HASTELLOY ALLOYS: SOLUTIONS TO SEVERE WET FLUE GAS DESULFURIZATION ENVIRONMENTS

Transcription

1 HASTELLOY ALLOYS: SOLUTIONS TO SEVERE WET FLUE GAS DESULFURIZATION ENVIRONMENTS H. J. White Haynes International 1020 West Park A venue Kokomo, Indiana

2 HASTELLOY ALLOYS: SOLUTIONS TO SEVERE WET FLUE GAS DESULFURIZATION ENVIRONMENTS H. J. White Haynes International 1020 West Park A venue Kokomo, Indiana Abstract Beginning with the 1968 invention of HASTELLOY C-276 alloy, Haynes International has for the past 30 years provided alloy solutions for aggressive Flue Gas Desulfurization (FGD) environments. Since then continued improvements in performance were achieved with the introduction of HASTELLOY C-22 (1985), ULTIMET (1990), and HASTELLOY C-2000 (1995). This manuscript discusses these alloys and other contributions (Wallpaper Concept, PEMT, AGD, etc.) Haynes International has made to the FGD industry over the years.

3 Introduction Nominal Composition in wt. % UNS US Patent Ni Cr Mo W Cu Co C 1,836,317 rem C-276 N ,203,792 rem <2.5 C-22 N ,533,414 rem <2 59 N ,906,437 rem N ,019,184 rem C-2000 N ,280,540 rem <2 ULTIMET R ,002, rem The initial commercial grade of the HASTELLOY C alloy (US Patent # 1,836,317) was a cast product which contained high carbon, silicon, and vanadium to facilitate production. Carbon was present in the charge material and at that time could not be removed by economical processes; silicon was a residual from deoxidation of the melt and contributed to fluidity during casting; and vanadium was present as a grain refiner. Developments in melting and processing practices, oxygen blowing of heats, made it possible to reduce the carbon content to ~0.05 wt. % making large scale production of wrought alloys possible. The wrought version of HASTELLOY C had excellent corrosion properties except near welds where carbide precipitation at grain boundaries promoted intergranular corrosion. The post weld heat treatment needed to dissolve the carbides and restore corrosion resistance made fabrication impractical. In 1965, researchers at BASF limited the carbon (< 0.01 wt. %) and silicon ((< 0.08 wt. %) content of the alloy and invented HASTELLOY C-276 alloy (US Patent # 3,203,792). Klein et al (US Patents 3,982,925 and 3,988,147) development work on Argon Oxygen Decarburization made wrought C-276 alloy

4 commercially viable. Despite its excellent corrosion resistance, HASTELLOY C-276 alloy is susceptible to preferential weld metal and heat affected zone attack due to precipitation of molybdenum and tungsten rich phases. The composition of the alloy was such that the electron vacancy number (Nv) was not controlled and mu phase, which reduces the corrosion resistance of the alloy, formed during prolong exposure in the range of C. Advances in corrosion science and better understanding of the specific roles of chromium (in the range of 16-25% it forms a stable passive film during corrosion in oxidizing media), molybdenum (amounts upto 18 wt. % provides corrosion resistance, amounts greater than 18 wt.% embrittle the alloy due to formation of undesireable secondary phases), tungsten (provides resistances to localized corrosion- pitting and crevice and degrades uniform corrosion), and copper (provides uniform corrosion resistance) play in imparting corrosion resistance to the nickel- base alloys have led to new classes of alloy: HASTELLOY C-22 alloy, 59 alloy, 686 alloy, and HASTELLOY C alloy. In the mid 1980 s D. C. Agarwal and Lee Flasche s promotion of the wallpaper concept, see Figure 1, was one of the main reasons for the rapid acceptance of C-276, C-22, C-2000, 59, 686, and Ultimet (a cobalt based alloy used in FGD application where erosion is problematic) alloys into the FGD industry. The first known commercial application of C-276 alloy wallpaper was in 1980 at R.D. Morrow Sr. Power Plant (Mississippi). The corrosion problems and their solutions at this plant have been widely publicized. 1 It was found that the inlet duct floor wet/dry zone of the absorber section was a severely corrosive area. The sludge was monitored for a period of time in the inlet zone area in order to assess the corrosivity of the environment. Samples taken at two different times showed wide fluctuation in fluoride and chloride concentrations. One sludge sample showed 80,000 ppm chlorides while the other was found to

5 contain 6,300 ppm chlorides and 87,000 ppm fluorides. Alloy G panels welded with C-276 alloy wire was installed to help resist the environment. After 6 months service, alloy G failed; however, the C-276 alloy welds performed reliably. As a result, C-276 alloy was selected to sheet line the corrosive areas. During a shutdown in 1987, seven years after its installation, a small section of alloy C-276 began to exhibit excessive crevice attack. The material was removed (about one square foot) and replaced during a normal shutdown. HASTELLOY C-22 alloy was tested in this difficult area to compare both alloys in the same environment. The C-22 alloy in the adjacent area showed minor etching in a 1 inch diameter area and no additional attack was found on the balance of the test panel after 6 ½ years of exposure. Figure 1. Wallpaper concept developed by Haynes Interntaional

6 Paul Edward Manning Test (PEMT) The most widely used susceptibility test for intergranular corrosion of HASTELLOY C- 276 is ASTM G28A (ferric- sulfate- sulfuric acid test). The corrosion rate obtained by this test has uniform and intergranular corrosion components. The uniform corrosion rate which is a function of minor variations in alloy chemistry may easily mask the intergranular corrosion component of the overall corrosion rate. In the early 1980 s Dr. Paul Manning identified, through laboratory tests, a new solution chemistry that resolves the problem and displays a step function response in corrosion rate when there are high levels of grain boundary precipitation. This new test consistently detects undesirable microstructural conditions in HASTELLOY C-276 alloy that, in many cases, are not detected by ASTM G28A (ferric- sulfate- sulfuric acid test). The use of the ferric- sulfate- sulfuric acid test (ASTM G28A) as a quality control test in production over many years has pointed out its relative insensitivity in detecting intergranular corrosion tendency for HASTELLOY C-276 alloy. The currently used test (50% H 2 SO g/l Fe 2 (SO 4 ) 3 ) results in a very high uniform corrosion rate on C-276 alloy. In fully, solution- annealed material, limited, if any, intergranular corrosion takes place. Therefore, the corrosion rates which results from these samples identifies the uniform corrosion rate. In past experiences, the uniform corrosion rate varies in a range of mpy ( mm/ year) and is a function of the heat chemistry. A small variation of chromium and/ or molybdenum contents within the chemistry range of the alloy dramatically changes the uniform corrosion component of the overall corrosion rate. In cases where grain boundary precipitation exists and intergranular corrosion occurs, a high uniform corrosion rate range may easily mask the intergranular corrosion component of the overall corrosion rate. The ideal susceptibility test for intergranular corrosion is one that will give a very high ratio of intergranular corrosion component to uniform corrosion component when

7 grain boundary precipitation is present in the alloy. Dr. Manning s goal was to identify a screening/ quality control test environment for HASTELLOY C-276 alloy which would result in a step function response in corrosion rate at a predetermined level of grian boundary precipitation in the alloy. The use of such an improved screening test would ensure a well defined standard quality product. It must be emphasized that the purpose for the screening test was to detect susceptibility to intergranular corrosion as influenced by variations in manufacturing process and/ or composition. Material shown to be susceptible may or may not be intergranularly attacked in another environment. This must be established independently by specific tests or service experience. Figure 2 shows microstructure and ASTM G28A and B corrosion data for HASTELLOY C-276 and C-276 from an outside source. The PEMT solution test (23% H 2 SO % HCl + 1% CuCl 2 + 1% FeCl 3 ) on HASTELLOY C-276 alloy results in minor pitting attack and low corrosion rates on material that is not sensitized and provides a step function increase in corrosion rates on sensitized material by an intergranular corrosion mechanism that results in rapid pit propagation. The PEMT solution corrosion test results have consistently detected undesirable microstructural problems in millproduced material that, in many cases, are not detected by the ferric- sulfate- sulfuric acid (ASTM G28A) test. Dr. Manning s significant contribution---- the PEMT----- is well known in the industry today as ASTM G28B and is used to properly screen wrought product as well as welded and annealed product. Macrosegregation of molybdenum in unannealed weldments may be sufficient to cause preferential weld metal attack. Therefore, the test should not be used for quality control of as- welded structures.

8 Figure 2. Alloy C-276 comparison. A STM G28A w as not able to detect a poor microstructure as w ell as PEM T (A STM G28B) and A GD (A ziz s Green death). History of AGD A thinned down, severely corroded HASTELLOY C-276 alloy specimen w as received by Cabot Corporation (Haynes International) for examination in This specimen was a part of the lower impingement tray stages of a scrubber treating gases from a copper

9 smelting process. The lower tray stages were in service for about six months. It was exposed to a circulating solution containing 15% sulfuric acid, 300 ppm fluorides, and 100 ppm chlorides. The operating temperature was reported to be 65 C; however, it was suspected that the unit had been operated, without a coolant, at higher temperatures. Dr. Aziz Asphahani was asked to check the composition of the received specimen and its metallurgical structure, to comment on the performance of HASTELLOY C-276 alloy and to examine the effects of suspect aggressive ions (metal ions, F -, and Cl - ) on the corrosion resistance of C-276 alloy in sulfuric acid solution. Both a dark black and a yellow scale were present on the specimen. The black scale showed the presence of sulfur, copper, and selenium. The yellow scale also contained sulfur and selenium, plus mercury. Metallographic examination of the specimen revealed that corrosive attacked was present on both sides of the sample; no deep, localized pitting type corrosion occurred (more like wide craters). The alloys microstructure appeared to be free of any secondary precipitates. However, slight banding and wavy striations, typical of thin gauge HASTELLOY C-276 alloy were observed. The grain boundaries did not appear to be preferentially attacked, and corrosion proceeded indiscriminately along the grain facets and boundaries. The observed severe corrosion attack on HASTELLOY C-276 is reminiscent of the accelerated dissolution of this alloy in oxidizing acidic solutions at elevated temperatures. The corrosion rate of C-276 alloy in sulfuric acid solution is almost tripled when oxidizing ions and chloride ions are present. Temperature is the most important factor affecting the corrosion process in acidic oxidizing environment. The corrosion rate of HASTELLOY C-276 at 102 C is about 10 x higher than that measured at 70 C. Resulting corrosion rates of HASTELLOY C-276 in H 2 SO 4 and 7 vol % H 2 SO vol % HCl +1 % CuCl 2 + 1% FeCl 3 are shown in Table 1. The latter solution (7 vol % H 2 SO vol % HCl

10 +1 % CuCl 2 + 1% FeCl 3 ) is a good laboratory simulation for severe flue gas desulfurization environments and is well known as the Aziz s Green Death (AGD) test. Table 1. Alloy performance in Aziz s Green Death Solution Alloy Temperature, C Solution Corrosion Rate (mpy) HASTELLOY C %H 2 SO 4 11 HASTELLOY C AGD* 3.1 HASTELLOY C AGD alloy 102 AGD L alloy 102 AGD Cb3 alloy 102 AGD alloy 102 AGD 2861 * AGD- 7 vol % H 2 SO vol % HCl +1 % CuCl 2 + 1% FeCl 3 Conclusions Since the late 1960 s Haynes International has numerous contribution to the FGD industry. Its alloys HASTELLOY C-276, HASTELLOY C-22, HASTELLOY C-2000, and ULTIMET, used in wallpaper form, are a cost effective method for mitigation of severe corrosion problems in FGD systems. Haynes International s PEMT (ASTM G28B) test procedure ensures the quality material is supplied for wallpaper installations. Haynes International s AGD test is a well know laboratory test procedure for simulating severe FGD environments. Research and development efforts are ongoing at Haynes International in continue support of FGD and other markets.

11 References 1. D. Froelich and M. Ware, Corrosion Problems with a Closed Loop Limestone FGD System, Paper No. 203, Corrosion/82. NACE, Houston, Texas 1982.