*2.1. Samples' Preparation*

Commercially available M30NW biomedical alloy (AUBERT & DUVAL, Paris, France), with composition and properties specified by X4CrNiMnMo21-9-4 standard, was used as a substrate. The alloy plates (22 mm in diameter) were ground with SiC papers down to 1200 grits, polished with alumina slurry (0.3 μm), ultrasonically cleaned in distilled water, etched in acid mixture, passivated in boiling distilled water, rinsed with ethanol, and dried with argon according to the procedure described elsewhere [38].

In this study several sols were used for surface modification of M30NW alloy: TiO2 sol without dopants, sol doped with calcium ions Ca2+, sol doped with silver ions Ag<sup>+</sup>, as well as sols containing both Ca2+ and Ag+ ions in di fferent molar ratios (Ca/Ag 3:1, 1:1, 1:3).

All sols were synthesized using the sol-gel method, in which titanium tetrabutoxide (TiBut, Ti[O(CH2)3CH3]4, 97%, Sigma-Aldrich) was used as a precursor for titania, ethanol (EtOH, C2H5OH, pure p.a., 96%, POCh) as a solvent, nitric acid (HNO3, pure p.a., 65%, POCh) as a catalyst, calcium nitrate (Ca(NO3)2 4H2O, pure, CHEMPUR) and silver nitrate (AgNO3, pure p.a., POCh) solutions at a concentration of 2.056 mol/L as dopant sources. The composition of all sol solutions used in this study is presented in Table 1.


**Table 1.** Composition of sol solutions used for surface modification of M30NW alloy.

A titania-based coating (as a single layer) was applied onto the alloy surface with the dip-coating technique using a DCMono 75 dip-coater (NIMA Technology Ltd., Coventry, UK). The substrate was immersed in the sol for 30 s and withdrawn at a speed of 20 mm/min. Such modified alloy samples were dried in the oven at 100 ◦C for 2 h and then annealed at 450 ◦C for 1 h.

## *2.2. Surface Characterization*

Each type of sample was characterized in terms of surface properties. A metallographic microscope MMT 800 BT (Mikrolab, Lublin, Poland) was used for preliminary assessment of the sol-gel coatings' quality, including detection of cracks and defects. An atomic force microscope (AFM) Dimension Icon (Bruker, Santa Barbara, CA, USA) was applied for investigation of surface topography and roughness of the prepared coatings within a scan size of 1 μm × 1 μm. The AFM measurements were made in tapping mode using standard silicon probes (TESPA, Bruker AFM Probes, Camarillo, CA, USA). The morphology of the samples was observed using a field emission scanning electron microscope (FE-SEM, FEI Nova NanoSEM 450 with EDS analyzer, Thermo Fisher Scientific, Hillsbro, OR, USA), operating with an accelerating voltage of 15 kV. The phase composition and thickness of prepared coatings were identified using an Empyrean X-ray di ffractometer (XRD, PANalytical, Malvern, UK) working with Cu K α radiation (λ = 0.15418 nm). The phase analysis was carried out using GIXRD (grazing incidence X-ray di ffraction) mode with an incident beam angle of 0.3◦, whereas the thickness was estimated with the use of X-ray reflectivity method (XRR). Further data processing was performed using HighScore Plus with ICDD PDF 4+ Database and X'Pert Reflectivity software with Fourier transform analysis, respectively.

## *2.3. Corrosion Tests*

The anticorrosion ability of prepared coatings was evaluated by electrochemical measurements in phosphate buffered saline (PBS, NaCl 8.0 g/L, KH2PO4 0.2 g/L, Na2HPO4 ·12 H2O 2.9 g/L, KCl 0.2 g/L, pH 7.4) solution using a PGSTAT 30 potentiostat-galvanostat (EcoChemie Autolab, Utrecht, The Netherlands). All electrochemical experiments were performed at 37 ◦C, similar to human body temperature. Degassing of the electrolyte was achieved by argon bubbling through the solution. A conventional three-electrode cell was used with a platinum gauze as a counter electrode, a saturated calomel electrode (SCE, E = 0.236 VSHE) as a reference, and sample with an exposed area of 0.64 cm<sup>2</sup> as a working electrode.

In order to establish the corrosion potential Ecor, each sample was kept in PBS solution (under open circuit conditions) for 2000 s. The linear polarization measurements were performed in a scanning range of ±20 mV versus Ecor potential, with a scan rate of 0.166 mV/s. Potentiodynamic polarization tests were conducted with a scan rate of 1 mV/s from the initial potential of −200 mV versus Ecor to the potential at which current density of 5 mA/cm<sup>2</sup> was reached, then, the potential sweep was reversed and the backward branch was registered up to the initial potential. The surface morphology of the samples after potentiodynamic polarization was analyzed using scanning electron microscopy in order to determine the type and scale of corrosion damage.

The results of linear polarization measurements and potentiodynamic polarization tests were analyzed using CorrView software (Scribner Associates Inc., Southern Pines, NC, USA) and several corrosion parameters were determined: Polarization resistance, Rp; corrosion rate, CR; pitting potential, Epit; and repassivation potential, Erep. Triplicate measurements were conducted to check the reproducibility of the results. Each data point presented here is given as mean ± standard deviation (SD). All the potentials reported here are with respect to a saturated calomel electrode.

## *2.4. Immersion Tests*

To evaluate the bioactivity (as apatite formation ability) of the M30NW samples with prepared titania-based coatings, the immersion tests in a simulated body fluid (SBF) solution were carried out according to the procedure reported by Kokubo [39]. The samples were immersed in SBF (at 37 ◦C, similar to human body temperature) for 28 days, the solution was renewed every week. Then the SEM-EDS technique was employed to characterize the morphology and surface composition of samples. Based on SEM-EDS results, their ability to apatite formation was evaluated.
