 0, ts and SP (Fig. 3d). The area density value halves when intensity reaches 1.9 W/cm2 comparing to non-sonicated conditions. This means that application of US halves the amount of calcite needed to form a protective layer on a steel surface. The further rise of US intensity reduces the deposited mass due to the surface cleaning with high-energy cavitation. This part of the curve agrees well with the corresponding part of the SP – US intensity plot (Fig. 3b).
Fig. 6. XRD patterns of surface deposits, formed on mild steel in tap water during cathodic polarization at different ultrasound intensities. – calcite; □ – iron.
Fig. 6 shows the XRD patterns of surface deposits for applied ultrasound intensities. In all patterns the peaks at 44.6° and 64.7° correspond to the steel substrate due to low thickness of deposits. All other peaks were attributed to the calcite phase of calcium carbonate according to the JCPDS PDF2 standard card (01-089-1304) (ICDD (PDF-2/Release 2011 RDB). These data indicate that the calcite phase was predominantly formed during crystallization [15]. US was found not to change the crystal phase of calcium carbonate. At the same time the intensity of peaks in the XRD patterns was observed to decrease with rise of ultrasound intensity from 3.2 to 7.5 W/cm2. At the highest applied US intensity of 7.5 W/cm2 calcite peaks are hardly distinguishable, in agreement with the SEM surface observation (Fig. 4f) and deposited mass measurements (Fig. 3d) due to cavitation ruination of the crystals. To summarize, three major factors determine properties of the scaling under US application: the rise of nucleation site density, mass transport acceleration and crystal ruination with cavitation. A rise of nucleation site density occurs even at the lowest tested US intensity 1.1 W/cm2 and is evident from the SEM image (Fig. 4b). At the same time, Ca2+ supply is insufficient and scale deposition requires longer time. Higher US intensities
enhance Ca2+ and O2 supply and the scale layer is built faster with low porosity and thickness (1–3 %). A further rise of US intensity leads to stronger cavitation so that crystals ruination prevails over their deposition. Strong cavitation causes destruction of single crystals and eventually at 7.5 W/cm2 clears the surface. Two practical points can be gained from the obtained results. US intensity of 1.9 W/cm2 applied to the mild steel equipment exposed to tap water may help to build a highly protective scale layer on the surface. If the full surface clean-up is required, the US intensity as high as 7.5 W/cm2 is to be applied. Thus, the application of US cavitation can simultaneously solve both scale and corrosion problems in heat-exchanging equipment also being an environmental friendly and easy-accessible method. Further investigations are to be aimed on ultrasound frequency influence and ultrasound field distribution around the horn to achieve a simple and efficient technique to control both scaling and corrosion in heat-exchangers.
Conclusions The influence of ultrasound intensity on both CaCO3 deposition kinetics and scale properties on the steel rotating cylinder electrode was investigated. The following conclusions can be drawn. 1.
The application of US to the surface crystallization process accelerates the mass transport and increases nucleation site density. This leads to the rise of the initial current density and reduces scaling time. The mass transport acceleration results from the efficient solution agitation with acoustic waves and cavitation.
2.
The ultrasound intensity of 1.9 W/cm2 was found to promote formation of a scale layer with porosity of less than 1 %, what is five times lower comparing to the corresponding value obtained in the absence of ultrasound. The layer crystallization time and area density halved. Further increase of ultrasound intensity does not change the crystallization kinetics significantly.
3.
The increase of ultrasound intensity above 1.9 W/cm2 enhances a cavitation power. The strong cavitation at ultrasound intensity of 7.5 W/cm2 ruins the scale crystal, thus raising the porosity and eventually cleaning the surface.
4.
The application of ultrasound simultaneously solves scale and corrosion problems in heat-exchanging equipment. This environmental friendly and easy-accessible technique can provide the foundation for the development a simple and efficient approach for scale control in industry.
Acknowledgements This work was supported by the Ministry of education and science of Ukraine [grant number 2044, 2017].
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Highlights -
Influence of the ultrasound (28 kHz, 1.17.5 W/cm2) on CaCO3 nucleation-growth was studied;
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Ultrasound affects nucleation sites density, mass-transport rate and cavitation erosion;
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The highest blocking properties were obtained under applied intensity of 1.9 W/cm2;
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Ultrasound intensity of 7.5 W/cm2 ruins the scale crystal, thus cleaning the surface.