corrosion mechanisms and models for flue gas corrosion in ... · corrosion should be: a better...

Corrosion mechanisms and models for flue gas corrosion in aluminum heat exchangers W. Wang, W. Fürbeth

DECHEMA-Forschungsinstitut, Frankfurt am Main, Germany e-mail: wang@dechema.de

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