Corrosion

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Surface & Coatings Technology 202 (2008) 3263 – 3271 www.elsevier.com/locate/surfcoat

Boroaluminide coatings on ferritic–martensitic steel deposited by low-temperature pack cementation
S.A. Tsipas a,⁎, H. Omar b , F.H. Perez a , D.N. Tsipas b
a

Grupo de Investigación de Ingeniería de Superficies - Dpto Ciencia de Los Materiales, UniversidadComplutense Madrid, Avenida Complutense s/n, Facultad de Ciencias Químicas, 28040 Madrid, Spain b Department of Mechanical Engineering, Physical Metallurgy Laboratory, Aristotle's University of Thessaloniki, 54124 Thessaloniki, Greece Received 15 October 2007; accepted in revised form 29 November 2007 Available online 5 December 2007

Abstract In this study, new boroaluminide protective coatingswere deposited on ferritic–martensitic steel substrates (P91) using the pack cementation technique, at moderate temperatures in order not to influence the substrates' mechanical properties. Extensive thermodynamic calculations were performed initially, using the Thermocalc Computer program, so as to optimize the process parameters. The most important gas-precursors for successful deposition of thecoatings were identified. The effect of pack composition on the formation and growth of boroaluminides at 715 °C, using pack powders containing Al and B as element depositing sources, two halide salts as activators, and Al2O3 as inert filler, was investigated. Three distinct regions were found in the coatings consisting of an outer Al-rich layer, a transition region containing Al, B and Fe and aninner layer containing mostly B, Cr and Fe. The layers were characterized by means of optical and Scanning Electron Microscopy (SEM) in terms of coating morphology and thickness. X-ray diffraction (XRD) was used in order to detect the phases formed and the presence of iron aluminide and boride phases in the coatings due to the boroaluminizing process. © 2007 Elsevier B.V. All rights reserved.Keywords: Thermodynamic calculations; Protective coatings; Boron; Steels; Pack cementation; Thermochemical processes; Diffusion barrier

1. Introduction Ferritic–martensitic steels are prone to corrosion whenever they are exposed to aggressive and/or corrosive environments, including aqueous, abrasive, high temperature, stress and other service parameters [1]. A way to enhance the resistance of suchmaterials, while preserving their mechanical properties is by depositing coatings with entirely different surface properties that meet the necessary performance requirements. The beneficial effects of intermetallic coatings in improving the corrosion and/or wear resistance of various substrates in aggressive atmospheres have been extensively reported in

⁎ Corresponding author. Tel.:+3491944215; fax: +34 91 39 44357. E-mail addresses: sofia.tsipas@quim.ucm.es (S.A. Tsipas), tsipas@eng.auth.gr (D.N. Tsipas). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.11.034

literature [2–4]. A convenient and effective technique for the deposition of protective intermetallic coatings on steels and other metal alloys is a thermochemical surfacetreatment such as pack cementation[5,6]. Processes such as alumizing [7,8], chromizing[9], carburizing[10] and boriding[11] have been successfully carried out by pack cementation, which is basically an insitu, self generated chemical vapor deposition (CVD) process [5]. Aluminizing, has been found to improve substantially the high temperature oxidation and corrosion resistance of steel [12–16]. Theintermetallic iron-aluminde coating deposited forms a protective Al2O3 oxide scale, through preferential oxidation of Al, which acts as a protective barrier separating the underneath material from the aggressive environment. Boriding is increasingly being used in various braches of engineering for protecting metal pieces due to the excellent combination of wear resistance and corrosion...