**6. Conclusions**

The large number of hypotheses concerning the electrochemical polishing mechanism results from the complexity of the process. Apart from the anodic di ffusion layer of high density and electric resistance, also called the viscous layer, thatt is formed in the proximity of the anode, an oxide layer is also formed on the surface of the anode itself. The presence of these layers limits the processes of di ffusion of water particles and migration of hydrated anions and cations in the electric field. A common phenomenon that results from the simultaneous formation of both these layers is the process of di ffusion of water particles and migration of anions towards the anode as well as the migration of anodic dissolution products away from the anode.

Water particles and anions, which are referred to as the acceptors of the anodic dissolution process, are necessary in the process of electrochemical formation and chemical dissolution of the oxides that constitute the passive layer. Due to the competitiveness of both these processes, even small di fferences in the rate of migration and di ffusion of the acceptors of the dissolution process may lead to significant di fferences in the rate of dissolution of individual elements of the anode surface structure.

For designing the composition of the electrochemical polishing bath solution and selecting the parameters of the electrolytic polishing process, it is beneficial to assume the occurrence of three, at the same time, independent physicochemical processes:


The most popular additions to phosphate–sulphate solutions used as electropolishing baths are glycerol (glycerine) or chromic acid anhydride, CrO3. Glycerol increases the viscosity and density of the solution, which increases the thickness of the di ffusion film and the greater distance of the anode potential from the thermodynamic value. Glycerol also reduces the relative concentration of water in the near-anode area and as a substance with active oxygen atoms forms an adsorption film on the surface of the anode. Chromic anhydride also increases the viscosity and density of the solution but also has oxidizing properties, which facilitate the formation of the oxide film. Of course, chromium anhydride and glycerine should not be added at the same time, as there would be a reaction between the oxidant (chromic acid) and the reducing agen<sup>t</sup> (glycerine).

The main method of modifying the surface (preceding mechanical polishing and electrochemical polishing) or the final processing of metal elements is abrasive processing. Mechanical processing (machining, blast cleaning, or tumbling) cannot ensure the desired smoothness and visual properties of elements of a complex internal structure. After electropolishing, samples are characterised by lower roughness *Ra* and higher corrosion resistance. Electrochemical polishing reduces the adhesion of cells and bacteria to the processed surfaces, positively influencing the usability and durability of the devices. Errors in designing and assembling devices made from 304 and 304L stainless steel used in constructing technological equipment installed in wastewater treatment plants may result in a fast rate of corrosion. The advantage of electropolishing is that it may be applied both as a finishing process and after previous mechanical or chemical processing.

**Author Contributions:** Conceptualization, E.Ł.-W.; formal analysis, E.Ł.-W.; investigation, E.Ł.-W., P.L. and G.N.; data curation, E.Ł.-W.; writing—original draft preparation, E.Ł.-W.; writing—review and editing, P.L. and G.N.; visualization, E.Ł.-W.; supervision, G.N. and P.L.; project administration, P.L.; funding acquisition, P.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Centre for Research and Development (NCBR) within the project IonsMonit as part of the Lider programme (LIDER/22/0187/L-7/15/NCBR/2016). The publication stage was supported by the Leading Reasarch Groups support project from the subsidy increased for the period 2020–2025 in the amount of 2% of the subsidy referred to Art. 387 (3) of the Law of 20 July 2018 on Higher Education and Science, obtained in 2019.

**Conflicts of Interest:** The authors declare no conflict of interest.
