**1. Introduction**

Stainless steels are used as structural and functional materials in products operating under aggressive conditions in the food, chemical, and thermal power engineering industries, among others. Products made of this material are characterized by high ductility, toughness, heat resistance and corrosion resistance but low strength and hardness. To harden stainless steels, surface plastic deformation technologies aimed at creating grain boundaries and substructural hardening are effectively used, for example, in ultrasonic strain engineering technology [1] and ultrasonic shot peening [2,3]. The disadvantage of deformation surface treatments is the significant increase in roughness and the need for subsequent finishing [1–3]. Laser shock peening increases the thickness of the hardened layer to 2 mm and have been proven to solve this problem [4–6].

**Citation:** Kusmanov, S.; Mukhacheva, T.; Tambovskiy, I.; Naumov, A.; Belov, R.; Sokova, E.; Kusmanova, I. Increasing Hardness and Wear Resistance of Austenitic Stainless Steel Surface by Anodic Plasma Electrolytic Treatment. *Metals* **2023**, *13*, 872. https://doi.org/ 10.3390/met13050872

Academic Editor: Francesca Borgioli

Received: 21 March 2023 Revised: 27 April 2023 Accepted: 28 April 2023 Published: 30 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Alternatives to the mechanical treatment of the metal surface include physical and chemical methods of surface hardening, such as plasma electrolytic treatment, which has a complex effect on the operational properties of metal products [7–9]. The majority of studies cover the cathodic variant of plasma electrolytic diffusion saturation variety. Thus, during the cathodic plasma electrolytic nitriding (PEN) of 316L steel in a solution of ammonium nitrate and potassium hydroxide, the formation of FeN0.076 and Fe3O4 in the surface layer was revealed [10]. After PEN, the 10–15 μm thick oxide layer containing 39–41% oxygen is formed on the surface of the steel in a carbamide electrolyte. The maximum nitrogen content in steel of 0.68% is no longer detected at a depth of 30 μm. The cathodic nitriding of austenitic stainless steel in solutions of ammonium carbonate leads to the formation of nitrides (Fe2N, Fe4N, CrN and Cr2N) and oxides (Fe3O4 and Cr2O3) of iron and chromium, and the microstructure of the nitrided layer contains a nitride zone and an internal nitriding zone [11]. As the temperature increases, more high-nitrogen nitride, Fe2N, is formed. The nitriding of stainless steel has shown a positive result for increasing wear and corrosion resistance. After the PEN of 316L steel in solutions of carbamide or ammonium nitrate with the addition of potassium hydroxide, the dry friction coefficient with a corundum ball decreases from 0.19 in the untreated sample to 0.13 with an increase in wear resistance of 4.4–10 times [10]. The PEN of 304, 316L and 430 stainless steels in sodium nitrite solution proved to be an effective method of inhibiting pitting corrosion in 0.5M sodium chloride solution [12].

After the cathodic plasma electrolytic carburizing (PEC) of 12Cr18Ni10Ti steel, FeO iron oxides were detected in the glycerin electrolyte [13]. An increase in the applied voltage increased the degree of grain grinding to an extent to which nanoscale crystals formed alongside the increase in surface roughness [14]. After the carburizing of 12Cr18Ni10Ti steel, the compression of ferrite and austenite crystal lattices was observed, which occurs due to the displacement of the lines (110) α-Fe and (111) γ-Fe. Additional phases of Fe3O4, (Cr,Fe)7C3, Fe15Cr4Ni2, CrN and CrFe were detected in the PEC of 304 austenitic steel in a chloride-glycerin electrolyte [15] and 403 stainless martensitic steel, including Fe3O4, CrFe, FeO and CrC [16]. Cathodic carburizing during pulse treatment (250~600 V; 1500 Hz) in an electrolyte of glycerin and sodium chloride on 1Cr18Ni9Ti steel forms a hardened layer with a thickness of 0.2 mm and a microhardness up to 513 HV for 3–5 min [17]. After the cathodic carburizing of 304 steel in an electrolyte of glycerin (80%) and potassium chloride at a voltage of 350 V for 3 min, the thickness of the layer hardened to 762 HV reaches 0.085 mm [18].

In the cathodic plasma electrolytic nitrocarburizing (PENC) of 316L stainless steel in a carbamide electrolyte, the main phase appears as austenite nitrogen [19]. Additionally, oxides and oxygen-containing phases, including NiFe2O4, FeCr2O4 [20], Fe2O3, Fe3O4, Cr2O4 [21] and Fe(Fe,Cr)2O4 [22,23], nitrides, including Fe3N [21], CrN and Cr2N [23], carbides, including Cr3C2 and Cr7C3 [23], and silicon dioxide [21] are detected. The cathodic PENC of 304 steel in carbamide electrolytes increased microhardness up to 1380 HV with an increase in surface roughness from 0.025 to 0.14 μm and a 4.2-fold increase in wear resistance [19]. In one study, 316L steel after cathodic PENC in carbamide electrolytes with various additives had an increase in microhardness up to 1200 HV and 50 times the wear resistance [22] and, in another, an increase up to 1600 HV and 4.5 times the wear resistance [23].

The disadvantage of cathode plasma electrolytic treatment is the low controllability of the technological process and consequent properties. In addition, the increase in surface roughness that accompanies the cathodic treatment option requires additional finishing. The anodic version of plasma electrolytic saturation as a way to increase the operational properties of stainless steel products has not been practically considered in the literature. This option of plasma electrolytic treatment, however, in addition to hardening and increasing wear and corrosion resistance allows us to reduce surface roughness and exclude subsequent finishing treatment [24–27]. In this paper, the possibility of increasing hardness

and wear resistance of the stainless steel surface with various types of anodic plasma electrolytic diffusion saturation (nitriding, boriding and carburizing) is considered.
