N', N'-dialkylhydrazides as inhibitors of acid corrosion of steel

Article abstract of DOI:10.1134/S1070427209010121

The effect of N',N'-dialkylhydrazides of aliphatic carboxylic acids of the general formula C₄H₉CH(C₂H₅)C(O) NHN(R)₂ on the corrosion behavior of St.20 steel in 0.1 and 1 M HCl was studied by electroch

Full text of DOI:10.1134/S1070427209010121

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 1, pp. 57−61. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © M.G. Shcherban’, T.D. Batueva, A.V. Radushev, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 1, pp. 58−62.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
N', N'-Dialkylhydrazides as Inhibitors of Acid Corrosion of Steel
M. G. Shcherban’, T. D. Batueva, and A. V. Radushev
Perm State University, Perm, Russia
Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russia
Received January 22, 2008
Abstract—The effect of N',N'-dialkylhydrazides of aliphatic carboxylic acids of the general formula
C4H9CH(C2H5)C(O)NHN(R)2 on the corrosion behavior of St.20 steel in 0.1 and 1 M HCl was studied by
electrochemical and gravimetric methods.
DOI: 10.1134/S1070427209010121
Corrosion of metal alloys causes enormous damage
to both the industry and the environment. Annual metal
loss reaches 12% of the total metal stock, or 30% of
the annual metal production [1]. Therefore, search for
new inhibitors remains an urgent problem. Among
N',N'-dialkylhydrazides of aliphatic carboxylic acids
(DAHCAs), N',N'-dialkylhydrazide of 2-ethylhexanoic
acid (DAH-2EHA) was tested as inhibitor of acid
corrosion. It is known that some hydrazides are used
as drugs [2], in polymer synthesis [3], as extractants
for nonferrous and rare metals [4], as collectors in KCl
flotation [5], and as corrosion inhibitors [6].
was washed with water and a sodium carbonate solution.
The products were recrystallized from acetone–water
mixtures (3 : 1 to 7 : 1). 2-Ethylhexanoic acid N',N'-
di(2-ethylhexyl)hydrazide was purified by transforming
it into salicylate, with the subsequent neutralization with
ammonia. The product purity was confirmed by TLC, IR
and 1H NMR spectroscopy, and analysis by nonaqueous
potentiometric titration [8] and volumetric heterophase
titration with sodium lauryl sulfate [9]. The main
substance contents and melting points of the hydrazides
are listed in Table 1.
Corrosion tests were performed with samples made
of St.20 steel. Gravimetric tests were performed with flat
samples of size 30 × 20 × 2 mm, and electrochemical
studies, with flat electrodes with a working surface
area of 1 cm2. Sample pretreatment involved trimming
with fine abrasive papers, degreasing with alcohol,
and washing with distilled water. In gravimetric tests,
the whole surface of the samples was exposed to the
solution, and in electrochemical tests all nonworking
surfaces were insulated with a wax–rosin mixture. The
tests were performed in 0.1 and 1 M HCl (chemically
pure grade). Working electrolytes were prepared in
distilled water. An inhibitor sample was dissolved in
the corrosion medium, after which the protective action
Z and inhibiting effect γ were estimated gravimetrically
[7] using the formulas
It was shown previously [7] that the inhibiting effect
of hydrazides of fatty carboxylic acids (HFAs) in 0.1 M
HCl was manifested with C7–C8 substituents, and at R =
C9–C12 it amounted to 82–96%, whereas in neutral media
hydrazides with R = C7H15 and C10H21 acted as corrosion
stimulants, and for C8H17 and C12H25 the protective effect
did not exceed 11%.
N',N'-Dialkyl- and N',N'-dibenzylhydrazides of
2-ethylhexanoic acid (DAH-2EHA) of the general
formula C4H9CH(C2H5)C(O)NHN(R)2, where R = C2H5,
C4H9, C5H11, C8H17, CH2C6H5, and C4H9CH(C2H5)CH2
were prepared by the reaction of the unsubstituted acid
hydrazide with a twofold (by moles) amount of the
corresponding alkyl bromide in the presence of KOH.
The synthesis was performed in i-PrOH with the addition
of benzene for azeotropic distillation of water. The
mixture was refluxed for 15 h. Then the KBr precipitate
was filtered off, the alcohol was distilled off, and the
residue was dissolved in chloroform. The organic layer
ρ0 − ρ
ρ0
i0 − i
i0
Z =
100 =
100,
(1)
(2)
ρ0
ρ
γ
57

58
SHCHERBAN’ et al.
where ρ0 and ρ are the rates of steel corrosion without
inhibitor and with its addition (g m–2 h–1), respectively,
and i0 and i are the corrosion current densities (A m–2)
without inhibitor and with its addition, respectively.
1 M HCl with additions of DAH-2EHA and without
them. The concentration of the hydrazides in solutions
was varied within 0.02–0.05 g l–1 in 0.1 M HCl and
0.02–0.1 g l–1 in 1 M HCl, which is determined by the
solubility of these compounds in aqueous media [9]. The
results of the tests are given in Table 2. As can be seen,
in 0.1 M HCl, addition of 0.02 g l–1 hydrazide does not
noticeably affect the rate of St.20 corrosion. The protective
effect becomes noticeable at a hydrazide concentration
of 0.05 g l–1, amounting to approximately 60% for R =
C4H9, C5H11, and CH2C6H5, which is associated with an
increase in the length of the hydrocarbon radical in the
case of R = C4H9 and C5H11 and with the presence of an
aromatic ring in CH2C6H5.
The maximal protective effect Zin1MHCl at 0.1 g l–1
concentration of the additives was 50–70%. The best
results at both HCl concentrations were obtained with
R= C2H5, C4H9, C5H11, and CH2C6H5, and therefore
these derivatives were taken for further electrochemical
studies.
To estimate the polarization resistance, examine the
mechanism of the action of organic additives and their
effect on electrochemical processes occurring on the steel
surface in acids, and determine the rate of steel corrosion
icor, we used the method of polarization curves. The curves
were recorded with a PI-50 potentiostat in a YaSE-2
standard electrochemical cell with a saturated silver
chloride reference electrode and a platinum auxiliary
electrode. The working electrode was placed in the cell
with the electrolyte being tested and kept there for
30 min to attain a constant corrosion potential Ecor. The
cathodic and anodic polarization curves were taken in
the potentiostatic mode with a step of 20 mV. The time
of keeping the system at each potential was 1 min. When
estimating the polarization resistance for taking the U–I
characteristics, we chose the potential interval of ±30 mV
relative to the corrosion potential. The curves were taken
in the potentiostatic mode with a step of 3–4 mV in the
direction from cathodic to anodic region, with keeping for
1 min at each potential. The polarization resistance Rp was
calculated as the slope of the tangent to the polarization
curves in the point corresponding to i = 0.
Figures 1a and 1b show the polarization curves (PCs)
taken on St.20 in 0.1 and 1 M HCl without inhibitor and in
the presence of DAH-2EHAwith R = CH2C6H5. It can be
seen that in all the cases a Tafel dependence is observed,
and introduction of hydrazides in various concentrations
inhibits both processes. The results of treatment of the
PCs taken in the presence of DAH-2EHA are given in
Table 3. It can be seen that the DAH-2EHA inhibitors
affect both steps of the corrosion process, shifting
the corrosion potential toward more positive values,
decreasing the corrosion rate, and favoring an increase in
the Tafel slopes of both the cathodic and anodic curves.
Intense polarization of the electrode gives rise to various
factors that can distort the results of corrosion tests:
dissolution of the electrode surface, leading to an increase
in the true surface area; formation of corrosion products;
change in the electrolyte composition; and additional
cathodic currents at intense polarization. This can be
avoided by using the polarization resistance method
The corrosion current was calculated by the
equation
bc
ba
B
Rp
(3)
,
icor
Rp
2.3
bc ba
where bc and ba are the coefficients of the Tafel
equation, determined from the results of the polarization
measurements.
The data presented in this paper were obtained by
averaging the results of three to four measurements.
In the first step of our study, we determined
gravimetrically the rate of St.20 corrosion in 0.1 and
Table 1. Main substance contents and melting points of hydrazides C4H9CH(C2H5)CONHNR2
O
O
O
O
O
O
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N', N'-DIALKYLHYDRAZIDES AS INHIBITORS OF ACID CORROSION OF STEEL
59
Table 2. Corrosion rate of St.20 in 0.1 and 1 M HCl and protective (Z) and inhibiting (γ) effects of hydrazides
C4H9CH(C2H5)CONHNR2
Table 3. Corrosion-electrochemical characteristics of St.20 in 0.1 and 1 M HCl in the presence of inhibitors
C4H9CH(C2H5)CONHNR2
[10]. Comparison of the corrosion currents obtained by
extrapolation of the Tafel portions and calculated by the
method of polarization resistance [Eq. (3)] (Table 3)
shows that the results are in good agreement.
Table 4.ProtectiveeffectofinhibitorsC4H9CH(C2H5)CONHNR2
in 1 and 0.1 M HCl from the results of gravimetric and
electrochemical measurements
The protective effects determined from the electro-
chemical and gravimetric measurements are given in
Table 4. It can be seen that the absolute values determined
gravimetrically and calculated from the electrochemical
measurements do not coincide. This is due to the
following facts. The gravimetric measurements give the
corrosion rate averaged over the experimental period
(24 h), whereas in electrochemical measurements these
parameters are determined at a specific instant of time.As
the action of an inhibitor involves the occurrence in time of
adsorption processes on the electrode surface, the results
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60
SHCHERBAN’ et al.
of the gravimetric measurements should be considered to
be more reliable. However, the general trends observed
in the electrochemical and gravimetric measurements
coincide. In both cases, the hydrazides with R = C5H11
and CH2C6H5 are the most effective. On the whole, the
low values of the inhibiting and protective effects of the
DAH-2EHAs are most probably due to their relatively
low adsorption. An increase in the length and degree of
branching of the hydrocarbon radical leads to an increase
in the inhibiting power (increase in protective effects in
going from R = C2H5 to R = C5H11 and CH2C6H5). The
compounds with R = C8H17 and CH2CH(C2H5)C4H9
might have better characteristics if they were better
soluble in HCl solutions. It is known [11] that, similarly
to unsubstituted aliphatic acid hydrazides [12], the
hydrazides under consideration (HL) are amphoteric and
form organic cations (H2L+) in acid solutions:
0.80
0.40
2
1
0.0
2.0
4.0
0.70
0.50
0.40
2
1
H
pKa2
pKa1
H
(4)
L
L
H2
HL
A study of the effect of organic cations on partial
electrode reactions determining the iron corrosion in
acids shows that the corrosion deceleration in this case
is due to an increase in the overvoltage of electrode
reactions as a result of an increase in the positive value
of the adsorption ψ' potential [13]. In view of the large
size of cations of protonated DAH-2EHAs, it can be
assumed that the mechanism of their action as inhibitors
is mixed, i.e., the contribution of mechanical shielding of
the surface in adsorption of these additives should also
be taken into account.
1.0
1.0
3.0
Polarization curves taken on St.20 in (a) 0.1 and (b) 1 M HCl.
(E) Potential and (i) current density. (1) No inhibitor and (2)
DAH-2EHAwith R = CH2C6H5 added in an amount of (a) 0.05
and (b) 0.1 g l–1.
Thus, the examined compounds are complex
inhibitors, and their protective effect is due to a number
of factors, including the ψ' effect and the blocking
effect.
increase in the length and degree of branching of the
radical.
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