Enhancing the pitting resistance of AISI 430 stainless steel by laser treatment

The paper proposes a method of laser treatment of the steel surface for protection against the formation of electrochemical corrosion. AISI 430 stainless steel plates were used in the work. The influence of laser structuring on the contact angle and the interaction of laser-structured steel surfaces with an aggressive environment during corrosion tests are considered. The composition of oxides formed after laser treatment, as well as the roughness of the modified surface, are considered. Positive dynamics in protection against pitting corrosion formation on the surface of metals have been revealed.


Introduction
Today, the corrosion destruction of metals and alloys remains one of the principal problems for modern materials science. Various steel grades, widely used in industry, are susceptible to corrosion under different operating conditions. One of the common types of corrosion is atmospheric corrosion of carbon steels in industrial and coastal (marine) areas with high humidity Panchenko et al. (2020Panchenko et al. ( , 2021. It is known that stainless steels are resistant to general corrosion, but are susceptible to local (pitting) corrosion in eroding liquids (for example, in chloride-containing solutions Baranidharan et al. (2021); Loto (2015)) transported in the petrochemical, oil refining and mining industries. There is also a need to protect medical devices that are sterilized and disinfected before use, and during operation come into contact with biological fluids, which are aggressive environments of the human body that accelerate corrosion processes.
There are many methods of corrosion protection, such as mechanical-creation of metallic (layers of copper, zinc, silver, and other metals) and non-metallic (organic, including epoxy and phenolic resins, inorganic, including paint coatings and silicate enamels) protective coatings Kausar (2019), chemical-adding alloying additives (for example, the use of more than 12% chromium in the composition of steel Barbara and Robert (2006)), electrochemical treatment Grundmeier et al. (2000), the introduction of corrosion inhibitors into the corrosive environment Raja and Sethuraman (2008), and thermal-heating in a furnace Bergant et al. (2014), plasma Xi et al. (2008).
However, existing methods have several disadvantages, for instance, uneven layer thickness when applying metal coatings, brittleness and cracking of non-metallic coatings under thermal and mechanical shocks, and toxicity of the inhibitor, which can have a dangerous effect on humans and other living beings Raja and Sethuraman (2008).
The main mechanisms that increase the corrosion resistance of metals can be summarized as follows: • The formation of a protective film on the surface of the metal, which prevents contact between the material and the environment (in the case of the using of an oxide film of metals, it must have a Pilling-Bedward factor ( ) from 1 to 2.5, which characterizes the ratio of the molecular volumes of oxide and metal Pilling (1923); Xu and Gao (2000)); • Formation of a hydrophobic surface in the Cassie-Baxter state (state of heterogeneous wetting) Marmur (2004) when there is an air gap between the surface of the material and the wetting liquid, which prevents contact between the material and the liquid.
Among the thermal methods, laser heating is of interest due to its ability to control the physicochemical properties of metals in a locally specified region. It is worth noting that laser processing does not require consumables, which eliminates the toxicity that can occur when processing with various corrosion inhibitors. The development of optical systems, such as scanning, projection, and combined optical methods, allows laser processing on relatively large areas.
Several scientific groups have already demonstrated studies of increasing the metals' corrosion properties by laser processing.
For instance, Shusen Zhuang and colleagues Zhuang et al. (2021) have created a multilayer film of iron oxides (gamma-Fe 2 O 3 /Fe 3 O 4 , FeO) on carbon steel by CW fiber laser treatment. It showed the highest corrosion resistance compared to an untreated steel surface or a surface containing only FeO oxides, the Pilling-Bedward factor of which is less than that of other iron oxides and is 1.76 ( (Fe 2 O 3 ) = 2.15; (Fe 3 O 4 ) = 2.10). It should be noted that in this way it is possible to protect not only various types of steels, but also alloys of other metals: copper alloys Emelyanenko et al. (2018), aluminum-magnesium alloys Boinovich et al. (2015), etc.
Yao Lu and colleagues Lu et al. (2020) showed an increase in corrosion resistance due to the formation of a hydrophobic surface on stainless steel with low liquid adhesion to the surface due to treatment with a nanosecond laser source and subsequent application of a hydrophobic substance. At the same time, a decrease in corrosion resistance due to the formation of a hydrophobic surface, but with high adhesion, was shown in another work Rafieazad et al. (2018). The stated above once again confirms that in order to increase corrosion resistance by imparting hydrophobic properties to the surface, it is necessary to form an air gap between the metal surface and the environment.
Thus, the published works show the possibility of increasing the resistance of metals and their alloys to corrosion due to laser treatment. All of the above has been demonstrated by various research groups using laser sources of different pulse duration's in completely dissimilar modes of laser action on different materials, which does not allow their simple reproduction and, moreover, industrial implementation.
It shoul be noted that the proposed methods for combating pitting corrosion on stainless steel are based on the formation of a hydrophobic layer or a protective layer of iron oxide on the steel surface, while oxides of other metals are not considered. Furthermore, many of the proposed methods are not one-stage and, in addition to laser processing, require the use of additional consumables such as fluoroalkylsilane Lu et al. (2020), FAS-17 Ma et al. (2018), fluorosilane Emelyanenko et al. (2018), and others Rafieazad et al. (2018).
The purpose of this work is to develop a method for laser surface treatment of stainless steel grades (for example, AISI 430 steel) to increase the resistance to pitting corrosion. As equipment for laser processing, the Minimarker 2 setup, widely used in industry, based on an ytterbium pulsed fiber laser source with a nanosecond pulse duration, was chosen.
Based on the request of the real economy sector and on the mechanisms for the formation of anti-corrosion coatings, the following criteria were formulated for the morphology of the steel surface after laser processing: (1) Laser processing should not have a significant impact on the appearance of the finished product (this criterion is not important for all types of products); (2) An oxide layer of metal with the Pilling-Bedward factor should be formed on the steel surface (from the review of the literature 1> >2.5); (3) The surface of the steel must be hydrophobic and have low adhesion to the liquid.

Materials and methods
AISI 430 stainless steel widely used in industry (10x50 mm plates, 0.5 ± 0.05 mm thick, Ra = 0.065 μ m) was chosen as metal alloy to increase resistance to pitting corrosion. Before laser treatment, the steel surface was degreased with ethanol.
Laser treatment of the steel surface was carried out under normal laboratory conditions in an air atmosphere using a technological setup based on a pulsed ytterbium fiber laser (Minimarker 2) with an average power of 50 W and a wavelength of = 1.064μ m (OOO Laser Center, Russia), which generates pulses with pulse repetition rate 50-100 kHz and pulse duration 100 ns.
To check the sufficiency of the above criteria, it was decided to consider the laser exposure regimes in wider temperature ranges (below and above the melting and evaporation temperatures) and at different values of laser pulse imprint overlapping-0% (no overlapping), 30% (slight overlapping) and 60% (significant overlapping). The power density thresholds for melting ( I melt ) and evaporation ( I ev ) of AISI 430 steel were calculated using the formulas: where T melt is the melting temperature (2073 K); T ev is the evaporation temperature (3418 K); T 0 is the starting temperature (300 K); R is the reflection coefficient (0,75 at =1.06 mkm); k is the thermal conductivity (37 W/m ⋅ K); is the thermal diffusivity (3 ⋅ 10 −6 m 2 /c); is the pulse duration (1 ⋅ 10 −7 c) and is equal to I melt = 42,5 MW/cm 2 , I ev = 74,7 MW/cm 2 . The following power densities have been chosen: 28,3 MW/cm 2 ; 56,6 MW/cm 2 ; 113,2 MW/cm 2 .
The morphology of steel surfaces and their chemical composition were studied using an EDX Zeiss Merlin scanning electron microscope. The composition of raw stainless steel AISI 430, obtained by us in the course of research: Fe−83.3%, Cr−16.34%, Si−0.33%. Surface roughness after laser treatment was measured using a Hommel Werke T8000 contact profilometer.
The contact angle was measured using the sessile drop method. A high-resolution digital camera ToupCam for imaging, ToupView software for processing the results were used. Distilled water was utilized as the test liquid. The volume of the drop for measurement in the experiment was 0.0035 cm 2 . Dosing and placement of the drop were carried out using a Satorius mechanical dispensary. After droplets were placed on the surface, the contact angle was measured. Droplet adhesion on the surface was measured using a mechanized table. The sample with a drop placed on it was tilted at angles from 0 • to 180 • to the horizon.
Hydrophobic properties enhancement to the surface of steels was carried out by laser structuring of the surface and subsequent use of the PM-10 muffle furnace at a temperature of 100 • for 3 hours Ngo and Chun (2017); Dinh et al. (2018).
The phase composition of the surface before and after laser treatment was obtained by X-ray diffraction analysis (XRD) using a Bruker D8 Advance X-ray diffractometer with Bragg-Bretnano focusing with a scanning range of 25 • -95 • (2Θ ) and a step of 0.02 • (2Θ).
Electrochemical testing of AISI 430 stainless steel for resistance to pitting corrosion was carried out by taking anodic potentiodynamic curves (0.2 mW/s) in an aerated aqueous solution of 3.5 wt.% NaCl in a three-electrode cell using an IPC-PRO-MF potentiostat. The reference electrode was a silver chloride electrode, and the auxiliary electrode was a platinum one. Before taking the curves, the free corrosion potential ( E cor ) of the steel was recorded for at least 1 hour, taking the potential at the end of exposure as E cor , provided that the potential change over the last 30 minutes was not more than 30 mW. (1)

Physical and chemical properties of AISI 430 steel surface before and after laser treatment
The surface morphology of AISI 430 steel after laser irradiation under various treatment modes, described in detail in the section Materials and methods, is shown in Fig. 1. It can be seen that there are no cracks on the surface in the images that can become a source of further corrosion. When the steel surface is heated below the melting threshold of the material, the surface morphology changes slightly. When the surface is heated above the melting threshold, a melt crater is formed, and with intense evaporation, a pit is formed, and melt drops become noticeable at the edges of the imprint.
After laser modification of the steel surface (with an increase in power density and/or overlapping of laser pulse imprints), the contact angle of the droplet decreases almost to zero. After laser structuring of a hydrophilic surface, it transfers to superhydrophilic state ( Fig. 2A) following the Wenzel equation Wenzel (1936). The contact angle of the steel surface before laser treatment was 69 • ± 3 • .
Since metal oxides adsorb organic compounds from the air on their surface, it is possible to form a hydrophobic surface of steel due to prolonged exposure of these samples to air. Additional heating of the surface (for example, in a muffle furnace) will accelerate the Fig. 1 Surface morphology of AISI 430 steel after laser treatment with laser imprint overlapping of 0, 30, 60 and power densities of 28.3; 56.6; 113.2MW/cm 2 formation of a hydrophobic surface (Fig. 2B), therefore, we chose this method as the least expensive in terms of time resources. It is also likely that various oxide film defects, such as cracks, are eliminated during annealing in a furnace, which makes it possible to positively influence the resistance of steel to pitting corrosion.
The mechanisms of the formation of a hydrophobic surface on steel are described in more detail in our previous article Shchedrina et al. (2020).
It should be noted that the steel surface, even after furnace treatment, has strong adhesion to the droplet surface. Even when the sample is tilted by 90 • or more, the drop does not lose contact with the surface and does not roll, which indicates a homogeneous hydrophobicity regime and the absence of an air gap between the sample surface and water.
The phase composition of the steel surface after laser treatment, obtained by X-ray diffraction analysis, is shown in Fig. 3 and Table 1.
During laser treatment of the steel surface in non-overlapping modes, only chromium (II) oxide ( (CrO) = 1.84) and -Fe are formed on its surface. In modes without laser imprint overlapping, oxygen access to the steel components is unrestricted. Since chromium has a greater affinity for oxygen than iron, chemically sorbed oxygen selectively interacts with chromium ions or atoms, forming a thin layer of CrO on the steel surface.
With an increase in laser exposure time (the overlapping of laser imprints is 30%), a redox reaction occurs, in which CrO is converted into chromium (III) oxide ( (Cr 2 O 3 ) = 2.02).
With a further increase in the exposure time (the overlapping of laser imprints is 60%), with a lack of oxygen, iron diffuses through the existing chromium oxides, which leads to the formation of iron oxides (II, III) ( (Fe 3 O 4 ) = 2.10) Wang et al. (2005). Table 2 shows the surface roughness (Ra) of AISI 430 steel before and after laser treatment. It can be seen that an increase in the power density of laser irradiation and overlapping of laser imprints leads to a growth in surface roughness and a corresponding increase in hydrophilic or hydrophobic effects. With an initial hydrophilic smooth surface, an increase in roughness leads to a decrease in the contact angle, and in the case of a hydrophobic initial surface, the contact angle increase (according to Fig. 2).

Pitting corrosion resistance study of AISI 430 steel before and after laser treatment
According to the results of electrochemical studies, free corrosion potential of steel ( E cor ) (Fig. 4), as well as the main indicators of pitting resistance, were determined: E b -potential for the formation of stable pittings (pitting potential); ΔE b -potential difference between pitting and free corrosion. The higher the values of ΔE b and E b , the higher the pitting resistance of steel Annual Book of ASTM (1999); Nascimento et al. (2022). It follows from Fig. 4 that E cor shifts to the negative side after all laser processing modes, except for mode 8. This indicates that the steel after such processing methods is in a state of active uniform dissolution of the entire surface and the resulting corrosion products do not passivate the steel surface. At the same time, a necessary condition for the development of pitting corrosion is the presence of E cor in the range of positive values due to the formation of a passive film from corrosion products with its further destruction in local areas. Thus, under conditions of free corrosion, the studied steel is not prone to the formation of pitting.
Next, electrochemical tests were carried out with anodic polarization of the samples in order to accelerate the oxidation of their surface and create conditions for the occurrence of pitting corrosion.
Based on the data obtained, an average assessment of the resistance of samples to pitting corrosion to the results of 3 measurements in the initial state and after laser surface modification is presented in Fig. 5.
The horizontal and vertical lines within the graphs show the pitting resistance of untreated (control) stainless steel.
It has been demonstrated that the best resistance to pitting corrosion ( E b , ΔE b values are higher than that of unstructured ones) is demonstrated by samples after laser treatment of minimum power density with the imprint overlapping of 0%, 60%, and heat treatment in a furnace (Fig. 5, points 1, 3 in green areas). The fact that samples with the lowest power density have better anticorrosion properties relative to samples with high power density can be explained by the fact that all of our samples exhibit Wenzel hydrophobicity, in which, with increasing roughness, the metal-liquid contact area also increases, which negatively affects the corrosion resistance.
Concerning chemical composition, the homogeneous film of chromium oxide of the sample treated with mode 1 shows greater resistance to corrosion than the heterogeneous film of the sample treated with mode 2, consisting of chromium and iron oxides. Figure 6 illustrates the dependence of the corrosion resistance on the roughness of lasertreated steel. The black line on the graph represents the general trend of corrosion resistance versus roughness, presented for a clearer perception. It can be seen that the level of protection against pitting corrosion decreases with increasing roughness. Mode 9 stands apart from the trend, showing an average level of corrosion protection with high roughness. Presumably, it results from the oxide film produced by Mode 9 being thicker relative to other modes. The higher intensity of the peaks for this sample in the XRD analysis illustrates this assumption.

Conclusion
Based on the results of electrochemical studies of AISI 430 stainless steel samples, we can conclude that laser treatment can increase the resistance of this steel grade to pitting corrosion. It was found that during laser treatment, various oxides of chromium and iron were formed on the surface of the steel, which had a positive effect on corrosion resistance. With an increase in power density and laser imprint overlapping, oxides formation becomes more active. Also, the research results make it possible to determine that in a homogeneous Pitting resistance of stainless steel compared to laser treatment, where the green area is the highest resistance to pitting corrosion, the yellow area is satisfactory resistance to pitting corrosion, the red area is not resistant to pitting corrosion wetting regime, the surface roughness negatively affects the corrosion resistance due to an increase in the contact area of the sample and the aggressive solution. It is the small roughness that explains the good results of modes with low power density relative to modes with high power density.
Modes with the lowest power density (and lowest roughness) showed the best resistance to pitting corrosion, which is associated with a homogeneous mode of surface wetting. Mode 1 proved to be the most optimal, in which a homogeneous chromium oxide film is formed. Surprisingly, according to the research results, mode 2, having the same composition and similar surface roughness, shows less resistance to pitting corrosion than mode 1.
In this article, to explain the correlation between the chemical composition of oxide films and resistance to pitting corrosion, only the Pilling and Bedworth criteria (which is used to assess chemical corrosion, does not take into account air humidity) and the heterogeneity of oxide coatings were considered. In the future, for evaluation, it is necessary to additionally take into account other characteristics of the film: adhesion to the metal surface, strength, elasticity, inertness, coefficient of thermal expansion of the film Lee et al. (2001).
Other factors affect the rate and degree of corrosion, such as the rate of diffusion and interaction reactions of corrosive particles, the presence of removal of corrosion products from the metal surface, the composition and velocity of the corrosive medium, mechanical stresses, etc. Probably, the results of processing modes 1 and 2 differ precisely due to these factors affecting pitting corrosion. Consideration of these factors, a more detailed analysis of corrosion resistance (pitting depth, an increase in the number of tests), and the stating of a correlation between them are our further goals and are planned for consideration in future works.