Hot corrosion behavior of CoWSi/WSi2 coating exposed to Na2SO4+NaCl salt at 900°CCeramics International

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Authors
M. Zarezadeh Mehrizi, Gh. Eisaabadi, R. Beygi
Year
2015
DOI
10.1016/j.ceramint.2015.11.064
Subject
Process Chemistry and Technology / Materials Chemistry / Electronic, Optical and Magnetic Materials / Surfaces, Coatings and Films / Ceramics and Composites

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CERAMICS

Available online at www.sciencedirect.com

Ceramics International ] (

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Si/ to etic as to the formation of oxides and spinels of silicon, cobalt and tungsten. Also, NaCl accelerated the degeneration of the coating because of solution is to develop alloys and compounds with optimum temperatures for its ability to form an adherent and continuous available on hot corrosion behaviors of the multi-phase metal 2. Experimental procedureCoWSi/WSi2 composite powder was synthesized by mechanical alloying according to the previous studies [11] http://dx.doi.org/10.1016/j.ceramint.2015.11.064 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. nCorresponding author. Tel./fax: ?98 863 4057003.

E-mail address: m-zarezadeh@araku.ac.ir (M.Z. Mehrizi).Please cite this (2015), http://dmechanical properties, then deposit a protective coating to improve their corrosion resistance [2,3]. It is reported that silicon-containing overlay coatings on nickel- and iron-based alloys improve high temperature oxidation and hot corrosion resistance in the temperature range of 700?1200 1C [4,5]. The most known silicides are based on tungsten disilicide (WSi2) which are resistant against hot corrosion in gaseous oxidizing media up to 1600?1700 1C [6]. Tungsten disilicide can be regarded as an attractive coating material for protection of alloys used in an oxidative and corrosion atmosphere at high silicides coating. In the present study, CoWSi/WSi2 composite coatings were fabricated on a substrate of a Ni by laser cladding using CoWSi/WSi2 nanocomposite powder (fabricated by mechanical alloying) [11] and the hot corrosion behavior was evaluated under cyclic high temperature conditions. Knowledge of the nature of the surface scales formed during hot corrosion is important for evaluating the alloys for their use and degradation characteristics in high-temperature applications.and sulfide. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: CoWSi/WSi2 coating; Cyclic hot corrosion; Molten salt 1. Introduction

For high temperature components serving in aero-gas turbine engines, the material must possess both excellent mechanical property and corrosion resistance. However, these two properties are usually incompatible [1]. A common

SiO2 film on its surface [7,8]. Previous studies have reported the general microstructure of the CoWSi?WSi2 coating by laser cladding. Similarly, the high temperature oxidation behaviors of this coating has also been studied in terms of weight change measurements and oxide scale characterization [9,10], but to the knowledge of the authors, little information isproducing the volatile CoCl2 and thereby oxygen and sulfur could easily penetrate into the coatings and caused the formation of internal oxideHot corrosion behavior of CoWSi/W salt at

M. Zarezadeh Mehrizin

Department of Materials Science and Engineering, Facul

Received 1 August 2015; received in revised f

Abstract

In this study, the high temperature hot corrosion behavior of a CoW performed on CoWSi/WSi2 coated nickel specimens after exposure conditions. Thermogravimetric technique was used to establish the kin the corrosion products. The oxide scale formed on the coating surface warticle as: M.Z. Mehrizi, et al., Hot corrosion behavior of CoWSi/W x.doi.org/10.1016/j.ceramint.2015.11.064INTERNATIONAL ]]]]) ]]]?]]] 2 coating exposed to Na2SO4?NaCl 00 1C h. Eisaabadi, R. Beygi f Engineering, Arak University, Arak 38156-8-8349, Iran 7 October 2015; accepted 11 November 2015

WSi2 composite coating was investigated. Hot corrosion studies were a molten Na2SO4?NaCl salt environment at 900 1C under cyclic s of corrosion. XRD and SEM/EDS techniques were used to analyze complex and the hot corrosion resistance of coating may be attributed www.elsevier.com/locate/ceramintSi2 coating exposed to Na2SO4?NaCl salt at 900 1C, Ceramics International and were used as the laser cladding precursor materials.

Commercial pure electrolyzed nickel plates, 60 mm 25 mm 0 mm in size, were selected as the substrate material.

The powders were placed on Ni surface samples with a powder-bed thickness of approximately 1 mm. Laser cladding was conducted on Nd:YAG pulsed laser materials processing system equipped with a 4-axis computer numerical controlled (CNC) machine under Argon shielding environment. After experimenting with different sets of processing parameters, the best laser cladding parameters were selected: laser power of 0.45 kW, an impulse width of 5.0 ms, a frequency of 30 Hz, a beam diameter of 1.8 mm, a defocusing distance of 5.5 mm and an overlap of 50% at a scanning speed of 2.5 mm/s.

Hot corrosion test of the coatings were performed in a muffle furnace in static air. The bare nickel (substrate) and

CoWSi/WSi2 composite coating samples were placed on a hot iron plate and then brushed with salt mixture Na2SO4/NaCl (3:1, by weight). The samples were weighed before and after importing salt to ensure a salt supply of about 2 mg/cm2.

Afterwards, the samples were placed inside alumina crucibles and loaded into muffle furnace to carry out the corrosion experiment at 900 1C. At regular intervals (17 h), the specimens were taken out, cooled down to room temperature and then washed in boiling distilled water to obliterate the remaining salt. Then, the samples were dried and weighed by electronic balance with sensitivity of 105 g. Subsequently, a uniform fresh salt coating was brushed again to continue the experiment. The corrosion test lasted for 51 h in the salt. Phase constituents of the coatings and all corrosion scales were analyzed by Philips X-ray diffraction (XRD) with a Cu K? tube operated at 30 mA and 40 kV and a scan rate of 0.51/min.

The cross-section for microstructure observations of both ascoated and hot corroded samples were examined in a Seron

AIS 2300 scanning electron microscope (SEM) equipped with

EDS. Etching of the cross-sectional microstructure was carried in limited cases using H2O-7% HF-43% HNO3 water solution