UDC 669.14.018.8

KINETICS OF GAS CORROSION OF AUSTENITIC STEEL 12Kh18N10T

E. Yu. Priimak,1 V. I. Gryzunov,1 and T. I. Gryzunova,1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 21 24, September, 2009.

The process of gas corrosion of steel 12Kh18N10T at a temperature of 900C is studied. The kinetic condi-

tions of formation and growth of the oxide layer are analyzed.

Keywords: stainless steel, gas corrosion, oxide layer, high-temperature strength.

INTRODUCTION

Chromium-nickel steels of type 18-10 are widely used as

stainless, heat-resistant and refractory materials. Steel

12Kh18N10T, which is single-phase in hardened condition,

becomes a two-phase one after heating due to segregation of

carbide particles. This is accompanied by a change in the

operating characteristics of the steel. The action of air on the

heated metal causes formation of scale. A film of the prod-

ucts of reaction between the constituent elements and the ad-

sorbed oxygen from the gas medium forms on the surface.

The oxidation reaction occurring on the boundary of the

metal is a heterogeneous one. For the most frequent process

of gas corrosion the reaction has the form

mMe +mn

4O

2= Me

mO

mn2 .

Modern physicochemical methods make it possible to

consider in detail the process of oxidation of the steel and its

mechanism. The aim of the present work consisted in analyz-

ing the structure and phase composition of oxidized layer on

high-alloy steel and studying the kinetics of its formation and

growth.

METHODS OF STUDY

We chose chromium-nickel austenitic steel 12Kh18N10T

preliminarily quenched in oil for solving our task. The steel

had the following chemical composition (wt.%): 0.12 C, 17.9

Cr, 10.2 Ni, 1.0 Mn, 0.8 Si, 0.4 Ti, 0.025 S, 0.03 P.

The steel was tested for high-temperature strength under

conditions of a still air atmosphere by standard methods

(GOST 613071). The high-temperature strength was evalu-

ated in terms of growth in the mass of the specimens during

the test. The specimens were measured, weighed, placed into

a crucible, and held for 5, 20, and 50 h at 900C in a muffle

furnace. The specimens withdrawn from the furnace were

cooled in air. The oxidized specimens were subjected to a

metallographic study using a JEOL JSM-6469LV scanning

electron microscope (Oxford Instruments) in the mode of

reflected electrons at accelerating voltage of 20 kV. X-ray

phase analysis of the oxidation products was performed by

the Debye-Scherer method also known as a powder method.

For this purpose scale was removed accurately from the oxi-

dized specimens and crushed in an agate mortar under a layer

of ethyl alcohol in order to obtain fine dispersed powder. Af-

ter drying, the powder was pressed at a pressure of

50 5 MPa in a hydraulic press into tablets 10 mm in diame-

ter and 5 mm thick. The x-ray phase analysis was performed

with the help of a DRON-2 diffractometer in copper K

radi-

ation in angle range 2 = 10 120. The diffractograms were

deciphered and the phases identified by the method of com-

parison of experimental data with tabulated ones.

RESULTS AND DISCUSSION

Figure 1 presents the results of measurement of growth

in the mass of specimens as a function of the time of hold in

oxidizing atmosphere. The form of curve m = f () plotted

from experimental data can hardly be used for determining

the law of growth of the oxide layer with time. It was ob-

tained by smoothing the curve with the help of a

log m = f (log ) functional grid. Then the plot was used for

determining the constant coefficients of the corresponding

empirical equation. In this way we established that the de-

Metal Science and Heat Treatment Vol. 51, Nos. 9 10, 2009

429

0026-0673/09/0910-0429 2009 Springer Science + Business Media, Inc.

1Orsk Liberal-Technological Institute, Orsk, Russia

(E-mail: elena-pijjmak@yandex.ru).

pendence of the rate of oxidation of steel 12Kh18N10T on

the duration of heating at 900C obeys a power law

m2.39

= 11.48. (1)

In high-temperature heating of the steel in air atmosphere

the oxidation of the surface is accompanied by internal oxi-

dation as a result of which regions in the form of oxide

chains over grain boundaries appear under the scale (Fig. 2).

The oxide film formed on steel 12Kh18N10T has a com-

posite structure and consists of several layers. An x-ray

phase analysis has shown the presence of the following

phases in the scale: Fe2O

3, Fe

3O

4, FeO, (Ni, Cr)

2O

4, and

Cr2O

3. The data of the x-ray phase and spectrum analyses

(Table 1) allow us to conclude that the external part of the

scale consists of three layers, namely, an external layer of

Fe2O

3hematite, a layer of Fe

3O

4magnetite, and then a layer

of FeO wstite. After the wstite layer goes an internal layer

adjoining the matrix metal, which consists of spinel with

complex composition, wstite, and chromium oxide. Chro-

mium and nickel concentrate only in the internal layer of the

scale and are absent in the external layers. When the speci-

men is cooled, the scale partially chips-off due to the appear-

ance of internal stresses. For this reason we do not observe

Fe2O

3and Fe

3O

4phases in the photographs of microstructure

of the oxidized layer (Fig. 2).

The mechanism of oxidation of the metal is complex and

develops in the following stages: (1 ) adsorption of oxygen

on the surface of the metal, (2 ) transition of oxygen from

molecular state into atomic one, (3 ) chemical reaction be-

tween the adsorbed oxygen and the metal, and (4 ) growth in

the thickness of the oxide layer.

The process of adsorption of oxygen on the surface of

the metal is describable by the Langmuir equation

=bC

bC1, (2)

where is the fraction of occupied sites, C is the concentra-

tion of oxygen, and b is a factor.

The rate of oxidation is proportional to the concentration

of oxygen in the adsorbed layer or to the fraction of the occu-

pied surface of the metal, i.e.,

C

t= k, (3)

where C is the concentration of oxygen, t is the time, and k is

a proportionality factor.

Substituting (2) into (3) we obtain

C

t=

kbC

bC1. (4)

At low concentrations the term bC in expression 1 + bC

can be neglected, because bC 1. Then it follows from

Eq. (4) that

C

t= kbC. (5)

430 E. Yu. Priimak et al.

0.015

0.010

0.005

0 20 40 60

, h

m, kg m 2

Fig. 1. Dependence of growth in the mass of a specimen on the hold

time at 900C in the process of oxidation of steel 12Kh18N10T in air. 60 m

1 2 3 4 5 6 78 9

Fig. 2. Structure of oxide layer on steel 12Kh18N10T after high-

temperature oxidation at 900C for 50 h. The numbers mark the

places of recording of x-ray spectra (see Table 1); the arrow points in

the direction from the surface of the specimen to its core, 500.

TABLE 1. Distribution of Elements in the Oxide Layer of a Speci-

men of Steel 12Kh18N10T after High-Temperature Oxidation

(900C, 50 h)

Spec-

trum*

Content of elements, at.%

O Si Ti Cr Mb Fe Ni

1 46.16 0.09 0.86 52.89

2 44.99 1.22 53.79

3 41.81 1.32 0.79 56.07

4 43.88 1.00 4.66 0.79 46.35 3.31

5 34.21 1.08 30.18 1.21 23.19 10.13

6 47.33 0.53 20.66 1.97 17.45 12.06

7 44.76 11.80 1.45 40.34 1.65

8 14.84 70.84 14.32

9 1.07 1.82 18.21 0.83 68.55 9.53

*See Fig. 2.

We denote kb = K. Then, with allowance for Eq. (5), ex-

pression (4) can be written as

C

t= KC, (6)

where K is a constant of the rate of the chemical reaction.

When a metal is oxidized, the reaction on the surface oc-

curs as a first-order one. Oxygen reacts simultaneously with

iron, nickel, and chromium forming Fe3O

4, (Ni, Cr)

2O

4, and

Cr2O

3. These reactions occur simultaneously but at different

rates, i.e.,

O2

2O

Fe O

Ni, Cr O

Cr O

2 3

2 4

2 3

( ) . (7)

Such reactions develop in parallel. Then we have the fol-

lowing system instead of Eq. (6):

K CC

t

K CC

t

K CC

t

1

1

2

2

3

3

, (8)

where C1

, C2

, and C3

represent the atomic fractions of oxy-

gen (in %) in the respective compounds.

With allowance for the law of conservation of substance

the rate of transformation in all the directions is equal to the

sum of the rates

(K1

+ K2

+ K3

)C =

C

t, (9)

whence

C = C0

e Kt

, (10)

where K = K1

+ K2

+ K3

.

Thus, the process on the surface of the metal is not only

adsorption of oxygen but also chemisorption. The thickness

of the resulting thin film can be evaluated on the basis of the

following considerations. The concentration of oxygen ad-

sorbed on the surface of the metal remains invariable. How-

ever, penetration of oxygen atoms into the depth gives rise to

a certain gradient of concentrations of the reacting sub-

stances. Oxygen atoms are transported by diffusion in accor-

dance with Ficks equation

DC

x

C

t

2

2 , (11)

where D is the diffusivity of oxygen in the metal and C is its

concentration.

Substituting the right-hand part of expression (11) into

(6) we obtain

DC

x

2

2= KC, (12)

whence we have

C = C0

e

K

D

x

. (13)

Using equation (13) we can evaluate the effective depth

of penetration of oxygen due to chemisorption

L =D

K. (14)

At 900C the diffusivity of oxygen in the metal

D = 1.1 10 18 m2sec [1]. The value of the constant of thechemical reaction is calculated from (10), i.e., K =

2.6 10 1 sec 1. Thus, the thickness of the chemisorbed

layer of oxygen in the metal is 162 nm.

The values of K1

, K2

, and K3

can be found by integrating

Eq. (8) and using the initial conditions t = 0, C01

= C02

=

C03

= 0. We have

C CK

Ke

C CK

Ke

C CK

Ke

Kt

Kt

Kt

1 0

1

2 0

2

3 0

3

1

1

1

( )

( )

( )

. (15)

Whence we obtain K1

= K3

= 3.8 10 6 sec 1, K2

=

1.84 10 5 sec 1.

After formation of a chemisorbed layer of oxygen the

oxidation process develops due to the feed of iron, nickel,

and chromium atoms, the diffusion mobility of which at this

temperature is an order of magnitude higher. The occurring

reactive diffusion yields individual layers of (Fe, Ni, Cr)2O

4,

FeO, and Fe3O

4+ Fe

2O

3.

Since the diffusivity of iron, nickel, and chromium atoms

is on the order of 10 15 m2sec [2] and the constants of therate of chemical reaction of formation of these compounds

are equal to 10 7 10 6 sec 1, we infer that the process oc-

curs in the diffusion range. However, the growth of layers in

time should differ from parabolic, because the components

affect each other in the oxidation process.

The effective diffusivity in the ith phase, which deter-

mines the rate of its growth, depends on the proportion of the

concentrations of the neighbor phases [3, 4], i.e.,

Di=

1

21

t

x

yx

i

i

ij i

j

n

, (16)

where Diis the diffusivity of atoms in the ith phase, x

iis the

Kinetics of Gas Corrosion of Austenitic Steel 12Kh18N10T 431

length of the layer with the ith phase, and yi

is the reduced

concentration determined from the equation

y =C C

C C

i

(17)

(here C and C are the initial concentrations of elements on

the boundary of scale). The value of ij

is determined from

the formulas

ij=

( )

[ ( ) ( ) ( )]

( )

1

1

41 1 2 1

1

2

y y

y y y y y y

y y

i j

ij i i i i i

i j

j i

j i

j i

(18)

If we introduce a constant of phase growth using a para-

bolic law, we arrive at

i

2=

x

t

i

2

, (19)

Then we find from Eq. (19) that

Di=

x

t y

x

xK

i ij

i

ij

i

i

jj

n

i

2

1

2

2

, (20)

where depends on the parameters of all the phases forming

in the oxidation process. The value of is not constant and

differs from unity. In this connection we cannot speak of di-

rect proportionality between the time and the squared thick-

ness of the layer, though the mechanism of growth of the oxi-

de film is determined by the diffusion feed of atoms to the re-

action zone.

We evaluated the length of each layer formed in scale

during a hold at 900C (Table 2). The diffusivity of the me-

tals in the corresponding oxides has been taken from [1, 2].

According to our computations the total thickness of the

scale is 117.8 m, which corresponds to the data of direct

measurements performed with the help of scanning electron

microscope.

CONCLUSIONS

1. An oxide film consisting of layers of (Fe, Ni, Cr)2O

4,

FeO, and Fe3O

4+ Fe

2O

4forms on the surface of steel

12Kh18N10T in the process of gas corrosion in air atmo-

sphere.

2. Considering the mechanism of oxidation of the steel

we established the occurrence of chemisorption on the sur-

face of the metal.

3. Growth of individual layers in the scale occurs by dif-

fusion mechanism though it differs from a parabolic law in

time.

4. We computed the thickness of the scale and of its indi-

vidual layers and its variation in the process of gas corrosion.

The computed values were close to the experimental results.

REFERENCES

1. O. Kubashewski and B. Hopkins, Oxidation of Metals and Alloys

[Russian translation], Metallurgiya, Moscow (1965).

2. N. M. Baron, E. I. Kvyat, and E. A. Podgornaya, A Brief Hand-

book of Physicochemical Quantities [in Russian], Khimiya,

Moscow Leningrad (1965).

3. K. P. Gurov, B. A. Kartashkin, and Yu. E. Ugaste, Interdiffusion

in Multiphase Metallic Systems [in Russian], Nauka, Moscow

(1981).

4. V. I. Gryzunov, S. V. Kirilenko, and V. I. Polukhina, The Kinetics

of Chemical Heterogeneous Reactions in Solid Phases [in Rus-

sian], OGTI, Orsk (2007).

432 E. Yu. Priimak et al.

TABLE 2. Parameters of Diffusion and Thickness of the Layer of

Oxide Film after High-Temperature Oxidation of Steel 12Kh18N10T

(900C, 50 h)

Compound D, m2sec K, sec 1 L, m

(Fe, Ni, Cr)2O

4 2.12 10 15

2.36 10 6

20.8

FeO 5.6 10 15

3.8 10 6

24.5

Fe3O

4+ Fe

2O

3 1.6 10 15

4.2 10 6

72.5

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