Formation of PEO coating on Al microelectrode under elementary anodic/cathodic pulses

Highlights

• Elementary role of sparking cathodic breakdown on PEO coating growth

• Sparking cathodic breakdown causes loss of coating materials.

• Sparking during cathodic breakdown does not alter its novel role to randomize anodic breakdown sites.

Abstract

Cathodic sweep and pulse polarization of bare and anodized Al microelectrodes were carried out in alkaline silicate solution. The sweep polarization causes a severe damage to as-polished Al, but reduced and localized damages to the anodized Al. The anodic film resists both cathodic sweep and pulse breakdown. Cathodic pulse polarization can cause capacitive charging and mechanical damage to the oxide film and the substrate depending on the degree of polarization. High cathodic pulse polarization causes sparking and subsequent modification of discharge channels formed by the prior anodic breakdown. The enhancement of plasma formation and sparking during cathodic pulse breakdown is associated with cathodic corrosion of Al, excessive H₂ gas formation and local high electric field. Although sparking cathodic breakdown explodes the discharge channels and causes the loss of coating materials, it does not alter its beneficial role to randomize the sites breakdown by succeeding anodic pulse. The sequential pulses form dense coating materials.

Introduction

Over the last three decades, plasma electrolytic oxidation (PEO) or micro-arc oxidation has been studied to a great extent for the surface treatment of light metal (Mg, Al, Ti) alloys. PEO coatings can functionalize the light metal alloys for a variety of purposes, and PEO coated alloys can find significant applications in automobile, aerospace, computer and biomedical industries where promising light-weight materials are required. PEO coatings can control or enhance corrosion resistance of Mg alloys, can enhance wear and corrosion resistance of Al alloys and can enhance biocompatibility of Ti alloys. The wide-ranging applications of PEO coatings attract researchers to study the fundamental processes of PEO, and to study the properties of PEO coatings. The present study focuses on the fundamental processes of PEO.

Since the coating material is mainly generated by micro-discharges, the micro-discharges create a lot of pores in the coating. These discharge pores are considered as the major defects of the coatings; however, these are potential micro-containers for corrosion-inhibitors [[1], [2], [3], [4]] or scaffolds for tissue growth on implant materials [5]. The porosity controls mainly the corrosion resistance of the coatings [6]. A great effort has been made to control the porosity of PEO coatings, and to study the evolution of porosity during the coating process. Coating process parameters like the composition of electrolytes [7], forms of the power supply (i.e., DC, unipolar, bipolar, AC) [8,9], temperature [10,11] and composition of alloys [12] are shown to have significant influence on morphology and properties of the coatings. AC or bipolar power supply is useful to form less defective and uniform coatings [[13], [14], [15]]. More pertinently, alkaline silicate solution and asymmetric bipolar or AC power supply are suitable to tune sparking to ‘soft sparking’ and hence to form denser coatings on Al and Mg alloys [16,17]. Formation of groove-like discharge channels is promoted by healing the anodic breakdown sites in alkaline silicate electrolytes [18]. Relatively larger cathodic current helps create ‘soft sparking’ which forms denser coatings. Due to rather strict requirements of electrolytes and power mode for ‘soft sparking’, promising corrosion resistant coatings by PEO are not usually formed. Therefore, filling the discharge pores/craters by a secondary method is widely considered. Electrophoretic deposition [19], dip-coat [20], hydrothermal treatment [21] and sol-gel [20] methods are mainly used to seal the pores of PEO coatings. The cost of bare PEO coating itself is higher [22]; the duplex (secondary) treatments of PEO coatings are no ways economical. As a very good application, the short-term PEO, popularly known as ‘flash PEO’ [23], is emerging as the new surface pretreatment.

As a hope that the single PEO treatment could form denser and protective coatings, fundamental study of PEO process was time-and-again carried out to understand the controlling parameters of micro-discharges and coating formation. Ultra-fast video imaging [[24], [25], [26]], spectroscopic analysis [[24], [25], [26], [27], [28]] and current transients of single or multiple pulses [18,29] are usually used to understand the details of the coating process. Capturing light emission both in anodic and cathodic half-cycles was empathetically carried out [30,[31], [32], [33]]. The micro-discharging in anodic half-cycle usually produces coating materials and light emission that features PEO. The micro-discharging elevates the local temperature exceedingly, triggers thermochemical reactions and forms ‘melt-quenched’ oxide in the cooled electrolytes [34]. Light emission during cathodic half-cycle was also reported in some specific conditions [33,35], this feature of PEO is of the prime interest of the present study. Cathodic spark discharge is generally considered destructive to the coating growth [31]. Cathodic spark is considered to be discharged only when the surface is positively charged or shielding effect of the electrical double layer is undermined by ion adsorption. The positively charged surface is usually formed in acidic electrolytes [35]. Presence of fluoride and relatively low pH solutions, less preferred parameters for PEO, are considered as the factors that trigger cathodic spark discharge [32]. Although fluoride contained electrolytes form protective PEO coatings [7], it is yet to understand the destructive roles of cathodic half-cycle in PEO. In this study, only the magnitude of cathodic voltage is found as a factor that triggers cathodic spark discharge.

The general concept for the action of cathodic polarization of anodized Al is associated with hydrogen gas generation and pitting of the substrate [36], and these changes were observed with polarization below −2 V (Ag/AgCl). In PEO of Al alloys, the operating cathodic voltage can reach −160 V in alkaline silicate solution [37]. In this study, the conditions for origin of spark discharge during cathodic polarization and the influence of sparking cathodic breakdown on coating formation have been addressed. For this purpose, Al micro-electrode is selected so that all the surface changes can be confined to a small micro-sized area that is easily observed at a time under the SEM stage. Cathodic sweep polarization up to −10 V and pulse polarization up to −120 V have been employed to figure out all possible roles of cathodic polarization during operation of PEO. In this study, a visual inspection and formation of ‘melt-quench’ oxides, rather than ultra-fast camera imaging or spectroscopic analysis, have been used to observe the light emission. Cathodic current exceeding 100 A cm⁻² on the microelectrode causes spark discharge, ‘melt-quenching’ of oxide and partial damages to anodic film or prior anodic breakdown sites. In lack of photographic evidence of sparking in the present study, formation of ‘melt-quenched’ oxide has been taken as the signature of spark discharge. From simple experimentation of the present study, it has been shown that although sparking cathodic breakdown is partially destructive, repeated anodic/cathodic cycles form a quite dense and protective coating.