Low temperature

corrosion in cement

plants

Christian Suchak, Volker Hoenig;

VDZ, Duesseldorf, Germany

Introduction

Corrosion describes a wide topic and

summarizes all the miscellaneous

corrosion types within one term. The

speed and severity of corrosion depend

on a number of factors, which in the

end define the actual corrosion

mechanism. Looking at the corrosion

problem in a cement plant, two basic

types of corrosion can be

distinguished. Corrosion at

temperatures above 400 °C is described

as high temperature corrosion and

beneath 400 °C as low temperature

corrosion. In case of low temperature

corrosion it is always implied that an

aqueous electrolyte takes part in the

electrochemical reaction, so that a

cathodic and anodic reaction takes

place. This rough classification allows

allocating the different components of

a cement plant to these corrosion types.

Basically all aggregates downstream of

the preheater, the bypass filter as well

as the clinker cooler filter are subject to

low temperature corrosion. Major

equipment parts such as dedusting

units or chimneys have often high

maintenance requirements related to

corrosion damages which cause

additional costs for the operating

companies.

Figure 1 & 2: Corroded sidewalls of a bag house

Possible corrosion mechanisms

The gas way of cement plants are

subjected to corrosion damage. In the

process gas are gaseous and solid

chloridic and sulphatic compounds

present which have corrosive

properties. The oxygen and water

vapour in the gas atmosphere provide

for oxidizing conditions. Carbon

dioxide, originating from the

decarbonisation reaction of calcium

carbonate and fuel combustion, has

both oxidizing and carburizing

characteristics. As soon as water and in

particular different kinds of acids are

condensing, the metal parts are subject

to corrosion. In table 1 hypothetical

acids present in the waste gases of

cement plants are summarized.

Table 1: Acids with corrosive potential

occurring in the waste gas path of cement

plants during running operation

Acid Gas Agent Relevance

Nitrous acid

HNO2

NO &

NO2

Low

Nitric acid

HNO3

NO2 Low

Hydrochloric acid

HCl

Cl2 Medium

Sulphurous acid

H2SO3

SO2 Low

Sulphuric acid

H2SO4

SO3 High

For the formation of nitrous, nitric and

sulphurous acid condensed water is

required. The gas agents are physically

dissolved in water and form the various

acids and could oppose a corrosive

threat to the metal.

Figure 3: SO3-Measurement in a low dust gas

environment

Corrosion during kiln operation

Inspections of different precipitators

during major maintenance stops

showed in general signs of both

extensive and uniform damages as well

as sign of pitting. Both these types of

corrosion damages can be caused by

acids (cf. Figure 1&2). A possible

explanation for this corrosion

occurrence could be the condensation

of sulphuric acid during the running

clinker production. If a considerable

concentration of SO3 is present in these

flue gases, it reacts with the water

vapour beneath 500 °C to H2SO4. Then

the acid can condensate on all open

metal surfaces by reaching the

sulphuric acid dew point. Until today,

the widely observed phenomenon of

corrosion and the hypothesis of the

existence of sulphuric acid corrosion in

cement plants have not been

investigated systematically yet.

Therefore, hardly any scientific studies

have been conducted in this matter so

far, also no systematically verified

measurements of SO3 in high dust

atmospheres at a cement plant have

been reported. Additionally, the

existing measuring techniques for

determining SO3 and H2SO4

respectively in flue gases are

exclusively designed for power plant

environments (cf. Figure 3). Such

measurements are regularly carried out

in the power industry at dust loads of

max 1 g/Nm³.

In comparison, the flue gases of

cement plants can have - depending on

the point of process - a dust load of 30

– 500 g/Nm³. The regular measuring

equipment is not usable for such

conditions. A new measuring technique

was therefore necessary to be able

firstly to measure and secondly assess

the possibility of the existence of

SO3/H2SO4 and furthermore to

determine whether the prevailing

concentrations are harmful or can be

neglected.

The determination of acidic components in high dust flue gases

All conventional measuring techniques

used to determine gas components in

flue gases consist of probes which are

equipped with ceramic and sintered

metal filters respectively. Over time a

considerable amount of dust builds up

on the filter surface and in filter pores

and a closed dust layer is formed. The

measured gas has to pass this dust layer

and is subject to the risk of chemical

reactions. This problem was confirmed

by several measurements done with a

conventional probe and a continuously

working SO2-analyzer. After five

minutes the dust layer starts to absorb

the major amount of SO2 (cf. Figure 5).

It is therefore not possible to rule out a

false measurement of highly reactive

gas components such as SO3/H2SO4.

Therefore, an electrostatic precipitator

was developed to avoid this problem.

With this equipment several

measurements in different cement

plants were successfully executed (cf.

Figure 4).

Figure 4: SO3-Measurement in a high dust gas environment

The measurements done with the

newly developed measuring probe

showed considerable amounts of SO3

(measured as H2SO4). In some cases

Figure 5: Continuous SO2-measurement with a

conventional gas probe

concentrations up to 90 mg/Nm³ were

found. Therefore qualitative and

quantitative proof of H2SO4 existence

in flue gases has been verified. This

underlines the possibility that H2SO4

can occur even in environments with

nearly pure carbonate particles,

particularly considering the common

dust loads of 30 - 500 g/m³. With

regard to the confirmation of the

H2SO4 existence in the clinker burning

process, it is suggested that this

compound is generated in the top

cyclone stages of the preheater. Then,

at temperatures of 400 – 600 °C the

geogenic sulphides are oxidizing to

SO2. Furthermore high amounts of

catalytic effective oxides like Al2O3,

SiO2 and Fe2O3 [1], as part of the kiln

feed, are suspended in the atmosphere

of the cyclones.

Figure 6: H2SO4 isothermal dew point curves for a cement plant important partial pressure ranges

and plant measurements

Through the heterogenic catalytic

chemical reaction SO2 is been oxidized

to SO3. During the cooling in the

conditioning tower or in the raw mill,

the SO3 reacts at temperatures beneath

500 °C with water in the gas

atmosphere to H2SO4 [2].

The calculations of the sulphuric acid

dew points of the performed SO3

measurements have been executed with

the following empirical determined

equation [3]:

422lg7.18lg6.274.122

SOHOHpp

With ϑ as the acid dew point in °C,

pH2O as the partial pressure of water

and pH2SO4 as the partial pressure of

sulphuric acid, both in mmHg. The

results of the plant measurements are

shown in figure 6. The graph displays a

selection of measurements in

comparison with the theoretical acid

dew point at different temperatures.

Several measurements are well below

the acid isothermal curve of 130 °C.

Since in many cement plants the

exhaust temperature is below 130 °C,

especially in mill on operation,

precipitation of acid takes place.

Correspondingly under these

conditions the sulphuric acid is present

as a condensate and can cause active

corrosion. This supports the earlier

mentioned hypothesis that the observed

corrosion could be partially caused by

sulphuric acid.

Measures against low temperature corrosion in cement plants

Considering the measured amount of

sulphuric acid and compared to the

observed extend of corrosion it is to

consider that also other factors could

cause these partially heavy damages.

Cement plants without considerable

amounts of H2SO4 in the flue gases

show in some cases also severe

damages. This leads to the assumption

that the observed corrosion is not

solely caused by the presence of

sulphuric acid. During periods of time

of short or long down-times the

temperatures of the flue gases are

dropping considerably and water

condensates freely. In all likelihood

other corrosive compounds and

corrosive agents in the flue gases are

also taking part in the active metal

corrosion. But these corrosion

mechanisms are not discussed any

further, because it goes beyond the

scope of this paper.

In general three possibilities to avoid

low temperature corrosion caused by

H2SO4 condensation can be defined as

following measures:

electrochemical protection

influencing the properties of the

reactants and/or changing the

reaction conditions respectively

separation of the metal from the

corrosive environment

However not all mentioned possible

actions are economically feasible.

Especially the first measure is much

too costly and practically not

achievable. In regard of the second

point not much can be done effectively

too. It is not feasible to lower the SO3-

generation by using different sources

of raw materials, merely if alternative

materials contribute significantly to

low volatile sulphur input.

Figure 7: Protective coating in a bag house [5]

Also the conditions in the waste gas

path are somewhat fixed and cannot be

varied much. It´s only in cases when

SO2 abatement is applied, that this

could be adapted to SO3 minimization

by means of Ca(OH)2 injections

directly after the preheater. The

neutralisation reaction of SO3 works

chemically as of SO2. So by applying

effective SO2-reduction systems

immediately after the preheater, the

absolute amount of acids in the flue gas

can be reduced and therefore the

corrosive potential.

Special attention could be given to the

temperature level of the exhaust gas.

As long the temperature is well above

the correspondent acid dew point, no

acid will condensate and cause

corrosion. In some cases the increase

of the waste gas temperature of 10 °C

would be sufficient to avoid any

condensations. It´s also important to

make sure that a proper insulation of

all affected aggregates is maintained.

Therefore it is crucial to avoid any

false air leakages so that no low gas

temperatures can occur locally, which

could lead to major water and acid

condensations.

With the application of a protective

coating of the affected areas by organic

coatings an effective corrosion

protection can be achieved (cf.

figure 7) [4]. But this measure is also

cost-intensive and requires a high

Figure 8: Spalling of an organic coating because of too high mechanical stress [5]

amount of preparation before

implementation. The different

coefficients of thermal expansion of

the coating and metal have to be

considered too. So the surfaces of all

areas which are to be coated have to be

especially prepared to avoid localized

mechanical stress and spalling of the

coating (cf. figure 8).

The application of high alloyed acid

resistant steels is a last possible

measure to avoid high material losses.

But it also implicates high material and

erection costs which are in most cases

not expedient.

References

[1] Wickert K.: Chemische

Umsetzungen im Feuerraum der

Schmelzkammerkessel; BWK 9

(1957); pp. 105-118

[2] Rolker J.: Zur elektrischen

Messung des Säuretaupunktes von

SO3-haltigen Rauchgasen;

Dissertation; Universität Stuttgart;

1973

[3] Haase R., Borgmann H.-W.:

Präzisionsmessungen zur Ermittlung

von Säuretaupunkten; Mitteilungen der

VGB 76 (1962) 2; S. 16-19

[4] Mazeika L., Sherin B., Cardwell B.:

Repairing a corroded baghouse filter at

the Durkee cement plant by coating the

surface; Cement International 5 (2004)

2; pp. 100-103

[5] Personal information: Solnhofer

Portland-Zementwerke GmbH & Co.

KG, Germany