Corrosion resistance of the modified surface of AISI 316L steel

The paper is devoted to the study of the effect of laser processing on the wettability and corrosion resistance of the surface of experimental samples made of AISI 316L sheet steel and samples made by printing from AISI 316L steel powder using selective laser melting technology. As a result of the studies, it was established that laser texturing of the relief with the subsequent formation of molecular layers of surfactants on a steel surface helps to achieve higher contact angle values, however, such treatment leads to a decrease in corrosion resistance.


Introduction
AISI 316 steel and AISI 316L with reduced carbon content is characterized by a better degree of weldability are the materials widely used in various fields of industry.For example, it is known that microchannel heat exchangers [1] and blades of steam and gas turbines [2] are made from AISI 316L steel powder using 3D printing.
One of the main causes of corrosion of steel surfaces is contact with aggressive gases dissolved in the working fluid (coolant), such as oxygen.Iron, which is the main element in the composition of steels, reacts quite quickly with dissolved oxygen, and the speed of the entire process is determined by the concentration of the latter.In the case when layers are formed that effectively prevent the penetration of oxygen through them, steel surfaces will be less susceptible to corrosion processes.
Studies [3][4][5] have shown that hydrophobic surfaces of various structural materials have increased corrosion resistance.The mechanism of corrosion protection in these studies is because upon contact between a hydrophobic surface and, for example, a liquid, an air layer is formed, which significantly reduces the area of contact of the aggressive environment with the metal and prevents the occurrence of corrosion processes [6].However, this effect, as a rule, is not reproduced at excess pressure, due to the fact that the heterogeneous wetting regime transforms into a homogeneous regime, i.e. the liquid is in contact with the entire surface area and there is no air layer.In this case, corrosion processes are intensified due to an increase around contact of the liquid with the surface.

Hydrophobization of metal surfaces
It is known that the main conditions, the joint fulfillment of which is necessary to create hydrophobic surfaces, are as follows:  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 both through the use of various coatings and through surface treatment.It is known that some modern anti-corrosion coatings have hydrophobic properties.Such coatings reduce the corrosion activity of the metal and are wear-resistant.To impart hydrophobic properties to a surface, various organic and inorganic coatings are usually used, which lead to a decrease in its surface energy.
Thus, in [7] it was shown that the surface of samples with a fluoroplastic coating and a contact angle of about 159° has less corrosion activity, which is expressed in a shift in the corrosion potential of the Tafel polarization curves towards more positive (more noble) values.
The research results presented in [8] show that the formation of a coating on the surface of steel based on a compound of propylene with maleic anhydride and the addition of graphene oxide made it possible to increase the contact angle to 166°.Due to the high dielectric properties of the coating, the corrosion resistance of the modified steel surface has increased significantly compared to the original one.
The listed examples of hydrophobic coatings used for corrosion protection have a greater safety margin than an oxide film, but their gradual degradation leads to a decrease in the effectiveness of corrosion protection [9].
Recently, more and more attention has been paid to the development of mechanical methods for modifying metal surfaces in order to change wettability properties, in particular to achieve a hydrophobic state.An analysis of research conducted in this area has shown that the use of hydrophobic surfaces created on the basis of mechanical modification of the surface with a subsequent reduction in surface energy helps to reduce hydraulic resistance during the transportation of liquid media [10][11], reduce the rate of ice formation [12], and intensify heat exchange processes [13] and other positive effects.
One of the promising methods for surface modification is laser ablation.During laser surface treatment, part of the material is removed, resulting in the formation of a structured relief with specified geometric properties.Laser surface treatment makes it possible to achieve high contact angles (up to 150° [14]), while the technological process is quite easy to implement and inexpensive.However, in the case of stainless steel surfaces, laser modification can lead to the removal of the oxide film, providing high corrosion resistance of the surface.To reduce surface energy and partially restore anti-corrosion properties, it is possible to form polymolecular layers of surfactants on the modified surface.

Manufacturing of experimental samples
To conduct comparative corrosion studies, samples were made from AISI 316L sheet steel using the guillotine cutting method and samples from AISI 316L steel powder using the printing method based on 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 manufactured experimental samples 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 corrosion testing process: 1 and 8 -samples with initial surface, 2 and 9 -samples with an initial surface 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 with subsequent formation of molecular layers of surfactants.

Research methodology
Before conducting comparative corrosion studies, the mass of the samples was measured, and the contact angle was measured on the surface of the experimental samples using a contact tensiometer.Corrosion studies were carried out using the gravimetric method and were carried out in a model environment containing 30 mg/dm 3 of NaCl and 70 mg/dm 3 of Na 2 SO 4 (chloride content -18.2 mg/dm 3 , sulfates -47.32 mg/dm 3 ), as a basis for prepared using distilled water.Each sample was placed in the middle of the volume of the model medium in a separate container (beaker), where it was kept for 890 hours.After a given time, the samples were removed from the containers and subjected to cleaning in an ultrasonic bath, after which they were rinsed with distilled water and dried in a desiccator for 24 hours.Next, the samples were re-weighed and the corrosion rate K (mg/m 2 •hour) was calculated using expression (1).
where  -initial mass of the sample, g,  -sample mass after research, g,  -sample surface, m 2 ,  -time, h.

Results and discussion
As a result of measuring the contact angle on the untreated surfaces of experimental samples manufactured using additive technologies from AISI 316L steel powder and AISI 316L steel sheet, it was revealed that the initial surfaces of the samples are characterized by a hydrophobic state, and the angle values differ slightly and are 96 and 95° respectively.
It has been established that reducing the surface energy by applying molecular layers of surfactants to the surface of experimental samples with the original surface does not lead to a significant increase in the degree of hydrophobicity, and the values of the surface contact angle after treatment are 98 and 101° for samples made using 3D printing and from sheet metal.steel accordingly.
The created relief consists of parallel grooves, the distance between the vertices of which is ~100 μm.The depth of the groove depends on the laser radiation density and varies from ~5 to ~70 μm at a density of 50 to 300 J/cm 2 , respectively.Figure 2, as an example, shows photographs of the surface and section of an experimental steel sample modified at a laser radiation energy density of 150 J/cm 2 .а) b) Figure 2. Electron microscopy photographs of the surface (a) and section (b) of an AISI 316L steel sample processed using laser equipment at a radiation density of 150 J/cm 2 .
As a result of the analysis of the data obtained by measuring the contact angle on the surface of experimental samples, the dependence of the contact angle on the value of the laser radiation density at which the steel surfaces of the samples was processed was revealed (see Figure 3).Figure 3 shows that an increase in the laser radiation density when treating the surface of samples leads to an increase in the contact angle.At the same time, the value of the contact angle of the surfaces of experimental samples made of AISI 316L sheet steel is higher than for samples manufactured using additive technologies.The maximum values of the contact angle were achieved during laser texturing APITECH-V-2023 Journal of Physics: Conference Series 2697 (2024) 012018 of steel surfaces with a laser radiation density of 300 J/cm 2 and are 136 and 156 degrees for samples made using 3D printing and from sheet steel, respectively.It is worth noting that at radiation densities in the range from 50 to 150 J/cm 2 , wettability conversion occurs, and, despite the formation of molecular layers of surfactants on surfaces treated using laser equipment, the samples exhibit hydrophilic properties.
Figure 4 shows images of the surface of experimental samples made of AISI 316L sheet steel (1-7) and samples made using additive technologies from AISI 316L steel powder (8-15) after corrosion tests: 1 and 8 -samples with the original surface, 2 and 9 -samples with an initial surface with formed molecular surfactant layers, 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 of surfactants.As a result of comparative corrosion studies, it was revealed that the corrosion rate of samples with an original sheet steel surface is 8.3 times lower than the corrosion rate of surfaces manufactured using additive technologies (0.11 and 0.93 mg/m 2 •h, respectively).The formation of molecular layers of surfactants on the surface of the initial samples of two types does not affect the corrosion rate; no significant changes in the surface were detected.
Figure 5 shows the dependence of the change in corrosion rate on the value of the contact angle on the surfaces treated using laser radiation with the subsequent formation of molecular layers of surfactants, experimental images made using additive technologies from AISI 316L steel powder (curve 1) and on the surface of experimental images from sheet steel AISI 316L (curve 2) after completion of corrosion tests.From the analysis of the dependencies presented in Figures 3 and 5, it was revealed that surface modification using laser equipment reduces the corrosion properties of two types of samples: the corrosion rate increases with increasing radiation density and, as a consequence, the degree of hydrophobicity from 1.2 to 3.3 mg /m 2 •h for the surface of sheet steel samples and from 1.2 to 4.2 mg/m2•h for the surface of samples made by 3D printing.Moreover, surface modification using laser equipment practically equalizes the corrosion rates of sheet steel samples and samples made by 3D printing from AISI 316L steel powder -the corrosion rate of sheet steel samples is less than the corrosion rate of samples made using additive technologies, on average by 1 ,2 times.

Conclusion
As a result of the research, the following was established:  the corrosion rate of samples with an original sheet steel surface is 8.3 times lower than the ones manufactured using additive technologies (0.11 and 0.93 mg/m2•h, respectively);  the 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 leads to an increase in the degree of hydrophobization.The maximum values of the contact angle were achieved during laser texturing of steel surfaces with a laser radiation density of 300 J/cm2 and are 136 and 156 degrees for samples made using 3D printing and from sheet steel, respectively;  increasing the degree of hydrophobicity by laser texturing of the relief on steel surfaces with the subsequent formation of molecular layers of surfactants leads to a decrease in corrosion resistance.However, such modification of surfaces practically equalizes the corrosion rates of samples made from sheet steel and samples made by 3D printing from AISI 316L steel powder -the corrosion rate of samples from sheet steel is less than the corrosion rate of samples made using additive technologies, on average, by 1.2 times.

Figure 1 .
Figure 1.Images 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 corrosion tests.

Figure 3 .
Figure 3. Dependence of the contact angle on the laser radiation density during surface treatment of experimental samples made using additive technologies from AISI 316L steel powder (curve 1) and from AISI 316L steel sheet (curve 2).

Figure 4 .
Figure 4. Images 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) after corrosion tests.

Figure 5 .
Figure 5. Dependence of the change in corrosion rate on the degree of wettability of the surfaces of 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 corrosion tests.