Investigation of corrosion behaviour of carbon steel coated by pulsed plasma electrolytic boronising technique in 3?5 wt-%NaCl aqueous solution S. H. Alavi* 1 , C. Dehghanian 1 and P. Taheri 2 Boride coatings were synthesised on AISI H13 steel using pulsed plasma electrolytic boronising technique in a modified aqueous solution of borax. Hard and strongly adherent coatings with various desirable properties were obtained by applying different frequencies and duty cycles during the treatment. The analytical characteristics of the coating were assessed using X-ray diffraction and scanning electron microscopy techniques. The thickness of boride layers was increased by increasing frequencies. The corrosion performances of these coatings were investigated by a potentiodynamic polarisation test and ac impedance spectroscopy. Equivalent circuits have been proposed to represent the boride phases, oxide film and electrolyte systems examined. The general corrosion resistance of the produced coatings has been found to increase with the frequency. Keywords: Pulsed plasma electrolytic boronising, Corrosion, Nanocrystals, Boride layers Introduction Carbon steel has been used widely in many industries because of its good strength and low production costs. 1 However, It cannot be used in marine media and outdoor atmosphere because of its low corrosion resistance. 2 In order to improve the corrosion perfor- mances of the carbon steel, surface treatment techniques have been applied. 3 The plasma electrolytic saturation technique is one of the plasma deposition methods which occurs in an aqueous system. The cathode is composed of uniform plasma environment in a super heated vapour sheath. 4,5 Nanocrystalline layers will be formed when a high voltage is applied to the system. During the process, a large amount of heat is produced. 6,7 In this process, the applied voltage determines the surface temperature of the workpieces during the treatment. 7–9 This method is relatively low cost and ecologically friendly and non-hazardous in comparison with other electrolytes which might other- wise be considered. 10 The boron diffusion into the surface of metal alloys creates a fully dense zone of metal borides. This layer effectively results in superior surface properties of metal. Boron is an element which has a relatively small size, and is allowed to diffuse into a variety of metals, including ferrous, nickel and cobalt alloys. 11 In plasma boriding of steel, the boride phases formed on the surface have high hardness and wear resistance. 12–14 In steel treatment, typical boronising process is performed from 1100 to 1300 K and, as a result, a hardened layer (2000–2500 HV) is formed. 15–17 The purpose of this study is to investigate the role of frequency and duty cycle on the corrosion properties of boride layers by means of electrochemical impedance spectroscopy (EIS). In the EIS technique measurements are usually done by an ac potential within an electrochemical cell establish- ing the current through the cell. This ac current signal contains an excitation frequency, and can be analysed as the sum of sinusoidal functions (Fourier series). The result of EIS is the impedance of the electrochemical system as a function of frequency. Usually impedance is described with an equivalent electrical circuit, con- structed preferably by circuit elements with the proper electrochemical background. 18,19 Experimental Coating synthesis The chemical composition of AISI H13 substrate used in this study is 0?35C–1?08Si–0?035Mn–4?95Cr– 1?25Mo–1?00V (wt-%). The disc substrates made of AISI H13 steel with a diameter of 25 mm and a thickness of 3 mm were polished by a silicon carbide abrasive paper from 400 reaching down 2000 and were cleaned by ethyl alcohol before the treatment. The sample was held in the solution via an AISI 316 rod. Plasma electrolytic boriding (PEB) treatments were 1 Faculty of Engineering, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran 2 Department of Materials Science and Engineering, Delft University of Technology, Postbus 5, 2600 AA, Delft, The Netherlands *Corresponding author, email alavii@gmail.com ß 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 10 June 2010; accepted 13 November 2010 DOI 10.1179/1743294410Y.0000000016 Surface Engineering 2011 VOL 27 NO 7 509