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

V. Fantini

CESI RICERCA, Milan, Italy

Abstract

In Municipal Waste Incinerators (MWI) considerable

corrosion problems of critical components, such assuperheater 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-to-

erosion 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 suchas 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 0.7 -1.0 mm. Thereforelaser technology enables to produce high quality coatings with

a considerable saving in feeding materials, when compared to

conventional GMAW welding where 2.5 - 3 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 420-440 °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 erosionproblems 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 during2005 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 thermalspraying (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.



On the other hand GMAW cladding for corrosion protection is

normally applied with thickness of 2-3 mm to boilerwaterwalls 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 maintainingthe 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 qualitycladdings that show low iron content even with thickness of

0.7-1 mm.

Laser Cladding Workstation

Figure 1 shows the laser workstation developed by CESIRICERCA 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-20052 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 20-100 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 1.3-1.4 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 3.2-3.8 % 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

(20.000-500.000 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.

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

measurement points (EDS)

F e c o n t e n t i n w e i g

h t )

Figure 3: Iron content into cladding of Figure 2.



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 differenttechnologies. Tubes samples coated by HVOF, sealed flame

spray and laser cladding have been exposed into superheaterzone 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 flamespray 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 corrosionattack at 500°C and it penetrates up the to interface.

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 processand 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 inTable 1. Campaign has been preformed from 2004 to early

2006. 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.

The skin temperature of the components in operation are

420°C - 440°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 operationtemperature. 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 14700 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

(a)

(b)

(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.



Table 1: Results of the tests of laser cladding performed in

various municipal waste incinerators.

MWI Component Temp.

(°C)

Test

duration

(h)

Test

result

MSB-

Germany

(grate)

Superheater 420 15560 OK

GKS –

Germany

(grate)


ACSM – Italy

(grate)

Boiler 1st fold 355 12960 OK

ACSM – Italy

(grate)

Soot blower 600 1490 Not OK

NRB – Italy

(FDB)


HERA – Italy(FDB)

Fluidized bed 550 5800 Not OK

REA – Italy

(grate)

Thermocouple

sheaths

900 11900 OK

Figure 5: Laser clad tubes of superheater on NRB incinerator

after 14700 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 15560 and11650 hours of operation.

Also in NRB plant the laser cladding of alloy 625 on

superheater tubes are still working and, after 14700 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.

Laser clad tubes mounted in flue gas first-fold wall of ACSM

municipal waste incinerator are still in operation. Due to therelative 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 sootblowers, operating at 600 °C and installed in the same plant,

are not able to withstand corrosion attack and test has beenstopped 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 mmof 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 withstandinghigh corrosion rate at 600 °C and Stellite 6 at 600°C shows asignificant 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 alloy625 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 circumferenceof 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.

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 11900 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.



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 weldingtechniques.

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 wallsoperated 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 combinedattack and a new protection materials must be developed forthis application.

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 processfor applying this material to components of large surface is

under development.

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)

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