The computer fitting results of Nyquist plots and Bode plots
(impedance and q) for AA5052 aluminium alloy in 4 M sodium
hydroxide-ethylene glycol solution containing 1mMADA are given
in Fig. 4(b) and (c). It is evident that the errors are within 10% for the
fitting results and the experimental data.
The reaction mechanism is studied according to fitting the
experimental data with the equivalent circuit. At higher frequencies,
the first capacitive semicircle is ascribed to the redox
reaction of Al4 Alþ. This charge transfer process is considered to
be the rate-controlling step. At medium frequencies, the first partinductive loop is formed due to the intermediates deposited on the
aluminium alloy surface. At lower frequencies, the second capacitive
semicircle is attributed to redox reaction of Alþ 4 Al3þ. This
process is the fast complementary step. The curves show that
increasing the concentration of dicarboxylic acids leads to increase
the size of the capacitive semicircles, indicating the increase of
charge transfer resistance. Compared with the blank solution, RS,
(Rct,1), R1, and (Rct,2) increase evidently. It shows that the presence of
ethylene glycol decreases the ionic conductivity of the electrolyte to
some extent. The largest Rp value is obtained for the AA5052 alloy
by the addition of the dicarboxylic acids. The presence of 1mMADA
has the best inhibition effect for the self-corrosion AA5052 alloy. It
is attributed to the formation of complex film via coordination bond
between dicarboxylic acid and aluminium ions. We also measured
the performance of dicarboxylic acids at higher concentration
(1.5 mM and 2 mM) in gasometry tests and electrochemical tests.
However, little change has been observed at higher concentration
compared with the 1 mM.
The computer fitting results of Nyquist plots and Bode plots(impedance and q) for AA5052 aluminium alloy in 4 M sodiumhydroxide-ethylene glycol solution containing 1mMADA are givenin Fig. 4(b) and (c). It is evident that the errors are within 10% for thefitting results and the experimental data.The reaction mechanism is studied according to fitting theexperimental data with the equivalent circuit. At higher frequencies,the first capacitive semicircle is ascribed to the redoxreaction of Al4 Alþ. This charge transfer process is considered tobe the rate-controlling step. At medium frequencies, the first partinductive loop is formed due to the intermediates deposited on thealuminium alloy surface. At lower frequencies, the second capacitivesemicircle is attributed to redox reaction of Alþ 4 Al3þ. Thisprocess is the fast complementary step. The curves show thatincreasing the concentration of dicarboxylic acids leads to increasethe size of the capacitive semicircles, indicating the increase ofcharge transfer resistance. Compared with the blank solution, RS,(Rct,1), R1, and (Rct,2) increase evidently. It shows that the presence ofethylene glycol decreases the ionic conductivity of the electrolyte tosome extent. The largest Rp value is obtained for the AA5052 alloyby the addition of the dicarboxylic acids. The presence of 1mMADAhas the best inhibition effect for the self-corrosion AA5052 alloy. Itis attributed to the formation of complex film via coordination bond
between dicarboxylic acid and aluminium ions. We also measured
the performance of dicarboxylic acids at higher concentration
(1.5 mM and 2 mM) in gasometry tests and electrochemical tests.
However, little change has been observed at higher concentration
compared with the 1 mM.
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