Kiln shell corrosion

Kiln shell corrosion

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Causes of kiln shell corrosion
• Composition of the kiln atmosphere

• Content of volatile components

• Alkalis ,chloidic,sulfur and vanadic compounds, water vapour
etc are the main volatile components predominating in the
kiln gas atmosphere of cement kiln systems.

• Alkalis and chloridic compounds are mainly taken into the kiln
by the kiln feed and the sulfur and vanadic compounds by the
fuel
• The combustion of alternate fuels and especially waste fuels
Increases the concentrations of volatile and further harmful
compounds within the kiln system accelerating shell corrosion.

• Attack by the back fly Ferrum Pedis.


Schematic mechanism of shell corrosion


Different shell corrosions

Three different kinds of kiln shell corrosion are observed in
Cement kiln systems

• Corrosion during kiln operation resulting in scaling of the
kiln shell by oxidation of the metallic iron at elevated
temperature ( so-called high temperature corrosion

• Rusting during longer kiln shut downs caused by condensation
or absorption of humid ( moist air , water)

• Wear of oxidised kiln shell during longer kiln shut downs caused
by small blackflies eating iron compounds ( so-called
Ferrum Pedis syndrome)


Corroded kiln shell samples ( high temperature corrosion)


Corroded kiln shell samples( rusting during longer kiln shut downs


Corrosion of corroded kiln shell by Ferrum Pedis


Corroded kiln shell


Kiln shell corrosion
• migration and efforescences of salts between brick work and kiln shell
• chemical attack of salts under kiln operating conditions (high thermal corrosion)
• depending on the alkali-sulfur ratio and oxygen partial pressure bi- and trivalent
iron oxides and / or iron sulfides are formed.


Pictures of corroded shell by
Optical microscope


Corrosion

1. Scaling : Oxidation by dry gases at
elevated temperature ( S , Cl )
2. Rusting : Oxidation by moist air or water
of condensation at low temperatures (
KCl )


Corrosion ( Study )

• 12 Kilns at four plants located relatively close
to one another have been studied.
• Clear indications were found of a relationship
between shell corrosion and the cement-making
process,
• Corrosion was found only in the eight
precalciner kilns studied.
• Suspension preheater kilns , showed no signs
of corrosion.


Macroscopic description of
the corrosion

• The fastest corrosion rates are found from
end of the coating over a distance of 5 * the
kiln dia towards the kiln inlet.
• The rate of corrosion then gradually
decreases towards the kiln inlet.
• There is hardly any corrosion on the burner
side of the coating.
• Fig. 1 illustrates the usual pattern of the
coating.


Corrosion scale morphology
and composition

Brick side : Brownish
Kiln shell side : Shiny dark brown or shiny
black


Corrosion scale morphology
and composition
Three types of corrosion scales were
observed
• Chloride-free, alkali-free scales
• Scales containing chloride without
alkalis
• Scales containing both chloride and
alkalis


Corrosion scale morphology and
composition

Chloride-free, alkali-free scales
• Alternating layers of iron oxides and sulphides
• The minerals found by X-ray diffraction were

1. Fe2O3 ( haematite )
2. Fe3O4 ( magnetite )
3. FeS2 ( pyrite )
4. FeS ( pyrrhotite )

Corrosion scale
morphology and
composition
Scales containing chloride without alkalis
• High sulphur layers alternate with high oxygen
layers
• With presence of chloride
• Quantitative microanalysis of corrosion powder
( % w/w ) : Iron : 74 , Sulphur : 15 , Chloride :
2.5,
Potassium : 0.5

Corrosion scale morphology and
composition

Scales containing both chloride and alkalis

•Quantitative microanalysis of corrosion powder
( % w/w ) : Iron : 64 , Sulphur : 14 , Chloride : 12
Potassium : 6.5


Mechanisms important to kiln shell
corrosion

1. Oxidation
2. Sulphidization
3. Hot corrosion


corrosion

Oxidation
In an oxidising atmosphere the iron from the
steel shell will react with oxygen to form
an oxide scale. Compound with highest oxygen
content , Fe2O3 being found at the scale-brick
interface , and the compound with highest iron
content , FeO at the metal-scale interface .


Mechanisms important to
kiln shell corrosion

Sulphidization
Where no oxygen is present , SO2 takes over
as the oxygen donor and a different reaction
occurs as follows :
4 Fe + 2 SO2 ( g ) = Fe3O4 + FeS2
Suphidization is enhanced by the presence of
chlorides , mainly because they affect the morphology
of corrosion scale ,hindering the formation of a strong
, protective oxide layer.

corrosion

Sulphidization
1. High temperature hydrolysis of the thermally unstable alkali
chlorides to form the more stable sulphates.
2. Followed by re-oxidation of hydrogen chloride gas ( by oxygen
or SO2 ) at lower temperatures to elemental chlorine which
attacks the metal.
2 KCl ( g) + H2O ( g ) + SO2 ( g ) + ½ O2 (g ) =
K2SO4 + 2 HCl ( g ) ( T < 900 0C )
2 HCl ( g ) + O2 ( g ) = Cl2 ( g ) + H2O ( T< 400 0C )


corrosion

Sulphidization
1. In preheater kilns , chlorides evaporates during
or shortly after calcination without substantial
formation of hydrogen chloride gas.
2. Kilns with tertiary air duct, show delayed alkali
chloride evaporation and, consequently,
evaporation will be followed by more extensive
hydrolysis of chlorides.

corrosion
Sulphidization
• Once Cl2 ( g ) is formed, it can reach the kiln shell
through the protective refractory bricks or through
cracks and fissures in the lining, and will react with
either the oxide-sulphide layers or, most likely,
directly with the kiln shell according to the following
reactions :
- reaction with the oxide-sulphide layers
FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 +
O2
- reaction with the kiln shell
Cl2 + Fe = FeCl2

corrosion

Hot corrosion
• Alkalis can probably only penetrate the lining
as part of a liquid potassium and/or sodium salt
melt.
• If corrosion products therefore contain
substantial quantities of potassium or sodium
the form of corrosion is termed as Hot
corrosion, indicating that a liquid phase takes
part in the corrosion reactions.

Measures to prevent kiln shell corrosion

1. Apply gas-tight coatings on the kiln shell.
2. Maintain a full size dense lining.
3. Keep a high oxygen surplus in the kiln.
4. To cut down on chloride and sulphur input to
the kiln.
5. To increase kiln chloride valve : to reduce
burning zone temperature.

Kiln shell corrosion

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Kiln shell corrosion