Plasma Electrolytic Polishing (PEP) is an advanced anodic process that enhances stainless steel surfaces through controlled electrochemical dissolution and plasma-mediated modification. This study demonstrates that PEP treatment of AISI 304 SS at 300 V in aqueous urea solution (3.0 wt.%)/NH₄NO₃ (0.25 wt.%) achieves remarkable improvements: surface roughness decreases by 54.6% (from 0.197 ± 0.023 μm to 0.0895 ± 0.0205 μm) with minimal mass loss (0.0035 g·cm⁻²) in just 20 min. Tafel analysis showed a 99% reduction in corrosion rate (0.00497 mm yr⁻¹) compared to untreated AISI 304 SS (0.094 mm yr⁻¹). Cyclic Potentiodynamic Polarization (CPDP) measurements indicated superior pitting resistance (E_pit = +0.423 vs. +0.486 V for PEP processing). XPS analysis elucidates the underlying mechanisms, showing a 91% increase in the Cr/Fe ratio (0.44 to 0.84) and complete transformation of surface oxides to protective Cr₂O₃ (57.34 wt.%) and Fe₃O₄ (55.88 wt.%), which collectively explain the enhanced corrosion resistance.

The electrochemical co-deposition of Cr-Co oxide coatings at room temperature on 304 stainless steel (SS) was studied using an electrolyte composed of a mixture of ethylene glycol (EG), hydrated metal chloride salts (MCln∙YH₂O), and water as a secondary hydrogen donor (HBD). Metallic Cu and Ni undercoats were applied to improve the adhesion of a posterior Cr-Co metallic and oxide layer. The electroactive events that took place during both electrodeposition processes were studied using cyclic voltammetry (CV) and chronoamperometry. The microstructure and composition of the surface layers were studied using scanning electron microscopy (SEM/EDS), X-ray diffraction (XRD) and cross-sectional elemental mapping via transmission electron microscopy (TEM). The surface of steel with the Cr-Co:EG-H₂O coating showed greater resistance to pitting corrosion (123.93 mV) compared to untreated stainless steel (62.3 mV). This sample showed a large hystere-sis area, which is associated with high resistance to pitting corrosion by the occurrence of a re-passivation of the sample at a Erep value of 24.31 mV. After the cyclic potentiodynamic polariza-tion (CPP) test, the lowest specific mass loss (0.001 mg/cm²) was achieved for the AISI 304 SS sample coated using EG-water solvents (Cr-Co:EG-H₂O), while the untreated AISI 304 SS reached a higher specific mass loss (0.01 mg/cm²). The Electrochemical Impedance Spectroscopy (EIS) tests showed that the uniform corrosion resistance varied significantly from the untreated AISI 304 SS (35 kΩ) to the coated sample (57 kΩ), which is attributed to the protection provided by the chromium and cobalt oxides coating. The best corrosion resistance achieved was correlated with a superhydrophobic character (with a contact angle of 158.41°) of the Cr-Co coatings. This was in turn a consequence of a needle-like morphology characteristic of these oxides.

This work studied the effect of self-protective paste nitriding (SPN) and ion plasma nitriding (IPN) on the surface chemistry, microstructure, and nanohardness of AISI 304 and 316L stainless steels, with both treated at 440 °C for 5 h. Surface modifications analyzed using SEM and nanoindentation revealed distinct outcomes. SPN induced an oxynitriding effect due to the oxidation properties of the pastes, forming Fe₃O₄ and Fe_xC phases, while IPN produced an expanded austenite layer. Both methods enhanced surface nanohardness, but SPN showed superior results. For 316L SS, SPN increased nanohardness by 367.81% (6.83 GPa) compared to a 133.5% increase (3.41 GPa) with IPN. For 304 SS, SPN improved nanohardness by 26% (2.23 GPa), whereas IPN reduced it by 48% (0.92 GPa). These findings highlight SPN’s potential as an effective anti-wear treatment, particularly for 316L SS. The SPN process utilized a eutectic mixture of sodium cyanate and sodium carbonate, while IPN employed a N₂:H₂ (1:1) gas mixture. SEM analyses confirmed the formation of γ-Fe(N) phases, indicating dispersed iron nitrides (FeN, Fe₃N, Fe₄N). SPN’s simultaneous oxidation and nitrocarburization led to an oxide layer above the nitride diffusion layer, enhancing mechanical properties through iron oxides (Fe₃O₄) and carbides (Fe_xC). Comparative analysis showed that AISI 316L exhibited better performance than AISI 304, underscoring SPN’s effectiveness for surface modification.