Laser Cladding: a New Technology for Corrosion and Erosion Protection of Boiler Tubes

Transcription

1 Laser Cladding: a New Technology for Corrosion and Erosion Protection of Boiler Tubes V. Fantini CESI RICERCA, Milan, Italy Abstract In Municipal Waste Incinerators (MWI) considerable corrosion problems of critical components, such as superheater or boiler tubes, are always reported. Especially in modern WTE plants the need of efficiency increase requires operation at higher temperatures, which in turn enhances the corrosion rates. Laser cladding technology was successfully used for the production of anticorrosion and resistant-toerosion coatings on tubes of superheaters and boilers. Compared to protective coatings produced by flame spraying devices, laser cladding is virtually porosity free and metallurgically bonded to the substrate, ensuring the possibility of bending the clad tubes without any damage such as cracks or spalling. This ability to sustain high deformation rate is absolutely necessary for the construction of superheaters serpentines, opening the door to the production of a whole superheaters assembly protected by a laser cladding. Due to the very low thermal load of the process, if compared to usual GMAW welding, laser cladding allows producing coatings with very low iron content (1-3%) even in a single pass with thickness lying in the range mm. Therefore laser technology enables to produce high quality coatings with a considerable saving in feeding materials, when compared to conventional GMAW welding where mm thick cladding is necessary to have the same iron content of a single pass laser cladding. In this paper are presented advantages of this new technology and CESI RICERCA facilities for industrial production of MWI superheater and boiler clad tubes by its new automatic diode laser workstation. Results of a campaign of in-plant tests and performances obtained in operation by several laser clad components installed in European MWI plants are also presented. Introduction Most of modern Waste to Energy plants require high steam pressure and temperature values for increasing the energy recovery efficiency. For this purpose the pressure and temperature of steam flowing into superheater tubes of many WTE plants in Europe moved in the last years from 350 C to the actual C and, in the design of the new plants, superheater steam temperature up to 520 C is considered in order to reach net electric recovery efficiencies above 31 %. Following the increase of the operating temperatures, corrosion problems in boiler and superheater are also dramatically increased. Moreover the most widely used systems of on-line cleaning of the supeheater tubes on flue gas side are built by soot blowers, which cause serious erosion problems in those zones where steam flow coming from blowers impinges. For that reason the corrosion/erosion attack is one of the most important and widely reported problems in many MWI. In Italy a recent investigation performed during 2005 showed that 65% of the existing incinerators report corrosion or erosion problems [1] independently on plant size. Many data from operation experience in modern WTE plants report that components made by the conventional carbon steel show corrosion rates that can reach 1.5 mm/year in boiler waterwalls and 2.5 mm/year in superheaters, values completely unacceptable [2]. The strategy often adopted for the corrosion and erosion protection is to apply coatings to the critical components, such as first superheater coil, soot blower zones, boiler ceiling and flue gas first-fold walls. Many coatings and cladding were developed for the specific use in MWI plants, using thermal spraying (HVOF, sealed flame spray) and welding (GMAW) technologies. Thermal spray coatings are normally used for protection of zones exposed to moderate corrosion (max. skin temperature 350 C) and sometime they are applied to flue gas first-fold walls of boilers, while GMAW cladding is generally used for protection of waterwalls in post-combustion zones and of superheater tubes. Thermal spray coatings cannot be applied to superheater tubes because of its porosity connected to the substrate and the lack of metallurgical bonding. The last feature also prevents the possibility of bending coated tubes during coil construction, because of the coating spalling or cracking.

2 On the other hand GMAW cladding for corrosion protection is normally applied with thickness of 2-3 mm to boiler waterwalls and superheater tubes. The reason of that relative high thickness is the necessity of reducing to a very low value the iron content inside the cladding, not changing the original chemical composition of feeding material and so maintaining the original corrosion protection performance of the welded material. Due to the high thermal load of GMAW process, iron content below 3% into the cladding can be normally reach only by 2-3 mm thickness in the case of an ordinary and most widely used feeding material like alloy 625. CESI RICERCA has adapted the laser cladding technology and built an equipment that enables to produce high quality claddings that show low iron content even with thickness of mm. Laser Cladding Workstation Figure 1 shows the laser workstation developed by CESI RICERCA for industrial production of claddings of boiler and superheater tubes of incinerators. The workstation is equipped by 6 kw diode laser, supplied by Rofin-Baasel Italiana s.r.l. Viale Lombardia 159, I Monza (MI)-Italy, having 6 mm x 2 mm rectangular beam spot size on the tube surface. Cladding material is fed in powder and it is injected into the molten pool on the tube surface by a proprietary cladding deposition head. Rectangular beam spot size has been selected for increasing the deposition rate compared to the usual circular spot sizes. Cladding seams typically 6 mm wide are generated on the tubes. Tubes having diameter of mm and length up to 13 m can be processed in the workstation, covering the whole sizes of components normally used in grate or fluidized bed WTE plants. Semiautomatic system for tube loading and unloading is also provided. Deposition rate is in the range Kg/h for conventional alloy 625. Figure 2 and Figure 3 show the typical characteristics of cladding of very thin thickness produced by the laser workstation in single pass process on superheater tubes. Alloy 625 is clad on a P22 tube of 42 mm in diameter with thickness of 0.65 mm. Iron content of % is obtained into the coating uniformly up to the interface between cladding and base material. Increasing cladding thickness up to 1 mm in single pass process, the iron content is reduced below 2%. In-Plant Characterization of Laser Cladding In order to characterize corrosion resistance of developed laser cladding when they are applied to critical components of municipal waste incinerators, an extensive campaign has been done in various WTE plants in Italy and Europe. Size plants ( ton/year), operating skin temperature of the components and different typology of burned waste (ordinary, solid recovery fuel, biomass) have been considered in order to Figure 1: Diode laser cladding workstation developed by CESI RICERCA. Figure 2: Alloy 625 laser cladding of 0.65 mm on P22 alloy tube (right) of 42 mm diameter. Fe content in weight) measurement points (EDS) Figure 3: Iron content into cladding of Figure 2.

3 perform tests representative of different and real operation conditions. First test has been performed in SAKO incinerator (Brno, Czeck Republic) comparing performances of alloy 625 coatings applied to superheater tubes by different technologies. Tubes samples coated by HVOF, sealed flame spray and laser cladding have been exposed into superheater zone at temperature of 400 C and 500 C. Figure 4 shows the results of the tests after 1880 hours of exposure. Samples were not cooled down during the test, so their skin temperature was equal to the flue gas temperature. Thickness of sealed flame spray coating, HVOF coating and laser cladding are respectively 0.45 mm, 0.2 mm and 0.6 mm. Sealed flame spray and HVOF coatings present relevant corrosion phenomena at the interface with base material, due to their intrinsic porosity. In the case of flame spray coating the sealing layer at the surface (upper in Fig. 4a), which is applied to reduce permeability, is not able to withstand corrosion attack at 500 C and it penetrates up the to interface. (a) On the contrary laser cladding shows initial corrosion in very thin layer (less than 50 µm) at the coating surface, while the rest of cladding thickness and base material interface are not attacked by corrosion. Islands at the surface of laser cladding of Fig. 4c are due to lack of complete melting of the powder grains of feeding materials during the laser deposition process and are not related to corrosion. On the basis of encouraging results obtained from the high temperature test in SAKO plant, a characterization campaign in various incinerators has been performed as displayed in Table 1. Campaign has been preformed from 2004 to early The critical components considered in the campaign are the first coil of superheater tubes (SH), first-fold boiler waterwalls tubes, soot blowers, fluidized bed (FDB) boiler tubes and high temperature thermocouple sheaths. (b) The skin temperature of the components in operation are 420 C C in the case of superheater tubes, 335 C in flue gas first-fold boiler wall tubes, 600 C in soot blowers, 900 C in thermocouple sheaths and 550 C in fluidized bed tubes. The cladding material is the ordinary alloy 625 in all the applications. Only in the case of thermocouple sheaths a Ni- Cr-Co alloy has been used, due to the very high operation temperature. Table 1 summarizes the results of these in-plant tests. Figure 5 shows the result of the test of a superheater tube with alloy 625 laser cladding after hours of operation in NRB incinerator. Discussion of the Results Laser cladding of alloy 625 present a very good resistance to the corrosion in the case of superheater tubes. Actually coatings are still in operation in superheater coils of two German large size and modern municipal waste incinerators (c) Figure 4: In-plant comparative test of alloy 625 coatings at 500 C-1880 h; base material in lower part of pictures - (a) sealed flame spray;(b) HVOF; (c) laser cladding.

4 Table 1: Results of the tests of laser cladding performed in various municipal waste incinerators. MWI Component Temp. ( C) MSB- Germany GKS Germany ACSM Italy ACSM Italy NRB Italy (FDB) HERA Italy (FDB) REA Italy Test duration (h) Test result Superheater OK Superheater OK Boiler 1 st fold OK Soot blower Not OK Superheater OK Fluidized bed Not OK Thermocouple sheaths OK Laser clad tubes mounted in flue gas first-fold wall of ACSM municipal waste incinerator are still in operation. Due to the relative low temperature of 335 C, alloy 625 cladding presents a very low corrosion rate of about 0.1 mm/year. On the contrary alloy 625 laser cladding applied on soot blowers, operating at 600 C and installed in the same plant, are not able to withstand corrosion attack and test has been stopped after 1490 hours. In order to improve corrosion and erosion resistance, two laser cladding types are produced by the workstation to try to get a solution for the specific application on soot blowers: the first cladding is a double pass alloy 625 cladding, 2mm thick, with very low iron content, while the second one is a 2 mm thick coating made by 1 mm of alloy 625 plus 1 mm of Stellite 6. The second type was developed for preventing combined corrosion and erosion attack on soot blowers. The test of these claddings in ACSM plant gave negative results yet. Probably a new cladding material alternative to alloy 625 is necessary for withstanding high corrosion rate at 600 C and Stellite 6 at 600 C shows a significant reduction of its microhardness compared to the conventional applications where it is used for erosion protection at low temperature. Negative results are also reported in HERA plant, where alloy 625 or alloy 625 plus Stellite 6 laser cladding have been applied on tubes in fluidized bed. The tubes are immersed into a strong flow of sand supporting the bed and are operated at 550 C. Also in this case the strong erosion produced by the sand flow destroys in a relative short time all types of cladding applied on the tubes and the base material. The cladding thickness appears destroyed only on the half of circumference of the tube where sand flow impacts, while the damage is negligible on the rest of the circumference. Also in this case materials used in the cladding can t withstand the combination of strong erosion and corrosion at high temperature. Figure 5: Laser clad tubes of superheater on NRB incinerator after hours of operation at 440 C. (MSB- Schwandorf and GKS-Shweinnfurt). In the case of MSB and GKS plants the initial thickness of the cladding was 1 mm +/- 7% and the measured average corrosion rate is 0.15 mm/year and 0.18 mm/year respectively after and hours of operation. Also in NRB plant the laser cladding of alloy 625 on superheater tubes are still working and, after hours of operation at the relative high temperature of 440 C; the measured average corrosion rate is about 0.22 mm/year. This plant has been selected for the particular typology of the burned waste, that is 20% of solid recovery fuel and 80% of biomass; then the corrosive flue gas atmosphere is expected to be different from that one of more conventional waste typology as in MSB and GKS plants. Finally an innovative laser cladding has been developed for the corrosion protection of thermocouple sheaths at REA plant. These components are operated at the very high temperature of 900 C. Of course at this temperature alloy 625 is not able to withstand corrosion attach, so a new Ni-Cr-Co laser cladding was developed by CESI RICERCA. The bare thermocouple sheathes installed in REA incinerator are completely damaged and substituted after 4-5 months of operation. The test of laser cladding shows a good result. Clad sheathes are still in operation after hour, that is about 3.5 times the lifetime of bare component. At the moment some problems remain for the deposition of this new feed material on large surfaces (like tubes), due to the cracks that can arise during the process, so further improvements in laser process must be achieved.

5 Summary and Conclusion Laser technology developed by CESI RICERCA allows producing high quality cladding of thicknesses ranging from 0.7 mm to 1 mm, so permitting a significant economical save of the expensive feed materials compared to other welding techniques. Laser cladding on critical components of municipal waste incinerators have been produced by CESI RICERCA laser workstation and installed in various European plants of different typology for in-field characterization. Results of long term tests show very good behavior of alloy 625 laser cladding applied to superheater and flue gas first-fold boiler walls operated up to 440 C. When operational temperature of the components exceeds 500 C and moderate or strong erosion is also present, alloy 625, eventually plus Stellite 6, are not able to withstand combined attack and a new protection materials must be developed for this application. Acknowledgments This work has been financed by the Research Fund for Italian Electrical System established with Ministry of Industry Decree DM 26/1/2000. References 1. V. Fantini, Outcome of an investigation on the needs of technological development in WTE plants, La termovalorizzazione dei rifiuti in Italia: l esperienza di esercizio e l applicazione delle nuove tecnologie, Oct 22, 2006, (Milan, Italy) 2. L. Paul, G. Clark, M. Eckhardt and B. Hoberg, Experience with Weld Overlay and Alloy Solid Tubing Materials in Waste to Energy Plants, 12th Annual North American Waste to Energy Conference Proc., May 17-19, 2004, (Savannah, Georgia, USA) Good results have instead been obtained for corrosion protection of small components operating at 900 C, using a new Ni-Cr-Co alloy as feeding material. A new laser process for applying this material to components of large surface is under development.