Addition of Organic Acids during PEO of Titanium in Alkaline Solution

Abstract

This research study describes recent advances in understanding the effects of the addition of organic acids, such as acetic, lactic, citric and phytic acids, on the process of plasma electrolytic oxidation (PEO) on Ti using an alkaline bath. As the plasma developed over the workpiece is central to determine the particular morphological and structural features of the growing oxide, the focus is then on the inter-relationships between the electrolyte and the resultant plasma regime established. In situ optical emission spectroscopy (OES) allowed us to verify a marked plasma suppression when adding low-molecular-weight anions such as acetates, resulting in short-lived and well-distributed discharges. Conversely, when more bulky anions, such as lactates, citrates and phytates, were considered, a less efficient shielding of the electrode caused the build-up of long-lasting and destructive sparks responsible for the formation of thicker coatings, even >30 µm, at the expense of a higher roughness and loss of compactness. Corrosion resistance was tested electrochemically, according to electrochemical impedance spectroscopy (EIS), and weight losses evidenced the coatings produced in the solution containing acetates to be more suitable for service in H2SO4.

Use of Organic Acids as Additives for Plasma Electrolytic Oxidation (PEO) of Titanium

“…Indeed, when describing the interaction between the treated Ti samples, the growing oxide, and the electrolytic species it is important to consider the possibility of anions adsorption onto the electrode surface, which may modify the charge transfer reaction immediately preceding the discharge event. The efficiency of the coverage of the surface provided by the anions strongly depends on the steric hindrance, the structure, the molecular weight, and the functional groups of the molecules that are involved in the PEO process [24,25].…”

Application of Biomedical Microspheres in Wound Healing

“…Some other POE cannot produce acids during degradation. Therefore, the degradation rate does not increase due to autocatalysis caused by acids produced during the degradation process [ 130 ]. Different POE materials or microspheres can be synthesized by combining different polymers or different feed molar ratios, so that microspheres have rich physical properties and different water solubility, biocompatibility, and degradability [ 131 ].…”

A green vanadium-based formulation for the conversion of steel

“…In order to get insight about the time scale of the processes, occurring on the tested surfaces, the −Z ′′ was plotted in Figure 11 against frequency allowing to extrapolate the characteristic frequency of the ongoing corrosion reactions. From the maximum of the−Z ′′ [36][37][38][39], it is possible to extrapolate the time constant governing the electrochemical reaction, corresponding to 0.06 Hz for the bare metal and 0.15 Hz for VCC. The different values are indicative of different electrochemical reactions, while a higher −Z ′′ modulus, obtained in case of VCC, expresses improved surface properties compatibly with the broader phase angle observed over the experimental frequency window for the surface converted at 45°C, corroborating higher capacitive phenomena ascribed to the presence of the coating [38,[40][41][42][43].…”

“…For the interpretation of the EIS spectra, an appropriate physical model of the electrochemical reactions occurring in the system is necessary. In both cases, for uncoated and VCC-coated Fe, a simple Randles equivalent circuit (insert of Figure 10(b)) is proposed to simulate the impedance data corresponding to a charge transfer process of a reactive electrode [35][36][37][38], describing a system governed by a single time constant. A constant phase element (CPE) was used instead of a pure capacitor to account for defects and nonhomogeneous current distributions, while R s is the solution resistance and R p has the usual meaning [44].…”