corrosion mechanisms and models for flue gas corrosion in ... · corrosion should be: a better...
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Corrosion mechanisms and models for flue gas corrosion in aluminum heat exchangers W. Wang, W. Fürbeth
DECHEMA-Forschungsinstitut, Frankfurt am Main, Germany e-mail: [email protected]
Funded by: M2i Materials Innovation Institute
Background
Condensing boilers are highly efficient heating boilers on the market. Due to the formation of acidic condensate, heat exchangers and drain systems are contaminated and/or clogged easily with corrosion products.
In the light of recent works [1], major factors affecting acid dewpoint corrosion should be:
A better understanding of the underlying processes leading to corrosion of such systems should be obtained.
Heat exchanger
Drain system
Flue gas
Condensing boiler
Flue gas
Occurrence of H2SO4 dewpoint corrosion
Condensation of H
2 SO4
Condensation of H
Cl
H2 O
Heat exchanger surface T
High T
Low T
120°C
55°C
Dewpoint corrosion in heat exchanger
• Temperature gradient • Flue Gas Composition • Material (Aluminum Silicon alloy) • Boiler Operation • Corrosion product formation • Cleaning procedures
Mass losses derived from immersion corrosion testing of Aluminum 230 at 40°C, 50°C, 55°C in various H2SO4 concentrations (0.1%, 1%), and the dissolution reactions and mechanism in acid solution.
[1] Slaiman, Q.J.M., Nassouri, H.J. (1985). Journal of Petrochemical Research 4 (2): 139-151. [2] Derek D Stuart, ISA 55th Analysis Division Symposium, 2010, section 3.3
Reference
SEM images of corroded surfaces after 4 hours (1) , 24hours (2) and 72 hours (3) in 0.1% sulfuric acid at 55°C with magnification of 5000x.
(1) (2) (3)
Si phase
Aluminum cast alloy EN AC 44300 (Aluminum 230)
EPMA micrographs from cross-sections of corroded Al230 after 5min in 30% sulfuric acid at 80°C.
Schematic representation of microstructure development of corroded surface during the immersion test.
Product formation pH < 4
nOHSOAlSOOHAl )()()( 23422
42 =+ −+
Product formation pH = 4
++ +↓=+ HOHAlOHAl 6)(262 323
3232 )(23 OHAlOHOAl =+
Hydration
Hydrogen ion adsorption
OHOHAlHOHAl 223 )()( +=+ ++
OHAlHOHAl 23
2 22)( +=+ +++
23 326 HAlHAl +=+ ++
Oxide layer
Substrate metal
OH2 OH2
+H
Substrate metal
+H
Substrate metal
+H
Substrate metal
+H3)(OHAl
Nyquist plots of Aluminum 230 exposed in 0.1 % H2SO4 solution at RT.
Dew Point Corrosion Test
The corrosion rate is governed by the rate of acid deposition and not by the rate of reaction between metal and acid [2].
Immersion Test
Electrochemical Impedance Spectroscopy
Condensation rate of sulfuric acid
Condensation rate of water
Dew point : around 150°C
• Dew point of sulphuric acid
• Condensation rate
• Heat transfer mg= mass flow rate of gas Cpg=specific heat of gas Tgin= Inside temperature of gas Tgo=Outside temperature of gas Tdew= dew point temperature of vapor m= condensation rate Q=heat transfer Hfg= latent heat of condensation
Conclusions