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Abstract
In the present study, chemical (Weight loss) and electrochemical
(EFM, Potentiodynamic polarization and EIS) methods were used to
study the ability of some amino acids as non-toxic type inhibitors to
inhibit the general corrosion of low alloy steel in aerated stagnant
Hydrochloric, Sulfuric and perchloric acid solutions.
The study includes the comparison of corrosion inhibition of low
alloy steel in 0.5M Hydrochloric, 0.5M Sulfuric and 0.5M Perchloric
acid solutions by Glycine and Tyrosine using several techniques include
the new technique Electrochemical Frequency Modulation (EFM),
besides Tafel extrapolation, Electrochemical Impedance Spectroscopy
(EIS) and weight loss methods. The measurements were carried out at
five temperatures ranging from (20-60°C) in the absence and presence
of various concentrations (1x10-3 to 5x10-2 M) of Glycine and Tyrosine.
Moreover, the anodic behaviour of low alloy steel in perchloric
acid solutions in the absence and presence of Glycine and Tyrosine was also
investigated.
The principle conclusions are:
01) The used amino acids Glycine and Tyrosine have been shown
good inhibiting properties for low alloy steel in 0.5M Hydrochloric,
0.5M Sulfuric and 0.5M Perchloric acids.
02) The inhibition efficiency increases with the increase of inhibitor
concentration but decreases with the increase of temperature.
03) Polarization curves in 0.5M HCl, 0.5M H2SO4 and 0.5M HClO4 in
absence and presence of Glycine and Tyrosine indicated that these
compounds act as mixed type inhibitors, they increase both the
anodic and cathodic overpotentials and shift parallely the position
of the Tafel lines to both directions. This means that these
compounds suppress both the cathodic evolution of hydrogen and
anodic dissolution of the alloy and have no effect on the corrosion
mechanism and so the adsorbed species exert their action by simple
blocking of the active sites on steel surface and hence act as
blocking type inhibitors.
04) The results obtained from (EIS) show that the corrosion reactions
in the absence and presence of both inhibitors proceed under charge
transfer control. The increase of concentration of the inhibitors
leads to an increase in the value of the charge transfer resistance
(Rct) i.e. a decrease of the corrosion rate of the steel. The double
layer capacitance (Cdl) of the corroding low alloy steel interface
decreases with increase in the inhibitor concentration, suggesting
an increase of the surface coverage of the inhibitor due to the
adsorption of the inhibitor species at the steel surface.
05) The new technique Electrochemical Frequency Modulation
(EFM) was used as a rapid and non-destructive technique for
corrosion rate measurements. Corrosion current densities (Icorr)
obtained with this technique were in good agreement with those
obtained from Tafel extrapolation technique. In addition the
Causality factors were good internal check for verifying the validity
of data obtained by this technique.
06) All the data obtained from the four different methods, namely,
weight loss, Electrochemical Frequency Modulation (EFM),
Potentiodynamic Polarization, and Electrochemical Impedance
Spectroscopy (EIS) measurements are in good agreements. The
results obtained from the four different methods show similar and
parallel trends. Based on these results, the Electrochemical
Frequency Modulation (EFM) technique appears capable of
monitoring the corrosion inhibition of low alloy steel/0.5M HCl,
low alloy steel/0.5M H2SO4 and low alloy steel/0.5M HClO4
systems in absence and presence of various concentrations of
Glycine and Tyrosine and at different temperatures.
07) Activation parameters for corrosion process of HCl and H2SO4 in
absence and presence of studied inhibitors were determined from
the data obtained from the above four techniques at five
temperatures and six different concentrations of the inhibitors. The
activation parameters showed higher activation energies and
enthalpies indicative of the high protection efficiency as
concentration of inhibitor increased. The activation energy ( o
a E )
values are less than (80 kJ mol-1) as an indicator of physical
adsorption. The entropy change (DSo ) values in the presence and
absence of inhibitors are large and negative, meaning that an
increasing in ordering takes place in going from reactants to the
activated complex. The sign of ( o
H ads ) is positive indicating that
the corrosion of low alloy steel in HCl and H2SO4 is endothermic.
08) Adsorption of Glycine and Tyrosine at the steel surface in these acid
solutions gave a good fit to Temkin isotherm. The equilibrium
constant of adsorption process (Kads) is relatively small, decreases
with temperature indicative of weaker binding to the metal surface
and hence a lower inhibition efficiency at higher temperatures. The
adsorption thermodynamic parameters showed the values of the
standard free energy change of adsorption ( o
G ads ) acquire a negative
sign ensure the spontaneity of the adsorption process and stability of
the adsorbed layer at the steel surface which accompanied with a high
efficient adsorption for such compounds. In addition the values of
( o
G ads ) are less negative than (-40kJmol-1) indicate a physical
adsorption which involves electrostatic interaction between the
charged inhibitor species and the charged metal surface. Also The
calculated values of ( o
H ads ) are negative, confirming the exothermic
nature of the adsorption of Glycine and Tyrosine at the steel surface.
The entropy ( o
S ads ) values of the inhibition process for the adsorption
of Glycine and Tyrosine at steel surface are large and negative, this
reflects the formation of an ordered stable layer of inhibitors at the
alloy surface.
09) The inhibition of Glycine and Tyrosine in acid solutions is due to the
adsorption of the amino acids (protonated and neutral molecules) at
the steel surface and blocking its reaction active sites. The difference
in inhibitor efficiencies is probably associated with a dissimilar
structural distribution of molecules on the substrate. The high
inhibitive force of Tyrosine more than Glycine may be ascribed to the
presence of a phenylic ring, which provides some of its electronic
density along with a steric effect to the inhibitor structure.
10) SEM and EDX examinations of the steel surface confirm the
existence of such a protective adsorbed film at the low alloy steel
surface. These data support the results obtained from chemical and
electrochemical methods that Glycine and Tyrosine are good
inhibitors for low alloy steel in in 0.5M HCl, 0.5M H2SO4 and 0.5M
HClO4 acid solutions.
11) The anodic behaviour of low alloy steel in perchloric acid
solutions devoid of and containing various concentrations of
Glycine and Tyrosine was investigated by cyclic voltammetric
technique.
12) In pure HClO4 acid solutions, the anodic polarization involves
one anodic peak (peak A). The peak current density of peak
increases with increasing HClO4 concentration. The peak A is
followed by passive region prior to oxygen evolution potential. The
passivity is due to existence of iron oxides on the anode surface.
The cathodic sweep involves one cathodic peak (peak C). This
cathodic is conjugated to (peak A).
13) The peak current density of peak increases with increasing the
scan rate and indicates that the anodic reaction under peak A is
under diffusion control.
14) The addition of either Glycine or Tyrosine inhibits the anodic
active dissolution of the low alloy steel in 0.5M HClO4. The
inhibition action of both compounds increases with increasing both
inhibitor concentration. The inhibition action of these inhibitors is
due to their adsorption at the steel surface.
15) The values of inhibition efficiency (IE%) of Glycine and
Tyrosine for low alloy steel in 0.5M HCl, 0.5 M H2SO4 and 0.5M
HClO4 solutions which obtained from the four various corrosion
monitoring techniques, assumed that the inhibition efficiency (IE%)
increase in the order H2SO4 > HCl > HClO4.