Effect of Ultrasonic Cleaning after Laser Texturizing of Surface of AISI 316L Steel on the Degree of Wetting and Corrosion Resistance

Abstract

One of the most common current processing methods in various scientific studies is the modification of surfaces of various structural materials via laser radiation (laser ablation technique). The laser texturizing of metal surfaces is one of the promising applications for the creation of hydrophobic surfaces with a high water contact angle, increased corrosion resistance, and other properties. This paper reports the results of experimental studies to determine the effect of ultrasonic surface cleaning after laser texturizing on the degree of wetting and corrosion resistance of AISI 316L steel. The results show that ultrasonic cleaning leads to the removal of micro-/nano-sized particles formed on the surface following the laser texturizing of roughness. This effect, in turn, helps us to obtain higher values for the water contact angle and to increase the corrosion resistance.

1. Introduction

Over the last few decades, the world scientific community has been actively studying the wettability properties of solid surfaces, as well as the influence of the degree of wetting on the functional properties of the surfaces of various structural materials.

Various studies have shown that achieving a hydrophobic or superhydrophobic surface state, when the value of the water contact angle is more than 90 or 150°, respectively, causes effects such as increasing the corrosion resistance of materials [1,2], enhancing heat exchange processes [3,4], reducing the hydraulic resistance during the transport of liquid media [5], reducing the rate of ice formation [6,7] and the formation of various types of deposits [8], and many others. Moreover, many studies claim that the degree of hydrophobicity has a direct influence on the magnitude of the above effects.

To date, many different techniques have been developed for the hydrophobization of metal surfaces, which are based on reducing the surface energy by chemically changing the composition of the surface layer or by applying a hydrophobic agent to the surface. Among the most widespread methods of applying a hydrophobic agent to the surface are methods of adsorption from solutions or vapors [9,10,11], dip coating [12], spin coating [13], and others. However, such methods do not create surfaces with superhydrophobic properties, which are the most promising for use in practice.

In a study [14], it was reported that, in addition to a significant reduction in the surface energy value, the texturizing of micro-/nano-sized ordered roughness on the material surface is also necessary to establish a superhydrophobic surface state. It should be noted that surface energy is the main factor in achieving a hydrophobic state, which is primarily determined by the chemical composition of the surface [15]. However, in [16], it is shown that the lowest surface free energy of a solid obtained through covering the glass surface with correctly aligned most-densely packed hexagonal groups (CF₃) was 6.7 mJ/m², but the achieved contact angle was only 119°. This, in turn, shows that the formation of roughness in the form of micro-/nanoscale hills and depressions, imitating the features of natural surfaces of a lotus leaf or insect bodies, is a necessary condition for achieving a superhydrophobic state [17].

A review of the scientific and technical literature has shown that one of the most promising methods for texturizing a metal surface of micro-/nano-sized relief is a method based on laser ablation [18]. By flexibly varying the laser radiation parameters, this technique allows us to control with sufficient accuracy the geometry of the relief created and, consequently, the degree of wetting. It should be noted that, in most of the known studies, femtosecond laser stations were used to modify the surfaces of various structural materials. However, in recent years, the interest in laser stations with nanosecond pulses has increased in this field.

As shown in [19], the formation of relief on a metal surface during laser texturizing occurs as a result of movement in the metal melted under the influence of laser radiation from the center of the contact spot to the periphery. In this case, a multimodal roughness is formed as a result of the partial return of highly dispersed ablated material. It is assumed that said multimodality of the formed roughness on the solid surface contributes to achieving higher values for the water contact angle of the surface and, as a consequence, to obtaining more significant effects attributable to superhydrophobic surfaces.

2. Effect of Laser Texturing on Corrosion Resistance

Many studies have investigated the corrosion behavior of hydrophobic metal surfaces processed using laser texturing. Most of these studies report improved corrosion resistance compared to untreated material [20,21,22,23]. However, the mechanism for increasing corrosion resistance with such treatment has not yet been identified.

Thus, in works [24,25], it was established that surface treatment using laser equipment leads to the achievement of a superhydrophobic state in the surface, as a result of which the corrosion resistance increases. It is assumed that the increase in corrosion resistance is caused by the fact that, when the surface comes into contact with an aggressive environment, air is captured in the cavities formed from the laser processing of the metal, which prevents the penetration of corrosive substances to the surface itself. In addition, the authors of these studies established a relationship based on which it follows that, with an increase in the contact wetting angle, an increase in corrosion resistance occurs.

The study [26] states that the increase in corrosion resistance for superhydrophobic metal surfaces may be due to the following mechanisms:

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a negatively charged region is formed at the boundary between the aqueous environment and the hydrophobic coating, which leads to a decrease in the content of corrosive anions;

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the hydrophobic coating serves as a corrosion inhibitor, preventing the adsorption of aggressive ions that initiate corrosion processes on the metal surface;

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textured multimodal roughness, like the hydrophobic coating, acts as a barrier to electron transfer between the metal surface and the electrolyte solution.

The authors of [27] proposed another hypothesis, wherein they report that the increase in the corrosion resistance of steel after surface treatment using the laser texturing method is due to the formation of a protective oxide layer enriched with chromium, which forms on the surface of the alloy during laser processing. At the same time, it has been established that the grain size, the distribution of which varies significantly over the thickness of the sample, does not have a significant effect on the corrosion resistance of steel.

It is worth noting that most of the studies that investigated the change in the corrosion resistance of metal surfaces as a result of modification using the laser ablation method were carried out using the electrochemical method [25,28,29].

In this work, comparative corrosion studies were carried out using the gravimetric method. The choice of this method is justified by a number of the following advantages over other methods of measuring corrosion rate:

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it is a direct measurement method, as it is based on measuring the mass loss of the material;

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it allows us to obtain fairly accurate results, since this method involves weighing the objects of study before and after corrosion tests. Despite the long testing time, this method is used as a verification method in arbitration analyses.

3. Materials and Methods

This paper is an experimental study to determine the effect of ultrasonic surface cleaning after laser modification on the degree of wetting and corrosion resistance of the surface of AISI 316L steel. To conduct the study, two series of test specimens were manufactured and processed under identical laser treatment parameters. One of the series was subjected to ultrasonic cleaning after laser surface modification. In order to increase the accuracy of the results, three samples were made for each test parameter and, thus, the total number of samples was 36, of which:

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samples with the original surface—3 pieces;

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samples on the surface of which molecular layers of surfactants were formed—3 pieces;

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samples modified using laser equipment and subsequently processed in a surfactant emulsion—15 pieces;

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samples subjected to ultrasonic cleaning after laser surface modification and subsequent treatment in a surfactant emulsion—15 pieces.

3.1. Manufacturing of Specimens and Surface Preparation

To conduct an experimental study, specimens with dimensions of 50 × 30 × 1 mm were made from AISI 316L sheet steel. This type of steel was chosen due to its wide and varied range of applications, including power engineering, chemical, oil, marine, and other industries. Before laser treatment, and to clean the surfaces from oil and fat, and abrasive and process contaminants, the test specimens were cleaned with isopropyl alcohol and then washed with deionized water. We should note that such cleaning of metal surfaces is not a mandatory step in laser treatment, but its absence can lead to the distortion of the results of the study.

Surface modification of test specimens was performed using a pulsed ytterbium fiber laser with the following characteristics: the wavelength was 1064 nm; the maximum laser power after focusing the system was 22.4 W. The laser pulse duration can be varied within the range from 4 to 200 ns, with a modulation frequency from 20 to 100 kHz. The laser beam passing through the deflecting system (collimator, galvanometric scanners, and focusing lens) is focused on the focal plane with a spot diameter of ~59 µm.

To create a surface relief in the form of 100 µm equidistant grooves (see Figure 1), the laser texturizing technique was used. The variable scanning speed leads to changes in the laser fluence when treating the surface, resulting in the formation of reliefs with different geometry. The laser treatment parameters and their corresponding laser fluence values are presented in

Table 1. Parameters of the laser treatment.

Laser Output Power, W	Modulation Rate, kHz	Scan Speed, mm/s	Laser Fluence, J/cm2
22.4	20	836	50
		387	100
		255	150
		191	200
		127	300

. The laser texturizing process was performed under atmospheric conditions.

After laser surface modification, a proportion of the specimens were subjected to ultrasonic cleaning for 30 min.

To reduce the surface energy, molecular layers of surfactant were formed on the surface of the specimens treated with the laser station. One of the effective and technologically quite simple current methods is the use of surfactants from the class of film-forming aliphatic amines C16–C20. The treatment in the surfactant emulsion was carried out according to the method presented in [30].

A Tescan Mira LMU high-resolution scanning electron microscope was used to visually analyze the laser-textured surface of the test specimens.

The surface wettability was determined using the OCA 20 optical tensiometer (DataPhysics Instruments GmbH, Filderstadt, Germany). To obtain the most accurate values for the water contact angle, measurements were carried out three times at different points of the modified surface of each specimen. The average angle was determined, and the value of this angle was used in the analysis of the results.

3.2. Selection of a Model Environment for Corrosion Research

The methodology for conducting corrosion research is based on modeling the working fluid, which contains corrosive compounds, and creating conditions that accelerate the process of corrosion in structural materials. When modeling test conditions, it is necessary to take into account the compositional features of a real corrosive environment. External factors affecting steel corrosion include the composition, temperature, speed, and pH of the coolant.

When modeling corrosion processes, it is necessary to be guided by the following principles:

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acceleration of the corrosion process should not be caused by a change in its mechanism;

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to most effectively accelerate the corrosion process, it is necessary to identify the main controlling factor and act on it. For example, if the corrosive activity of one of the components of the medium clearly prevails, in model environments, it is advisable to increase its concentration, while controlling the preservation of the unchanged mechanism of the corrosion process.

The effect of CO₂ comes down to counteracting the formation of a protective film of corrosion products on the surface of the steel at high coolant temperatures, which inhibits the dissolution of steel. The higher the CO₂ content, the lower the protective properties of the oxide film. When steel surfaces come into contact with water with a low pH, a comparative increase in pH can reduce damage by reducing the concentration of CO₂ in the water. It is worth noting that the corrosive activity of CO₂ is 7–10 times lower compared to an equivalent amount of oxygen.

Oxygen has a dual effect on corrosion. On the one hand, oxygen as a passivator reduces corrosion due to the improvement of the properties of the protective film on the metal surface, the oxidation of exposed surface areas, and the formation of passivating adsorption layers on the metal surface; on the other hand, as an active depolarizer, it causes increased corrosion due to the depolarization of the cathode areas. Depending on the concentration of oxygen, the composition of the solution, and other physicochemical parameters, one or the other action of oxygen predominates. For example, increased access to oxygen reduces the corrosion rate of iron by 0.1 N. sodium chloride solution at relatively high speeds of solution movement as a result of passivation. In the presence of a sufficient amount of oxygen, steel corrosion occurs with oxygen depolarization. The corrosion rate is higher with mixed depolarization, for example, the corrosion of iron in tap water containing CO₂ and O₂.

If the corrosion rate is controlled by oxygen diffusion, then, for a given concentration of O₂, the rate approximately doubles for every 30 °C increase in temperature. In an open vessel, from which dissolved oxygen can escape, the corrosion rate increases with temperature up to 80 °C and then drops to a very low value as the water boils. This sharp decrease is associated with a noticeable decrease in the solubility of oxygen in water, and this effect ultimately suppresses the accelerating effect of temperature. At a temperature of 100 °C, the corrosion rate is the same as at ordinary temperature. In a closed system, oxygen cannot escape, so the rate of corrosion continues to increase with increasing temperature until all the oxygen is used up.

Within the pH range from 4 to 10, the corrosion rate is determined only by the rate of oxygen diffusion to the metal surface. The main diffusion barrier, the iron oxide (II) film, is constantly renewed during the corrosion process. Regardless of the pH value of the water within these limits, the surface of the iron is always in contact with an alkaline solution saturated with hydrated iron oxide (pH $\gg$ 9.5). In an acidic environment (pH $<$ 4), the iron oxide film dissolves, the pH value of the iron surface decreases, and the metal is in more-or-less-direct contact with the aqueous environment. In this case, the increase in the reaction rate is the result of both a significant rate of hydrogen evolution and oxygen depolarization. An increase in the alkalinity of the environment (pH $>$ 10) causes an increase in pH on the surface of the iron. The corrosion rate decreases as the iron becomes more and more passivated in the presence of alkalis and dissolved oxygen.

In general, the rate of reaction is determined by the oxygen concentration, temperature, or the mixing rate of the water, since the pH of almost all natural waters ranges from 4 to 10. Therefore, any iron immersed in fresh or sea water—whether low- or high-carbon steel, low-alloy steel, ductile iron, cast iron, or cold-rolled low-carbon steel—will have almost the same corrosion rate.

The content of sulfates and chlorides in water is a factor that determines the intensity of local metal corrosion. The most aggressive are chloride ions, which form highly soluble compounds in aqueous media that can easily penetrate the protective surface films of the metal and contribute to the appearance of point defects (such as pitting) in the passive film on the metal surface. Chloride and sulfate ions are adsorbed on the film and displace oxygen; pores appear in the film, facilitating corrosion.

In this work, a model corrosion test environment was developed, taking into account the characteristics of the chemical composition of the tested material. For example, the model environments recommended as standard for determining the susceptibility of corrosion-resistant steels to pitting corrosion cannot be used when testing carbon and low-alloy steels, since the latter are not passivated in them.

Thus, to study the corrosion rate of steels, model environments of distilled water, tap water, and directly working waters of thermal stations (district water, etc.) can be used:

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model of river water containing 30 mg/dm³ NaCl and 70 mg/dm³ Na₂SO₄ (chloride content—18.2 mg/dm³, sulfates—47.32 mg/dm³);

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model of demineralized water with a pH equal to 9.8–9.9—for heaters of heating supply systems operating on softened or demineralized water;

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model of district water used in heating systems contaminated with chlorides and sulfates, containing 14.8 mg/dm³ NaCl and 56.2 mg/dm³ Na₂SO₄ (chloride content—9 mg/dm³, sulfates—38 mg/dm³), with a solution pH of 9.1 ± 0.1 when testing low-alloy and low-carbon steels.

3.3. Corrosion Studies

Corrosion studies were conducted in a model medium containing 30 mg/dm³ NaCl and 70 mg/dm³ Na₂SO₄ (chloride content of 18.2 mg/dm³, sulphate content of 47.32 mg/dm³). Distilled water was used as a base for the preparation of the model solution.

The specimens were immersed in the corrosive medium in the middle of the beaker, with each specimen placed in a separate beaker. The studies were conducted in static conditions under free aeration at a temperature of 22 ± 2 °C. The soaking time of the specimens was 890 h. After the specified time, the specimens were subjected to mechanical and ultrasonic cleaning and subsequent drying.

The rate of corrosion processes $g$ was determined according to Relationship (1):

$$g=\frac{m_1-m_2}{S·\tau },$$

(1)

where $m_1$ and $m_2$—mass of experimental specimen before and after corrosion tests, mg; $S$—area of entire surface of experimental specimen, cm²; $\tau$—time of soaking of specimens in modelling medium, hours.

The mass of test specimens was determined using OHAUS analytical scales (OHAUS corporation, Parsippany, NJ, USA).

4. Results

Figure 2 shows the image of the surface of the experimental specimen immediately after laser treatment (see Figure 2a), as well as the surface of the specimen treated with similar laser radiation parameters, with subsequent cleaning in an ultrasonic bath (see Figure 2b). Image analysis shows that ultrasonic cleaning removed micro- and nano-sized particles formed during laser texturizing from the surface of the experimental specimen.

To calculate the mass removal as a result of the ultrasonic cleaning of surfaces treated with the laser texturizing technique, the mass of specimens before and after cleaning was determined. The obtained dependence of the specimen mass removal on the fluence of laser radiation (see Figure 3) shows that the increase in the fluence of laser radiation leads to the formation of a greater number (by mass) of particles forming multimodal roughness, which are removed during ultrasonic cleaning, and, therefore, have poor adhesion to the surface.

To determine the effect of ultrasonic cleaning on the degree of wetting of steel surfaces treated using a laser station with subsequent treatment in surfactant emulsion, the values for the water contact angle of the surface of the test specimens were measured. Figure 4 shows the results of the measurements, as well as the obtained dependences of the water contact angle on the laser fluence.

The analysis of the results presented in Figure 4 shows that the original surface of the specimen of AISI 316L steel is hydrophobic without additional modification, but the water contact angle is quite low and is 95°. Treatment of the original surface with a surfactant, which is a hydrophobic agent, leads to a slight increase in the water contact angle, to 101°.

Laser treatment of the surface leads to significant changes in the value of the water contact angle, which has a well-defined dependence on the laser fluence in the range from 50 to 300 J/cm². At a laser fluence of 50 J/cm², the transition to a hydrophilic state with a water contact angle of 67° was discovered. However, increasing the laser fluence during treatment leads to an increase in the water contact angle and the achievement of a hydrophobic state, with a maximum value of 153° achieved at a laser fluence of 300 J/cm².

For the specimens subjected to additional ultrasonic cleaning after laser texturizing, a similar trend of an increasing water contact angle with rising laser radiation density was observed. However, the minimum laser fluence followed by ultrasonic cleaning resulted in the achievement of a hydrophobic state with a water contact angle of 121°. The maximum water contact angle of 161° was also obtained at a laser fluence of 300 J/cm². We can summarize that ultrasonic cleaning after laser texturizing of the relief on a steel surface promotes the achievement of higher values for the water contact angle.

After conducting studies to determine the effect of ultrasonic surface cleaning after laser texturizing on the degree of wetting, corrosion tests were performed. Figure 5 shows the dependencies obtained through analysis of the results of the corrosion tests.

Figure 5 shows that the increase in laser fluence during the laser texturizing of the AISI 316L steel surface leads to an increase in the corrosion rate. In addition, the correlation of the results presented in Figure 4 and Figure 5 shows that, as the degree of hydrophobicity of the steel surface increases, there is also an increase in the corrosion rate. It is important to note that this contradicts the theory of increased corrosion resistance in hydrophobic surfaces. However, it is supposed that it can be explained by a significant increase in the contact area of the steel surface with the aggressive medium due to the formation of roughness via laser texturizing.

The analysis of the dependences presented in Figure 5 indicates that additional ultrasonic cleaning of steel surfaces modified by laser texturizing leads to a decrease in the rate of the processes of up to six times. It is suggested that this may be due to the removal, via ultrasonic cleaning, of particles formed by the return of some of the ablated material with poor adhesion to the surface. This, in turn, leads to a decrease in the surface contact area with the aggressive medium and, as a consequence, to a decrease in the corrosion rate.

5. Conclusions

Experimental studies to determine the effect of ultrasonic cleaning after laser surface modification on the wettability and corrosion resistance of AISI 316L showed:

• Modification of the steel surface via the laser texturizing of its relief and, with subsequent treatment, with surfactants promotes the achievement of a hydrophobic state. In this case, an increase in the value of the water contact angle occurs with increasing laser fluence. Within the range of laser fluence from 50 to 300 J/cm², the value of the water contact angle changes from 67 to 152°, respectively.
• Ultrasonic cleaning of steel surfaces leads to the removal of nano-/micro-sized particles formed during laser texturizing and forming multimodal roughness. It is noted that the mass fraction of these particles increases with increasing laser fluence during laser surface treatment. Thus, at laser fluences between 50 and 300 J/cm², the mass removal ranges from 17 to 109 ppm. It is suggested that this is caused by the increasing number of returning particles ablated during laser texturizing, which have a low adhesion to the surface.
• Ultrasonic cleaning after the laser texturizing of steel surfaces contributes to achieving a higher degree of hydrophobicity. The increase in the water contact angle ranges from 6 to 81% in the studied range of laser fluence. The maximum value of the water contact angle was 161° at a laser fluence of 300 J/cm². This effect can be explained by the fact that, as a result of ultrasonic cleaning, the textured relief of the steel surface becomes more structured, resulting in a decrease in the proportion of the contact area of the steel surface with the liquid. This thesis is consistent, among other things, with the Cassie–Baxter model of surface wettability.
• Additional ultrasonic cleaning of steel surfaces modified by laser texturizing leads to a decrease in the rate of processes of up to six times. It is suggested that this may be due to the removal, via ultrasonic cleaning, of particles formed by the return of some of the ablated material with a poor adhesion to the surface. This, in turn, leads to a decrease in the surface contact area with the aggressive medium and, as a consequence, to a decrease in the corrosion rate.