HUNGARIAN JOURNAL
OF INDUSTRIAL CHEMISTRY
"VESZPREM
Vol. 30. pp. 37 - 39 (2002)
THE INFLUENCE OF WATER CONCENTRATION ON THE CORROSION OF
LOW ALLOY STEELS IN THE SYSTEM METHANOL- ETHYLENE GLYCOL -
N-BUTIRIC ACID
D. SUTIMAN and A. CAILEAN
(Faculty of Industrial Chemistry, Technical University "Gh. Asachl", D.Mangeron 71A, Iasi 6600, ROMANIA)
Received: AprillO, 2001
The behaviour of three types of steel with a variable carbon content (from 0,2 to 0,4 % ) is studied in medium of
methanol- 10% ethylene glycol- 5% n-butiric acid with water concentration between 1% to 5 % . The weight loss are
measured and the gravimetric figure K (g·m·2·h-1) and the penetration figure P (mm • m·2·year-1) are calculated. Also, the
kinetic corrosion parameters are established. The methods used for identification and analysis of the corrosion
compounds are: elemental analysis, I.R. spectra and X-ray diffraction. Also, by electron microscopy, a corrosion
mechanism is assigned.
Introduction
This paper is a continuation of studies of the corrosion
behaviour of low alloy steel types in methanol medium
and/or ethylene glycol, having as corrosion reagent,
saturated mono- and di-carboxyl organic acids [1-5].
This corrosion system and also the studied acids are the
main responsible agents for the corrosion process that
appears in the synthetic fiber industry.
Experimental
The three steel types studied for the corrosion are: OL
37, OL 50, and OL 60 having the chemical composition
presented in Table].
The metallic samples used for corrosion were cut up
from a cylindrical bar with 5 cm2 active surface. A
single basis of the cylinder was corroded after
preliminary grinding and polishing. Dyeing protected
the other basis and the lateral surface.
The corrosion system contained methanol, 10 %
ethylene glycol and 5 % n-butiric acid, the water
concentration varied between 1 % and 5 %. We used
Merck reagents and the water was bidistilled, having
electrical conductivity of 12 JIS·cm-1• Karl-Fischer
method was used to determine the water content. The
variation of pH - values in the corrosion anhydrous
system and at different water concentration was
measured with a Hach pH-meter.
Before introducing the samples in the corrosive
system, they were submitted to a degreasing process in
boiling benzene for 30 minutes and then degreased in a
solution of hydrochloric acid (3 %) for 3 minutes. The
corrosion system was open, allowing the permanenr
access of oxygen from the atmosphere, reproducing the
industrial process conditions. The final weight loss was
converted to gravimetric figures K (g·m ·2·h · 1) and
penetration figures P (mm·m ·2·year'1).
For establishing the corrosion type. the metallic
surface was visualised by electron microscopy on a
TESLA B 300 microscope.
For the values of 1 % , 3 % and 5 % water
concentration, the polarisation curves were plotted on a
TACUSSEL S8R potentiometer with input impedance
of 1012 n. From the shape of these curves, the kinetic
corrosion parameters (Sst, Scor and icor) were calculated.
The corrosion final compounds, for every value of
water concentration, were soluble in the system. They
were obtained in solid state by evaporating the corrosive
solutions at 40 °C, in inert atmosphere for avoiding the
oxidation process that could take place because of the
increasing temperature. After separation and drying.
these compounds were analysed by X~ray diffraction on
a HZG 4C Karl Zeiss Jena diffractometer using Co(l{J
radiation, by IR spectroscopy on a SPECORD M82. The
chemical composition (C. H. 0 and Fe) of the final
compounds was afso determined.
38
Table I The composition of steels used for corrosion test
Steel
OL37
OL50
OL60
%C
0.20
0.30
0.40
%Mn
0.80
0.80
0.80
%S
0.06
0.05
0.05
%Si
0.40
0.40
0.40
%P
0.06
0.05
0.05
Table 2 The values of indices K and P for the studies steels
1
2
3
4
5
OL37
KIP
0.303/0.27
0.312/0.28
0.325/0.29
0.306/0.27
0.301/0.27
OL50
KJ!P
0.293/0.26
0.307/0.28
0.315/0.28
0.301/0.27
0.295/0.26
OL60
KIP
0.286/0.26
0.293//0.26
0.305/0.27
0.287/0.26
0.273/0.25
Fig.] The metallic surface ofOL 60 in the system methanol-
ethylene glicol- n-butiric acid- 3% water (x 1200)
Results and discussion
Studying the pH - values, we observed that in
anhydrous system. the pH value is 2.7 and adding water,
it is very slowly decreased to 2.6, at 5% water
concentration. This practically constant pH-variation
showed that n-butiric acid ionisation occurred with the
help of the two solvents. Water had not a significant
importance in modifying the dissociation of the acid.
In Table 2. the values of weight losses converted
into K and P figures are presented. From these values,
we can observe a significant decrease the corrosion rate
between 3 % and 4 % water concentration. This
observation leads us to the conclusion that for values
higher than 3 %~ water molecules participate in the
formation of passive oxyhydroxyde layer establishing.
The metallic surface~ visualised by electron microscope
had the same aspect for an three types of steel,
indicating a generalised corrosive process with a
corrosive compound .layer on the entire surface. This
layer is not uniform~ presenting holes~ and cannot
accomplish an efficient anticorrosive protection. In
Fig./. the aspect of the metallic surface of OL 60 in the
system with 3 % water concentration. magnified by
1200 times is presented.
The polarisation curves were plotted for the values
of 3% and 5<.i- water concentration. They had the same
Table 3 The values of the corrosion parameters in the system
methanol - 10% ethylene glycol - n-butiric acid - water
Types of
EsbmV Ecor, mV icov ~cm
2
3% 5% 3% 5% 3% 5%
steels
H~O H20 H20 H20 HzO H20
OL37 -483 -427 -510 -440 2.03 1.32
OL50 -467 -403 -480 -415 1.86 1.28
OL60 -449 -395 -465 -410 1.73 1.05
Fig.2 The polarization curves in the system methanol -
ethylene glicol- n-butiric acid- 3% water (• OL 37; x OL 50;
o OL 60)
15 xt12an·'
7
Fig.3 The IR spectra of the corrosion compounds obtained in
the system methanol - ethylene glicol - n-butiric acid - 3%
water
shape, as presented in Fi'g.2. From the shape of these
curves. the kinetic corrosion parameters were calculated
and presented in Table 3.
The value of the corrosion current density, that
practically shows the corrosion rate, decreases after the
3 % water concentration in the system. This observation
is in concordance with the determined value for the
corrosion rate, observed by the weight loss.
Studying the values of weight losses and also of the
corrosion current density, we observed that the steel
with the best behaviour is OL 60.
In order to establish a corrosion mechanism. the
final corrosion compounds were analysed.
The X - ray diffraction spectra of the corrosion
compounds, of all types of steel, are the same and are
being characteristic to the amorphous compounds.
Table 4 The chemical composition of the corrosion
compounds in the system methanol - 10% ethylene glycol - n-
butiric - 3% water
% OL37 OL50 OL60
c 42.17 42.86 42.56
H 7.04 7.35 6.87
0 28.56 28.47 23.03
Fe 22.23 21.32 21.54
The IR - spectra are also identical, meaning that
they present the same absorption bands at the same
wave number; the IR-spectrum of the corrosion
compound in the system containing 3 % water is
presented in Fig.3.
In this spectrum, as it is mainly observed, the
displacement of the characteristic peak of n-butiric acid,
for the group coo-, from the value 1729 cm-1 to 1690
cm-1• This fact is explained by a stronger bonding of
carboxylic oxygen. Also, the splitting in two peaks at
this value explains the existence of two types of bonds,
one covalent and the other coordinative [6]. The peak
from 1550 cm-1 shows that this group is asymmetrically
bonded. The absorption bands of HO- are placed at 3400
cm-1 and 2900 cm·I, case in which water is coordinative
bonded through oxygen and also, they indicate that
these groups participate to a bridge linkage Fe+--0-Fe
[7].
In the IR-spectrum, a peak at 1080 cm-1 is also
observed, characteristic for the OH groups~ from alcohol
[6-8]. The remaining maxima from the spectrum
correspond to the vibration and rotation movements of
C-C and C-H bonds.
The elemental chemical composition of the
corrosion compounds is similar for all water
concentrations (variation of 2 %). In Table 4, the
chemical composition of the corrosion compound
derived from the system with 3 % water is presented.
From the presented data the following mechanism is
assigned:
Fe0 -> Fe3+ + 2e·
Hz0+1120z+2e-> 2Ho-
Fe2+ + Ho- -7 Fe(OH)2
Fe(OH)z + 2 C3H7- COOH -7 Fe(C3H~OO)z + H20
2Fe(C3H~OO)z + H20+1/2 0z ~ 2 [Fe(C3H~OO)zOH]
n[Fe(C3H~OO)zOH} POUMERIZATION ) [Fe(C3H7C00)20H]n
The structure of iron(III) polybutirate is presented in
Fig.4.
Also, it is very important to mention that in the
studied references, were not found the data related to the
compounds of iron with n-butiric acid.
The hexacoordination of iron and the bond of the
final chain molecules are realised through CH30- and
HO- CH2 - CH2- o- groups.
Conclusions
• Ail three types of steel there is a given water
concentration where the corrosion rate has a
maximum value.
39
Fig.4 The structure of the polymer compound of the iron with
the n-butiric acid
• The molar ratio water/acid, from which the
corrosion rate decreases is 2.39, is smaller than
the one presented in literature [9 - 11] for
achieving an oxyhydroxylic passive layer. In
literature, the value for this molar ratio is 4. The
smaller value can be justified by the fact that the
acid is dissociated in ions and, through the two
organic solvents, water molecules are able to
participate at smaller concentrations in the
formation of the passive layer. This is in
concordance with the minimum variation of pH,
if into the system, more water is added.
• The steel with the highest stability in all the
cases is OL 60, which has the highest carbon
content; the stability may be connected to the
existence in the steel structure of a solid iron-
carbon solution, that is formed when the carbon
concentration is grater than 0.3 % [12]. The
corrosion mechanism is a complex one involving
oxygen from the atmosphere.
REFERENCES
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Hung. J. Ind. Chem.J999, 27, 107-110
3. SUTIMAN D., CRETESCU I. and NEMTOI G.: Rev.
Chim., 1999,50, 10,766-770
4. SUTIMAN D., CRETESCU I. and CAILBAN A.: Rev.
Chim., 2000,51, 11, 889-892
5. SUTIMAN D., CRETESCU I. and VIZITIU M.: Rev.
Chim. 2000,51, 12,986-989
6. AVRAM M.: Infrared Spectroscoy, Ed. Tehnica,
Bucuresti, p. 15-86, 1960 (in Romanian)
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Physical Methods Applied in Organic Chemistry,
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Romanian)
8. The Sandler Handbook of Infrared Spectra, Sandler
Hayden, London, 1978
9. BANAS J.: Electrochim Acta, 1987,32,871-875
10. BANAS J.: Mat. I Odlew, 1990, 16, 73-80
11. HOOR T. P.: J. Ele.ctrochim. Soc., 1990, 16,73-77
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