A CORROSION MANAGEMENT AND APPLICATIONS ENGINEERING MAGAZINE FROM OUTOKUMPU 1–2/2013
Corrosion of austenitic and duplex stainless steels in flue gas cleaning systems for waste combustion processesPage 2
A new developed Ni-Cr-Mo alloy with improved corrosion resistance in flue gas desulfurization and chemical process applicationsPage 11
2
AbstractCondensing flue gases are highly corrosive and present severe challenges in terms of materials selection. The sulphuric acid condensation which occurs in coal-fired power plants has been long recognised as a cause of corrosion in flue gas desulphurisa-tion (FGD) units, but the more complex conditions which can arise with increasing combustion or co-combustion of biomass and waste have been less thoroughly investigated. In this paper results are presented from laboratory tests designed to simulate the corrosive conditions in flue gas cleaning systems and based on analysis of actual condensate. The major focus is on waste combustion, where the main condensate component is hydrochloric acid but varying amounts of H2SO4, HBr, HNO3, HF H2SO4, HBr, HNO3 and metal ions may also be present. These data are correlated to field tests of creviced and welded coupons in power plants. The range of steels investigated spans from UNS S31726 to the superaustenitic grades S34565, S31254 and S32654 and the superduplex S32750. Results are presented in terms of the measured uniform corrosion rates, and the contributions from crevice corrosion are considered. Comparisons are made to earlier data from coal fired plants.
Keywords: stainless, testing, corrosion, flue gas cleaning,
Corrosion of austenitic and duplex stainless steels in flue gas cleaning systems for waste combustion processesRachel Pettersson, Arne Bergquist, Sophia Ekman, Outokumpu Stainless AB, Avesta Research Centre, Avesta, Sweden.
Jesper Flyg Swerea KIMAB, Kista, Sweden
Gary Carinci, TMR Stainless, Pittsburgh, USA
IntroductionCorrosion in condensing flue gases represents a complex situation and is of great importance in power plants, since the first condensate from flue gases is very corrosive, and can often give rise to component failure at very specific locations in the flue gas handling system. There are many decades of experience of corrosion issues in the flue gas desulphurisation (FGD) systems of coal-fired power plants, but the situation for waste-fired plants is rather different in that there are usually higher levels of halides and low levels of sulphur compounds. A European project [1] has therefore been directed specifically towards examining corrosion issues in the flue gas cleaning systems of waste-fired power plants. There are a number of challenges which have to be met in order to conduct relevant testing on which to base materials selection. Firstly, conditions vary widely between plants and over time, since the fuel mix shows seasonal variation. Secondly, condensation occurs on cooled surfaces, either on walls, pipes etc. which are externally cooled or on heat transfer surfaces such as heat exchangers and condensers. The results in this paper focus on the use of in-plant tests and laboratory simulations, which are correlated to electrochemical testing in synthetic condensates.
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3
Materials and environmentsMaterials The materials included in this investigation are listed together with their chemical compositions in Table 1. The superaustenitic grades S31254 (254 SMO®) and S32654 (654 SMO®) were supplied by Outokumpu, S31726, S31803, S34565 and the nickel-base alloy N10276 were provided by Industeel and S32750 by Sandvik Materials Technology. All materials were in the form of flat products with a thickness in the range 4 – 8 mm. For both the laboratory and field exposures, specimens with dimensions 60 x 60 mm were used. A TIG bead-on-plate weld with appropriate filler was located approximately 10 mm from the lower edge, and a crevice washer with 12 slots was mounted with a bolt passing through a hole 20 mm from the upper edge. Specimens were thoroughly pickled after welding and either bolted onto an N10276 baseplate through a thin PTFE film for electrical insulation or mounted in test racks, Figure 1.
Exposure sites Exposures of test panel or test racks were done in the flue gas handling system in four waste-fired plants denoted A-D. These are described in Table 2 and a generic outline of the process steps described is given in Figure 2. The exposure periods ranged from 4 to 11 months. Comparison is also made to results originally reported in [2] for the flue gas desulfurisation unit of a coal-fired plant. For the laboratory testing a furnace was run at 90° C with a gas atmosphere as detailed in Table 2. The panels were placed on cooling coils so that the metal temperature was approximately 80 °C and condensation occurred on the specimens. Further experimental details are given in [3]. For the in-plant testing the base plate was bolted onto available structural components in the
UNS EN Type Cr Ni Mo N W PREN*
S31726 1.4439 Austenitic 17.2 12.9 4.1 0.14 33
S31803 1.4462 Duplex 22.32 5.2 3.2 0.17 36
S32750 1.4410 Duplex 25.3 7.1 3.9 0.26 42
S31254 1.4547 Austenitic 19.8 17.9 6.0 0.204 43
S34565 1.4565 Austenitic 24.1 18.0 4.4 0.44 46
S32654 1.4652 Austenitic 24.3 21.8 7.3 0.52 57
N10276 2.4819 Ni-base 16 57.6 16.2 0 3.3 75
*PREN = %Cr+3.3(%Mo+½*%W)+16(%N) Table 1 Materials investigated and chemical composition in wt %; alloys are listed in order of increasing PREN which is a rough predictor of pitting corrosion resistance.
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flue gas cleaning systems. This gave a certain degree of cooling, but less than that which occurs on heat transfer surfaces in flue gas condensers.
Figure 1 Specimen dimensions and mounting on either a N10276 panel (center) for exposure in gaseous condensing gases or on a test rack for immersion.
60 mm
60 m
m
Figure 2 Typical components in a power plant flue gas system.
Boiler Reactor(active C, lime etc)
Econ
omis
er
Filt
er
Con
dens
or
Sta
ck
Scr
ubbe
r
Elet
rost
atic
pre
cipi
tato
r
Reh
eat
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Condensate Fuel, location, Dewpoint collection Plant (duration) Test Position (°C) (°C)
A Domestic waste, Aa After filter, before scrubber 91 37– 63
Sweden A1 Neutral zone at top of scrubber
(8 months) A2 Condenser inlet
B Domestic Ba After economiser, before filter 67 35 – 55
building waste, B1 Inlet to pre-cooler (scrubber)
Sweden B2 Top of precooler (scrubber). Water saturated
(7 months) B3 Top of condenser, Sprayed with wash liquor
C Domestic+other Ca-1 Before economiser 80 27 – 45
waste, Sweden Ca-2 After economiser 61 45 – 55
(4 months) C1-C2 Condenser inlet, duplicate racks. Highly corrosive – no cleaning before condenser
D Household + D1 Washer 1 inlet, directly after filter, facing industrial waste + gas stream
sludge, Germany D2 Washer 1 inlet, facing dripping condensate
(11 months) D3, D4 Washer 1 outlet, condensate ~2% HCl
S Coal, USA S1 Spray tower
(9 or 8 months) S2 Outlet duct
L Lab. exposure L2 Air +90% H2O + 1.6% HCl + 0.09% SO2
(500–550 hours) L3 Air +90% H2O + 3.2% HCl + 0.09% SO2
Table 2 Analysis and coupon exposure positions, together with measured dewpoints and condensate collection temperatures.
Table 3 Environments used for laboratory evaluation of corrosion in synthetic condensates.
Environment HCl conc. Temp. Other
1 1.6 70°C
2 0.8 70°C
3 1.6 50°C
4 0.8M 50°C
5 0.2M 50°C
HCl+H2SO4 ( sulphate addition) 0.8M 70°C 0.2M H2SO4
HCl/H2SO4 (sulphate replacement ) 0.4M 70°C 0.2M H2SO4
HCl/HNO3 0.79M 70°C 0.01M HNO3
HCl/HBr 0.79M 70°C 0.01M HBr
HCl/HF 0.79M 70°C 0.01M HF
Complex 0.79M 70°C 0.01M HNO3+0.01M HBr+0.01M HF
Metal 0.79M 70°C 0.025M AlCl3 + 0.025M FeCl3 + 0.025M NaCl + 0.025M ZnCl2
In-situ dew point measurements and removal of condensate for analysis was done at plants, A, B and C at the analysis points specified in Table 2. Details of the dew point and condensate extraction probes are given in [1]. Analysis was done using ICP-TOFMS (Inductively-coupled plasma time of flight mass spectroscopy) for metal ions, ion chromatography for anions and a pH electrode for the hydrogen ion concentration. There were few complete gas analyses available from the plants, but data from Plant A during the test period gave maximum levels of 17% H2O, 9% O2, 21 ppm SO2, 58 ppm HCl, 55 ppm NO and 24 ppm CO. No CO2 analysis was available but this was taken as 14% from an analysis in the same plant presented in [4].
Synthetic condensate solutions were prepared according to Table 3 and used for laboratory tests. Specimens were wet surface ground to 600 mesh SiC paper and mounted in a flushed-port cell to avoid crevice effects. The solutions were aerated, since this was considered more representative of the application. The corrosion potential was first measured for 5 minutes then the potential scanned in the anodic direction at 20 mV/min starting at a potential 100 mV below the corrosion potential. Further experimental details are given in [5].
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RESULTS AND DISCUSSIONDewpoint and condensate composition The dewpoints measured in the plants were in the range 61–91 °C, see Table 2, with the higher values being recorded in the earlier stages in the flue gas cleaning systems, before additional water was introduced via quenchers and scrubbers. For Plant A, a value of 91 °C was recorded directly after the filter, before the scrubber, and for Plant C a dewpoint of 80 °C was measured for the hot flue gases before the economizer. The nature of this first condensate is open to some degree of speculation, since it was not possible to
Con
cent
ratio
n (M
)
A
C
B
Condensate temperature (°C)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.020 40 60 80 100
A (Cl-)B (Cl-)C (Cl-)
Figure 3 Results of analyses of collected condensate from plants A, B and C, bold line shows the range of measured dewpoints
are five circled crosses which indicate the compositions selected for the synthetic condensate. Empirical relationships are widely-used to predict the dewpoints of flue gases. A number of these have been published, one fairly comprehensive collection by Kiang [6], cited by Huijbegts and Leferink [7] contains the following equations (with Td in K and pressures in mmHg.) Inserting values of 30 or 60 vppm HCl and 30 or 60 vppm H2SO4, both of which are in the range measured in the plants, yields the dewpoints in Table 4. The first measured dewpoint indication with the condensation probe was higher than the
1000/ Td (HCl) = 3.7368 – 0.1591 ln(pH2O) – 0.0326 ln(pHCl) + 0.00269 ln(pH2O)ln(pHCl)
1000/ Td (H2SO4) = 2.276 – 0.0294 ln(pH2O) – 0.0858 ln(pH2SO4) + 0.0062 ln(pH2O)ln(pH2SO4)
% H2O HCl HCl H2SO4 H2SO4
30 vppm 60 vppm 30 vppm 60 vppm
12 50 51 150 157
16 55 57 153 160
20 59 61 155 162
Table 4 Predicted dewpoints (in ºC) of a gas containing 60 vppm HCl and 30 vppm H2SO4 as a function of water content.
collect a sufficient amount of condensate for analysis until the temperature dropped by at least 5 °C and in some cases over 30 °C. The analysis results showed that the condensate was predominantly HCl and that the condensate concentration increased with conden-sation temperature, reaching a measured peak of 1.8M HCl. The measured chloride levels as a function of temperature are shown in Figure 3, where the range of measured dewpoints is shown as a bold extension of the regression line for each plant. The maximum recorded sulphate levels were 0.18M, 0.02M and 0.05M for plants A, B and C respectively. The concern that there could have been appreciable levels of HF proved unfounded, as the fluoride levels were in all cases below the detection limits. Also included in Figure 3
expected condensation point of HCl, but could be affected by the presence of other contaminants. Condensation of sulphuric acid can occur below the azeotropic point (98.3% and 340 °C at 1 bar) and it is clear that even the low levels present in flue gas can condense at appreciably higher temperatures than HCl. In a second approach, a chemical database (Outotec HSC Chemistry 6.1) was used to assess the dewpoint in Plant A. for which a gas analysis was available. The results in Figure 4 indicate condensation of H2SO4 at 220 °C and HCl at around 60 °C and a condensate primarily comprising HCl and reaching a peak concentration of 1.9M for Plant A. This is in good agreement with the analysis results and the empirical predictions in Table 4. It is also consistent with the dewpoints of 67 °C and 61 °C measured after the economiser in Plants B and C respectively, but below the topmost levels of 91 °C in Plant A and 80 °C in Plant C. It may be that the complex chemistry in the plant flue gas, possibly in conjunction with practical aspects such as inhomogeneity, gas flow and fly ash condensation have contributed to cause earlier condensation than predicted. It is also conceivable that very low levels of H2SO4 and/or NH4Cl formed as a result of ammonium additions for NOX control, may contribute to the first dewpoint indication. It appears that calculation of dewpoint and condensate composition may be a feasible way to assess the corrosive conditions in flue gas cleaning systems, but unfortunately gas analyses were not available from all the plants and positions. As an illustrative example, the lower part of Figure 4 shows the effect of a ten-fold increase in the SO2 level to 210 ppm. Such a level is quite extreme for waste combustion, even if sulphate additions to the boiler are made to mitigate chloride-induced superheater corrosion in waste boilers. However, it is actually in the lower range for coal fired plants, where levels of 3000 ppm have been reported. The influence of this ten-fold increase in SO2 on the H2SO4 dewpoint is appreciable, increasing it from 220 to 270 °C, as is the effect on the low temperature condensate, shifting it from HCl-dominant to H2SO4 dominant. Conversely, dropping the SO2 level to 0.01 ppm is predicted to decrease the H2SO4 dewpoint to 120 °C. However, it is relevant to point out in this context that the scrubbing liquor in desulfurisation units in coal-fired power plants is nevertheless dominated by chloride. The chlorides originate from fuel, make-up water and lime additives and accumulate due to recirculation. For example in Plant S chloride concentrations of 6 – 8% were measured in the spray tower.
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Figure 4 Predicted equilibria and mole fractions and condensate concentrations based on the gas analysis of 17% H2O, 9% O2, 14% CO2, 21 ppm SO2, 58 ppm HCl, 55 ppm NO, 24 ppm CO for Plant A (top) and an illustration of the effect of varying the SO2 level on dewpoints and condensate composition (below)
Figure 5 Appearance of panels after testing B1 (left) D1 (center) and L3 (right). Specimens of S31726, S31254, S31803 and S32750 were missing after test D1.
In-plant corrosion testing resultsExamples of the appearance of the test panels B1, D1 and the laboratory exposure L3 are shown in Figure 5. In plant B, the specimens were intact but covered with brown corrosion products. However, the conditions in Plant D were remarkably corrosive, as
evidenced by the fact that specimens of S31726, S31254, S31803 and S32750 were all completely corroded and missing from D1, in the position facing the flue gas. Conditions were slightly less corrosive in position D2, facing the dripping condensate, so only S31803 and S32750 had been lost due to extensive corrosion.
mol
Temperature (°C)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.0020 40 60 80 100 120
Plant A Plant A, 21 ppm SO2
Mol
e fr
actio
n or
mol
ar c
ocen
trat
ion
Temperature (°C)
0 50 100 150 200 250 300
1.E+01
1.E-01
1.E-03
1.E-05
1.E-07
1.E-09
M HCI M H2SO4H2OH2SO4CI-
Mol
e fr
actio
n or
mol
ar c
once
ntra
tion
Temperature (°C)
0 50 100 150 200 250 300 350
1.E+01
1.E-01
1.E-03
1.E-05
1.E-07
1.E-09
M HCI M H2SO4H2OH2SO4CI-
Mol
e fr
actio
n or
mol
ar c
once
ntra
tion
Temperature (°C)
0 50 100 150 200 250 300
1.E+01
1.E-01
1.E-03
1.E-05
1.E-07
1.E-09
M HCI M H2SO4H2OH2SO4CI-
N2(g) H2O(g)CO2(g)H2OO2(g)
21 ppm SO2 0.01 ppm SO2
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The weight losses are plotted in Figure 6 for all tests and the maximum measured depths of crevice attack listed in Table 5. Both illustrate the point that a broad range of conditions exist. A line is drawn at 0.1 mm/year because this is often considered the limit under which an alloy can be regarded as corrosion-resistant [8]. The degree of uniform corrosion at position A1 was negligible, but there was corrosion under the crevice washers for all the stainless steels with a typical maximum depth in the range 200–300 µm. The exception was S32654, which showed no crevice attack, and was in this respect on a par with the nickel-base alloy N10276. At position A2, in less aggressive conditions at the inlet of the condenser, uniform corrosion rates were negligible for all materials. However, S31726 suffered some crevice corrosion, with a maximum depth of 100 – 200 µm and there was slight crevice attack on one of the duplicate specimens of S34565. In plant B the corrosion rates were below 1 mm/year for all materials and below 0.1 mm/year even in the most corrosive position, B1, for the superaustenitic grades S31254 and S32654 and the nickel-base N10276. However, all of the alloys suffered some degree of crevice corrosion, with the depth of crevice attack in the range 100 – 200 µm. It was not considered meaningful to evaluate the extent of crevice attack for the alloys showing higher corrosion rates. In addition to the highest corrosion rates, of ~ 0.5 mm/year, both duplex grades also showed preferential attack at the fusion line of the welds which basically discounts these grades for this
environment. At position B2 the uniform corrosion rates were lower, below 0.1 mm/year for all grades and crevice attack was also seen for all grades. The greatest depth of attack was seen for S31726 (306 µm) and S31803 (340 µm), and the least for S32654 (7 µm) and N10276 (44 µm). At position B3 at the top of the condenser the uniform corrosion rate was negligible. Only S31726 showed a crevice corrosion depth above 100 µm and no crevice corrosion at all was seen for S31254, S32654 or N10276. In Plant C the two racks C1 and C2 were at the same location and showed good reproducibility. Evaluation, reported by Sandvik Materials Technology in [1], showed that only the superaustenitic S32654 and the nickel-base N10276 exhibited low or no corro-sion. S31726 showed very extensive corrosion with one specimen missing. Almost no crevice corrosion was seen, with only attack to a depth of 70 µm for S31803. In Plant D the conditions were basically too aggressive for all the stainless steel grades in positions D1 and D2 at the washer inlet, although S32654 showed the best performance. Even the nickel-base alloy N10276 showed a corrosion rate of almost 0.5 mm/year; this was also the only case in which the corrosion rate was higher for the specimen facing the dripping condensate. For the stainless steel grades, the corrosion rate was higher by a factor of two or more for the specimens facing the flue gas flow. In the upper part of the washer, positions D3 and D4 corrosion rates for all materials were below 0.01 mm/year, but appreciable crevice
Figure 6 Results of field testing [1] with corrosion rate expressed in mm/year. A value of 4mm indicates cases where specimens had completely corroded.
Cor
rosi
on (m
m/y
ear)
N10
276
S326
54
S312
54
S345
65
S327
50
S318
03
S317
26
0.1
Duplex AusteniticNi-
base4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
L2 L3 C1 C2 D1 D2 A1 A2 B1 B2 B3 D3 D4 S1 S2
Cor
rosi
on r
ate
(mm
/yea
r)
N10
276
S326
54
S312
54
S345
65
S327
50
S318
03
S317
26
0.1
Duplex AusteniticNi-
base0.5
0.4
0.3
0.2
1.0
A1 A2 B1 B2 B3 C1 C2 D1 D2 D3 D4 S1 S2 L1 L2
S31726 315 220 x 306 115 0 0 x x 608 21 373 117 0 0
S31803 21 0 x 340 92 0 70 x x 92 120 215 116 0 0
S32750 879 0 x 290 68 0 0 x x 153 351 87 157 0 0
S31254 367 0 220 241 0 0 0 x x 0 0 83 100 0 0
S34565 290 73 x 232 55 0 0 x x 31 0 - - 0 0
S32654 0 0 118 7 0 0 0 x x 0 0 - - 0 0
N10276 0 0 100 44 0 0 0 x x 0 0 - - 0 0
Table 5 Maximum observed depth of crevice attack [µm]. (x) denotes no evaluation due to a high rate of uniform corrosion and (–) that the materials were not tested.
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corrosion was shown for S31726 facing the gas flow (608 µm) and for S32750 facing the condensate (351 µm). In the coal-fired plant S only the lower-alloyed grades, ranging from S31726 to S31254, were tested and all showed acceptably low levels of uniform corrosion. Issues instead arose with localised corrosion under the crevice washers for all four tested alloys in the outlet duct and for S31726 and S31803 in the spray tower. However, the crevice corrosion depth is limited, so it is reasonable to assume that the higher alloyed grades such as S32654 and N10276 should be able to withstand these conditions. In the laboratory tests L2 and L3 the corrosion behaviour was dominated by uniform corrosion, and the area under the crevice formers appeared to have been protected by the crevice former, rather than causing more aggressive localised conditions. There
was little pitting and no indication of selective attack of the welds except for grade S31803 in Test 3. The increase in HCl concentration for L3 more than doubled the corrosion rate, pushing all the alloys including the nickel-base N10276 over the limit of 0.1 mm/year. Nevertheless, all three superaustenitic stainless steel grades (S34565, S31254 and S32654) showed corrosion rates which may well be acceptable in service. An attempt to summarise all the corrosion data is given in Table 6. Red is used to denote a corrosion rate above 0.1 mm/year, or a depth of crevice corrosion attack in excess of 100 µm. The comparison shows how S31726 is basically insufficient for almost all the conditions tested, while the superaustenitic S32654 exhibited the best performance of the stainless steel grades, on a par with that of the nickel-base alloy N10276.
Electrochemical testing resultsA simple hydrochloric acid environment was used in the first series of laboratory tests, and the polarisation curves were employed to give a rapid overview of the corrosion behaviour. In most cases the solution was purged with air since this was considered most relevant to the flue gas cleaning application, but in borderline conditions the effect of mild argon deaeration was also investigated. In the majority of cases there was no significant difference between the two types of conditions but one borderline example is shown in Figure 7. The summarised results in Figure 8 show that in the mildest environment, 0.2M HCl at 50 °C, all steels were passive, with no active peak. Increasing the concentration to 0.8M caused S31726 to shift to active corrosion, and a further step up to 1.6M caused S31254 and S31803 to activate. S32750 showed unstable passivity and could be shifted to active corrosion behaviour using mild deaeration as shown in Figure 7 so this result is included in Figure 8. Only the superaustenitic stainless steel S32654 and the nickel-base alloy N10276 remained passive. Increasing the test temperature to 70 °C yielded the same type of behaviour, with similar levels of active corrosion for S31726 and S31803. Once again the superaustenitic S32654 and the nickel-base N10276 remained passive. The superaustenitic grades S31254 and S34565 showed a moderate degree of corrosion, while the superduplex S32750 maintained an unstable passivity, so mild dearation could induce active corrosion. This gave maximum corrosion currents in both 0.8M HCl and 1.6M HCl at 70 °C which are included in Figure 8.
A1 A2 B1 B2 B3 C1 C2 D1 D2 D3 D4 S1 S2 L1 L2
S31726
S31803
S32750
S31254
S34565
S32654
N10276
–
–
–
–
–
–
–
–
–
–
–
–
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Table 6 Ranking of grade performance in the environments tested. Red indicates a corrosion rate above 0.1 mm/year (first column) or crevice corrosion penetration above 0.1 mm (second column), where x denotes no evaluation due to a high rate of uniform corrosion.
Figure 7 Polarisation curves for S32750 in 1.6M HCl at 50 °C. Under these conditions passivity was unstable, with a high corrosion potential and no active peak if the solution was aerated, but an active corrosion peak in deaer-ated conditions. Data from the latter is included in Figure 8.
log
curr
ent
(A/c
m2)
Potential (V-SCE)
0
-1
-2
-3
-4
-5
-6-0.6 -0.1 0.4 0.9 1.4
S32750 - 1.6 HCI, 50 C
DeaeratedAerated
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Pitting corrosion was observed for S31726 in all five HCl environ-ments, while the duplex steel S31803 exhibited pitting in 0.8M HCl and 1.6M HCl 70 °C, and also showed some repassivating pitting at 50 °C. This is in partial agreement with the alloy performance predicted from the empirical PREN relationship between pitting resistance and alloy composition, included in Table 1. Overall the alloy performance is in good agreement with the general trends seen in the field testing. The effect of the presence of other ionic species is included in Figure 8 for duplicate specimens of S31726 and S31254. Partial replacement of HCl by H2SO4 had a beneficial effect, causing S31254 to passivate (higher corrosion potential), reducing the maximum current density for S31726 and also raising the pitting potential. Addition of sulphuric acid to the 0.8M HCl base solution induced passivation for one of the two specimens of S31254 and increased the pitting potential for S31726, again indicating an inhibitive effect of sulphate. However, it also gave a minor increase in the corrosion rate of S31726. The effect of sulphur is an
interesting one in view of the use of sulphur additions to mitigate high temperature corrosion in waste-fired power plants. It means that this can also have a beneficial downstream effect under acid condensation conditions. The disadvantages are that the presence of sulphur may cause condensation to commence at higher tempera-tures and that the sulphur must naturally then be removed in the FGD system before reaching the stack, but multiple beneficial effects of a single addition nevertheless are an unexpected bonus. This naturally does not apply to oxidising sulphur species such as persulfate which could potentially increase the redox potential to a level above the pitting potential. Partial substitution of chloride by bromide, nitrate or fluoride had only a marginal influence on uniform corrosion, but nitrate increased the pitting potential for S31726. Cation addition had a beneficial effect, causing both materials to shift from active to passive behaviour because of the increase in redox potential, but also decreased the pitting potential for S31726.
i max
(mA/
cm2)
N10
276
S326
54
S312
54
S345
65
S327
50
S318
03
S317
26
-2
2090 90 35
18
16
14
12
10
8
6
2
4
0
Cut
tent
max
imum
(mA/
cm2)
0.8M
HC
l
HC
l+H
2SO
4
HC
l/H
2SO
4
HC
l/H
NO
3
Com
plex
Cat
ions
HC
l/H
F
HC
l/H
Br
-2
8
6
4
2
0
0.2M 50 °C0.8M 50 °C1.6M 50 °C0.8M 70 °C1.6M 70 °C
S31726S31254
Figure 8 Maximum current densities in polarisation curves in the five basic environments (left) and the effect of other environmental additions in 0.8M solution at 70 °C (right).
102/2013 |
Reproduced with permission from NACE International, Houston, TX.All rights reserved. Paper 2695 presented at NACE CORROSION2013, Orlando, Florida, USA, © NACE International 2013.
CONCLUSIONSCondensation of flue gases in waste-fired power plants leads to very corrosive conditions in flue gas cleaning systems, with a condensate which is primarily hydrochloric acid. Evaluation of test panels and racks exposed in four waste-fired power plants showed that there was an immense variation in corrosion rates, varying from negligible to so severe that a number of specimens were completely corroded and therefore missing after 8 months of testing. In general the performance followed the PREN and correlated well with laboratory tests under condensation conditions and electrochemical tests in synthetic condensates. The behaviour of the alloys in the laboratory simulated condensation environment and the plant exposures was very similar, although more crevice corrosion was observed in the field exposures, which were of longer duration. The lowest corrosion resistance was exhibited by the austenitic S31726 and the duplex S31803, so the superaustenitic grades S34565 and S31254 may be useful in conditions in which these lower alloyed stainless steels are inadequate. Duplex grades should be used with some caution due to a number of instances of high corrosion rates. The best performance of all the stainless steel grades investigated was shown by the superaustenitic S32654. This may be a cost-effective alternative to the nickel-base alloy N10276 in some flue gas cleaning systems.
ACKNOWLEDGMENTSThis work was supported by a financial grant from the European Research Fund for Coal and Steel under contract 7210-PR-305. The partners of the project are gratefully acknowledged for collaboration, including supply of materials and data.
REFERENCES[1] M. Linder, R. Gubner, U. Kivisäkk, J. Peultier, A. Bergquist,
B. Beckers, R. Pettersson, J. Flyg: Stainless steels for wet conditions in municipal incineration and combustion plants.RFCS final report EUR 22082EN.
[2] B. Beckers, A. Bergquist, M. Snis, E. Torsner: Modern Materials in Flue Gas Cleaning applications; corrosion properties inartificial scrubber environment, field tests & service experience NACE Corrosion 2007, paper 07351.
[3] R. Pettersson, A. Bergquist, J. Flyg, Corrosion of stainless steels in condensing flue gases from waste-fired power plants. Proc Eurocorr 2012.
[4] P. Steinmetz, C. Rapin: Corrosion of metallic materials in waste incinerators. Materials Science Form 251 – 254 (1997) 505 – 518.
[5] R. Pettersson, J. Flyg, A. Bergquist, B. Beckers Stainless steels in waste and biomass power plant applications. Proc Stainless Steel World conference, Maastricht, November 2009
[6] Y-H. Kiang: Predicting dewpoints of acid gases, Chemical Engineering, Feb 9th (1981) p. 127.
[7] W. M. M. Huijbregts, R. G. I. Leferink: Latest advances in the understanding of acid dewpoint corrosion: corrosion and stress corrosion cracking in combustion gas condensates. Anti Corrosion Methods and Materials 51:3 (2004) 173 – 188
[8] Outokumpu Corrosion Handbook, 10th Edition, 2009.
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IntroductionAs is to be expected, nickel-chromium-molybdenum alloys combine the good resistance to corrosion of nickel-molybdenum alloys under reducing conditions with the good resistance to corrosion exhibited by nickel-chromium alloys under oxidizing conditions [1]. The most important for engineering purposes are the so-called C-type alloys, the first of which was introduced as long ago as the 1930s. This was a nickel alloy with typically (figures in mass-%) 16 % chromium and 16 % molybdenum which also contained 4 % tungsten and 6 % iron together with 0.7 % silicon and 0.05 % carbon; this alloy is no longer in common use [2]. With the discovery that to reduce precipitation-readiness it is important to reduce both the carbon and the silicon content of such materials to very low levels, it was developed into the alloy UNS N10276 which is in use today, as shown in Table 1. Low silicon contents of typically 0.04 mass-% and very low carbon contents of typically 0.005 mass-% are nowadays characteristic of all the commonly used C-type nickel-chromium-molybdenum alloys [2], which also include the alloys UNS N06455, UNS N06022 and UNS N06059 in Table 1. Whereas according to Table 1 the chromium contents of nickel-chromium-molybdenum alloys are in a range between 16 and 23 %, the molybdenum contents are in most cases at about or near to 16 %, with exception of alloy UNS N06625 which contains only 9 % molybdenum. There is no alloy in common use where the molybdenum content is essentially above 16 % which would allow handling more corrosive reducing acids.
AbstractAlloy UNS N06058 is a new Ni-21%Cr-19%Mo alloy which has been recently developed. Alloy UNS N06058 is the first Ni-Cr-Mo alloy to contain nitrogen as an alloying constituent. Together with the chromium and the increased molybdenum content the alloying of nitrogen results in a pitting resistance equivalent number (PREN) of about 86, surpassing the nickel-chromium-molybdenum alloys being commonly in use so far. As a result of the specific combination of its alloying constitu-ents the new alloy is featuring an excellent balanced corrosion resist-ance in oxidizing as well as in reducing environments. In general the new material is characterised by superior resistance to general corrosion in acids and acidic mixtures even when contaminated with chlorides as well as to localised corrosion in regard to all other C-type alloys. Good fabricability including weldability is also given. This work reports on the main properties of alloy UNS N06058 and its suitability for interesting areas of application like flue gas desulfurization and severely corrosive chemical processes.
Keywords: Ni-Cr-Mo alloy, C-type alloy, UNS N06058, corrosion resistant alloy, localised corrosion, pitting, chemical process industry, FGD, acids.
Alloy EN UNS Ni Cr Mo Fe Other
625 2.4856 N06625 62 22 9 3 3.4 Nb 52
C-4 2.4610 N06455 66 16 16 1 69
C-276 2.4819 N10276 57 16 16 6 3.5 W 69
22 2.4602 N06022 56 22 13 4 3 W 65
686 2.4606 N06686 58 21 16 1.5 3.8 W 74
2000 2.4675 N06200 57 23 16 1.5 1.6 Cu 76
59 2.4605 N06059 59 23 16 1 76
2120 2.4700 N06058 59 21 19 1 0.075 N 86
Designation PREN = %Cr + 3.3 %Mo
+ 30 %N
Main alloying elements, typical values, % by mass
Table 1 Commonly used nickel-chromium-molybdenum alloys in comparison to the new alloy UNS N06058 with (Cr + Mo) in ascending order.
A new developed Ni-Cr-Mo alloy with improved corrosion resistance in flue gas desulfurization and chemical process applicationsH. Alves ThyssenKrupp VDM GmbH, Werdohl, Germany
D. Kurumlu, R. Behrens,ThyssenKrupp VDM GmbH, Altena, Germany
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properly manufactured materials are correlating to the ratio of chromium to molybdenum plus tungsten content [3] as shown in Figure 1. As to be seen in Figure 1 the corrosion rate obtained on a sample of the new developed alloy UNS N06058 is following exactly the correlation which had been developed previously in testing of the other nickel-chromium-molybdenum alloys. When examining the behavior of alloy UNS N06058 in the ASTM-G 28 B intergranular corrosion test solution (23 % H2SO4 + 1.2 % HCl + 1 % CuCl2 + 1 % FeCl3, boiling) the corrosion rate obtained on alloy UNS N06058 is in an intermediate position when compared to the corrosion rates of the other Ni-Cr-Mo alloys as indicated in Table 2. However, when looking at the results obtained in the boiling HCl test solutions, the corrosion rates of alloy UNS N06058 are markedly lower than those obtained on the other nickel-chromium- molybdenum alloys indicated in Table 2. Obviously the low corrosion rates of alloy UNS N06058 in the HCl test solutions reflect the high molybdenum content of the alloy. Finally, the corrosion rates obtained for alloy UNS N06058 in 90 % H2SO4 at 90 °C and 105 °C show that the alloy’s limit of corrosion resistance (≤ 0.1 mm/a) in this environment is somewhat above 90 °C, higher than explored for the other nickel-chromi-um-molybdenum alloys indicated in Table 2. This is an indication of alloy UNS N06058’s usefulness as a material for plate type heat exchangers to cool down H2SO4 of high concentration.
The last column of Table 1 gives the pitting resistance equivalent number (PREN) as calculated from the alloy composition figures in Table 1. Indeed, for resistance to pitting and crevice corrosion, the sum of the chromium content plus approximately 3.3 times the molybdenum content, the so-called pitting resistance equivalent number, is the decisive factor. In many cases, 30 times the nitrogen content may also be added to this [1], as expressed by the following Equation (1):
PREN = %Cr + 3.3 %Mo + 30 %N (1)
Therefore alloys with higher molybdenum contents are used in corrosive environments contaminated with sulfuric acid, halides and especially chlorides, for example in flue gas scrubbers. As Table 1 shows, the commonly used nickel-chromium-molybdenum alloys from alloy UNS N06625 up to alloy UNS N06059 are covering a range of PREN-values between about 52 and 76.
THE NEW DEVELOPED Ni-Cr-Mo-ALLOY 2120Composition Typical values of composition of the new developed alloy 2120 – Nicrofer 5821 hMoN(1) – UNS N06058 are indicated in the bottom line of Table 1. When compared to alloy UNS N06059 the chromium content is slightly reduced and the molybdenum content is considerably increased. As comes out from Table 1, alloy UNS N06058 is the first Ni-Cr-Mo-alloy to contain nitrogen as an alloying constit-uent. Together with the increased molybdenum content this results in a PREN-value of about 86 which is the highest PREN-value of all Ni-Cr-Mo-alloys being commonly in use so far. Whereas Table 1 indicates the chemical composition of alloy UNS N06058 as it is now the experimental data presented in the following have been established over the last years on materials with a somewhat higher molybdenum content which was more close to 19.5 % than today’s 19 %. This has to be taken in consideration where necessary.
Corrosion resistance Table 2 shows a comparison of corrosion rates of Alloy UNS N06058 when tested in a variety of environments. The ASTM(2)-G 28 A medium is an oxidizing sulfuric acid test solution (50 % H2SO4 + 42 g/l Fe2(SO4)3 x 9 H2O, boiling) where the corrosion rates of
Medium UNS UNS UNS UNS UNS UNS N06058 N06686 N06200 N06022 N10276 N06455
ASTM-G 28 A boiling 2.0 2.63 0.68 0.91 6.03 3.0
ASTM-G 28 B boiling 0.24 0.26 0.11 0.18 0.86 3.5
10 % HCl boiling 3.4 7.1 7.6 9.0 6.5 6.4
5 % HCl boiling 1.6 6.2 4.4 6.9 3.6 3.79
2 % HCl boiling 0.02 0.15 0.07 1.26 0.61 1.23
90 % H2SO4
3 x 7 days at 90 °C 0.04 1.20 0.18 – 1.17 –
3 x 7 days at 105 °C 0.49 2.5 0.42* – 1.20 –
Table 2 Comparison of corrosion rates of alloy UNS N06058 when tested in a variety of environments (mm/a).
Cor
rosi
on lo
ss (m
m/y
ear)
6
5
4
3
2
1
01 2 3 4
ASTM G-28 A
N10276(15.5)
N06455(15.9)
Alloy (chromium, wt. %)
N06022(21.3)
N06059(22.7)
N06625(22.15)
N06985(22.6)
N06058(20.8)
CRMo + W
Figure 1 Corrosion loss of nickel-chromium-molybdenum alloys in boiling 50 % H2SO4 with 42 g/l Fe2(SO4)3 x 9 H2O in accordance with ASTM G-28 A as a function of their ratio of chromium to molybdenum plus tungsten, from [3] with addition of a corrosion rate obtained on a sample of alloy UNS N06058.
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Figure 2 shows a comparison of data for the critical pitting temperature (CPT) of various nickel-chromium-molybdenum alloys when tested in a solution 7 vol. % H2SO4 + 3 vol. % HCl + 1 % CuCl2 + 1 % FeCl3, called Green Death. This test is based on observa-tions in flue gas desulfurization systems of coal-fired power stations [4]. A gradual temperature increase of 5 °C increments has been applied after a testing time of 24 hrs till the samples failed by pitting. As to be expected from the PREN values indicated in Table 1 the critical pitting temperature of alloy UNS N06058 in this test is very clearly ahead of the other nickel-chromium- molybdenum alloys which suggests a superior resistance to pitting and crevice corrosion of alloy UNS N06058 in demanding application Figure 3 depicts the isocorrosion lines of alloy UNS N06058 in slightly aerated industrial grade sulfuric acid, determined in
immersion tests over at least 120 h. The 0.1 mm/a line is running well above 80 °C across nearly the whole concentration range, in contrast to the 0.1 mm/a isocorrosion line of alloy UNS N10276 which goes down to about 80 °C already at about 20 % H2SO4 [4]. The 0.5 mm/a isocorrosion line of alloy UNS N06058 is running clearly above 100 °C across the whole concentration range of sulfuric acid, on the average on a somewhat higher level than the 0.5 mm/a isocorrosion line [4] of the alloy UNS N10276. In accordance with the sulfuric acid data of Table 2 this indicates a somewhat better corrosion resistance of alloy UNS N06058 in sulfuric acid than that of the other nickel-chromium-molybdenum alloys being in use today.
Mechanical Properties Figure 4 shows the mechanical properties of alloy UNS N06058 in the solution annealed condition as obtained in the tension test on sheet of 3 mm thickness at ambient temperature (left) and at 450 °C (right), in comparison to alloys UNS N10276 and UNS N06059. Due to alloying with more molybdenum and some nitrogen the strength values of alloy UNS N06058 are somewhat above the values of the other two alloys tested. Brinell hardness increases from alloy UNS N10276 over alloy UNS N06059 to alloy UNS N06058. Impact energy measured as ISO V-notch average values for the solution annealed condition are highest for alloy UNS N10276 and lowest for alloy UNS N06058, but still above 150 J at -196 °C.
Figure 2 Critical Pitting Temperature (CPT) when tested in a solution 7 vol. % H2SO4 + 3 vol. % HCl + 1 % CuCl2 + 1 % FeCl3, called “Green Death”. A gradual temperature increase of 5 °C increments has been applied after a testing time of 24 hrs till the samples failed by pitting. Some data (*) have been taken from Material Data Sheets. With respect to the data shown on the right it has to be kept in mind that the Green Death solution chemically becomes unsta-ble above 120 °C, making the test less reliable at the higher temperatures [4].
Tem
pera
ture
(°C
)
N06
058
N06
022
N06
022
N10
276
N06
625
N06
455
0
150
100
50
100*
*from material data sheet
100*
120* 120*
135>145
Figure 3 Isocorrosion diagram of alloy UNS N06058 in slightly aerated indus-trial grade sulfuric acid, determined in immersion tests over at least 120 h. The diagram is showing the isocorrosion lines as a function of the sulfuric acid concentration and the temperature.
Tem
pera
ture
(°C
)
Concentration of H2SO4 (%)
140
120
100
80
60
40
20
00 10 20 30 40 60 7050 80 90 100
> 0,5 mm/a
< 0,1 mm/a
0,1 – 0,5 mm/a
Figure 4 Mechanical properties of alloy UNS N06058 in comparison to the nickel-chromium-molybdenum alloys UNS N10276 and UNS N06059 obtained on one heat (alloy UNS N06058) or as average values from current production (alloys UNS N10276 and UNS N06059).
(MPa
)
N10276 N06059 N060580
200
400
600
800
1000Rp0,2 RmRT tensile tests/d = 3 mm
(MPa
)
N10276 N06059 N060580
200
400
600
800
1000Rp0,2 Rm450°C tensile tests/d = 3 mm
(J)
N10276 N06059 N060580
50100150200250300
-196°C 20°CImpact energy
(-)
N10276 N06059 N060580
20
40
60
80
100Hardness in HRB
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Manufacturing Alloy UNS N06058 is melted and cast easily in making use of the available technologies. A strand cast ingot of alloy UNS N06058 is shown in Figure 5. Plate, strip and longitudinally welded tubes made of alloy UNS N06058 are shown in Figure 6. These and other kinds of semi finished products of alloy UNS N06058 can be shaped into the various final forms required about as easily as with the other commonly used nickel-chromium-molybdenum alloys. Due to the high molybdenum content of alloy UNS N06058 some attention is recommended to make sure that especially in thicker sections centerline segregation will not be present or
surpass an undesirable extent. This is best done in specifying a homogenization heat treatment not on the cast ingots but after a certain degree of hot rolling or forging, because the annealing time being required for leveling out any segregations is dependent not only on the temperature dependent coefficient of diffusion but on the square of the distance the atoms have to go by diffusion. So in case the ingot thickness is brought down by hot deformation by a factor of 10 the time required for leveling out segregations in a subsequent diffusion annealing treatment is reduced by a factor of 100 [5]. An example of the microscopic grain structure of alloy UNS N06058 after a final solution annealing treatment, with equiaxed grains and a multitude of twinned crystals, is demon-strated in Figure 7. Depending on the wall thickness, the welding of alloy UNS N06058 can be done using a matching filler metal or without filler metal as in the case of the tubes shown in Figure 6. Testing of such weldments in a calcium chloride test medium resulted in an excellent resistance to pitting and crevice corrosion. Thermal stability at intermediate temperatures in terms of time-temperature-sensitization diagrams [3] is shown in Figure 8. The lines in Figure 8 indicate the annealing time after which the intercrystalline penetration is expected to exceed a depth of 50 µm. The more the lines are shifted to the right the better is the thermal stability. The orange dots indicate where the line for alloy UNS N06058 is to be positioned: more or less close to the line that has been established for alloy UNS N06686, in between the lines for alloy UNS N10276 and alloy UNS N06059. For this and other reasons of similarity manufacturing of alloy UNS N06058 can follow rules that are not too much different from the rules being valid for these two Ni-Cr-Mo alloys where a wealth of manufacturing experience is available.
Applications Alloy UNS N06058 is intended for applications where a high resistance to corrosion in very aggressive media is required, as much under oxidizing as under reducing conditions, and where an excellent resistance to pitting and crevice corrosion in acidic chloride containing media is a need. Typical applications of alloy UNS N06058 include scrubbers and other components in flue gas desulfurization and waste incinera-
Figure 5 Strand cast ingot of alloy UNS N06058.
Figure 6 Fabricated products of alloy UNS N06058,plate, strip and welded tube.
Figure 7 Microscopic grain structure of alloy UNS N06058 after final solution annealing.
Figure 8 Time-temperature-sensitization diagrams of various nickel-chromium- molybdenum alloys, determined by means of tests in accordance with ASTM G-28 A [3]. The lines indicate the annealing time after which the intercrystalline penetration is expected to exceed a depth of 50 µm. The orange dots indicate where the line for alloy UNS N06058 is to be positioned.
Tem
pera
tur
(°C
)
Time (h)
1400
1000
900
800
700
600
500
4000.3 0.1 0.3 1 3 10 30 100
N06058 N06686 N06200 N10276 N06059 N06022 N06625 N06455
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Reproduced with permission from NACE International, Houston, TX.All rights reserved. Paper 2325 presented at NACE CORROSION2013, Orlando, Florida, USA, © NACE International 2013.
tion plants as e.g. heat recovery systems for indirect heat transfer. Advantageous applications include also heat exchangers for sulfuric acid coolers, production and processing of organic chloride containing chemicals, handling of mineral acids like hydrochloric acid and sulfuric acid, handling of pharmaceutical intermediates and fine chemicals. Figure 9 illustrates the recent application of alloy UNS N06058 as material of construction for heat exchanger inside walls of a waste incineration plant. Generally the application of alloy UNS N06058 is to be consid-ered where the resistance to pitting and crevice corrosion of the nickel-chromium-molybdenum alloys being in current use today is approaching the limits and alternative materials as zirconium and graphite have to be used. An application of alloy UNS N06058 for down hole equipment in sour-gas production appears to be promising due to the alloy’s combination of high nickel, chromium and molybdenum content which by experience allows to expect a high resistance to chloride induced stress corrosion cracking in the presence of hydrogen sulfides [1]. This and other applications like in geothermal environments are still to be explored.
Figure 9 Alloy UNS N06058 in use in the inside walls of a heat exchanger of a waste incineration plant.
C-Steel
UNS N06058
CONCLUSIONS1) Alloy UNS N06058 is the first nickel-chromium-molybdenum
alloy to contain nitrogen as an alloying constituent.
2) The increased molybdenum content of alloy UNS N06058 and alloying with nitrogen results in a PREN value of about 86 which is the highest PREN value of all Ni-Cr-Mo-alloys being commonly in use so far.
3) As a consequence of the high PREN value alloy UNS N06058 is ahead of the other Ni-Cr-Mo-alloys with respect to its unique resistance to pitting and crevice corrosion in acidic chloride containing media as they occur e.g. in flue gas desulfurization.
4) Generally the application of alloy UNS N06058 is to be considered where the resistance to pitting and crevice corrosion of other Ni-Cr-Mo alloys is approaching the limits and alternative materials like zirconium or graphite had to be selected in the past.
5) In boiling hydrochloric acid test solutions the corrosion resistance of alloy UNS N06058 proved to be markedly better than with other Ni-Cr-Mo alloys, obviously due to the alloy’s high molybdenum content.
ACKNOWLEDGEMENTThe welded tubes shown in Figure 6 are a courtesy of Schoeller Werk GmbH & Co KG. The heat exchanger photos shown in Figure 9 are a courtesy of Wallstein Ingenieur GmbH.
REFERENCES[1] U. Heubner: Nickel alloys and high-alloy special stainless
steels – Material overview and metallurgical principles, in U. Heubner, J. Klöwer and –5 co-authors: Nickel alloys and high-alloy special stainless steels – Properties-Manufactur-ing-Application, 4th completely revised edition, expert verlag, 2012, pp. 1 – 39.
[2] D.C. Agarwal, W.R. Herda: The “C” family of Ni-Cr-Mo alloys’ partnership with the Chemical Process Industry: the last 70 years, Materials and Corrosion 48 (1997) 542 – 548.
[3] U. Heubner: Corrosion resistance of nickel alloys and high-alloy special stainless steels, in U. Heubner, J. Klöwer and 5 co-authors: Nickel alloys and high-alloy special stainless steels - Properties-Manufacturing-Application, 4th completely revised edition, expert verlag, 2012, pp. 40 – 75.
[4] V. Wahl: The use of nickel alloys and stainless steels in environmental engineering, in U. Heubner, J. Klöwer and 5 co-authors: Nickel alloys and high-alloy special stainless steels - Properties-Manufacturing-Application, 4th completely revised edition, expert verlag, 2012, pp. 203 – 237.
[5] U. Heubner: Wärmebehandlung von Nichteisenmetallen, in G. Spur, T. Stöferle: Handbuch der Fertigungstechnik, Band 4/2 Wärmebehandeln, Carl Hanser Verlag, 1987, pp. 985 – 988.
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