By Larry Paul and Gregg Clark, ThyssenKrupp VDM USA Inc.
The coal-fired boiler environment produces corrosive conditions in both the furnace region and in the boiler convection pass. Corrosion-resistant weld overlay materials are widely used to manage these corrosion issues. In the furnace region, a mixed sulfidation/oxidation mechanism causes corrosion rates of up to 80 mpy (milli-inch per year) on boiler tubes (2 mm/y). The highest corrosion rates are seen where low NOX combustion is combined with burning high-sulfur fuels. This results in the formation of corrosive hydrogen sulfide (H2S) gases and ferrous sulfide (FeS) tube deposits.
Higher chromium materials can support a protective chromium oxide scale even under severe conditions; therefore, they corrode less. The industry approach to date has been to restrict the chromium level of historical overlay materials to the upper end of their specification limits.
One example of this approach is the common practice to restrict the chromium level of UNS N06022 (Alloy 622) to from 22.0 percent to 22.5 percent chromium (Cr), which is at the upper end of the specification limit of 20.0 percent to 22.5 percent chromium. This practice by its nature can only have a small influence on the level of chromium in the weld overlay and can only slightly increase corrosion resistance. Higher chromium contents in the weld overlay can be much more easily attained simply by switching to a higher chromium alloy. This was the basis for introducing Alloy 33 for use as a weld overlay on waterwall tubes in coal-fired boilers.
Another concern in high pressure supercritical boilers is furnace wall tube cracking. This is a corrosion fatigue mechanism called circumferential cracking and has been observed on furnace tubes for many years. The root cause of this cracking is still under investigation by the Electric Power Research Institute (EPRI) and others, but higher design stresses, dirtier coals and running the boiler harder all make the problem worse. Some weld overlay materials have better resistance to cracking, but all materials have cracked under the most severe conditions.
In the convection pass, coal ash corrosion can attack re-heater and superheater tubes. This corrosion mechanism involves molten alkaline iron tri-sulfate salt deposits on the boiler tubes and rates of 250 mpy (10 m/y) have been observed. Some recent studies suggest a mixture of mechanisms including oxidation, sulfidation and carburization may also now be involved. Recent changes in boiler operation to minimize NOX in the flue gas may be changing the corrosive conditions in the convection pass region and therefore the corrosion mechanisms. In convection pass tubes, the use of a high chromium weld overlay material has provided the best protection. FM-72, with 45 percent chromium, has been the preferred material to combat coal ash corrosion in convection pass tubes.
Table 1 gives the nominal chemical compositions of the most common weld overlay materials now being used in coal- fired boilers.
Alloy 33 Performance
Field testing was widely used to supplement laboratory corrosion data generated in earlier studies. Several sites were chosen to represent a wide range of boiler design and operating practice.
The field tests were also used to evaluate the cracking resistance of Alloy 33, since the cracking mechanism is not fully understood, is quite complex and difficult to reproduce under laboratory conditions. Several boilers where circumferential cracking has occurred on other materials were intentionally selected to ensure a proper evaluation of Alloy 33’s cracking resistance.
The boilers used in the field testing are shown in Table 2 along with a description of the location and type of sample used.
After five years, the corrosion resistance of Alloy 33 has been demonstrated in all of the boilers used in the test program. The corrosion rate of Alloy 33 has been less than 1 mpy in all cases. This is similar to the long-term corrosion rate measured in the laboratory studies for Alloy 33.
Table 3 compares corrosion rates measured in the laboratory and field and projects the life of a weld overlay for Alloys 33 and 622. The laboratory measured corrosion rates are higher than those from actual boiler tubes, so the life projections in Table 3 are conservative estimates.
Another goal of the field test program was to evaluate the resistance of Alloy 33 to circumferential cracking. After five years, there are no reported cases of cracking in any of the test panels.
In one supercritical boiler, the Alloy 33 weld overlay sample was removed after 34 months and the tube panel’s bare ends were found to have circumferential cracks. While the SA213-T11 (1/¼ Cr-½ Mo) base tubing had cracks, the Alloy 33 weld overlay used on these same tubes was free from cracks. Cracks in the bare alloy tubes were about 0.020” (0.5 mm) deep and are filled with an oxide wedge with a sulfur “spine” down the middle; this is typical of circumferential cracking. Alloy 33 overlay directly adjacent to this area was investigated and found to be crack-free.
Figure 1 shows micrographs of the cracks in the bare SA213-T11 tubing alongside the uncracked Alloy 33 weld overlay from this same region. Therefore, the Alloy 33 was shown to resist cracking even when in an area known to produce circumferential cracking. The same result was again observed in samples removed from this same unit in spring 2009. So, after five years of exposure in a supercritical boiler that has produced cracks in other boiler tube materials, the Alloy 33 remains unaffected.
Approximately 8,000 square feet of furnace tubes were covered with Alloy 33 overlay at Western Kentucky Energy (WKE) in spring 2007. After two years of operation, the Alloy 33 is essentially unaffected and looks as good as the day it was installed. Several additional boilers have also used Alloy 33 weld overlay on the waterwall tubes (all field-applied so far).
Combating Coal Ash Corrosion
Because of the high chromium content of Alloy 33 and the ability to apply it to tubes as a weld overlay, interest was developed in using this material for protection from coal ash and oil ash corrosion in the convection pass of coal-fired boilers.
The benefit of high-chromium content materials in combating coal ash corrosion has been recognized for years. An example of a recent field test that shows the general effect of chromium upon coal ash corrosion is shown in Figure 2. Conclusions drawn from Figure 2 are that increasing the chromium content of an alloy increases the resistance to coal ash corrosion and above about 30 percent chromium, diminishing effects can be expected of further chromium additions to the alloy.
Alloy 33, which contains 33 percent chromium, clearly has sufficient chromium to impart excellent resistance to coal ash corrosion. In addition, Alloy 33 has excellent weldability and fabrication qualities.
Superheater Replacement Project
In summer 2007, an eastern utility used Alloy 33 as the overlay material for a superheater replacement project.
Approximately 7,000 linear feet of tubing was covered with Alloy 33, requiring about 28,000 pounds of Alloy 33 weld wire. A 360-degree spiral overlay covered the tubing’s entire outside surface; the base tubing was a mixture of SA213-TP347 and SA213-T22. The Alloy 33 weld overlay tubes were subsequently bent into superheater platens, as shown in Figure 3. After two years, these tubes are still in service and showing little corrosion.
Figure 3 overlay tubes bent into superheater platens
A 360° spiral overlay of Alloy 33 on a boiler tube. This is the weld overlay method used for superheater and reheater tubes. (Photo courtesy of Alstom Power.)
Interest in Alloy 33 is Growing
After five years of testing and full- scale commercial use of Alloy 33, positive experiences continue. When used as corrosion protection on waterwalls, low corrosion rates are seen on all boiler types burning a wide variety of fuels. Cracking has not been observed on any shop-produced tube panel or any automated field-applied overlay of Alloy 33. In addition, Alloy 33 is less costly than Alloy 622, FM-52 or 625.
Because of the corrosion resistance in the lower furnace, the use of Alloy 33 has been extended into the convection pass area of the boiler as well.
A large superheater replacement project demonstrated that Alloy 33 could be applied to tubes as a weld overlay and subsequently bent into appropriate shapes to form full superheater platens.
The combination of technical advantages and significant cost savings makes a compelling argument for the use of Alloy 33 as a weld overlay material in coal-fired, oil-fired or black liquor-fired boilers. As a result, it is gaining interest in the marketplace.
Authors: Larry Paul has worked on material issues in power generating systems for more than 25 years. His early work with The Babcock & Wilcox Co. focused primarily on corrosion protection of heat transfer equipment; he holds a U.S. patent in this area. For the last 12 years his work has focused on the use of nickel-based alloys to increase reliability and extend the life of power plant equipment. Paul is currently a technical marketing manager with ThyssenKrupp VDM, responsible for introducing new alloys and overseeing technical issues involving power generation systems, including weld overlay applications. He holds an M.S. in metallurgical engineering and materials science from the University of Notre Dame.
Gregg Clark has been involved in sales, marketing and technical support functions in the high performance alloy industry serving various industries. He has co-written several papers in peer-reviewed journals and is an active member of the National Association of Corrosion Engineers. He holds a B.S. in marketing and business administration from the University of Illinois.