On reducing the rate of deposit formation on the AISI 316L steel surface

The article is devoted to the investigation of the rate of deposit formation on the hydrophobic modified surface of experimental samples made of AISI 316L sheet steel and samples of AISI 316L steel powder printed using selective laser melting technology. A saturated solution of CaCO3 was used as a model medium during the research. The research established that hydrophobization of the surface of AISI 316L steel based on laser texturing with the subsequent formation of molecular layers of surfactants can significantly reduce the rate of deposit formation.


Introduction
One of the ways to save energy in the energy sector and industry is to reduce the rate of deposit formation on the functional surfaces of heat exchange equipment.The formation of layers of deposits occurs because of the deposition and accumulation of substances, for example, suspended particles and insoluble salts, on the internal or external surfaces of heating equipment.This in turn leads to a decrease in the efficiency and reliability of equipment, an increase in hydraulic resistance and, consequently, an increase in operating and capital costs.
Modern products for energy and industry can be manufactured using traditional sheet or injection molded materials, as well as using additive technologies -3D metal printing.It is known that microchannel heat exchangers [1] and blades of steam and gas turbines [2] are manufactured using 3D printing.
One of the materials widely used in the chemical and food industries, as well as for the manufacture of heat exchange equipment, is stainless steel AISI 316 and a modified version of AISI 316L with a reduced carbon content, as a result of which it is characterized by a better degree of weldability, although both types have significant resistance to corrosion.AISI 316L steel powder is used for the manufacture of microchannel heat exchangers used for cooling electronic devices using additive technologies [3].
Typical deposits on heat exchange surfaces of thermal power equipment are corrosive (metal oxides and hydroxides) and salt (CaCO 3 , CaSO 4 , CaSiO 3 and others) deposits.In this case, corrosive products initiate and accelerate the formation and formation of layers of deposits.The methods and methods used today to combat the formation of deposits on functional surfaces are aimed at restoring the original thermohydrodynamic characteristics, but do not eliminate the cause of deposit formation.In this regard, there is a need to develop a method to reduce the rate of deposit formation or completely eliminate the need to clean both the functional surfaces of traditional products made from AISI 316L steel and the surfaces of products made using additive technologies from AISI 316L steel powder.

Conditions for reducing the rate of deposit formation
The mechanism of formation of deposits on the surface can be described using the theory of heterogeneous nucleation and crystal growth [4,5].According to the classical theory of nucleation [6], due to stochastic density flotation in an aqueous medium, caused by the chaotic movement and collision of clusters of impurity molecules, a crystal of a metastable phase is formed.Further stable growth of the crystal upon aggregation with molecules from the bulk of the aqueous medium is possible only when it reaches a critical size.The barrier to heterogeneous nucleation of a stable phase crystal on a smooth surface is the surface energy at the phase interface.Moreover, the lower the surface energy, the higher the value of the potential barrier that prevents heterogeneous crystallization.This determines the achievement of a positive effect when using hydrophobic surfaces to prevent the formation and formation of layers of deposits.
It is known that the main conditions, the joint fulfillment of which is necessary to create hydrophobic surfaces, are:  changing the morphology of the material surface in order to establish a heterogeneous wetting regime;  reduction of surface energy.Changing the morphology of a metal surface is possible using different coatings or through mechanical modification of the surface.The effective use of hydrophobic coatings to reduce the rate of deposit formation was confirmed, for example, in [7], in which a hydrophobic composite surface based on carbon nanoparticles was kept in a supersaturated CaCO 3 solution together with samples with hydrophilic properties.After exposure for a given time, calcite structures turned out to be predominant on hydrophilic surfaces, and aragonite structures were dominant on the hydrophobic surface.At the same time, it is known that needle-shaped structures characteristic of aragonite, a metastable crystalline structure of CaCO 3 , have less adhesion to the surface [8,9].However, coating processes are usually quite expensive and technologically complex, and the resulting coatings are characterized by low mechanical properties.
Mechanical methods that ensure the creation of a relief structure by surface deformation (knurling and cutting) do not have these disadvantages.However, the use of the deformation method allows the formation of a relief of constant geometry, without the possibility of varying the characteristics of the relief.Currently, modification of the surfaces of various structural materials using laser equipment -the laser ablation method -is being intensively developed [10].During laser texturing of a relief, various thermal processes occur on the metal surface, leading to melting, evaporation and recrystallization of the surface material, as a result of which a multimodal relief is formed.
The low surface energy characteristic of hydrophobic surfaces also contributes to a decrease in the rate of deposit formation on the metal surface.Lower surface energy is characterized by lower adhesion of deposits on the metal surface [11] due to a decrease in the strength of bonds at the solid-liquid interface.It was also noted in [12,13] that the growth rate of crystallization centers on surfaces with low surface energy is lower than for surfaces with high surface energy.Thus, a reduction in surface energy is possible, for example, using surfactants, in particular octadecylamine.

Manufacturing of experimental samples
To study the rate of deposit formation, samples were made from AISI 316L sheet steel by cutting and samples from AISI 316L steel powder by printing using selective laser melting technology.
The main characteristic of laser radiation that affects the geometric properties of the formed relief is the energy density of laser radiation [14].To form the relief in this study, the following laser radiation energy densities were chosen: 50, 100, 150, 200 and 300 J/cm 2 .
After laser texturing of the relief, molecular layers of surfactants were formed on the surface of the experimental samples due to the sorption of molecules onto the metal surface from an aqueous medium.
Figure 1 shows images of the surface of experimental samples made from AISI 316L sheet steel (1-7) and samples made using additive technologies from AISI 316L steel powder (8-15) before the start of the deposit formation process: 1 and 8 -samples with initial surface, 2 and 9 -samples with formed molecular layers of surfactants, 3-7 and 10-15 -samples subjected to laser texturing at laser energy densities of 50, 100, 150, 200 and 300 J/cm 2 , followed by the formation of molecular layers Surfactant. ) and samples manufactured using additive technologies from AISI 316L steel powder (8-15) before the deposit formation process.

Research methodology
Before the deposit formation process, the mass of the samples was measured, and the contact angle was measured on the surface of the experimental samples using a contact tensiometer.
To carry out the research, an experimental stand was used, the schematic diagram of which is presented in Figure 2. The process of deposit formation was carried out as follows.A measuring container (1) with the required amount of model medium ( 2) is placed in a container (3) with ice (4).Experimental samples (5) are placed in the model environment, fixed parallel to each other at a distance of 70 mm.The samples are heated by an electric current passed directly through them.To do this, wires are attached to the ends of the samples, which in turn are connected to an alternating current source (6).A saturated solution of calcium bicarbonate was used as a model medium for deposit formation.The exposure time of experimental samples in a model environment is 30 minutes.During this time, the main part of the formed calcium carbonate precipitates and heating stops.After a given time, the samples were removed from the container, hung vertically to dry, and kept in a desiccator for 24 hours.Next, the samples were weighed again and the rate (g/m 2 •hour) of deposit formation was calculated using equation (1). ⋅ ( 1 ) where m 0 -initial sample mass, g, m 1 -sample weight after testing, g, S -sample surface, m 2 , t -time, h.

Results and discussion
The created relief consists of parallel longitudinal grooves, the depth of which varies from a minimum value of ~5 µm for samples processed with a radiation intensity of 50 J/cm 2 to ~70 µm for samples processed with a radiation intensity of 300 J/cm 2 .The distance between the tops of the grooves is ~100 μm. Figure 3, as an example, shows sections and images of the surface of experimental samples modified at laser radiation energy densities of 100 and 300 J/cm 2 .The appearance of the experimental samples after completion of the deposit formation process is shown in Figure 4. Figure 4 shows that the amount of deposits on the surface of the samples made both from sheet steel and using additive technologies decreases with increasing laser radiation energy density.At energy densities of 200 (samples 6.7) and 300 (samples 14.15) J/cm 2 , visually there are practically no deposits on the surface, which confirms the theory that the rate of deposit formation on a rough surface with low surface energy decreases.Figure 5 shows the dependence of the change in the rate of deposit formation on the surface of experimental samples made using additive technologies from AISI 316L steel powder (curve 1) and on the surface of experimental samples from AISI 316L sheet steel (curve 2) after completion of the deposit formation process. Figure 5 shows that the rate of deposit formation on the surface of experimental samples, both made of sheet steel and samples made by 3D printing, decreases with increasing contact angle.At the same time, the rate of deposit formation on samples made of sheet steel and samples manufactured using additive technologies differs slightly, the range of the difference is from 1 to 11%, except for samples in the delivered state, where a maximum difference in the rate of deposit formation of 27% is observed.
It was revealed that the reduction in surface energy due to the use of surfactants for initial samples made of sheet steel has virtually no effect on the rate of deposit formation (37.7 g/m 2* h before surfactant treatment and 38.5 g/m 2* h after surfactant treatment), in this case, surfactant treatment of the surface of the original additive samples sharply reduces the rate of deposit formation from 51.4 to 43.30 g/m 2* h.An increase in the contact angle due to surface modification using laser exposure and surfactants leads to a significant reduction in the rate of deposit formation.Thus, modification of the surface of samples made from AISI 316L sheet steel reduces the rate of deposit formation to 33%, and when modifying the surface of samples made from AISI 316L steel powder, the rate of deposit formation decreases to 49%.

Conclusion
As a result of the study conducted to determine the rate of formation of deposits on the surface of AISI 316L steel, the following was established:  creation of a structured relief on the surface of steel using laser irradiation with a subsequent decrease in surface energy due to the formation of molecular layers of surfactants makes it possible to reduce the rate of deposit formation;  the rate of formation of deposits on the surface of sheet steel differs slightly from the rate of formation of deposits on the surface obtained using additive technologies;  possible reduction in the rate of deposit formation on the surface of sheet steel compared to the original surface is up to 33%;  possible reduction in the rate of deposit formation on the surface created using additive technologies from AISI 316L steel powder is up to 49%.

Figure 1 .
Figure 1.Image of the surface appearance of experimental samples made of AISI 316L sheet steel (1-7) and samples manufactured using additive technologies from AISI 316L steel powder (8-15) before the deposit formation process.

Figure 2 .
Figure 2. Schematic diagram of the experimental stand for the formation of deposits on the modified surface of experimental samples.

Figure 3 .
Figure 3. Images of samples made of AISI 316L steel obtained using electron microscopy: (a) -top view at a radiation intensity of 100 J/cm 2 , (b) -side view at a radiation intensity of 100 J/cm 2 , (c)top view at a radiation intensity 300 J/cm 2 , (d) -side view at radiation intensity 300 J/cm 2

Figure 4 .
Figure 4. Image of the appearance of the surface of experimental samples made of AISI 316L steel sheet (1-7) and manufactured using additive technologies from AISI 316L steel powder (8-15) after completion of the deposit formation process.

Figure 5 .
Figure 5. Dependence of changes in the rate of deposit formation on the surface of experimental samples made using additive technologies from AISI 316L steel powder (curve 1) and on the surface of experimental samples from AISI 316L sheet steel (curve 2) after completion of the deposit formation process.