Anticorrosion Behaviour of Calcareous Deposits Formed on Steel Heat-Exchange Surfaces

A new electrode to study both scaling and corrosion processes of mild steel in tap water was developed. Two identical steel rings are placed on the outside of a glass tube which is heated from inside with an electric spiral; the rings are connected to a corrometer to form a two-electrode corrosion probe. ,e corrosion rate variations with scale thickness, scale deposition time, and solution composition are measured using the linear polarization resistance technique. ,e deposited scale was formed of calcite crystals of 50–100 μm as established with SEM and XRD. ,e scale layer of 0.2mm formed in tap water within 90 hours reduces the steel corrosion rate from 0.8 to 0.1mm/year and serves as a barrier layer to prevent further corrosion.


Introduction
Scale and corrosion remain the most crucial problems of industrial heat-exchanging equipment [1][2][3]. Scale formation leads to heat losses and reduction of hydraulic pressure while corrosion causes fast shortening of the operation time.
e formation of scale can lead to additional damage and increase of thermal fatigue [4]. e inhomogeneous accumulation of scale leads to the localization of corrosion processes and the formation of corrosion pits [5]. Equipment functioning in the scale and corrosion conditions requires regular clean-up and on-line water treatment, as well as more frequent equipment replacement due to corrosion losses. e expenses related to scale and corrosion problems cause billion dollars of losses [6][7][8]. e search for new efficient techniques to solve simultaneously scale and corrosion problems is both a challenging and actual task.
Promising approaches are based on water treatment with a complex reagent which enables prevention of both scaling and corrosion in heat exchangers [9,10]. e challenge of the task is that the scale and corrosion processes are reversely related: diminishing of scaling leads to corrosion aggression, whereas the dense scale layer reduces corrosion rate but causes high heat losses [11,12]. e selection of an optimal way of scale and corrosion reduction in industry is based on tedious laboratory investigations. For this purpose, different techniques are applied; however, their results often appear controversial.
e scaling tendency of water with added reagent is analysed by chemical methods considering changes in solution during the test. e model solution is thermostated at 80°C, and the hardness salts concentration is titrated before and after the test [13]. e scaling tendency is calculated from the difference between the hardness contents before and after the test. e lower the difference, the higher the antiscaling efficiency of the reagent. e main drawback of this technique is that the properties of a heat-exchange surface towards scaling are ignored. e scale appearance is the result of change in water composition under heat; however the scale growth on the heat-exchange surface depends on the crystals' nucleation, their adhesion, and deposit removal [14].
To study the scale crystals growth on a surface, pH or carbonic equilibrium shift methods are applied [15][16][17]. Seed crystals are introduced to the saturated solution of calcium carbonate, and alkali is added drop-by-drop to induce crystallization. e amount of added alkali indicates the solution stability.
A more precise technique, electrocrystallization, applies electrochemical measurements [18][19][20][21][22][23][24][25][26][27][28][29]. A working electrode, which is usually made of steel, is polarized cathodically to a potential of 1.1 V/SCE to cause oxygen reduction. e pH value of electrolyte adjacent to the steel surface is raised to 9.5, so that carbonic equilibrium is shifted to the formation of carbonates. Calcium carbonate precipitates thus reducing cathodic current. From the current-time dependence scaling, time and blocking properties of the layer can be estimated. While the method deals with both scaling potential of the solution and blocking properties of the scale layer, it works only until the bare surface of the electrode is fully covered. Further growth of the scale layer is not possible.
An efficient experimental procedure that estimates both bulk and surface scaling coupled with corrosion rate measurements is highly desired [30]. e aim of the present work was to develop a simple technique to study temperature scaling and corrosion rate variations of mild steel under the scale layer. To this aim, a working electrode (Figure 1), which models a heat-exchange surface, has been designed that enables instantaneous corrosion rate determination during the scale deposition.

Working Electrode Construction.
A scheme of the working electrode is given in Figure 1. Its body was made of Pyrex glass tube (1) of 8 mm diameter. A heating spiral (2) made of a 0.3 mm nichrome wire was placed inside the glass tube. e electrical resistance of the spiral was 8 Ohm. e space between the wire and inner glass wall was filled with 2-4 μm SiO 2 powder (3) to achieve better heat transfer between the spiral and the glass wall. A pair of identical steel rings (4) (10 mm length and 19 mm in diameter) were placed coaxially on the outer wall of the glass tube. Mild steel St3 (European analogue of Fe 3 CN) containing 0.20% C, 0.43% Mn, 0.23% Si, 0.016% S, and 0.02% P was used as a material for the rings. e heating spiral was connected to the current transformer with the output voltage of 24 V, electrical power of a spiral was 72 W, and heat load of the electrode was 60 kW/m 2 .

Experimental Procedure.
e steel rings surface was mechanically polished with SiC emery paper of P600 grade, degreased with organic solvent, and rinsed with tap and distilled water. e working electrode was placed inside a glass beaker of 1 l, filled with water solution. e beaker was closed on the top with a teflon cover, and a reverse cooler was placed in the cover to prevent solution evaporation during the experiment. A thermometer was also placed on the top of the teflon cover in order to monitor temperature variations during the test. e working electrode heated the solution with its heating spiral. After approximately 3 hours, the water in the beaker reached the temperature of 100°C and started boiling, and the electrode working conditions were reached. e test continued for 6 hours; afterwards, heating was turned off, and the water was replaced with a fresh portion to keep hardness salts concentration constant. e procedure was repeated daily. e total boiling duration reached 100 hours. All the tests were performed in tap water, and its composition is given in Table 1. e hardness salts content was determined titrometrically before and after the test as total concentration of calcium and magnesium ions expressed as calcium carbonate. A solution sample was buffered to pH 10.1, the indicator was added, and ethylenediaminetetraacetic disodium salt (EDTA) was used as the titrant. e total dissolved solids content was determined with a TDS-meter based on conductivity measurements. e pH value of the solution was measured using an I-160MI pH-meter.

Scale Characterization.
e linear polarization resistance technique (LPR) was used for corrosion [31][32][33]. A pair of the steel rings were electrically connected to a corrometer R5126 (Ukrainian manufacture) and served as a two-electrode corrosion probe. LPR measurements were performed every 15 min. e corrometer utilizes galvanostatic polarization mode: the current density of 5 μA/cm 2 is applied during 30 sec to the pair of the electrodes [34]. e initial potential difference and solution resistance were automatically compensated, and the polarization resistance value was recalculated into steel corrosion rate with Stern's equation using the conventional B value of 26 mV [35]. e B value of 26 mV was e scale growth rate during its deposition was characterised by the scale thickness. Every 20-40 hours of boiling, the diameters of the steel rings were measured using a micrometer MK-25 with 0.01 mm accuracy. e X-ray diffraction phase analysis (XRD) of the scale deposits was performed using a Shimandzu LabX XRD 6000 powder X-ray diffractometer. Full X-ray diffraction patterns were recorded for the scan angles (2θ) from 5 to 90°with a step size of 0.1°. By using the ICDD-PDF database, individual crystalline phases were identified from their observed XRD patterns. e morphology of the surface was studied with the use of a SEM-106I Selmi microscope (Ukraine) operated at 20 kV.

Results and Discussion
e typical time dependence of the temperature and water parameters is presented in Figure 2. Water temperature rises during the first 3 hours and remains constant when the water is boiling (Figure 2(d)). It is clearly seen that the major hardness salts deposition occurs between 1.5 and 4.5 hours of heating (Figure 2(d)). e deposition starts when the water temperature reaches 70°C and continuous while hardness salts are present in the water. However, it is not reasonable to continue the experiment after 6 hours, because the crystallization rate becomes very low. After this period, further boiling does not change water hardness appreciably meaning all available carbonates are deposited. At this point, the solution should be replaced with a fresh one. e same results are obtained with TDS measuring during the test (Figure 2(c)). Comparing the TDS and total hardness dependences vs. time, it can be clearly seen that the calcium carbonate is the only precipitate in these conditions. e pH of the water (Figure 2(b)) rises during the first 4.5 hours, due to CO 2 desorption from the water. When the main part of CaCO 3 is deposited, pH value becomes constant. e scale thickness vs. time is given in Figure 3, curve 1. e deposition of hardness salts results in scale formation on the electrode surface. e surface scale layer grows steadily (21 μm/h) during the test meaning the scaling conditions remain constant. e deposition of carbonate scale forms a barrier layer on the electrode surface reducing oxygen ingress and so steel corrosion rate (Figure 3, curve 2). When a freshly prepared steel surface was immersed in tap water, the corrosion rate reached significant values up to 0.8 mm/ year. After the first 20 hours of boiling, the corrosion rate starts decreasing and finally stabilizes below 0.1 mm/year in 70 hours. Further scale deposition does not cause any further corrosion rate reduction. e rate of steel corrosion in tap water is known to be controlled by the rate of oxygen diffusion to the surface. When a freshly prepared steel surface is immersed into the solution, the corrosion rate is high. As scale deposition progresses, the scale crystals partially cover the surface and the oxygen ingress to the covered parts is limited.
us, corrosion rate decreases. e size and morphology of single scale crystals are determined by water composition and scaling rate, while the number of crystal nuclei is determined by the surface roughness. After the initial nuclei are formed on the surface, further scale growth occurs on these crystals and appearance of new crystallization sites is less probable. When all the surfaces are covered with scale crystals of finite size, some pores remain and corrosion continues, but the corrosion rate is significantly lower. e formation of a sediment layer on the metal surface is known to influence the corrosion process. As investigated for the 316L and ND steels in the conditions of a gas deep cooling device operation, the metal surface was covered by an ash deposit layer, coupling layer, and corrosion layer. Layered structure of the deposits leads to the accumulation of the acids in the resurface layer, and the corrosion rate rises significantly, especially when the wall temperature is lower than the dew point [36,37]. Such mechanism is not relevant to the corrosion in fluids; however, the formation of a nonuniform scale layer may lead to the localization of corrosion. Parts of the surface, covered with scale crystals, become anodic, and in meantime, pores in the scale layer supply oxygen to the surface and regions near the pores become cathodic. e pairs of differential aeration occur, accelerating local metal dissolution under the scale crystals. In order to prevent localization of corrosion process, the scale layer needs to be thin and uniform. e formation of such a protective layer is the aim of our further investigations.
e morphology of the formed scale layer was investigated with SEM ( Figure 4). e rings surface is fully covered with calcite crystals easily recognisable by their Advances in Materials Science and Engineering 3 rhombohedral shape. e growth of the crystals is spontaneous, and no specific directions are found. Crystals are grouped in agglomerates with the size of 50-100 μm. e crystalline structure of calcite was confirmed with XRD ( Figure 5). e XRD pattern clearly demonstrates characteristic peaks of calcite according to JCPDS PDF2 standard card (01-086-2335). e deposition of calcite occurs on a partially corroded metal surface because the corrosion rate on freshly prepared steel is high. When the surface is covered with the scale layer, scale growth rate becomes constant and no changes in the layer structure are observed.
e presence of Mg is detected in the crystals because of magnesium incorporation into the calcite lattice [38]. Incorporation of magnesium in the calcite crystals changes their morphology and crystals become damaged and nonuniform. Mg 2+ ions are known to adsorb on the growing sites of the calcite crystal, thus blocking its regular growth.  e scale layer appears to be an efficient barrier on the steel surface, being able to suppress oxygen diffusion and thus to reduce corrosion rate. Such a barrier can play a significant role in corrosion prevention in industrial equipment that operate in the scaling conditions; however, excess scale thickness impedes heat transfer and flow rate. e designed working electrode successfully models a heat-exchanger surface and is a convenient tool to study both scaling and corrosion processes. Its use allows studying the dependence of steel corrosion rate on scale morphology and thickness. e main advantage of the proposed electrode design is that scaling continues when the electrode surface is fully covered with the scale crystals.
is gives a possibility to study controllable scale removal and its influence on corrosion rate.

Conclusions
A simple and reliable technique to study scale formation and corrosion processes concurrently on the steel surface has been developed. e designed working electrode can be used both in laboratory and in industry to determine the scale and corrosion inhibitors efficiency. e developed electrode and technique were used to establish the protection properties of a scale layer on the steel surface. e scale layer of 0.2 mm formed from tap water reduces the corrosion rate from 0.8 to 0.1 mm/year in 100 hours and can serve natural corrosion protection. e developed technique can be further used to investigate the influence of the scale removal processes on corrosion of steel heat-exchange surfaces.
Data Availability e LPR and XRD data used to support the findings of this study are included within the article.

Advances in Materials Science and Engineering
Conflicts of Interest e authors declare that they have no conflicts of interest.