Results

 A.V.Andrei, V.Malinovschi, Gh.Albu, I.M.Borascu, E.Coacă – ELSSA LABORATORY, Patent: “INSTALLATION AND METHOD FOR OBTAINING ANTICORROSIVE PROTECTION LAYERS ON CARBON STEEL BY OXIDATION IN ELECTRIC PLASMA, granted by OSIM under no. 132201/2020
 Patent application pending for:
„INSTALLATION AND METHOD FOR OBTAINING ANTICORROSIVE PROTECTIVE SURFACE LAYERS ON STEEL BY SATURATION IN ELECTROLYTIC PLASMA, IN ANODIC REGIME”
 Patent application pending for: METHOD FOR OBTAINING ALUMINUM OXIDE CERAMIC LAYERS ON 316L STAINLESS STEEL BY CATHODIC DEPOSITION IN ELECTROLYTIC PLASMA
 Newly results published in ISI rated journals related on:

• Obtaining thin ceramic films based on aluminum oxide on 304 L and 316 L austenitic steels through PEO [1], [2];

• Characterization of microstructure and microcomposittion of protective structures developed on 316L steel by plasma electrolytic techniques [3]

• Stabilization of the tetragonal phase of ZrO2 in layers deposited on Zr 2.5 Nb by PEO in sodium aluminate electrolytes [4]

• Improving the mechanical, tribological and electrochemical properties of Ti and Ti alloys, by PEO.

To improve the corrosion resistance of steels selected for use in Heavy Liquid Metal (HLM) systems at temperatures above 500 ° C, one possibility is to alloy the superficial steel layer with Al, using the pulsed electron pulse beam - GESA; it is obviously interest in developing cheaper and more permissive surface engineering techniques with the geometry of treated components, such as PEO.
In [1], [2], [3] results are described regarding the development on austenitic steel 316L, of ceramic films based on aluminum oxide, using Plasma Electrolytic Oxidation; an overall description of the electrochemical processes involved in the growth of aluminum coating, the surface properties improvement, together with some consideration about the new materials development for energy technologies are presented.
The tetragonal phase of ZrO2 provides superior corrosion behavior under specific CANDU pressure tube operating conditions. In [4], [5] are presented results about Tetragonal ZrO2 phase stabilization in coating layers prepared on Zr–2.5%Nb alloy during plasma electrolytic oxidation in sodium aluminate electrolyte. Layers containing polycrystalline zirconia and amorphous aluminum oxide were obtained by plasma electrolytic oxidation (PEO) of Zr–2.5%Nb alloy in aqueous electrolyte solutions comprising NaAlO2 (pH = 13.0), NaAlO2 and NaOH (pH = 13.2), respectively. The PEO coatings were analyzed by x-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and potentiodynamic polarization tests. The deposited coating layers contain mainly tetragonal ZrO2 polycrystalline phase and amorphous aluminum oxide (Al2O3) phase. They have thicknesses between 9.0 and 53.0 µm and are uneven, heterogeneous and porous. Zirconia tetragonal phase stabilization by the aluminum oxide matrix was achieved through the ‘constraint’ mechanism. The corrosion current density values for the PEO layers are lower by 1–2 orders of magnitude than those of the untreated Zr–2.5%Nb alloy.
Titanium and its alloys have remarkable properties including high corrosion resistance, good mechanical behavior, good fatigue strength and toughness, low elastic modulus, high strength-to-weight ratio, relatively low density, high melting point, good biocompatibility. These properties have led to the development of a wide range of industrial applications in various fields as nuclear, aerospace, chemical, biomedical, automotive, energy, military, and marine. Some drawbacks related to low wear resistance, low hardness, high coefficient of friction and poor corrosion resistance in aggressive environments such as sulfuric acid, hydrochloric acid and phosphoric acid are among the factors that can limit the applications of these alloys. For this reason, it is necessary to cover the metal surface with a coating layer to further enhance wear and mechanical performance of pure titanium and titanium alloys, by various deposition processes

In [6], [7] are presented results regarding the improvement of the tribological, mechanical properties and the electrochemical behavior of CP-Ti, by PEO treatments.
In [6], we investigate the effects of acidic electrolyte solution containing NaH2PO4·2H2O and of electric current density on phosphorus incorporation, microstructure, mechanical properties and corrosion resistance in Ringer's physiological solution of the PEO layers deposited on pure Ti. The novelty of our approach is to enhance rutile phase formation and to increase the rutile/anatase fraction present in the surface layers by tailoring the deposition parameters inside the electrolytic tank during film growth, both potentiostatically and galvanostatically under various sodium phosphate concentrations and treatment times.

In [7] are presented results regarding the PEO-coated samples of CP-Ti grade 2 wich were processed in an electrolytic solution consisting of NaAlO2 at concentrations of 10 to 20 g/L and fixed NaOH of 2 g/L under galvanostatic conditions (0.36 A/cm2) for 10, 20 and 30 min growth time.
- The coatings are polycrystalline showing titanium‑aluminum oxides in the form of TiAl2O5 and/or Ti2Al6O13 as dominant phases, combined with γ-Al2O3 and α-Al2O3 forms and a small amount of NaAl6O9.5 crystal structure;
- High cooling rates achieved at the surface inhibit phase transition of Al2TiO5 into a mixture of aluminum and titanium oxides and thus, Al2TiO5 is stable at room temperature;
- The excess of aluminum oxide favors initial development of aluminum titanate and the remaining oxides crystallize in α- and γ- phases of Al2O3;
- Increased duration of the PEO process plays the key role in promoting α-Al2O3 formation on Ti substrate, regardless of electrolyte concentration involved;
- Surface morphology of the oxide layers appears to be porous, covered with crater-like holes and microsized particles dispersed throughout the film. By applying an identical spark voltage, the micropore dimensions were kept constant during the coating growth (5–15 μm);
- The oxygen found at the surface and inside the films promotes the oxidation of metal atoms that forms the PEO oxide layers;
- During the PEO process, as both the concentration of NaAlO2 in the electrolyte solution and deposition interval increase, the newly formed layers with thicknesses between 9.0 and 53.6 μm exhibited dramatic improvement of mechanical performances, in terms of hardness and scratch resistance, compared to the uncoated Ti, yielding values ranging from 5.5 to 24.5 GPa and 21 to 45 N, respectively. The excellent adhesion and hardness properties can be linked to the α-Al2O3 and γ-Al2O3 phases in the obtained coatings;
- The corrosion resistance in 3.5 wt% aqueous NaCl solution at room temperature for all treated samples was found to be two orders of magnitude higher when compared to pure Ti.

[1] V. A. Andrei, E. Coaca, M. Mihalache, V. Malinovschi, M. Pătraşcu (Mincă), "Study of ceramic-like alloy oxide thin films developed using plasma electrolytic oxidation applied on austenitic steels, Surface and Interface Analysis, Vol. 48 , Issue 7 (2016)

[2] Victor Aurel Andrei, Cristiana Radulescu, Viorel Malinovschi, Alexandru Marin,
Elisabeta Coaca, Maria Mihalache, Cristian Nicolae Mihailescu, Ioana Daniela Dulama, Sofia Teodorescu, Ioan Alin Bucurica, „Aluminum Oxide Ceramic Coatings on 316l Austenitic Steel Obtained by Plasma Electrolysis Oxidation Using a Pulsed Unipolar Power Supply”, Coatings, 2020, 10, 318

[3] V.A.Andrei, E. Coaca, I. Ionita, G. Torok, O.A. A. Rusin, A. Marin, M. Mihalache, L. Velciu, V. Malinovschi, T. Visan, "Microstructures and Micro Composting Developed by Plasma Electrolysis Processing of 316L Austenitic Steels to Achieve Al-containing Surface Layer” Materials Today: Proceedings, 4, (2017), 6990-6999

[4] V.Malinovschi, A. Marin, D. Negrea, V. Andrei, E. Coaca, "Tetragonal ZrO2 phase stabilization in coating layers prepared on Zr-2.5% Nb alloy during plasma electrolytic oxidation in sodium aluminate electrolytes”;, Materials Research Express, 4, (2017), 095702

[5] V. Malinovschi, A. Marin, D. Negrea, V. Andrei, E. Coaca, C. N. Mihailescu, Cristian P. Lungu, „ Characterization of Al2O3/ZrO2 composite coatings deposited on Zr-2.5Nb alloy by plasma electrolytic oxidation”, Applied Surface Science 451 (2018) 169-179

[6] V. Malinovschi, A. Marin, V. Andrei, E. Coaca, C.N. Mihailescu, Cristian P. Lungu,
Cristiana Radulescu, Ioana Daniela Dulama, „Obtaining and characterization of PEO layers prepared on CP-Ti in sodium dihydrogen phosphate dihydrate acidic electrolyte solution” , Surface & Coatings Technology 375 (2019) 621–636

[7] V. Malinovschi, A. Marin, C. Ducu, V. Andrei, E. Coaca, Valentin Craciun, Mihail Lungu,” Influence of sodium aluminate concentration and process duration on microstructure, mechanical and electrochemical behavior of PEO coatings formed on CP-Ti”, Surface & Coatings Technology, Volume 418, 25 July 2021, 127240