56 ACMM Condensate SCC www.hbscc.nl - 1 - LATEST ADVANCES IN THE UNDERSTANDING OF ACID DEWPOINT CORROSION:
CORROSION AND STRESS CORROSION CRACKING IN COMBUSTION GAS CONDENSATES
W.M.M. Huijbregts*, R. Leferink**
Anti-Corrosion Methods and Materials, Vol. 51, 3 (2004), pg 173-188.
SUMMARY
Corrosion failures very often occur becau of condensing flue gass containing H2O, SO3, NO x and
HCl. The corrosion failures can be of the type: general corrosion, pitting and stress corrosion cracking.
The chemistry of the condensing gass is discusd and some examples of corrosion in Blast
Stoves, Heat Recovery Steam Generators and waste incineration boilers are described. Mounting of
the insulation inside the casing is a main cau for stress corrosion cracking. Nitric acid can react with
carbon steel and insulation material forming ammonium nitrate and calcium nitrate, both very
hygroscopic materials and very corrosive for Stress Corrosion Cracking, even above the water dew
point.
1 INTRODUCTION
In the recent years many corrosion problems aro from condensing gass. When there is a risk of condensation the designer should have the answers on the following questions:
•Which condend liquid can be formed (the dewpoints of the various gass should be calculated)?
•Which amount of condend liquid can be expected?
•What concentration of corrosive liquid can be expected?
•What is the corrosion resistance of the material in the to be expected environment?
The method of calculation dew points, description and calculation of condensation phenomena and
the concentrations of the conden have been published rather well (Handbook of Chemistry and Physics1, Yen Hsiung Kiang2,Land3 , Hoftyzer4 and Ullman5).
* Huijbregts Corrosion Consultancy, Renkum, The Netherlands, www.hbscc.nl
** KEMA BV, Arnhem, The Netherlands
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In particularly stress corrosion cracking failures on carbon steels is a common mechanism in condensing nitrate gass. Much rearch on general aspects odf stress corrosion cracking has been
done (McEvily6, Dahl7 , Drodtenl8 , Krautschickl9, Mazille10 , Bunning11, Bohnenkamp12, Krautschick13)
More specific problems on stress corrosion cracking in blast stoves and in Heat Recovery Steam Generators has been studied since 1967 (Blekkenhorst14, Harp15,16, Ullman17, Kalfa18, Sucker19, Buhler20 and Leferink21).
The chemistry of condensation will be explained, after which the specific problems in blast stoves of
blast furnaces and in Heat Recovery Steam Generators (HRSG's) will be discusd. At last a specific problem of a corrosion problem in HCl containing flue gas in a waste incineration unit will be discusd
(de Weijer22).
2 DEW POINTS OF H2O, NO2 AND SO3
Bad on data from literature, the dew points of various gas compositions can be calculated and plotted. The dew point is the temperature at which the first liquid starts to conden from the gas pha. The water dew point can be plotted from the water vapour pressures tables1.
Figure 1. The water vapour pressures from the water vapour table. A gas with 6.5 v% H2O has a vapour pressure of 49.7 mm Hg (100 v% water has a vapour pressure of 758 mm Hg) and
a dewpoint of 38 °C.
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Dewpoints (°C) of the gass SO3, SO2, HCl and NO2 can be calculated by means of the
equations of Verhoff, Perry and Kiang2.
A: Dewpoint equation of SO3 according to Verhoff:
T d=1000/{2.276 - 0.0294ln(P H2O) - 0.0858*ln(P SO3) + 0.0062*ln(P H2O*P SO3)}
B: Dewpoint equation of SO2 according to Kiang:
Td=1000/{3.9526 - 0.1863*ln(P H2O) + 0.000867*ln(P SO2) - 0.00091*ln(P H2O*P SO2)}
C: Dewpoint equation of HCl according to Kiang:
Td=1000/{3.7368 - 0.1591*ln(P H2O) - 0.0326*ln(P HCl) + 0.00269*ln(P H2O*P HCl)}
D: Dewpoint equation of NO2 according to Perry:
T d NO2= 1000/(3.664 - 0.1446*ln(v%H2O/100*760) - 0.0827*ln(vppmNO2/1000000*760) +
0.00756*ln(v%H2O/100*760)*ln(vppmNO2/1000000*760)) - 273
Pressures (P) in the equations B, C en D are given in mm Hg; in equation A in atmosphere.
The Figures 2 up to 5 give the dew points of the gass SO3, SO2, HCl and NO2. When the calculated dewpoints are lower than the water dew point a straight water dew point line is given (for HCl and
NO2).
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Figure 2. Dew points of SO3at various water contents of the gas, calculated from the formula of Verhoff.
Figure 3. Dew points of SO2at various water contents of the gas, calculated from the formula of Kiang. The SO2 dew points for all gass are lower than the water dew point of the
gass.
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Figure 4. Dew points of HCl at various water contents of the gas, calculated from the formula of Kiang and the water vapour table.
Figure 5. Dew points of NO2at various water contents of the gas, calculated from the formula of Perry and the water vapour table.
3 AMOUNT OF CONDENSED ACID
When the temperature drops below the dew point, sulphuric acid, hydro nitric acid and water start to
conden either as small fog droplets or as a film onto the walls (Land3). Condensation droplets will be formed very easily on particles in the flue gas, acting as condensation nuclei. For instance fly ash particles (coal fired boilers, waste incineration boilers) can cau the mist condensation. On the other
hand in a rather clean gas, as in gas fired units, super-saturation will take place, resulting in film condensation on the cool walls or heat exchanger tubes.
If a mist is formed, most of the droplets are carried away with the flue gas and, in ca of acid droplets, the corrosion rate of steel will be low. However, in the ca of super-saturation, film condensation will occur and a liquid film will be formed on the bundle tubes or on the flue gas line walls. Besides, in the ca of mist condensation, high gas velocities or local high flow disturbances will
push the droplets onto the metal walls and a thin liquid film is formed as well by this mechanism
Acid deposition can be quite high under certain conditions. To get some idea of the deposition rates to
be expected Land3 produced Figure 6. The calculation method of Land was bad on heat transfer
