00258 corrosion erosion corrosion protection by mo

Corrosion & Erosion/Corrosion Protection by Modern Weld Overlays
in Low NOx, Coal-Fired Boilers
George Lai and Philip Hulsizer
Welding Services Inc.
2225 Skyland Court
Norcross, GA 30071 U.S.A.
ABSTRACT
This paper describes modern automatic overlay technology for applying weld overlay for surface protection of the waterwall
and superheater/reheater tubes against corrosion or erosion/corrosion in utility coal-fired boilers. The discussion also
includes the general characteristics and properties of modern weld overlays and the successfiil applications of the overlays to
protect the lower furnace against sulfidation in boilers equipped with low NOx burners as well as boiler tubes against soot
blower erosion/corrosion. Applications of weld overlay composite tubes for solving coal ash corrosion problems and
erosion/corrosion problems associated with superheaters and for solving carburization problems encountered in reheaters are
also discussed.
INTRODUCTION
In many large industrial plants, major plant equipment, such as boilers, is manufactured from carbon steels or low alloy steels
for pressure containment. These components are generally designed and constructed based on strength requirements
following codes and standards, such as ASME Codes. Although most of these components have corrosion allowance built
into their initial wall thickness, wastage rates due to corrosion or erosion/corrosion can be excessive for carbon steels or low
alloy steels. Following are four areas where surface protection of boiler tubes are often required in coal-fired boilers in order
to ensure an efficient and economical operation.
In an effort to reduce NOx emissions, coal-fired boilers are being equipped with low NO~ burners. As a result, the
combustion condition in the lower furnace of the boiler has changed ffi'om an oxidizing atmosphere to that of a reducing
condition. This results in increased waterwall wastage in many boilers. Wastage rates of 50-60 mpy or higher on carbon
steel or Cr-Mo steel waterwalls have been observed. The increased wastage rates are believed to result from sulfidation
attack. (1,2)
The slag deposits on the waterwalls or the tubes in the convection section, such as superheaters, can cause significant heat
transfer problems particularly when coal with high ash contents is burned. The industry practice for removing these
tenacious slag deposits from boiler tubes is to use steam from the soot blowers strategically located within the boiler. This
high temperature, high pressure steam can cause significant erosion damage to carbon or Cr-Mo steel tubes.
Because of higher metal temperatures, superheater tubes can experience coal-ash corrosion. Coal-ash corrosion is related to
ash/salt deposits containing low melting-point salts which become molten and flux away the protective oxide scales from
metal surfaces, thus resulting in higher wastage rates. (3) The wastage can be excessively high even for austenitic stainless
steels in some boilers.
Copyright
©2000 by NACE International.Requestsfor permission to publish this manuscript in any form, in part or in whole must be in writing to NACE
International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this
paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in U.S.A.
Selim Mahmoud Selim - Invoice 77764 downloaded on 1/7/2019 4:18:56 PM Single-user licence only, copying/networking prohibited
00258
CORROSION2000Paper No.

In coal-fired boilers equipped with low NOx burners, an increasing number of boilers have been reported to have experienced
carburization problems in their reheaters. (4)
Weld overlay has been used in the past as a temporary, "band-aid" type repair in the field until a somewhat permanent fix
could be developed to address the corrosion problem. Thanks to advances in automatic welding systems, application
techniques, welding process control, welding metallurgy and QA/QC programs, among others, modem weld overlays have
now become a long-term fix to corrosion problems in power generation and refinery/petrochemical processing.
The present paper discusses the application of modem weld overlays for providing a long-term solution to the corrosion or
erosion/corrosion problems encountered in coal-fired boilers. Discussion focuses on the overlay protection against lower
furnace waterwall sulfidation, soot blower erosion/corrosion, superheater and reheater corrosion. General characteristics and
properties of modern weld overlays are also described.
MODERN WELD OVERLAYS
Modem weld overlay technology offers a systematic approach to provide surface protection for major equipment using an
automated weld overlay system to deposit a corrosion or erosion/corrosion resistant weld overlay over a large area with
consistent quality and properties. This is accomplished with automatic overlay machines, which are equipped with real time
display of welding parameters, such as, voltage, current, travel speed, wire feed speed, torch oscillation, etc. The Unifuse®
overlay welding system described in this paper utilizes the pulse spray gas metal arc welding (PSGMAW) process.
The quality of the weld overlay is strongly dependent upon welding contractor, welding system, welding method, welding
parameters, weld bead sequence and profile, overlay microstructure, weldability of the overlay alloy, QA/QC programs, and
welding personnel, among others. Extensive experience has been gained on applications of weld overlays in waste-to-energy
boilers, coal fired boilers, kraft recovery boilers, and refinery/petrochemical/chemical plants for the past 15 years. The
technology has been proven to provide a reliable, cost effective, long-term solution to corrosion or erosion/corrosion
problems for the aforementioned industrial plants.
Weld Overlay Approach to Corrosion and Erosion/Corrosion Protection for Boilers
Field Overlay Application. The waterwall overlay technology was initially developed in the mid-1980s to solve severe
chloride corrosion problems in waste-to-energy boilers. The field overlay welding using alloy 625 (Ni-21.5Cr-9Mo-3.7Nb
alloy) was first applied to the waterwaU of a waste-to-energy boiler in Lawrence, MA in 1984. The alloy 625 overlay proved
to be so successful against fireside corrosion that approximately 280,000 Ibs of alloy 625 weld metal was applied during the
first five years for 15 waste-to-energy plants. (5) Today, alloy 625 overlay continues to play a very significant role in
managing the erosion/corrosion and corrosion problems in waste-to-energy boilers. So far over 800,000 lbs of alloy 625
overlays have been applied in waste-to-energy boilers in the U.S.
The overlay welding system is fully automatic and capable of depositing weld beads in a vertical down mode starting
typically from the membrane and then moving to the tube section following a preprogrammed weld bead sequence to achieve
a uniform coverage of the waterwall (i.e., membranes and tubes). Each weld bead is overlapped by subsequent weld bead to
insure a full coverage with no missing spots. The thickness of the overlay applied to the waterwall is typically 0.070"
minimum.
In modem overlay welding, large areas (i.e., thousands of square feet) of the waterwaU are routinely overlaid during a
maintenance shutdown. It is, thus, a common practice to use many welding machines, for example, 10 or more, at the same
time at different locations or elevations in the boiler to complete the project. The advantage of using the weld overlay
approach to the waterwall restoration and protection is that many weld overlay machines can be operating at the same time
inside the boiler. A modem weld overlay machine can deliver a welding "speed"of approximately 1.5 to 2.0 ft2 per hour.
Thus, with 10 machines, a total area of about 180 to 240 ft2 of the waterwall can be overlaid over a 12-h shift. The new
machine is now equipped with two welding torches, thus doubling the welding speed. The field overlay job generally
follows the following sequence of tasks: mobilization, site set-up, scaffolding, gritblasting, tube wall mapping, overlay
welding, final inspection, site clean-up, and demobilization. A waterwall area of about I000 ft: can be routinely overlaid in
seven days using a two 12-hour shift per day schedule. Figure 1 shows an automatic weld overlay machine "performing"
overlay welding on the waterwall of a large coal-fired boiler. Note the "hands-off' welder watching the machine in
operation.
Unifuse is a registered trademark of Welding Services Inc.

Distortion is an important issue in overlay welding of the waterwall. As the overlay area gets larger, the issue becomes more
important. With significant experience gained in recent years for working on large utility coal-fired boilers, application
related problems, such as waterwall distortion, cracking of buckstays, etc., have been significantly minimized. Some of the
on-going work on residual stresses and distortion involving finite element analysis and modeling will further contribute to the
understanding and minimization of the problems. A number of approaches have been taken to reduce heat input through
weld bead sequence and other strategies in order to achieve minimal waterwall distortion. One effective means is to perform
overlay welding with water in the tube.
Panel Replacement Using Shop-Overlaid Panels. When the waterwall is badly damaged and is beyond repair by field
overlay, the damaged section can be removed and replaced with shop-fabricated overlay panels. Replacement of waterwall
panels is generally considered to be the last resort. This is done only when the tube wall is too thin or badly cracked and it is
no longer possible to use weld overlay of a matching filler metal to build up the wall thickness to the ASME Code allowable.
Overlay panels are fabricated by simply applying overlay to regular carbon (or Cr-Mo) steel panels in a stand using a vertical
down welding mode. Panels of sizes up to 4 ft wide and 40 ft long can be readily handled in the shop. Application
techniques for overlay welding of panels in the shop are essentially same as field overlay, except panel overlay in the shop
has much better dimensional control.
The overlay panel can also be consmacted out of individual overlay composite tubes. The process involved in manufacturing
overlay composite tubes will be discussed in the next section. Panels constructed out of overlay composite tubes are
commonly used for construction of the floors in kraft recovery boilers.
Overlay Composite Tubes for Convection Sections. A proprietary welding process was developed for depositing an alloy
overlay on the outer diameter (OD) of a carbon steel or Cr-Mo steel tube for corrosion or erosion/corrosion protection. The
weld overlay deposit is produced by a fully automatic welding system with real-time digital readouts of major processing
parameters to ensure consistent welding parameters during the production run. This unique patented tube overlay process
deposits a uniform weld overlay with unique microstructure and properties.
Tubes with diameters from sAto 5 inches, wall of 0.100 inches or thicker, and length up to 45 ft can be processed on a routine
production basis. The overlay thickness depends on individual requirements for corrosion or erosion/corrosion protection,
with typical thickness being about 70 mils. Typical cross-section of an overlay composite tube is shown in Figure 2. These
overlay composite tubes are typically used in superheaters, reheaters, generating banks and economizers. Figure 3 shows an
example of a superheater constructed out of alloy 625 overlay composite tubes.
ASME Code & NBIC Considerations
Boilers are typically constructed to the American Society of Mechanical Engineers (ASME) Section I Code requirements.
However, any repairs and alterations of pressure-retaining components, such as boiler tubes, are permitted by the National
Board Inspection Code (NBIC) under specific conditions. These conditions are highlighted in the NBIC and the contractor
who performs the welding work is required to hold a valid "R" certificate issued by NBIC. NBIC allows for repairs of
wasted areas on the boiler tubes by welding, provided the welding procedures and the welder who performs the repair work
are qualified in accordance with ASME Section IX.
When the boiler tube wall is corroded to below the minimum wall thickness allowed by the ASME Code, the Code allows the
subject area to be restored to the required thickness using weld metal buildup with a filler metal composition matching that of
the boiler tube. This is routinely performed in waterwall restoration. Frequently, this weld metal buildup is performed to
restore the thin wall areas before applying a corrosion-resistant weld overlay, such as type 309 SS and alloy 625, for
corrosion or erosion/corrosion protection. However, the overlay of a corrosion-resistant alloy is not considered to be part of a
structural material in stress calculations.
General Characteristics & Properties of Modern Weld Overlays
Overlay Chemistry. The chemistry of a weld overlay is strongly dependent upon the weld wire chemistry, welding
method, weld bead sequence and other factors. There will be some dilution in chemistry for the weld overlay. Typically, the
dilution of the chemistry from the weld wire is about 10%. Chromium ranges for ER309 (AWS type 309 specification) and
ERNiCrMo-3 (AWS alloy 625 specification) are 23.0 to 25.0% and 20.0 to 23.0%, respectively. Type 309 SS overlay
typically exhibits about 21 to 22% Cr, and alloy 625 overlay typically has about 20% Cr . In general, weld wires with
chromium at the high end of the chemistry range are often used for overlay welding in an effort to achieve higher chromium
contents in overlays.

Using the PSGMAW process, the chromium concentration has been found to remain very constant across the overlay, except
within 10 mils (0.025 ram), or less from the fusion line. Table 1 shows the concentration gradient for chromium as a function
of distance from the fusion line for an alloy 625 overlay with a T-2 substrate. The chromium concentration was determined
by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS).
Coefficient of Thermal Expansion. The coefficient of thermal expansion (CTE) of a weld overlay alloy is another factor
considered by engineers when considering the weld overlay approach. It is ideally to have a weld overlay alloy with a
thermal expansion coefficient that matches that of carbon steel or Cr-Mo steel substrate. However, for utility boilers which
are normally shutdown once or twice a year, the difference in thermal expansion coefficients between the overlay and the
substrate steel may not play an important role in the overlay's performance. A brief discussion is given below for two
successful overlays, type 309 SS and alloy 625, in the application ofwaterwalls in coal-fired boilers.
The coefficients of thermal expansion of type 309 SS, alloy 625, carbon steel and two Cr-Mo steels are tabulated in Table 2.
Type 309 weld metal data was obtained from a weld overlay sample built up to about 3/4" thick. The weld overlay data are
essentially same as those of the wrought alloy reported in the literature (9.2 x 10-6 and 9.6 x 10-6 in/in F for 70-600 F and
70-1000 F, respectively). Alloy 625 overlay is believed to exhibit the same thermal expansion coefficient values as those of
alloy 625 wrought alloy.
Table 2 shows that alloy 625 exhibits coefficients of thermal expansion that are essentially the same as those of carbon steel
and Cr-Mo steels. As a result, little or no stresses are likely to be produced between the alloy 625 overlay and carbon steel or
Cr-Mo steel substrate due to temperature fluctuations. Type 309 SS overlay, on the other hand, has higher thermal expansion
coefficients than those of carbon steel or Cr-Mo steels. However, utility coal-fired boilers do not operate with frequent
startups and shutdowns, except for regular maintenance shutdowns, which are typically performed once or twice a year.
Thus, low cycle fatigue cracking is not likely to be an important issue for the waterwall overlaid with type 309 SS.
Some people have mistakenly compared the behavior of the dissimilar metal weld of a 309 SS overlaid carbon (or Cr-Mo)
steel waterwall to the dissimilar metal weld joint which connects a ferritic (carbon or Cr-Mo) steel tube to an austenitic
stainless steel tube in a superheater or reheater. These two cases are completely different in nature. The latter case of the
dissimilar metal weld, which joins a ferritic steel tube to an austenitic stainless steel tube in a superheater or reheater, is part
of the pressure boundary, and carries the full mechanical and thermal loading as does the tube. In the overlay case, the
relatively "thin" overlay (minimum of 0.070" thick) essentially expands and contracts following the base carbon (or Cr-Mo)
steel tube. Furthermore, the overlay "weld" is not part of the pressure boundary and does not carry the full mechanical and
thermal loading from the operation, even though the 625 or 309 SS overlay does provide high temperature strengthening to
the overlaid tubing.
As for thermal fatigue cracking due to temperature gradients through the tube wall and temperature fluctuations during
operation, the difference in temperature changes is not likely to be significant enough to cause thermal fatigue cracking due
to the thermal expansion mismatch between the 309 SS overlay and the substrate steel. This will be illustrated in a case
history in a later section and by numerous successful cases for type 309 SS overlays. Furthermore, some of the metallurgical
atu'ibutes of type 309 SS overlay may also contribute to the successful performance of the overlay. For example, excellent
ductility of type 309 SS overlay, which exhibits more than 40% room temperature tensile elongation, when produced by
PSGMAW, is likely to help contribute to its thermal fatigue resistance.
Crown Bead Profile. The weld metal profile is very important for its resistance to thermal fatigue cracking. This is
particularly true for the crown bead since the crown portion of the tube is subject to the most severe thermal gradient. A
good crown bead profile should exhibit a "convex" feature. A concave weld profile should be avoided, since stress
concentration can be generated when thermal expansion and contraction takes place.
Overlay Microstructure. The pulse spray GMAW process along with fast oscillation and optimum welding parameters
produces an overlay of an austenitic alloy, such as type 309 SS and alloy 625, with unique microstructure and properties.
Both type 309 SS and alloy 625 overlays exhibit a unique multi-layered microstructure with very fine dendritic subgrains.
These microstructural features are described in detail elsewhere. (6) Typical dendritic subgrain structure for a Type 309 SS
overlay is shown in Fig. 4. The overlay microstructure is also free of eutectic phases and carbides. All these microstructural
features are believed to be responsible for the excellent ductility of the overlaid products. Furthermore, the formation of lack
of fusion defects and internal voids is minimized; and solidification cracking is essentially eliminated.
The overlay microstructure has also been characterized by microhardness measurements across the overlay, fusion line and
heat-affected zone (HAZ). In general, no significant hardening was observed in the HAZ due to overlay welding, even when
water was used during the welding process. Table 3 shows the Vicker microhardness results for a type 309 SS overlay

applied to a 1-1/4Cr-1/2Mo (T-I 1) waterwall while water was in the tubes to minimize the distortion of the waterwall. The
HAZ is typically 40-50 mils deep. Lacking excessive hardening in the HAZ as well as a relatively mild HAZ depth helps
keep the substrate steel relatively ductile.
Overlay Ductility. Due to the aforementioned microstructural features, the overlay generally exhibits good ductility, as
illustrated by a type 309 SS overlay. A weld overlay sample was prepared by building up to about 3/4" thick sample blank.
Tensile specimens were machined from the all-weld-metal sample for room temperature tensile testing. Test results showed
excellent tensile ductility for the weld overlay with about 43% elongation and 66% reduction in area. This is also illustrated
by the results of room temperature tensile tests on specimens obtained from an alloy 625/SA 210 overlay composite tube.
The results, as shown in Table 4, showed that alloy 625 overlay not only strengthened the overlay tubing but also retained its
ductility.
Effect of Service Exposure. Since the waterwall metal temperatures in a coal-fired boiler are expected to be around 700-
850F, the overlay properties are expected to improve with service exposure. This is because the temperatures are high
enough to relieve some residual stresses present in the overlaid product and to temper bainite, or martensite (if it forms), in
HAZ. Furthermore, the service temperatures are too low to cause carbide precipitation in either type 309 SS or alloy 625
overlay. The beneficial effect of the service temperature is clearly revealed by the laboratory aging studies, with results
summarized in Table 5.
Evaluation of several overlaid waterwall samples obtained from boilers after long-term services has shown that the HAZ
hardening became significantly less pronounced as a result of the long-term exposure to the operating temperatures. One
example was a type 309 SS overlay on a T-11 waterwall after service for seven years in a supercritical boiler. The HAZ
including the fusion line was found to exhibit about 200 HVN (an equivalent of Rc 22). Another example was an alloy 625
overlay on carbon steel waterwall after service for more than five years in a waste-to-energy boiler. The HAZ hardening was
completely eliminated.
APPLICATIONS & PERFORMANCE OF MODERN OVERLAYS IN COAL-FIRED BOILERS
Waterwall Protection Against Sulfidation
Combustion of coal takes place in the lower furnace surrounded by the waterwall (typically a tube-membrane-tube
construction), typically under highly oxidizing conditions. Under these conditions, waterwalls made of carbon steels or Cr-
Mo steels form Fe304 and Fe203, and generally exhibit relatively low wastage rates. As a result, tube wall wastage has been
manageable without the need for additional surface protection.
However, recent installation of low NOx burners in many utility coal-fired boilers has resulted in significantly higher wastage
rates for the waterwall. In these units, ash deposits were found to contain unburnt carbon and pyrite (FeS). This is indicative
of reducing conditions generated under low NOx combustion. Carbon steel or Cr-Mo steel tubes were found to suffer
sulfidation attack, (1,2,7) with wastage rates of 50-60 mpy or higher observed.
Both type 309 SS and alloy 625 overlays have chromium contents high enough to provide adequate long-term sulfidation
resistance at the furnace waterwall locations, since the metal temperatures in those areas are generally well below 1000 F. At
such low temperatures, stainless steels and nickel-base alloys with 20% Cr, including alloy 625, exhibit good sulfidation
resistance.
In late 1980s, boiler trials were performed to test overlays of type 309 SS and nickel-base alloy 625. Initial results were very
encouraging and large scale applications of type 309 SS and alloy 625 overlays began in the 1993-1994 period. More and
more utility coal-fired boilers are now relying on either type 309 SS or alloy 625 overlays for waterwall protection against
sulfidation attack under low NOx combustion. Table 6 lists some coal-fired boilers with waterwalls overlaid with 309 SS,
alloy 625 and alloy 622.
There has been some interest in the industry recently for a stainless steel overlay with a much higher chromium content than
type 309 SS for better sulfidation resistance. One such stainless steel that has received some attention is type 312 SS. The
weld wire contains 28-32% Cr and 8.0-10.5% Ni, and is widely available. Furthermore, in terms of weld wire cost, type 312
SS is slightly more expensive than type 309 SS but significantly less expensive than alloy 625. Type 312 SS has been widely
used as an overlay alloy for batch digesters in the pulp and paper industry for the past three years with great success. (8) It is
a duplex stainless steel, consisting of ferrite and austenite. However, there is one concern for this alloy when it is being
considered for high temperature applications. It is well known that ferritic stainless steels are susceptible to the phenomenon
frequently referred to as "885 F (475 C) embrittlement". When exposed to temperatures between 750 and 1000 F, ferritic

stainless steels can become hardened and exhibit a significant ductility loss. (9) Duplex stainless steels are also susceptible to
this type of embrittlement because of the significant amount of ferrite present. Recent studies on the 312 SS overlay have
confirmed that the overlay was hardened to Rc 40 after exposure to 900 F for 500 hours. With such as a high hardness, the
overlay is expected to suffer a significant ductility loss. Experimentally it is difficult to quantify the ductility of the overlay
with a thickness of only about 70 mils. A thick 312 overlay sample built up by multiple weld passes was found to have lost
almost all of its ductility after aging for 500 hours at 900 F. The microstructure of the duplex stainless steel overlay is
strongly dependent upon, among other factors, cooling rates and reheating. Thus, a thick weld overlay using multiple weld
passes to build up a ½" or 3A"thick sample is likely to have a microstructure different from that of a single layer of typical
overlay. One should, however, proceed with caution in exploring the application of type 312 overlay for potential waterwall
protection in large boilers where the waterwall metal temperatures are high (e.g., 750 F or higher). On the other hand, the
hardening characteristics for the 312 overlay may be utilized to its advantage for lower temperature applications where
erosion/corrosion may be dominating the wastage mode.
There are more Ni-base alloys that are available for high Cr overlays. INCONEL® filler metal 52 (Ni-30Cr-9Fe), G-30®
(Ni-30Cr-15Fe-5.5Mo-2.5W), 45 CT or INCONEL 72 (Ni-44 Cr) and ALLCORR® (Ni-30Cr-9Mo) are good candidates for
providing higher Cr in the overlay than that of type 309 or alloy 625 overlay.
Soot Blower Erosion/Corrosion Protection
In coal-fired boilers, slag deposits on boiler tubes are common, and those deposits can affect the heat transfer of boiler tubes.
A common practice in the industry is to use soot blowers to remove those deposits periodically. The soot blowing steam often
causes severe erosion/corrosion problems for carbon steel or Cr-Mo steel tubes. Without any surface protection, these tubes
may only last for 1-1/2 year to 2 years. The damage mechanism is believed to be erosion/oxidation. The damage process is
due to steam impingement, which removes the scales and deposits from the tube, exposing the fresh tube surface to flue gas
stream, and thus more oxidation, followed by removal of the oxide scales by soot blowing steam. Therefore, tube wastage is
accelerated by this erosion and oxidation interaction. Soot blowers are used to remove slag deposits from the waterwall as
well as the tubes in convection sections, such as, superheaters, reheaters, generating banks and economizers. Both type 309
SS and alloy 625 overlays have been successful for this application in both the waterwall and convection areas. For
superheaters and reheaters with metal temperatures high enough to experience coal ash corrosion problems, Ni-44 Cr weld
wire (45CT or alloy 72) should be considered for weld overlays to resist soot blower erosion/corrosion attack.
The following case history illustrates the successful performance of type 309 SS overlay in protecting boiler tubes against
soot blower erosion/corrosion (10):
In a supercritical unit with 904 MW(e) at Georgia Power's Plant Wansley, the waterwall was made of 1-1/4" diamater, 0.220
MWT, ASME SA-213 T-11 tubes spaced at 1-3/4" center to center. The fuel was initially an Illinois basin coal, containing
about 2.5% sulfur and 0.12% chlorine. In 1994, the fuel was switched to an Eastem Appalachian low sulfur coal with less
than 1.4% sulfur and no detectable chlorine. Low NO,, burners and Separated Overtired Air (SOFA) were installed between
1992 and 1995. Up to 1989 before weld overlay was applied, the waterwall tubes were replaced every 1-1/2 years at the
areas subjected to soot blower erosion/corrosion. The wastage rates were approximately 80-90 mpy. The tube OD metal
temperature was estimated to be approximately 850 F. The soot blower steam used was at 700 F and 600 psi. In 1989, weld
overlay of type 309 SS was applied to the soot blower areas. After 18 months of operation, the overlay panel was inspected
visually, revealing no evidence of erosion/corrosion attack. The original weld bead ripples were still clearly visible. During
the spring 1997 shutdown, a tube sample was cut and removed from this original overlay panel for metallurgical examination.
Figure 5 shows the appearance of the crown weld beads on the tube sample removed after seven years of operation. The
original weld bead ripples were still clearly visible. The cross-section of the overlay over the boiler tube is shown in Fig. 6.
The sample was examined by optical microscopy and scanning electron microscopy, revealing no cracking at either interface
or the overlay surface. The overlay was in excellent condition with no evidence of erosion/corrosion attack. Typical overlay
surface condition at the crown bead is shown in Fig. 7. Furthermore, no thermal fatigue cracking was developed in the
overlay which had been subjected to periodic soot blowing for seven years, with metal temperatures fluctuating presumably
between 850 F to 700 F.
INCONEL is a registered trademark of Special Metals,
ALLCORR is a registered trademark of Allvac.
G-30 is a registered trademark of Haynes International, and

Coal-Ash Corrosion, Carburization & Erosion/Corrosion Protection for Superheaters & Reheaters
Type 309 SS and alloy 625 overlay composite tubes have also been successfully used to replace carbon steel or Cr-Mo steels
in superheaters and reheaters for resisting high temperature corrosion. In addition to resisting high temperature corrosion
attack, the overlay of either 309 SS or alloy 625 provides elevated temperature strengthening to the carbon or Cr-Mo
substrate steel tube. Thus, the overlay composite tube can withstand overheating much better than the bare steel tube.
Another major advantage for using overlay tubes for part of the superheater or reheater bundle is the continued use of carbon
or Cr-Mo steel tubes, thus eliminating the dissimilar metal weld joint when connecting them to the existing carbon or Cr-Mo
steel tube bundles. For superheaters and reheaters that operate at higher temperatures, with metal temperatures high enough
to cause severe coal ash corrosion to austenitic stainless steels, it is recommended that Ni-44Cr alloy be considered. The Ni-
44Cr alloy is a weld wire version of alloy 671. Co-extruded tubes with alloy 671 outer cladding over an alloy 800H substrate
have provided excellent resistance to coal ash corrosion in superheaters. (11,12) It is expected that the overlay of Ni-44Cr
should perform equally well in resisting coal ash corrosion, carburization and erosion/corrosion because of its high chromium
content.
SUMMARY
Modem weld overlay technology applied to provide corrosion and erosion/corrosion protection to coal-fired boilers was
described. The technology was developed in the mid-1980s to solve chloride corrosion problems encountered in waste-to-
energy boilers. It has been so successful that the overlay welding of alloy 625 has now become an industry practice to
protect the waterwall against fireside corrosion of waste-to-energy boilers. The technology has now been adopted in much
larger utility coal-fired boilers to protect the waterwalls against sulfidation attack under low NOx combustion conditions. An
equally successful application and performance has been obtained with overlays of Type 309 SS and alloy 625 in providing
corrosion and erosion/corrosion protection in coal-fired boilers. Many of these boilers have had four/five years or more of
operations with overlaid waterwalls. The use of weld overlay composite tubes for convection sections, such as superheaters
and reheaters, was also discussed, and general characteristics and properties of modem weld overlays used in coal-fired
boilers described.
REFERENCES
1. S.C. Kung and L. D. Paul, Paper No. 65, Corrosion/91, NACE, Houston, Texas (1991)
2. S.C. Kung and C. F. Eckhart, Paper No. 242, Corrosion/93, NACE, Houston, Texas (1993)
3. G.Y. Lai, High-Temperature Corrosion of Engineering Alloys, ASM International, Materials Park, OH (1990)
4. D.N. French, "Carburization Corrosion in Superheaters and Reheaters", Presented at International Conference on Boiler
Tube Failures in Fossil Plants, Nashville, Tennessee, Nov. 11-13, 1997
5. P. Hulsizer, Paper No. 246, Corrosion/91, NACE, Houston, Texas (1991)
6. G. Lai, M. Jirinec and P. Hulsizer, Conference Proceedings, 1998 TAPPI International Engineering
Conference, TAPPI, Atlanta, Georgia (1998), p. 417
7. B. Dooley, et al., Presented at International Conference on Boiler Tube Failures in Fossil Plants, Nashville, Tennessee,
Nov. 11-13, 1997
8. G.Y. Lai and P. N. Hulsizer, Conference Proceedings, 1999 TAPPI Engineering Conference, Vol. 1, TAPPI, Atlanta,
Georgia (1999), p. 399
9. D. Peckner and I. M. Bemstein, Handbook of Stainless Steels, McGraw-Hill Book Company, New York (1977)
10. G. Lai, P. Hulsizer and R. Lee, Presented at 1999 EPRI Fossil Plant Maintenance Conference, June 21-23, 1999, Atlanta,
Georgia
11. T. Flatley and C. W. Morris, in UK Corrosion '83, Conference Proceedings, Birmingham, UK, 15-17 Nov. 1982,
Institution of Corrosion Science & Technology, Birmingham, UK, 1982, p. 71.
12. Inco Alloys News on INCOCLAD 671/800HT, Special Metals Company, Huntington, WV (1998)

Table 1 Chromium Concentration Gradient for Alloy 625 Overlay
as a Function of Distance fi'om Fusion Line.
Location Distance from Fusion Line, mils Cr (wt.%)
Overlay 80 22
Overlay 60 22
Overlay 40 22
Overlay 20 21
Overlay 10 21
Overlay 2.0 21
Fusion line 0.0
Substrate 2.0 <1
Substrate 10 < 1
Table 2 Mean Coefficient of Thermal Expansion (10-6in/in F)
Temperature Carbon Steel
70-600 F 7.2 7.2 -
70-800 F 7.6 7.5 7.7
70-1000 F 8.0 7.9 7.8
* Weld overlay data
** Wrought alloy data
1/2Cr-1/2Mo 1-1/4Cr-1/2Mo 309 SS* Alloy 625**
9.3 7.4
9.5 7.6
9.7 7.8
Table 3 Microhardness Profile (500 g load) Across Overlay, Fusion Line and HAZ for a Type 309 Overlaid
T-I 1 Waterwall Tube. Overlay was Applied with Water in the Tubes
Location Distance fi'om Fusion Line, mils VHN (Rb or Rc)
Overlay 80 200 (Rb92)
Fusion line 0.0 257 (Rc23)
Substrate 2 276 (Rc27)
Substrate 40 176 (Rb88)
Table 4 Room Temperature Tensile Properties of Alloy 625 Overlaid SA 210 A 1Carbon Steel
in Comparison with ASME Specification for SA 210 A 1 Steel
U.T.S., ksi (MPa)
El (%)
* ASME Specification
625 Overlay/SA 210 A1 SA 210 AI*
88.6 (611) 60 (414) min.
32 30 min.

Table 5
Specimen Condition
As-Overlaid
As-Overlaid
Aged for 200 hrs
Aged for 200 hrs
Effect of Aging at 842 F on Tensile Ductility of Alloy 625 Overlaid Carbon Steel
Room Temperature Tensile Elongation (%)
31
28
40
38
Table 6 A list of U.S. Coal-Fired Boilers with Waterwalls Overlaid with Mostly Alloy 625 and Type 309 SS
(Jobs Performed in 1999 are not listed here)
Location Boiler Overlay Area ft2 Alloy Date Performed (m/v)
Alabama "A" 1000 309 2/94
"B" 460 625 3/97
Florida "A" 400 309 3/95
"B" 500 309 6/96
"C" 1165 309 3/97
"D" 998 309 7/97
"E" 5450 309 4/98
"F" 1328 309 4/98
"G" 150 309 2/99
Georgia "A" 800 309 4/92
"B" 700 309 4/93
"C" 1000 309 12/94
"D" 300 309 9/96
"E" 360 309 3/99
Illinois "A" 500 625 10/93
"B" 450 309 3/99
Indiana "A" 1000 309 10/94
"B" 1000 309 10/94
"C" 5000 625 3/96
"D" 850 625 5/96
"E" 10000 625 10/96
"F" 4600 625 10/98
Kentucky "A" 1000 625 4/97
Maryland "A" 500 309 10/93
"B" 500 309 10/96
Michigan "A" 1078 625 4/99
New Jersey "A" 1600 625 5/96
Ohio "A" 1000 309 10/96
"B" 350 309 1/97
"C" 800 309 4/97
"D" 300 309 4/97
"E" 400 309 4/97
"F" 450 309 3/98
"G" 500 309 6/98
"H" 4000 309 10/98
"I" 800 309 2/99
"J" 500 309 2/99
"K" 3000 625 4/99
"L" 500 309 4/99

Pennsylvania
Texas
"A"
"B"
~C~
"D"
"E"
"F"
"G"
~H~
"K"
"t"
"A"
500
600
6400
5000
900
1500
10000
1500
118
4500
1800
4200
1000
622
625
625
309
625
625
625
625
309
625
625
625
309
9/87
9/95
10/95
11/95
3/96
5/96
9/96
3/97
4/97
10/97
4/98
10/98
3/98

Fig.1. A "hands-off" welder watching
an automatic weld overlay machine
overlaying the waterwall of a large
coal-fired boiler.
Fig. 3. A superheater constructed out of
alloy 625 overlay composite tubes.
Fig. 2. Typical cross-section of a weld
overlay composite tube.
Fig. 4. Typical fine dendritic subgrain
structure for Type 309 SS overlay.

Fig. 5. Appearance of the crown weld beads
on the 309 SS overlay tube sample after
seven years of operation.
Fig. 6. Cross-section of Type 309
SS overlay after seven years of
operation.
Fig. 7. Overlay surface condition of
Type 309 SS overlay waterwall after
seven years of operation°

00258 corrosion erosion corrosion protection by mo

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to 00258 corrosion erosion corrosion protection by mo

Similar to 00258 corrosion erosion corrosion protection by mo (20)

Recently uploaded

Recently uploaded (20)

00258 corrosion erosion corrosion protection by mo