SEALING OF THERMALLY SPRAYED COATINGS M. Oksa, E. Turunen and T. Varis The use of coatings in fluidised bed boilers is increasing owing to the harsh conditions that modern boilers meet. High temperatures, combined with inhomoge- neous, high chlorine and alkali containing fuels such as biomass, cause severe material wastage to metallic parts in boilers. Thermally sprayed coatings have been reported to provide protection to boiler tubes. However, thermally sprayed coatings have encountered serious problems, such as corrosion on the substrate material owing to voids and oxides in lamella boundaries. Sealing of the coatings can solve these problems. This paper investigates the sealing of coat- ings. Thermally sprayed metallic coatings were sealed with different commercial sealing agents and laser treatment. The coatings were tested after sealing by simulating fluidised bed boiler superheater conditions. Specimens were examined by optical microscopy and SEM and analysed by energy dispersive X-ray analysis. Some of the tested sealants protected the coatings adequately in a short term alkali chloride– alkali sulphate exposure test. The best sealant con- tained aluminium oxide and aluminium phosphate. Laser treated coatings had good corrosion resistance in a short term test. SE/S293 Mrs Oksa (maria.oksa@vtt.fi), Turunen and Varis are at Metallimiehenkuja 8, PO Box 1703, 02044 VTT, Finland. Paper originally presented at the Eurocorr Conference in Budapest, Hungary, 28 September–2 October 2003. Manuscript received 5 December 2003; accepted: 22 December 2004. Keywords: Thermal spray, HVOF, Metallic coating, Sealing, Sealant, Laser, Corrosion protection, Chlorine, Alkali, Fluidised bed boiler # 2004 Institute of Materials, Minerals and Mining. Published by Maney on behalf of the Institute. INTRODUCTION The harsh environments in modern power plants, such as in biomass boilers and waste incinerators, demand better corrosion and wear resistance of boiler components. 1 One problem area is super- heaters. Combining high temperature (.500uC) and aggressive, volatile Cl compounds may lead to active oxidation, which induces serious material wastage of tubes. 2,3 Alkali chlorides are especially detrimental, because many of them have low melting points and may induce Cl corrosion ,500uC. 4 To overcome material wastage, more alloyed materials, such as chromium containing alloys and nickel based alloys, have found use instead of low alloy steels. 3,5 Most corrosion resistant materials in waste incineration plants (temperature range 450–550uC) are reported to have Cr and Ni content of (CrzNi)580 wt-%. 4 However, the high cost and difficult fabrication of these materials have favoured coatings as an alter- native to producing more corrosion and wear resistant tubing. 6 Thermal spray coatings are reported to have good adherence, and their corrosion resistance has been shown to be excellent in laboratory tests. 7 Reporting of coating endurance in actual boiler conditions has been minimal. Most testing of coat- ings has been performed in simulated laboratory tests or, in some cases, pilot plants. However, a detonation gun sprayed 50Ni–50Cr coating has been reported to remain in excellent condition after 7 years of opera- tion in a refuse incineration plant. 8 Ni–Cr coatings are easy to manufacture by spray- ing, and their corrosion resistance is good. However, owing to the porosity of thermal spray coatings, corrosion resistance is lowered in critical applications such as power boilers burning highly corrosive fuels. By sealing the thermally sprayed coating, porosity can be minimised from 1–2% to zero, therefore preventing the corrosive agents from penetrating to the substrate material. Sealing can either be done with different sealants, which are applied by painting onto the substrate, closing the open porosity of the coating surface, or by laser treatment. In laser treatment, a thin surface of the coating is melted and resolidified into a compact, homogeneous, non- porous layer. 9 The morphology and composition of the laser treated cladding are transformed from the original sprayed structure. 10 Applying sealing agent increases the cost of the coating. However, if the protective effect of the sealant is long enough, it can be economically applied for critical boiler parts subject to corrosion. EXPERIMENTAL In the present study, the enhanced corrosion of ther- mally sprayed coatings was examined. The corrosion medium was an alkali chloride–alkali sulphate melt simulating superheater corrosion. The tested coatings were high velocity oxy fuel (HVOF) sprayed Ni–45Cr and 80Cr 3 C 2 –20NiCr. Thermal spraying was done with a Praxair HV-2000 gun using DS-469 powder and H.C.Starck Amperit 586 . 1 powder. The approxi- mate layer thickness was 300 mm for the Cr 3 C 2 –NiCr coating and 250 mm for the NiCr coating. Coatings were dense and well adhered to the Fe37 substrate material. Five different sealing treatments were per- formed, including different commercial paints and laser fusion. The tested specimens are presented in Table 1. Sealant 1 is composed of silicate solutions, and its maximum operating temperature is 1370uC. Corrosion resistant sealant 2 contains aluminium oxide and aluminium phosphate and is suited for applications up to 1760uC. Sealant 3 consists of DOI 10.1179/026708404225016346 Surface Engineering 2004 Vol. 20 No. 4 251