Development of PEO in Low-Temperature Ternary Nitrate Molten Salt on Ti6Al4V

Abstract

Titanium alloys are frequently subjected to surface treatments to enhance their biocompatibility and corrosion resistance in biological environments. Plasma electrolytic oxidation (PEO) is an environmentally friendly electrochemical technique capable of forming oxide layers characterized by high corrosion resistance, biocompatibility, and strong adhesion to the substrate. In this study, the PEO process was performed using a low-melting-point ternary eutectic electrolyte composed of Ca(NO₃)₂-NaNO₃-KNO₃ (41-17-42 wt.%) with the addition of ammonium dihydrogen phosphate (ADP). The use of this electrolyte system enables a reduction in the operating temperature from 280 to 160 °C. The effects of applied voltage from 200 to 400V, current frequency from 50 to 1000 Hz, and ADP concentrations of 0.1, 0.5, 1, 2, and 5 wt.% on the growth of titanium oxide composite coatings on a Ti-6Al-4V substrate were investigated. The incorporation of Ca and P was confirmed by phase and chemical composition analysis, while scanning electron microscopy (SEM) revealed a porous surface morphology typical of PEO coatings. Corrosion resistance in Hank's solution, evaluated via Tafel plot fitting of potentiodynamic polarization curves, demonstrated a substantial improvement in electrochemical performance of the PEO-treated samples. The corrosion current decreased from 552 to 219 nA/cm², and the corrosion potential shifted from -102 to 793 mV vs. the Reference Hydrogen Electrode (RHE) compared to the uncoated alloy. These findings indicate optimal PEO processing parameters for producing composite oxide coatings on Ti-6Al-4V alloy surfaces with enhanced corrosion resistance and potential bioactivity, which are attributed to the incorporation of Ca and P into the coating structure.

Keywords: Ti alloy; biocompatibility; calcium phosphates; corrosion resistance; plasma electrolytic oxidation; ternary molten salt; titanium oxide.

Figure 1

Current–time and voltage–time curves for the PEO process in ternary eutectic electrolyte: (a) series with varying voltage and frequency; (b) series with different ADP concentrations.

Figure 2

SEM images of surface morphologies obtained by PEO with varying ADP concentrations: (a) 0.1, (b) 0.5, (c) 1, (d) 2, (e) 5, and (f) 5 wt.% with higher magnification. All coatings were synthesized at 400 V and 50 Hz.

Figure 3

SEM images of surface morphologies obtained by PEO under varying electrical parameters: (a) 200 V/50 Hz; (b) 300 V/50 Hz; (c) 400 V/50 Hz; (d) 400 V/500 Hz; (e) 400 V/1000 Hz. All coatings were synthesized in molten electrolytes with 1 wt.% ADP.

Figure 4

XRD patterns showing the phase composition of ceramic coatings obtained by PEO with varying ADP concentrations: (a) 0.1, (b) 0.5, (c) 1, (d) 2, and (e) 5 wt.%. All coatings were synthesized at 400 V and 50 Hz.

Figure 5

XRD patterns showing the phase composition of ceramic coatings obtained by PEO under different electrical parameters: (a) 200 V/50 Hz; (b) 300 V/50 Hz; (c) 400 V/50 Hz; (d) 400 V/500 Hz; (e) 400 V/1000 Hz. All coatings were synthesized in molten electrolytes with 1 wt.% ADP.

Figure 6

The anatase-to-rutile (A/R) ratio of PEO ceramic coatings: (a) synthesized with 1 wt.% ADP under varying voltages and frequencies; (b) formed with different ADP contents with fixed electrical parameters of 400 V and 50 Hz.

Figure 7

Elemental composition of Ca, P, and Ca/P ratio in the ceramic coatings obtained by PEO with the following: (a,c,e) an addition of 1 wt% ADP under varying voltages and frequencies; (b,d,f) different ADP contents under electrical parameters of 400 V and 50 Hz.

Figure 8

The contact angle of the ceramic coatings obtained by PEO with the following: (a) an addition of 1 wt.% of ADP under varying voltages and frequencies and (b) different ADP contents under electrical parameters of 400 V and 50 Hz.

Figure 9

Corrosion behavior (Tafel plots) of ceramic coatings synthesized by PEO conducted in ternary eutectic electrolyte with the following: (a) an addition of 1 wt.% ADP under varying voltages and frequencies; (b) different ADP contents at fixed electrical parameters of 400 V and 50 Hz.