The effect of substrate composition on the electrochemical and mechanical properties of PEO coatings on Mg alloys Emrah Cakmak a , Kadir C. Tekin a , Ugur Malayoglu a, ⁎, Suman Shrestha b a Dokuz Eylul University, Department of Metallurgical and Materials Engineering, Tinaztepe Campus, 35160 Izmir, Turkey b Keronite International Ltd, Cambridge CB21 6GP, United Kingdom abstract article info Article history: Received 13 May 2009 Accepted in revised form 9 October 2009 Available online 17 October 2009 Keywords: Plasma electrolytic oxidation Electrochemical corrosion Scratch test Magnesium alloys Plasma electrolytic oxidation (PEO) is a unique surface treatment technology which is based on anodic oxidation forming ceramic oxide coatings on the surface of light alloys such as Mg, Al and Ti. In the present study, PEO coatings prepared on AZ91D, AZ31B, AM60B and AM50B Mg alloys have been investigated. Surface morphology and elemental composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). SEM results showed that the coating exhibited a porous top surface layer and a subsequent dense layer with micro-pores and shrinkage cracks. Phase analysis of coatings was carried out by X-ray diffraction (XRD). XRD analyses indicated that PEO coatings on AZ alloys had higher amount of Periclase (MgO) followed by the presence of Spinel (MgAl 2 O 4 ) e.g. on the AZ91D alloy compared to that on AM series alloys. In order to examine the effect of substrate composition on adhesion strength of PEO coating scratch tests were carried out. Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization technique in 3.5% NaCl solution at room temperature (20 ± 2 °C). Corrosion test results indicated that the corrosion rates of coated Mg alloys decreased by nearly two orders of magnitude as compared to bare Mg alloys. PEO coatings on AZ series alloys showed better corrosion resistance and higher adhesion properties than AM series alloys. In addition to the PEO processing parameters, such are mainly attributes of the compositional variations of the substrate alloys which are responsible for the formation, phase contents and structural properties of the PEO coatings. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The use of Mg based materials provides advantage of reduced mass while offering improved mechanical and physical properties. How- ever, the characteristics of Mg based materials make them highly susceptible to corrosion. There are a number of coating technologies that have been studied for protecting magnesium and its alloys from corrosion including electrochemical plating [1,2], gas-phase deposition process [3], conversion coating [4–6], anodizing [7,8], laser surface alloying [9], and organic coating [10]. The conversion coating and anodizing are the most popular methods which are based on Cr compounds that exhibit good corrosion resistance, also been proven to be highly toxic carcinogens. Thus, development of an environmentally-friendly process is required to meet the more stringent environmental protection laws currently in effect or being proposed. Plasma electrolytic oxidation (PEO) is a unique surface treatment that transforms the surface of light alloys such as Mg, Al and Ti into a hard ceramic layer with excellent wear and corrosion resistant properties [11]. This method does not involve hazardous electrolyte components such as hexavalent-Cr or heavy metals. PEO process differs from conventional anodizing with respect to its high operating voltages which cause dielectric breakdown of the passive film formed on the component surface. When dielectric breakdown takes place due to high electrical field reached in the passive film, micro- discharges appear as lighting dots moving on the surface. Local temperatures in discharge regions can reach up to 10 4 K [12]. Metal atoms ionize in discharge channel via thermal ionization mechanisms to form metal oxides which then erupt into the electrolyte due to high pressure [12] and solidify on the surface. The process period must be clearly identified as coating build-up proceeds by micro-arcing until reaching a point at which deterioration of the coating is observed [13]. Many factors affecting the PEO process such as current density, voltage, electrolyte additives, pre- and post-treatments were inves- tigated [14–21]. The growth mechanism of PEO coatings on metallic materials has also been discussed [22]. Several studies about the electrochemical behavior of Mg alloys that were PEO coated in silicate [23–26] and phosphate [23,27–29] electrolytes were published. Variations in electrolyte compositions and the power supply cause Surface & Coatings Technology 204 (2010) 1305–1313 ⁎ Corresponding author. Tel.: +90 232 412 74 75; fax: +90 232 412 74 52. E-mail address: ugur.malayoglu@deu.edu.tr (U. Malayoglu). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.10.012 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat