Unipolar plasma electrolytic oxidation: Waveform optimisation for corrosion resistance of commercially pure titanium

Abstract

This study deals with the anodisation of titanium grade 2 in 0.5‐M sulphuric acid using a pulsed signal in a unipolar regime. The electrical parameters investigated are voltage, frequency and duty cycle. The use of duty cycles with a high percentage of anodic polarisation (90%), combined with high frequencies (1000 Hz) and the higher voltage tested (220 V), favoured the establishment of a plasma regime involving strong dielectric discharges, allowing the growth of thicker oxides but with rough architecture. The corrosion resistance of the formed film has been characterised by potentiodynamic tests in 0.5‐M NaBr for localised corrosion resistance and by immersion tests in 10% v/v sulphuric acid solution for a uniform corrosion assessment. Current–time curves, visual observations and electron microscope analysis (scanning electron microscopy, energy‐dispersive X‐ray spectroscopy) were the tools selected to provide a correlation between technological parameters and oxide growth mechanism. For localised and uniform corrosion, anodisation at 220 V with a high level of anodic polarisation (90%) and frequency (1000 Hz) was verified to be particularly advantageous.

Addition of Organic Acids during PEO of Titanium in Alkaline Solution

“…Figure 12 delineates the evolution of the PEO process appearing strongly affected by the different organic acids added to the alkaline bath. In particular, PEO-AC presented numerous small and homogeneously distributed white discharges very similar to light emissions produced during PEO performed in sulfuric acid electrolyte [3,7,34], where the absence of an orange-like hue accounts for the relatively low thickness of the growing layer. In fact, in all the other cases, orange-like sparks manifested, even in correspondence of 4 min of treatment, as localized spots with lower electrical resistivity.…”

“…In particular, when using acetic acid, a porous surface, common [35] to PEO treatments performed in low pH environments [3,7], developed with pore diameters of ~0.98 ± 0.3 µm. The pores' number was substantially reduced when using lactic (Figures 6 and 7) and citric (Figure 6m, even if their size was markedly increased (~4.64 ± 2.1 µm for PEO-LA and ~18.5 ± 3.4 µm for PEO-CI).…”

“…Even if the natural protective layer shows an outstanding chemical stability and adhesion to the substrate, its maximum thickness is limited to ~10 nm, making surface treatments necessary to provide a more effective and reliable protection. One of the most promising electrochemical techniques to achieve this purpose is plasma electrolytic oxidation (PEO), which allows one to increase the oxide thickness well beyond one micron [3,4]. PEO consists in a high-voltage anodic oxidation applied on valve metals such as Al, Mg and Ti, generally in alkaline solutions [5,6], even if, quite recently, PEO on Ti using sulfuric acid was verified to be highly effective for corrosion resistance enhancement [7].…”

Sealing of porous titanium oxides produced by plasma electrolytic oxidation

“…Being the solubility of calcium hydroxide in water an exothermic process, its stability can be enhanced in service conditions, exploiting relatively high temperatures, thereby making the sealing procedure more efficient. In particular, knowing the high aggressivity of hot concentrated sulfuric acid on grade 2 titanium [11,12] inducing corrosion rates of several mm/year even in presence of anodic coatings [13] the reaction between Ca(OH) 2 and H 2 SO 4 can favor the formation of a calcium sulfates skin. Depending on its uniformity and formation kinetics the reaction between the Ca(OH) 2 deposits and the acidic environment can be eventually stopped.…”

A comparison between corrosion performances of titanium grade 2 and 7 in strong reducing acids

“…[7] However, titanium corrosion resistance can be enhanced by creating a stronger and thicker film by means of anodic oxidation, an electrochemical technique used to build a passive layer on titanium. [8][9][10] Among all the mineral acids, sulfuric acid has probably the more diversified and established market ecosystem. Its production volume is so relevant that it can be used to forecast the wellness of a country's industry.…”