Accelerated hot corrosion studies of cold spray Ni–50Cr coating on boiler steels Niraj Bala a , Harpreet Singh a, * , Satya Prakash b a Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab, India b Indian Institute of Technology, Roorkee, Uttrakhand, India article info Article history: Received 19 February 2009 Accepted 14 June 2009 Available online 24 June 2009 Keywords: Hot corrosion Cold spray coatings Ni–50Cr Boiler steels Oxide scale abstract In the current investigation Ni–50Cr powder was deposited on two boiler steels SA-213-T22 and SA 516 (Grade 70) by cold spray process. The hot corrosion performance of coated as well as bare boiler steels was evaluated in an aggressive environment of Na 2 SO 4 –60% V 2 O 5 under cyclic conditions at an elevated temperature of 900 °C. The kinetics of the corrosion was approximated by the weight change measure- ments made after each cycle for a total period of 50 cycles. Each cycle consisted of 1 h heating in a tube furnace followed by 20 min cooling in ambient air. X-ray diffraction (XRD), scanning electron micros- copy/energy dispersive X-ray analysis (SEM/EDAX) techniques were used to analyse the corrosion prod- ucts. Both the uncoated boiler steels suffered intensive spallation in the form of removal of their oxide scales, which may be attributed to the formation of unprotective Fe 2 O 3 dominated oxide scales. The Ni–50Cr coated steels showed lesser weight gains and the oxide scales remained intact till the end of the experiment. The phases revealed in the oxide scales of the coated specimens were mainly oxides of chromium and nickel and their spinels which are reported to be protective against the hot corrosion. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The phenomenon of cold gas-dynamic spraying (cold spray) was discovered in the early 1980s at the Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Acad- emy of Sciences (ITAM of RAS) while studying models subjected to a supersonic two-phase flow (gas + solid particles) in a wind tunnel [1]. The fundamental concept of the cold spray process is that the coating is formed by a high-velocity flow of ‘‘cold” particles on a ‘‘cold” substrate. Cold spray process uses a high-pressure, high- velocity gas jet to impart the velocity for the coating particles. A high-pressure jet, preheated to compensate for the adiabatic cool- ing due to expansion, is expanded through a converging/diverging nozzle to form a supersonic gas jet. Powder particles, transported by a carrier gas, are injected into this gas jet. Momentum transfer from the supersonic gas jet to the particles results in high-velocity particle jet. These powder particles, on impact onto the substrate surface, plastically deform and form interlinking splats, resulting in a coating [2]. The cold spray process eliminates the harmful ef- fects of high-temperature on coatings and substrates, which result into significant advantages like avoiding oxidation and undesirable phases, retaining properties of initial particle materials, conducting heat and electricity easily through the coatings and providing high density, high hardness and cold worked micro-structure [3]. The high resistance of high-chromium, nickel–chromium alloys to high-temperature oxidation and corrosion makes them widely used as welded and thermally sprayed coatings in fossil fuel-fired boilers, waste incineration boilers and electric furnaces [4,5]. Nick- el–chromium alloys have been used as coatings to deal with oxi- dizing environments at high-temperature. When nickel is alloyed with chromium, this element oxidizes to Cr 2 O 3 , which could make it suitable for use, up to about 1200 °C. However, in practice its use is limited to temperatures below 800 °C [6]. The thermal sprayed 50/50 nickel–chromium alloy is usually recommended as an ero- sion–corrosion protection for boiler tubes in power generation applications [7]. As observed by Yamada et al. [8] the Ni–50Cr alloy coating produced by the detonation spray method exhibited the most corrosion resistance in a laboratory test. The hot corrosion test results of the Ni–50Cr alloy detonation spray coated boiler tubes showed excellent performance in the actual refuse incinera- tion plant. This plant operated for about 7 years without any prob- lems and the coated tubes were expected to have longer life [8]. The aim of the present work is to investigate the usefulness of cold sprayed Ni–50Cr coatings to control hot corrosion of T22 and SA 516 boiler steels at 900 °C for 50 cycles. It has been learnt that no information is available on the high-temperature behavior of these cold spray coatings in open literature. The cyclic oxidation conditions have been selected keeping in view that most of the ac- tual industrial components work under cyclic conditions of opera- tion. The information arising out from the investigation will be useful to explore the possibility of use of the cold spray coatings on the boiler tubes. 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.06.033 * Corresponding author. E-mail address: nirajbala1@rediffmail.com (H. Singh). Materials and Design 31 (2010) 244–253 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes