Corrosion Resistance of Steels Used in Alcohol and Sugar Industry

Article (PDF Available)inChemical Engineering & Technology 34(9):1393 - 1401 · September 2011with 984 Reads
DOI: 10.1002/ceat.201000542
Abstract
Evaporators used in power plants for sugar production are vessels with two boxes of heat exchangers. These heat exchangers made of a low-corrosion resistance material are frequently replaced during the alcohol production process. In view of the growing production of cane, sugar, and alcohol, and the corrosion of materials used in power plants, it was necessary to find alternative materials for carbon steel applied in the evaporators. A comparative study of the corrosion resistance of stainless steel as well as unalloyed and alloyed carbon-manganese steels in neutral and acid sugar cane juice electrolytes is presented. The alloyed carbon steels showed the best corrosion behavior in an acid environment compared to a neutral medium. Among the alloyed carbon steels, ASTM T11 and ASTM T22 steels provided the highest corrosion resistance in sugar cane juice.
Research Article
Corrosion Resistance of Steels Used in
Alcohol and Sugar Industry
Evaporators used in power plants for sugar production are vessels with two boxes
of heat exchangers. These heat exchangers made of a low-corrosion resistance
material are frequently replaced during the alcohol production process. In view of
the growing production of cane, sugar, and alcohol, and the corrosion of materi-
als used in power plants, it was necessary to find alternative materials for carbon
steel applied in the evaporators. A comparative study of the corrosion resistance
of stainless steel as well as unalloyed and alloyed carbon-manganese steels in neu-
tral and acid sugar cane juice electrolytes is presented. The alloyed carbon steels
showed the best corrosion behavior in an acid environment compared to a neu-
tral medium. Among the alloyed carbon steels, ASTM T11 and ASTM T22 steels
provided the highest corrosion resistance in sugar cane juice.
Keywords: Alloys, Corrosion, Electrochemical methods, Material science, Metals
Received: December 11, 2010; revised: February 15, 2011; accepted: May 26, 2011
DOI: 10.1002/ceat.201000542
1 Introduction
The Brazilian sugar and alcohol industry experienced two great
developments in the last decades. The first occurred from 1975
to 1985, associated with the increase in ethanol production,
while the second one is related with the increase in sugar pro-
duction and export since 1995 until today. The evaporators
applied in power plants for sugar production are vessels with
two boxes of heat exchangers whose function is to evaporate
water in the sugar cane juice, which contains 85 % of water.
The water content is reduced to 40 % using multiple-effect
evaporators.
Sugar cane contains water (65–75 wt-%), solids (11–18 wt-%),
fibers (8–14 w t-%), organic and inorganic acids, proteins,
starch, wax, and dyes [1]. The solids include saccharose, glu-
cose, fructose, and salts. The vapor comprises natural organic
acids from sugar cane and other acids produced in the process.
The condensed water contains dissolved gases, aldehydes, alco-
hols, organic acids, and nitrogen compounds. Sugar cane juice
is acidic with a pH value of 5.6 at extraction. This acidic prop-
erty is accounted for by the presence of a variety of acids,
namely aconitic, citric, malic, oxalic, glycolic, mesaonic, tartic,
succinic, fumaric, and syringic acid in sugar cane juice [1]. In
addition to these acids, there are approximately 50 different
kinds of microorganisms present in the green cane which are
very active. These microorganisms will contribute to a rather
quick drop in pH (3.1) of the sugar cane juice once extracted
[1]. Moreover, several minerals like water, salt, sulfate, and sili-
ca are also present throughout the process and promote sur-
face degradation of equipments in the alcohol industry by a
wear mechanism.
The steel tubes of evaporators are frequently substituted due
to corrosion. The corrosion process increases with higher
temperature and acidity. Several researchers studied the corro-
sion performance of steels in acid and neutral organic media
[2–11]. The inhibitive capabilities of some organic compounds
on the electrochemical corrosion of mild and stainless steels in
acid and alkaline media were reported [2–11]. Adsorption of
organic compounds on the surface of steels has usually a
chemisorption mechanism [7].
Here, the corrosion resistance of alloyed steels for substitu-
tion of carbon steel by using autoclaving tests, Tafel analysis,
linear polarization technique, and electrochemical impedance
spectroscopy (EIS) is evaluated.
2 Experimental
The studied steels were: a ferritic stainless steel (AISI 444;
AISI = American Iron and Steel Institute), a carbon steel (AISI
1020), a steel with 13 % chromium (Cr; API 5CT L80 13Cr;
API = American Petroleum Institute), and the alloyed carbon
steels ASTM A423 Gr1, API 5CT L80 Type 1, ASTM A213
Gr T11, and ASTM A213 Gr T22 (ASTM = American Society
for Testing and Materials). The steels are denominated in this
Chem. Eng. Technol. 2011,34, No. 9, 1393–1401 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Camila A. Farias
1
Vanessa F. C. Lins
2
1
Vallourec & Mannesman
Tubes, Belo Horizonte, Minas
Gerais, Brazil.
2
Federal University of Minas
Gerais, Corrosion and Surface
Laboratory, Minas Gerais,
Brazil.
Correspondence: Dr. V. F. C. Lins (vlins@deq.ufmg.br), Federal
University of Minas Gerais, Corrosion and Surface Laboratory, Antonio
Carlos Avenue 6627, CEP 31270090, Belo Horizonte, Minas Gerais,
Brazil.
Corrosion 1393
paper as AISI 444, 1020, L80 13Cr, A 423, L80 T1, T11, and
T22. The carbon contents of steel were analyzed applying the
combustion technique with detection by infrared (LECO
444 LS equipment, Leco Corp.). Molybdenum, nickel, chrome,
and copper concentrations were evaluated by inductively
coupled plasma optical emission spectrometry (ICP-OES;
Spectroflame Modula of the Spectra). Manganese, silicon,
phosphorus, and niobium were analyzed with the SRS 3000
Sequential X-Ray Spectrometer (Siemens).
The steel samples were submitted to a hard drawing using
an acrylic resin. A copper wire was welded onto the steel sam-
ple in order to promote an electrical contact. The electrical
tensile was evaluated in the steel sample by a MINIPA voltam-
meter, model ET-1002. Two electrolytes were tested: a com-
mercial sugar cane juice, pH 3.5, and a sugar cane juice with
addition of 40 g L
–1
sodium hydroxide aqueous solution in
order to achieve a neutral pH of 7.0. The equipment was an
Autolab PGSTAT128N with Frequency Response Analyzer soft-
ware. The reference electrode was Ag/AgCl and the counter
electrode was platinum. The area of the steel sample and the
area of platinum exposed to the electrolyte were 1 cm
2
and
2cm
2
, respectively.
The measurement of open circuit potential was performed
up to potential stabilization. The linear polarization technique
was applied by using a scan rate of 0.167 mV s
–1
and a pola-
rization of ±10 mV in relation to the corrosion potential
(Ecorr). The parameters of Tafel analysis were a scan rate of
0.167 mV s
–1
and a polarization of ±250 mV (Ecorr). Electro-
chemical impedance spectroscopy was performed by using a
potential amplitude of 10 mV and the frequency ranging from
100 kHz to 1 mHz. The conditions of the autoclave tests are
summarized in Tab. 1.
The mass loss was measured after the
autoclave test, and the corrosion rate of the
steel samples was calculated by:
CR = (KW)/(ATD) (1)
where CR is the corrosion rate (mm A
–1
),
K= 8.76 · 10
4
,Wis the mass loss (g), Ais
the sample area (cm
2
), Tis the test time
(h), and Dis the density of the steel (g cm
–3
).
The autoclave test apparatus was
mounted in a way that the samples had
contact with the liquid and vapor phase.
After the test, the specimens were cleaned
by using an aqueous solution of hydrochloric acid, tin chlo-
ride, and antimony trioxide (pickling solution), then dried
and weighed according to the ASTM G1 Standard.
3 Results and Discussion
The chemical compositions of the steels studied are listed in
Tab. 2 .
The stainless steel studied was ferritic, with 1.84 wt-% Mo
and 18.13 wt-% Cr. A423 was an alloyed carbon steel with a
higher concentration of nickel and copper than the other steels
under study. A213 had a higher content of Mo than the alloyed
carbon steels studied. The steels L80, T11, and T22 are carbon
steels alloyed with Mo and niobium (0.01 wt-%). The corro-
sion rates of steels obtained by using the autoclave test are
indicated in Fig . 1.
The most resistant steels are the ferritic stainless steel with
18 wt-% Cr and the steel with 13 wt-% Cr, as expected due to
the effect of Cr in alloys. A Cr content higher than 11wt-%
promoted the formation of a passive layer of chromium oxide/
hydroxide that reduces the corrosion rate of steel in aqueous
media [12]. Among the alloyed carbon steels, L80 T1 and T22
had the lowest corrosion rates for samples exposed to the
steam phase and for both pH values. The T11 and T22 exposed
to the liquid phase provided the highest corrosion resistance
for both pH values. The higher Mo and Cr contents of these
alloyed carbon steels promote a beneficial effect on the corro-
sion resistance of steels mainly in the liquid phase. Cr as
alloying element in steel produced a protective chromium
oxide/hydroxide layer, which reduced the corrosion rate of
steel. Cr contents higher than 11 wt-% produced a protective
and continuous oxide/hydroxide layer, but even lower contents
of Cr than 11 wt-% promote a decrease of corrosion rate in
aqueous solution. Mo increased the stability of the passive
layer [12, 13].
For samples exposed to the liquid phase, the stainless steels
and the alloyed carbon steels T11, T22, and L80 T1 showed
lower corrosion rates at acidic pH than at neutral pH. How-
ever, the carbon steel and A423 steel exhibited higher corro-
sion rates at acidic pH than at neutral pH. This behavior was
the same for the steel samples exposed to the vapor phase,
except for the steel A423 which had a lower corrosion rate at
acidic pH than at neutral pH.
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Table 1. Parameters of autoclave tests.
Electrolyte: Sugar cane juice
Volume: 1500 mL
Initial pH: 3.5 and 7.0
Temperature: 120 °C
Pressure: 689 MPa
Test time: 168 h
Table 2. Elemental chemical composition of the steels studied (wt-%).
CSiPS MnMoNiCrCuNb
1020 0.13 0.02 0.02 0.016 0.45 – 0.01 0.02 0.008
AISI 444 0.02 0.43 0.03 0.002 0.15 1.84 0.20 18.13 0.03 0.18
A423 0.13 0.21 0.07 0.002 0.46 0.02 0.27 0.42 0.31 –
T11 0.10 0.55 0.01 0.001 0.40 0.45 0.02 1.05 0.02 0.01
T22 0.09 0.25 0.01 – 0.45 0.90 0.02 2.10 0.03 0.01
L80T1 0.27 0.23 0.01 – 1.40 0.10 0.02 0.16 0.01 0.01
L8013Cr 0.19 0.32 0.02 – 0.66 0.05 0.14 12.80 0.08 0.01
1394 C. A. Farias, V. F. C. Lins
Concerning the electrochemical tests, the corrosion poten-
tial was obtained for steels in electrolytes pH 3.5 and 7.0 (see
Tab. 3). The stainless steel AISI 444 had the same corrosion
potential for both pH values. The other steels showed a higher
corrosion potential for the acid environment. The corrosion
behavior of steels obtained by using the open circuit potential
measurements is similar to that obtained by autoclave tests.
The results of polarization resistance of steels received by the
linear polarization technique are indicated in Tab. 3. At pH 7,
the most resistant steels are the steels with higher Cr contents
(18 wt-% and 12.8 wt-%). The carbon and alloyed carbon
steels demonstrated a similar corrosion behavior in the neutral
medium with values of polarization resistance ranging from
2.0 to 4.2 Xcm
2
. At pH 3.5, T11 and T22 provided a higher
corrosion resistance than the carbon and the other alloyed car-
bon steels studied.T11 and T22 steels had a higher polariza-
tion resistance in the acid environment than in the neutral
medium.
The corrosion resistance values of steels obtained by using
the Tafel analysis (see Tab. 3, Figs. 2 and 3) are consistent with
the results from the linear polarization technique. The alloyed
carbon steels showed a similar corrosion current in the neutral
electrolyte. In the acid and neutral sugar cane juice, the most
resistant steels are the steels with high Cr contents with passi-
vation current densities between 10
–6
and 10
–5
Acm
–2
. In the
acid electrolyte, among the alloyed carbon steels, T11 and T22
showed the lowest corrosion current. The T11 and T22 steels
provided lower values of corrosion current in the acid environ-
ment than in the neutral medium. The carbon steel (1020)
produced the same corrosion current in the acid and in the
neutral electrolyte.
The results of the polarization resistance obtained by EIS of
alloyed carbon steels are summarized in Tab. 3. A423 had a
higher polarization resistance among the alloyed carbon steels
in neutral sugar cane juice. The Nyquist diagram for A423 in
neutral sugar cane juice is illustrated in Fig . 4. A 423, T11, T22,
Chem. Eng. Technol. 2011,34, No. 9, 1393–1401 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
a)
b)
Figure 1. Corrosion rate of steels in sugar cane juice obtained by
the autoclave test.
Table 3. Corrosion potential, corrosion current, and polarization resistance for steel samples in sugar cane juice.
Steel Ecorr
[mV
Ag/AgCl
]
pH 3.5
Ecorr
[mV
Ag/AgCl
]
pH 7
Icorr
[lAcm
–2
]
pH 3.5
Icorr
[lAcm
–2
]
pH 7
Rp
[Xcm
2
]
pH 3.5
Rp
[Xcm
2
]
pH 7
Rp (EIS)
[Xcm
2
]
pH 3.5
Rp (EIS)
[Xcm
2
]
pH 7
1020 –595 –662 1.1 · 10
–5
1.1 · 10
–5
2.7 2.5 2674.0 490.8
AISI 444 16 15 8.7 · 10
–8
5.9 · 10
–8
206.0 1390.0 19570.0 93940.0
A423 –529 –641 2.7 · 10
–6
5.4 · 10
–6
4.9 4.2 1089.0 1908.0
T11 –532 –653 6.7 · 10
–7
8.9 · 10
–6
24.8 2.8 3812.0 800.7
T22 –535 –662 7.7 · 10
–7
6.7 · 10
–6
16.6 3.2 3027.0 603.2
L80 T1 –560 –659 1.3· 10
–6
3.0 · 10
–6
2.9 2.0 360.7 499.0
L80 13Cr –449 –504 3.8 · 10
–7
5.1 · 10
–7
67.6 227.0 12000.0 31040.0
Figure 2. Anodic potentiodynamic polarization curves of steels
in acid sugar cane juice.
Corrosion 1395
and L80 T1 steels displayed a similar electrochemical behavior
at pH 7. Nyquist diagrams of these steels in neutral medium
showed one capacitive and one inductive arc in the low-fre-
quency region, which indicates the occurrence of surface
adsorption as demonstrated in Fig. 4.
The simulated electrical circuit, which represents the corro-
sion process of these steels in neutral sugar cane juice, is de-
picted in Fig . 5. The circuit is a resistor (electrolyte resistance)
connected to a parallel circuit with three branches. Q is the
constant phase element in parallel to the polarization resis-
tance, and L is the inductance. Surface adsorption of sacchar-
ose molecules on the steel surface can occur. The reduction of
the anode area can explain the lower polarization resistance
values (and higher corrosion rates) obtained for T11 and T22
steels in neutral media in comparison to acid media. The
Nyquist diagram of 1020 steel did not present the inductive
arc, and the equivalent circuit consists of the electrolyte resis-
tance connected to a parallel circuit with the constant phase
element in parallel to the polarization resistance. The polariza-
tion resistance Rp of A423 steel in neutral medium was twice
the Rp of T11 and three times the Rp of T22, as indicated in
Tab. 3. A423 steel has a higher content of nickel and copper
than the other alloyed carbon steels. Copper and nickel pro-
duced basic salts, such as basic sulfate, that reduce the corro-
sion rate of steels.
In both electrolytes, the most resistant steels are AISI 444
with high Cr content due to production of a passive layer of
chromium oxides/hydroxides. The Nyquist diagram of AISI
444 steel at pH 3.5 and pH 7 demonstrates two capacitive arcs,
confirmed by the presence of two maximum peaks in the Bode
diagram (phase angle versus frequency curve), as indicated in
Figs. 6 and 7 for neutral medium. The capacitive arc in the
high-frequency region is associated to a passive layer. The
equivalent circuit consists of the electrolyte resistance con-
nected to two parallel circuits: in the first, the constant phase
element is in parallel to the charge transfer resistance, and in
the second, the constant phase element is in parallel to the
polarization resistance.
L80 13Cr steel presented one capacitive arc and a Warburg
impedance at pH 7 in the Nyquist diagram (Fig. 8). The pres-
ence of Warburg impedance indicated a mass-transport con-
trol of the corrosion process. The Bode diagram depicted one
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Figure 3. Anodic potentiodynamic polarization curves of steels
in neutral sugar cane juice.
Figure 4. Nyquist diagram obtained for A 423 steel in sugar cane juice at pH 7.0.
Figure 5. Electrical circuit simulating corrosion process of A 423
steel in neutral sugar cane juice.
1396 C. A. Farias, V. F. C. Lins
maximum peak in phase angle versus frequency curve, and the
passive layer was not identified at pH 7. The equivalent circuit
consists of the electrolyte resistance in parallel to the circuit
with the constant phase element in parallel to two resistances:
polarization resistance and Warburg impedance.
In the acid electrolyte, among the alloyed carbon steels, T11
and T22 steels presented the highest polarization resistance
(Tab. 3). These two steels had hig her polarization resistances in
the acid environment than in the neutral electrolyte (Tab. 3).
1020, A423, T11, and T22 steels showed only one capacitive
arc in the acid sugar cane juice. Fig. 9 illustrates the Nyquist
diagram for T11 steel in acid sugar cane juice. An inductive arc
did not appear in the Nyquist diagram, indicating the absence
of adsorption phenomena. The equivalent circuit consists of
Chem. Eng. Technol. 2011,34, No. 9, 1393–1401 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Figure 6. Nyquist diagram obtained for AISI 444 steel in sugar cane juice at pH 7.0.
Figure 7. Bode diagram obtained for AISI 444 steel in sugar cane juice at pH 7.0.
Corrosion 1397
the electrolyte resistance connected to a parallel circuit with
the constant phase element in parallel to the polarization resis-
tance.
L80 13Cr steel had two capacitive arcs in the Nyquist dia-
gram, indicating the presence of a passive layer at 3.5 pH
(Fig. 10). The equivalent circuit consists of the electrolyte resis-
tance connected to two parallel circuits: in the first, the con-
stant phase element is in parallel to the charge transfer resis-
tance, and in the second, the constant phase element is in
parallel to the polarization resistance.
Some hypotheses explain the best corrosion behavior of
steels T11 and T22 in acid electrolyte compared to the neutral
medium. The sugar cane juice contains 65–75 wt-% water
and 11–18 wt-% sugar. Among the sugar compounds, the
main constituents are saccharose or sucrose. The chemical
formula of saccharose is 2-[3,4-dihydroxy-2,5- bis (hydroxy-
methyl) tetrahydrofuran-2-yl] oxi-6- (hydroxymethyl) oxano-
3,4,5-triol [12]. The hydrolysis of saccharose is an acid-cata-
lyzed reaction, generating smaller molecules, namely D-glucose
and D-fructose.
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Figure 8. Nyquist diagram obtained for L80 13Cr steel in sugar cane juice at pH 7.0.
Figure 9. Nyquist diagram for T11 steel in acid sugar cane juice.
1398 C. A. Farias, V. F. C. Lins
C
12
H
22
O
11
+H
2
OC
6
H
12
O
6
+C
6
H
12
O
6
(2)
Yusof et al. [14] reported on the occurrence of saccharose
hydrolysis and found that during storage of sugar cane juice
the saccharose contents decreased while fructose, glucose, and
titratable acidity increased.
In a neutral environment, the cathodic reaction generates
hydroxyl ions which are attracted to the anode region, where
ferrous and ferritic ions are produced:
Fe Fe
2+
+2e
(3)
Fe
2+
Fe
3+
+e
(4)
2H
2
O+O
2
+e
4OH
(5)
In the neutral electrolyte, the saccharose molecules can be
adsorbed on the anode surface, generating microcells. The
anode area reduces due to the adsorption of saccharose, and
the corrosion rate increases.
In the acid electrolyte, the adsorption phenomenon was not
observed by using electrochemical impedance spectroscopy.
The acid environment accelerates saccharose hydrolysis, gener-
ating lower molecules of fructose and glucose. Under these
conditions, adsorption of sugar molecules was not identified
by EIS. The anodic area did not decrease and the corrosion
rate was lower than in the neutral medium. In acid electrolyte,
the cathodic reactions are:
2H
+
+e
H
2
(6)
4H
+
+O
2
+e
2H
2
O (7)
The chemical composition plays an important role in the
corrosion behavior of the steels studied. The carbon content
ranged from 0.02 w t-% for AISI 444 to 0.27 wt-% for L80 T1.
In literature it is reported that the carbon content of steel is
not a significant factor in corrosion [15].
The highest corrosion resistance of AISI 444 steel in these
electrolytes was due to the chemical composition of this
steel, mainly the high contents of Cr (18.13 wt-%) and Mo
(1.84 wt-%). Several researchers reported the effect of Cr and
Mo on the corrosion resistance of stainless steels [16–19].
Sekine et al. [16] observed that AISI 434, 444, and XM27 ferri-
tic steels are more stable than AISI 410L and 430 steels in
aqueous solutions, since increasing Cr and Mo enhance the
passivity. Hermas and Ogura [20] described that Mo inhibits
the active dissolution of austenitic stainless steels in an aque-
ous solution of sulfuric acid.
T11 steel had the highest corrosion resistance in an acid
electrolyte. This steel includes elements which contribute to
improve its corrosion resistance in an aqueous solution such
as Mo (0.46 wt-%) and Cr (1.05 wt-%). In acid and neutral
electrolytes, T11 steel with lower contents of Cr and Mo exhib-
ited a higher polarization resistance than T22 steel with
0.90 wt-% Mo and 2.10 wt-% Cr. However, T11 steel has a
higher silicon content than T22 steel. Melchers [15] described
the beneficial effect of silicon as an alloying element on the im-
mersion corrosion resistance of steels. Murayama et al. [21]
studied the atmospheric corrosion of a silicon-added carbon
steel and reported that the inner corrosion layer consists of
an iron-rich, coarse-grained b-FeOOH matrix and a silicon-
enriched narrow band composed of a mixture of a-FeOOH
and a nanometer-sized silicon oxide. The results of detailed
microstructural analyses indicated that silicon plays an impor-
tant role in modifying the rust layer structure. Nishimura [22]
Chem. Eng. Technol. 2011,34, No. 9, 1393–1401 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Figure 10. Nyquist diagram for L80 13Cr steel in acid sugar cane juice.
Corrosion 1399
also proved by using EPMA (electron probe micro-analyzer)
and TEM (transmission electron microscopy) analyses that
silicon and aluminum mainly exist in nano-scale iron complex
oxides in the inner rust layer formed on the developed weath-
ering steel. Si was identified as an intermediate state such as
Si
2+
in the complex oxides of the inner rust. Nishimura [22]
stated the beneficial effect of silicon and aluminum additions
on the atmospheric corrosion resistance of steel.
A 423 steels provided the highest corrosion resistance in
neutral sugar cane juice. These steels have higher contents of
nickel (0.27 wt-%) and copper (0.31 wt-%) than the other
alloyed carbon steels. The study of the corrosion mechanism
of the nickel-copper-chromium added steels by means of elec-
trochemical techniques such as potentiodynamic anodic polar-
ization indicated that the addition of copper to rimmed or alu-
minum-killed steels results in a sensible decrease in the critical
current density (i
cr
), demonstrating the smaller tendency of
these steels to suffer anodic dissolution [23]. It was also veri-
fied that the current density for the reduction of hydrogen ions
(i
H+
) increased from residual contents of copper up to
0.10 wt-%. The addition of copper accelerates the reduction of
hydrogen ions, thus improving the dissolution of A423 steel in
the acid electrolyte. Polarization resistance increased from
1089 Xcm
2
in the acid sugar cane juice to 1908 Xcm
2
in the
neutral juice. The effect of copper and nickel on corrosion
resistance of carbon steels is similar [23]. In environments
containing sulfate ions, the protection mechanism of the alloy-
ing elements nickel and copper can be related to the sealing of
pores of the corrosion layer due to the formation of basic sul-
fates, which are incorporated in the corrosion layer. An inner
rust layer with basic sulfates of copper and nickel protective
and less porous than the outer layer is produced [23].
In alcohol plants, evaporators are made of carbon steel. The
corrosion resistance of mild steel in alcohol plants is low and
its service life is limited to one year. AISI 444 steel has a higher
corrosion resistance in sugar and alcohol industries but causes
higher costs. One alternative is the use of alloyed carbon steels
in sugar and alcohol plants.
The autoclave testing is performed at 120 °C and 689 MPa,
simulating the service conditions in evaporators. This test pro-
vides a more real evaluation of corrosion behavior of steels
applied in evaporators. The electrochemical techniques were
performed under the same electrolyte and pH conditions of
autoclave testing, but at room temperature and pressure. The
advantage of electrochemical techniques such as EIS is that
additional information about corrosion mechanism and corro-
sion rate of steels is obtained.
The results of autoclave and electrochemical tests (see
Tab. 4) provided information to select T22 or T11 steels for
application in evaporators. Considering economical reasons,
T11 steel was selected due to the lower content of Mo and Cr
in relation to T22 steel.
4 Conclusions
AISI 444 steel followed by L8013Cr provided the highest corro-
sion resistance in sugar cane juice as evaluated by autoclave
testing and electrochemical techniques. The results of linear
polarization, Tafel analysis, and EIS proved that T11 steel
exhibited the highest corrosion resistance among the alloyed
carbon steels in acid sugar cane juice. The results of autoclave
tests in acid sugar cane juice indicated that the most resistant
steel was T22 followed by T11 steel.
In neutral sugar cane juice, the best corrosion behavior
among the alloyed carbon steels was obtained for A423 steel by
electrochemical techniques. The results of autoclave testing at
pH 7 indicated the best corrosion behavior among the alloyed
carbon steels for T22 steel followed by T11 and A423 steel. The
alloyed carbon steels T11 and T22 presented a better corrosion
behavior in an acidic environment than in a neutral medium.
The results of autoclave testing, which simulated the service
conditions of steels, and the support of electrochemical tests
provided information to select T22 or T11 steel for application
in evaporators. For economical reasons, T11 steel can be
selected due to the lower content of Mo and Cr in relation to
T22 steel.
The authors have declared no conflict of interest.
Symbols used
A[cm
2
] sample area
CR [mm a
–1
] corrosion rate
D[g cm
–3
] density of the steel
i
cr
[A cm
–2
] critical current density
K[–] constant (8.76 · 10
4
)
T[h] test time
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Table 4. Best corrosion behavior among the alloyed steels in
each test and condition.
A423 T11 T22 L80 T1
Autoclave test
liquid phase, pH 3.5
X
Autoclave test
liquid phase, pH 7
X
Autoclave test
steam phase, pH 3.5
X
Autoclave test
steam phase, pH 7
X
Linear polarization
pH 3.5
X
Linear polarization
pH 7
X
Tafel analysis
pH 3.5
X
Tafel analysis
pH 7
X
EIS
pH 3.5
X
EIS
pH 7
X
1400 C. A. Farias, V. F. C. Lins
References
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Chem. Eng. Technol. 2011,34, No. 9, 1393–1401 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Corrosion 1401
  • Article
    Full-text available
    In this study, corrosion behaviors of materials used in diffusion units and equipment used in juice clarification steps and tubes in evaporators at Ankara Sugar Factory were investigated in terms of juice production and juice clarification processes as well as juice medium at evaporation stages. The measurements have been performed by comparing steel types used in these units and alternative types of steels that can also be used during the study. For this purpose, pH and Brix (Bx, refractometric dry matter) values of raw juice, thin juice and juice taken from evaporator have been measured during 2009–2010, 2010–2011, and 2011–2012 campaign periods of Ankara Sugar Factory. In addition to these measurements, traditional weight loss and electrochemical tests such as Linear Polarization Resistance (LP), Tafel Extrapolation (TP), Electrochemical Impedance Spectroscopy (EIS) were performed to measure and compare the corrosion rate of the metals used in different juice mediums. The metals included in the study were AISI 316L, AISI 304L grade stainless steel, St 37.2 grade carbon steel and nickel-coated St 37.2. The highest and the lowest corrosion rates were recorded for raw juice and thin juice, respectively. St 37.2 steel had the fastest corrosion rate, whereas the stainless steel AISI 316L has the slowest corrosion rate. However, AISI 316L shows only slightly higher corrosion resistance compared to the corrosion resistance of AISI 304L in different juices. Therefore, AISI 304L steel, which is cheaper than AISI 316L, can be selected as a substation of St 37.2 steel.
  • Article
    Severe plastic deformation by high pressure torsion (HPT) is used to process and refine the grain structure of commercial purity magnesium and AZ31, AZ91, and ZK60 magnesium alloys. Transmission electron microscopy shows that the microstructure of pure magnesium is characterized by a bi‐modal grain size distribution with grains in the range of a few microns and ultrafine grains after HPT, whereas the magnesium alloys display a homogeneous ultrafine grain structure after processing. X ray diffraction analysis reveals that the AZ91 alloy displays the largest lattice microstrain and this alloy also exhibits the highest hardness after processing. The processed AZ31 and the ZK60 alloys show similar microstructures and maximum values of hardness. Contrary to earlier reports of significant improvements in the corrosion resistance of magnesium alloys in biological environments, the present results show that processing by HPT has no significant effect on the corrosion behavior of magnesium alloys in a 3.5% NaCl solution. By contrast, pure magnesium exhibits an increased corrosion resistance after HPT.
  • Article
    It is well established that magnesium has a considerable potential for use as a biodegradable material. This report describes the effect of processing by severe plastic deformation (SPD) on the grain refinement, mechanical behavior, biocompatibility and corrosion behavior of commercial purity magnesium. The material was received as cast slabs and processed by rolling, equal-channel angular pressing and high-pressure torsion to produce samples with average grain sizes in the range of ~0.5–300 μm. The results show that severe plastic deformation does not affect the biocompatibility. However, the corrosion behavior is affected by the processing route. Specifically, SPD processing leads to general corrosion as opposed to localized corrosion in the as-cast and hot-rolled condition.
  • Article
    Zeolite NaA and zeolite NaY coating on mild steel has been assessed in the current study. The synthesized zeolite NaA and NaY samples have also been studied using X-ray powder diffraction (XRD) method and scanning electron microscopy (SEM), and the samples with different concentrations of zeolite have been tested for its corrosion resistance property for against H2SO4 acid at temperatures up to 60 °C under stagnant as well as stirred conditions by polarization/impedance method. Zeolite NaA and NaY were dispersed in cyanoacrylate-based adhesive in the ratio 1:20 and coated on mild steel for the formation of membrane. XRD analysis revealed the formation of zeolite coating on mild steel substrate. Microstructure analysis using SEM images of the coated samples showed the formation of cubic structure. The coated zeolite layer consisted of top layer with thickness of 2.4–3.6 µm. Results from polarization/impedance revealed that zeolite NaA is more corrosion resistant than zeolite NaY having inhibition efficiency of 89.2 %. Graphical Abstract
  • Article
    Full-text available
    Stainless steel (SS) is successfully used in food industries due to the characteristics of its surface, which is essential for the maintenance of the properties of food. Tanks and stainless steel pipes, which come into direct contact with food and drink, undergo a process of cleaning and disinfecting called CIP (Clean-in-Place). This method consists of automatic recirculation of a detergent solution for a controlled time followed by a rinsing with water, and finally a cleaning and sanitizing solution immersion. This work consists of investigating the corrosion resistance of stainless steels used in the food industry using sanitizing solutions employed in the CIP procedure, and using electrochemical techniques such as potentiodynamic anodic polarization and electrochemical impedance spectroscopy.
  • Article
    Zeolite ZSM-5 coating on mild steel had been assessed for its corrosion resistance property against organic acids, namely, acetic acid, formic acid and citric acid of varying concentrations at temperatures up to 60 °C under stagnant as well as stirred conditions by weight-loss and polarization methods. Biocidal activity of the zeolite coating against Gram-negative bacteria Escherichia coli, Pseudomonas putida, Salmonella typhi and Gram-positive bacteria Staphylococcus aureus by using minimum inhibitory concentration (MIC) was also studied. With zeolite coating, corrosion inhibition efficiency up to 98% was achieved for 6 h of duration of contact between coating and acid solutions. Similar resistance persisted for the duration of 7 days too. Results obtained showed that extent of corrosion of mild steel decreased in the order, formic acid > citric acid > acetic acid. Also as expected, corrosion resistance though only slightly, decreased with solution temperature and concentration. High anti-microbial activity was observed at very low values of MIC (100 μg/mL). In light of reported literature, the presence of the structure directing agent within the channels of the zeolite has been attributed for the high corrosion resistance as well as anti-microbial activity observed here. Thus, zeolite offers a “greener” alternate to chromium and epoxy polymers based corrosion resistance coating.
  • Article
    The effect of chromium concentration in the surface film formed by surface treatments on the corrosion resistance of Type 430 stainless steel was studied. It was found that the chromium content depends on the surface treatment method. HNO3 treatments result in a significant enrichment of chromium in the surface film on the Type 430 stainless steel, and the chromium enrichment increases with increasing HNO3 concentration. The pitting potential (Ep), corrosion potential (Ec) and polarization resistance (Rp) are changed by the chromium enrichment (SCr) in the surface film formed by treatment. The higher the SCr value, the higher the Ep, Ec and Rp values.
  • Article
    A newly synthesized glycine derivative (GlyD1), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to control mild steel corrosion in 4.0M H2SO4 solutions at different temperatures (278–338K). Tafel extrapolation, linear polarization resistance (LPR) and impedance methods were used to test corrosion inhibitor efficiency. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. Results obtained were compared with an available glycine derivative (GlyD2) and glycine (Gly). Tafel polarization measurements revealed that the three tested inhibitors function as mixed-type compounds. The inhibition efficiency increased with increase in inhibitor concentration and decreased with temperature, suggesting the occurrence of physical adsorption. The adsorptive behaviour of the three inhibitors followed Temkin-type isotherm and the standard free energy changes of adsorption (ΔGadso) were evaluated for the three tested inhibitors as a function of temperature. The inhibition performance of GlyD1 was much better than those of GlyD2 and Gly itself. Results obtained from the different corrosion evaluation techniques were in good agreement.
  • Article
    Full-text available
    Intergranular corrosion (IGC) and precipitation of ferritic stainless steels (FSS) were investigated with change in Cr content from 11 wt.% to 17 wt.%. The increase in Cr content improved IGC resistance as temperature and time for the sensitization became higher and longer, respectively, but it did not prevent IGC. The analysis on the intergranular precipitates revealed that Cr segregation around fine intergranular TiC in developed all FSS regardless of Cr content. This Cr segregation is proposed to explain the Cr depletion for the cause of IGC in Ti-stabilized Cr FSS.
  • Article
    Corrosion inhibition of C-steel in 2M HCl was investigated in the absence and presence of different concentrations of some thiosemicarbazide derivatives namely, 1-ethyl-4(2,4-dinitrophenyl) thiosemicarbazide (I), 1,4-diphenylthiosemicarbazide (II), 1-ethyl-4-phenylthiosemicarbazide (III). Weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques were employed. Impedance measurements showed that the double-layer capacitance decreased and charge-transfer resistance increased with increase in the inhibitors concentration and hence increasing in inhibition efficiency. Potentiodynamic polarization study showed that all the inhibitors act as mixed-type inhibitors. Results obtained reveal that the inhibition efficiency (%IE) follows the sequence: I>II>III. The adsorption of these compounds on C-steel surface obeys Temkin’s adsorption isotherm and has a chemisorptions mechanism.
  • Article
    Corrosion products of the rust layer formed on a 2% Si-bearing low alloy steel during atmospheric exposure at a coastal area in Japan for three years were characterized. The inner layer of the rust, i.e., near oxide/metal interface, has a layer structure and the average grain size of corrosion products is finer than that in the upper region that is few micro meter above the interface. A series of nano probe energy disperse X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) analyses performed using a field emission gun (FEG) transmission electron microscope (TEM) revealed that the inner layer consists of an iron rich, coarse grained β-FeOOH matrix and the silicon enriched narrow band composed a mixture of α-FeOOH and nano-meter size silicon oxide. On the other hand, the α-FeOOH single phase area exists where the layer structure did not develop. The result of detailed microstructural analyses indicated that the silicon plays an important role in modifying the rust layer structure.
  • Article
    A high Si, Al type ultrafine-grained (UFG) weathering steel was produced by the multi-pass warm rolling method, and its corrosion resistance was estimated by a cyclic wet/dry corrosion test using chloride ions. The Si- and Al-bearing UFG steel exhibited excellent corrosion resistance in comparison with Si–Mn carbon steel (SM).EPMA and TEM analyses showed that Si and Al mainly exist in nano-scale iron complex oxides in the inner rust layer formed on the developed steel. The Al K X-ray spectrum of Al2O3 and metallic Al had different peak positions due to chemical shifts, and that of the test sample was the same as that of Al2O3 in the EPMA analysis. This result suggested that Al was present in the complex oxides as Al3+. In the same way, Si was identified as an intermediate state such as Si2+ in the complex oxides of the inner rust.EIS (electrochemical impedance spectroscopy) measurement of the corrosion test samples revealed that the rust resistance (Rrust) and corrosion reaction resistance (Rt) of the developed steel were much larger than those of SM. It was found that nano-scale complex iron oxides formed in the lower layer of iron rust in the developed steel, resulting in increased Rrust and Rt, and finally suppressing corrosion.
  • Article
    Small changes in the composition of mild and low alloy steels can effect their immersion corrosion behaviour. A number of comprehensive test programs for coupons immersed at different locations and recovered at different times have been reported. Comparison between them has also been attempted with modest success as well as leaving some apparent inconsistencies in the effect of some alloying elements.In this paper, a new comparative analysis of previously reported observations is reported. It employs a recently reported multi-phase phenomenological corrosion–time model, with different corrosion phases governing corrosion behaviour. Each phase is a function of time.The analysis shows that metal composition can influence the first, kinetically controlled corrosion phase and also the long-term anaerobic corrosion phases. However, during the phase controlled by oxygen diffusion through the corrosion product, metal composition is largely irrelevant, in agreement with theoretical predictions. It is shown that the several observations in the literature about the effect of particular alloys can be reconciled, including apparently conflicting observations about the effect of chromium content.
  • Article
    The influence of tungsten on the passivity of Fe-29Cr stainless steels was compared quantitatively with that of molybdenum by examining the effects of these alloying elements on passivation parameters such as the pitting potential (Epit), the primary passivation potential (Epp) and the critical anodic current density (Ic), which were determined from anodic polarization responses of the alloys in a chloride or an acid solution. As the content of molybdenum and/or tungsten increased, the passive region of the alloys was expanded by increasing Epit, and simultaneously by decreasing both Epp and Ic for the alloys. Quantitative relationships were drawn between the passivation parameters and the content of tungsten or molybdenum in the alloys. It was demonstrated that the effects of tungsten on the passivation parameters of the alloys are almost equivalent to those of molybdenum when compared on the basis of atomic per cent, a result of the similarities in chemical and electrical properties between the two elements.
  • Article
    Two new organic compounds were tested experimentally as inhibitors for mild steel in NaOH in presence of NaCl by electrochemical and hydrogen evolution techniques. Results demonstrated that the two inhibitors show an adsorption on steel surface according to Langmuir adsorption isotherm. The inhibition efficiency increases with increasing inhibitor concentrations to attain a maximum value at 1.0 mM for compound I and at 6.0 mM for compound II, respectively. The results were confirmed by surface examination via scanning electron microscope.
  • Article
    The effect of polyethyleneimine (PEI) as a corrosion inhibitor for ASTM 420 stainless steel in 3% aqueous NaCl solution was studied. The results of linear polarization and cyclic polarization measurements indicate high inhibiting effectiveness of the selected organics. Moreover, from cyclic polarization measurements it can be deduced that PEI acts as an inhibitor against pitting corrosion. Immersion tests in the presence of PEI showed remarkable corrosion protection against uniform corrosion. Film persistency immersion testing indicated that once the protective layer is formed, it is very stable in a non-inhibited NaCl solution. X-ray photoelectron spectroscopy measurements showed that PEI binding is mediated by electrostatic interactions between PEI and the substrate. A dense layer of PEI might be effective either in preventing diffusion of ionic species from the film or in preventing attack by chlorine from the salt water.