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Portugaliae Electrochimica Acta 25 (2007) 237-248
PORTUGALIAE
ELECTROCHIMICA
ACTA
Pitting Corrosion of Some Stainless Steel Alloys
Preoxidized at Different Conditions
S.S. Mahmoud
*
and M.M. Ahmed
a
Chemistry Department, University College of Girls for Arts, Science and Education,
Ain Shams University, Heliopolis, Cairo, Egypt
Received 8 February 2006; accepted 30 April 2007
Abstract
The pitting corrosion of some stainless steel alloys (preoxidized at different conditions)
in 3.5% NaCl solution was studied. The alloys are: one ferritic (15.05% Cr) (alloy1) and
two austenitic stainless steel alloys (17.9% Cr,7.08% Ni) (alloy2) and (20.45% Cr, 8.3%
Ni) (alloy3). Potentiodynamic anodic polarization and galvanic current-time
measurements were used in these investigations. The susceptibility of the alloys to
pitting corrosion decreases with the increase of chromium content of the alloy and with
the presence of nickel in the alloy. The preoxidation of the alloys in different media
improves their resistance to pitting corrosion in NaCl solution. The resistance to pitting
corrosion for the investigated alloy increases according to the order: no oxidation <
oxidation in air < oxidation in molten alkali nitrates < oxidation in molten alkali
carbonates. This resistance to pitting corrosion may be due to the formation of a
protective oxide film on the alloys’ surface. The composition of this film greatly
depends on the chemical composition of the alloy, on the condition of the preoxidation
process, and on the temperature.
Keywords: stainless steel alloys, pitting corrosion, molten alkali nitrates, molten alkali
carbonates.
Introduction
The corrosion resistance of stainless steel is due to a 20-40 Å
passive protective
film which consists mainly of Cr
2
O
3
. It has been reported that the high
temperature oxide film produced during soldering and stress relieving operations
has reduced localized corrosion resistance. Bianchi et al. [1] observed that there
is a close relationship between susceptibility to pitting and electronic properties
of the oxide film produced at lower temperature. The n-type semiconductor
oxides are more susceptible to pitting than their p- type counter parts. However,
*
Corresponding author. E-mail address: : drsohairr@hotmail.com
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
238
Kearns [2] attributed the higher pitting susceptibility of oxides formed at higher
temperatures to chromium depletion in the alloy substrate beneath the oxide.
Rastogi et al [3]
attributed the increased pitting susceptibility of oxidized
stainless steel specimens at 500
o
C to the change of chemical composition of the
oxide film i.e. lowering of Cr/Fe concentration ratio.
Pitting corrosion can be minimized by means of many ways. One of these ways
is the modification of the metallic surface by its oxidation and formation of
passive oxide films. These films can be formed by oxidation in air at high
temperatures and /or in molten oxyanion salts (such as molten alkali nitrates and
alkali carbonates).
In this paper the effect of composition of stainless steel alloys and the conditions
of preoxidation process on the pitting corrosion of some stainless steel alloys
were studied. The stainless steel alloys are: one ferritic and two austenitic
stainless steel alloys. The preoxidation was performed in air at high temperatures,
in molten alkali nitrates and in molten alkali carbonates. After the preoxidation of
the alloys their pitting corrosion behaviour in 3.5% NaCl was tested.
Experimental
Three specimens of the two types of stainless steel have been prepared and used
in these investigations. The first one is from ferritic steel (15.05%Cr) (alloy1).
The other two are from austenitic steel, one with (17.9%Cr + 7.08%Ni) (alloy2)
and the other (20.45%Cr + 8.3%Ni) (alloy3). The samples were treated at1050
o
C
for 15 minutes in the presence of N
2
gas followed by rapid quenching in water to
prevent any sensation to intercrystalline corrosion, and all carbides have then
gone into solution [4]. After this treatment the samples were pickled in a mixture
solution of 100 mL of concentrated HCl, 100 mL of distilled water and 10 mL of
concentrated HNO
3
at 60
o
C for 20 minutes. The samples were cleaned, washed
thoroughly with distilled water and dried.
The samples were treated at temperatures of 500 and 550
o
C in each of the
following media: 1-in air, 2- in molten nitrate and 3- in molten carbonate.
In air the samples were heated at temperatures of 500 and 550
o
C for one hour and
cooled to room temperature.
The NaNO
3
-KNO
3
eutectic (AR grade, molar ratio 50:50; melting point of 225
o
C) was prepared and dried as previously described [5-7]. Experiments of sample
treatment were carried out as previously described [5-7]. The experiments of
sample treatment in nitrate melt at temperatures of 500 and 550
o
C were
prolonged till reaching the steady- state potential for the samples under open-
circuit conditions.
The ternary Li
2
CO
3
-Na
2
CO
3
-K
2
CrO
3
(43.5 : 31.5 : 25 mole%, respectively)
mixture was prepared and dried as previously recommended [8-10]. This mixture
of alkali carbonates is of interest in oxidation studies because of its low melting
point (397
o
C) and good electrical properties. The experiments of sample
treatment in molten carbonate were carried out as previously described [8-10].
These experiments were carried out at temperatures of 500 and 550
o
C till
reaching the steady-state potential under the open-circuit conditions.
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
239
The pitting corrosion tests in 3.5%NaCl were carried out on the samples which
are: without oxidation, oxidized in air, oxidized in nitrate melt, oxidized in
carbonate melt. The techniques of measurements used in these tests were: i-
potentiodynamic anodic polarization with scan rate of 1 mV/sec., ii- galvanic
current-time. A platinum electrode and a saturated calomel electrode were used
as auxiliary and reference, respectively. The anodic polarization measurements
were carried with a Wenking potentiostat (model POS 73). The galvanic current
was measured on digital multimeter(model 1008,Kyoritsu Electrical Instruments
work, LTD Japan).
Results and discussion
Fig. 1 represents the potentiodynamic anodic polarization curves for the
investigated alloys (1,2,3) immersed in 3.5% NaCl solution. It is clear from these
plots that no active dissolution of oxidation peak was observed during the anodic
scan. The current flowing along the passive region increases suddenly and
markedly at some definite potentials denoting the initiation of visible pits on the
electrode surface. These potentials are called pitting potentials, E
pit
; the value of
E
pit
was deduced from the plots of Fig. 1 for the alloys (1,2,3) and listed in table
1. The value of E
pit
shifts to the more positive direction according to the order:
321
→
→
.
Figure 1. Potentiodynamic anodic polarization curves of the stainless steel alloys in 3.5
% NaCl solution: 1- alloy 1, 2- alloy 2, 3- alloy 3.
Fig. 2 represents the potentiodynamic anodic polarization curves for the alloys
(1,2,3) preoxidized in air at 500
o
C and immersed in 3.5% NaCl solution.
Similar curves are obtained for the alloys preoxidized at 550
o
C, but not shown.
The values of E
pit
are deduced from the plots of Fig. 2 and similar ones and listed
in table 1. It is clear from these results that the values of E
pit
for the alloys
preoxidized at 550
o
C are more positive than for preoxidation at 500
o
C. Also, the
value of E
pit
shifts to the more positive direction according to the order:
321
→
→
.
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
240
Table 1. Values of pitting potential, E
pit
, for the alloys in 3.5% NaCl solution at
different conditions.
E
pit
(mV)
Oxidation in air Oxidation in
nitrate melt
Oxidation in
carbonate melt
Alloy
No
oxidation
500
o
C
550
o
C 500
o
C 500
o
C 500
o
C 550
o
C
1 50 250 300 400 460 600 650
2 150 350 390 500 580 720 790
3 200 400 440 560 620 820 900
Figure 2. Potentiodynamic anodic polarization curves of the stainless steel alloys
(preoxidized in air at 500 ºC) in 3.5 % NaCl solution: 1- alloy 1, 2- alloy 2, 3- alloy 3.
Fig. 3 represents the potentiodynamic anodic polarization curves for the
investigated alloys (1,2,3), preoxidized in nitrate melt at 500
o
C, immersed in
3.5% NaCl solution. Similar curves are obtained for the alloys preoxidized at 550
o
C, but not shown. From the plots of Fig. 3 and similar ones the values of E
pit
are
deduced for the three alloys and listed in table 1. These results indicate that the
values of E
pit
for the alloys preoxidized at 550
o
C are more positive than for the
preoxidation at 500
o
C. Also, the value of E
pit
for the alloys shifts to the more
positive direction according to the order:
321
→
→
.
Fig. 4 represents the potentiodynamic anodic polarization curves for the
investigated alloys (1,2,3) preoxidized in carbonate melt at 500
o
C, immersed in
3.5% NaCl solution. Similar curves are obtained for the alloys preoxidized at 550
o
C in carbonate melt, but not shown. From the plots of Fig. 4 and similar ones the
values of E
pit
for the alloys are deduced and listed in table 1. The obtained results
indicate that the values of E
pit
for the alloys preoxidized at 550
o
C in carbonate
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
241
melt are more positive than for the preoxidation at 500
o
C. Also, the value of E
pit
for the investigated alloys shifts to the more positive direction according to the
order:
321
→
→
.
Figure 3. Potentiodynamic anodic polarization curves of the stainless steel alloys
(preoxidized in molten alkali nitrate at 500 ºC) in 3.5 % NaCl solution: 1- alloy 1, 2-
alloy 2, 3- alloy 3.
Figure 4. Potentiodynamic anodic polarization curves of the stainless steel alloys
(preoxidized in molten alkali carbonate at 500 ºC) in 3.5 % NaCl solution: 1- alloy 1, 2-
alloy 2, 3- alloy 3.
Fig. 5 shows a schematic presentation for the results of E
pit
obtained for the alloys
without oxidation and preoxidation in each of air, nitrate melt, carbonate melt at
500
o
C. Similar plots are obtained for the alloys preoxidized at 550
o
C, but not
shown. The results of table 1 and the plots of Fig. 5 and similar ones indicate that
the value of E
pit
for the investigated alloys shifts to the more positive direction
according to the order: no oxidation
→
oxidation in air
→
oxidation in nitrate
melt
→
oxidation in carbonate melt. Also, at the given conditions the values of
E
pit
for the alloys (1,2,3) preoxidized at 550
o
C are more positive than those
preoxidized at 500
o
C.
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
242
Figure 5. Pitting corrosion potential, E
pit
, for the stainless steel alloys (1, 2 and 3) in 3.5
% NaCl solution, at different conditions of treatment: A- no oxidation, B- oxidation in
air, C- oxidation in molten alkali nitrates, D- oxidation in molten alkali carbonates.
Fig. 6 represents the variation of the galvanic current as a function of exposure
time for the investigated alloys (1,2,3) immersed in 3.5% NaCl solution. These
plots indicate that there is an induction period required for the initiation and
propagation of the pitting corrosion on the surface of the electrode, as indicated
by a sudden increase in the galvanic current and appearance of fluctuations on
the curves. The value of the induction was deduced from the plots of Fig. 6 for
each alloy and listed in table 2. It is clear form the obtained results that the value
of the induction period greatly deqends on the type of the alloy and increases
according to the order: 1 < 2 < 3.
Figure 6. Galvanic current-time plots for the stainless steel alloys in 3.5 % NaCl
solution: 1- alloy 1, 2- alloy 2, 3- alloy 3.
Fig. 7 represents the variation of the galvanic as a function of exposure time for
the alloys, preoxidized in air at 500
o
C, immersed in 3.5% NaCl solution. Similar
plots are obtained for the alloys preoxidized in air at 550
o
C, but not shown. The
values of the induction period for the alloys were deduced from the plots of Fig.
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
243
7 and similar ones and listed in table 2. It is found that the induction period for
the investigated alloys increases according to the order: 1 < 2 < 3.
Fig. 8 represents the variation of the galvanic current as a function of exposure
time for the investigated alloys (1,2,3) immersed in 3.5% NaCl solution after
their oxidation in the nitrate melt at a temperature of 500
o
C. Similar plots are
obtained for the alloys (1,2,3) preoxidized at 550
o
C, but not shown. The values
of the induction period are deduced from the plots of Fig. 8 and similar ones and
listed in table 2. The obtained results indicate that the value of induction period
increases according to the order: 1 < 2 < 3.
Table 2. Values of induction period for the alloys in 3.5% NaCl solution at different
conditions.
Induction period / min
Oxidation in air Oxidation in
nitrate melt
Oxidation in
carbonate melt
Alloy
No
oxidation 500
o
C 550
o
C 500
o
C 550
o
C
500
o
C 550
o
C
1 430 600 685 700 785 830 920
2 500 680 760 780 860 930 1000
3 550 730 820 830 920 990 1090
Figure 7. Galvanic current-time plots for the stainless steel alloys (preoxidized in air at
500 ºC) in 3.5 % NaCl solution: 1- alloy 1, 2- alloy 2, 3- alloy 3.
Fig. 9 represents the variation of the galvanic current as a function of exposure
time for the investigated alloys (1,2,3) immersed in 3.5% NaCl solution after
their oxidation in carbonate melt at a temperature of 500
o
C. Similar plots are
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
244
obtained for the alloys (1,2,3) preoxidized at 500
o
C, but not shown. The values
of the induction period are deduced from the plots of Fig. 9 and similar ones and
listed in table 2. The obtained results indicate that the value of the induction
period increases according to the order: 1 < 2 < 3.
Figure 8. Galvanic current-time plots for the stainless steel alloys (preoxidized in
molten alkali nitrates) in 3.5 % NaCl solution: 1- alloy 1, 2- alloy 2, 3- alloy 3.
Fig. 10 shows a schematic representation of the results of the induction period
obtained for the alloys (1,2,3) without oxidation and preoxidation at the
temperature of 500
o
C in air, molten nitrate and molten carbonate. Similar plots
are obtained for the alloys (1,2,3) preoxidized at 550
o
C,but not shown. The
results listed in table 2 and the plots of Fig. 10 and similar ones indicate that the
value of induction period increases according to the order: no oxidation <
oxidation in air < oxidation in nitrate melt < oxidation in carbonate melt. Also at
those conditions the value of induction period for the alloys (1,2,3) preoxidized
at 550
o
C are longer than that for the alloys preoxidized at 500
o
C.
Pitting corrosion is a localized attack of metals and alloys which occurs as a
result of the breakdown of the otherwise protective passive film on the metal
surface. Many engineering metals and alloys such as stainless steel are useful due
to the formation of passive oxide films. This passive state in damaged and pitting
corrosion occurs when these alloys are polarized above some potentials in
environments containing certain aggressive agents such as chloride. Pitting
corrosion can be minimized by means of many ways. One of these ways is the
the modification of the metallic surface by its oxidation and formation of passive
oxide films. These films can be formed by oxidation in air at high temperature
and/ or in molten oxyanion salts (such as molten nitrates and molten carbonates).
Molten alkali nitrate eutectics are simple and convenient media for oxidation of
metals and alloys because the oxide ion activity at temperature > 500 K is
sufficiently high [11]. In molten nitrate the oxygen ions are originated by self-
dissociation according to the equilibrium:
NO
−
3
NO
+
2
+ O
2-
(1)
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
245
Figure 9. Galvanic current-time plots for the stainless steel alloys (preoxidized in
molten alkali carbonates) in 3.5 % NaCl solution: 1- alloy 1, 2- alloy 2, 3- alloy 3.
Figure 10. Induction period for the stainless steel alloys (1, 2 and 3) in 3.5 % NaCl
solution, at different conditions of treatment: A- no oxidation, B- oxidation in air, C-
oxidation in molten alkali nitrates, D- oxidation in molten alkali carbonates.
The concentration of oxygen ions can be calculated from the equilibrium
constants. Marchiano and Arvia have worked out Pourbaix-type potential versus
pO
2-
(pO
2-
= - log [O
2-
]) diagrams for molten nitrates and nitrites [12]. It is seen
from these diagrams that the various cathodic reactions leading to the formation
of oxygen ions occur at negative potential – 1.0 V. It was found that the
corrosion potential during the oxidation of the investigated stainless steel alloys
(1,2,3) in molten nitrate melt ranged from –1.10 to –0.25 V depending on
temperature [13]. Therefore the oxygen ions,O
2-
, and nitryl ions, NO
+
2
,play an
important role in the oxidation process.
The reaction leading to the oxidation of these alloys thus:
M M
n+
+ ne (2)
M
n+
+ n/2 O
2-
MO
n/2
(3)
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
246
while the cathodic reaction is:
NO
+
2
+ e NO
2
(4)
According to the postulates of electrochemical theory, the anodic processes may
be metal dissolution and / or anodic barrier layer formation beside the oxidation
of the solvent. Barrier layer may be formed by a dissolution-precipitation
mechanism and/or a solid state mechanism.
The results of x-ray diffraction analysis [6] carried out on stainless steel alloys
immersed in molten nitrate under open-circuit conditions indicate that the
composition and percentage of different phases formed on the surface greatly
depends on the type of the stainless steel alloy. For alloy (1) the phases formed
are: FeO, Fe
3
O
4
, FeCr
2
O
4
spinel. While for alloys (2,3) the phases formed are:
FeO, Fe
3
O
4
, FeCr
2
O
4
, NiCr
2
O
4
and NiFe
2
O
4
spinels.
In molten carbonate the oxide ions are originated by self-dissociation according
to the equilibrium:
CO
−2
3
CO
2
+ O
2-
(5)
this reaction is responsible for the presence of oxide ions in the carbonate melt .
In Lux– Flood acid-base properties, CO
2
is the acid and O
2-
is the base. It can be
assumed that the oxide ions, O
2-
, and carbonate ions, CO
−2
3
, play an important
role in the oxidation process of the stainless steel alloys in the carbonate melt.
Thus the reaction leading to the oxidation of different metals in the alloy may be
represented by the following equations:
M +n/2 O
2-
MO
n/2
+ ne (6)
and / or
M + n/2 CO
−2
3
MO
n/2
+ n/2 CO
2
+ ne (7)
in the case of carbonate melt and the investigated alloys (1,2,3), it can be proposed, as in
previous works [8,14,15], that the oxidation and passivation of the alloys may proceed
according to the following reactions:
Cr + Li
+
+ 2CO
−2
3
→
LiCrO
2
+ 2CO
2
+ 2e (8)
Fe + CO
−2
3
→
FeO
ss
+CO
2
+ 2e
(9)
Fe + Li
+
+ 2CO
−2
3
→
LiFeO
2 (ss)
+ 2CO
2
+ 3e (10)
A cubic solid solution (ss) Fe and LiFeO
2
is formed. However FeO is stable only
above 570
o
C, therefore FeO may decompose to Fe
3
O
4
and, apparently Fe
2
O
3
at
temperatures lower than 570
o
C as :
3FeO + O
2-
= Fe
3
O
4
+ 2e (11)
3Fe
3
O
4
+ O
2-
= 2Fe
2
O
3
+ 2e (12)
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
247
The possible cathodic reactions in molten carbonate may be:
3CO
2
+ 4e
→
C + 2CO
−2
3
(13)
CO
−2
3
+ 4e
→
C + 3O
2-
(14)
and /or
CO
2
+ 2e
→
CO + CO
−2
3
(15)
CO
−2
3
+ 2e
→
CO + 2O
2-
(16)
As above mentioned and according to the postulate of the electrochemical theory,
the anodic process may be the metal dissolution and / or anodic barrier
formation. Barrier layer may by formed by dissolution-precipitation mechanism
and / or solid state mechanism.
Also, the formation of LiFe
5
O
8
and α-Fe
2
O
3
may occur according to the reactions
[9]:
LiFeO
2
+ 4Fe + 6CO
−2
3
→
LiFe
5
O
8
+ 6CO
2
+ 12e (17)
2Fe + 3O
2-
→
Fe
2
O
3
+ 6e (18)
The stainless steel alloys (2,3) containing Ni are characterized by the formation
of NiO according to the reaction:
Ni + CO
−2
3
→
NiO + CO
2
+ 2e (19)
From the above mentioned results and discussion it was indicated that the
susceptibility of the investigated alloys (1,2,3) to pitting corrosion in NaCl
solution greatly depends on their chemical composition and conditions of their
surface treatment. The increase of chromium content of the alloy and the
presence of nickel in the alloy increase the resistance of the alloy to pitting
corrosion. This may be due to the presence of higher content of chromium oxide
in the surface film. Also, the preoxidation of the alloys in different media offers
good resistance of their surface to pitting corrosion. It seems that the composition
of oxide film formed on the alloys surface greatly depends on the preoxidation
medium and its temperature. For this reason the resistance of the alloys (1,2,3)
greatly depends on the conditions of the preoxidation of their surface.
Conclusions
1 -The resistance of the alloy to pitting corrosion in NaCl solution greatly
depends on its chemical composition. This resistance increases with the increase
of the chromium content of the alloy and the presence of nickel.
2- The preoxidation of the alloys in different media greatly improves the
resistance of the alloys to pitting corrosion.
S.S. Mahmoud and M.M. Ahmed / Portugaliae Electrochimica Acta 25 (2007) 237-248
248
3- Preoxidation of the alloys in each of molten alkali nitrates and molten alkali
carbonates overs more resistance to pitting corrosion than preoxidation in air.
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