(19) The purpose of this article is to determine the influence of 3-methyl-4-amino-5-mercapto-l,2,4-triazole (MAMT) on the corrosion of 6061 Al alloy in sodium hydroxide solution, using potentiodynamic polarization and electrochemical impedance techniques.
Table 1: The chemical composition of the 6061 Al alloy Element Cu Si Mg Cr Al Composition (wt%) 0.25 0.6 1.0 0.25 balance Synthesis of MAMT
The MAMT was synthesized and recrystallized as per the reported procedure in a one-step reaction of thiocarbohydrazide and glacial acetic acid.
The polarization studies of the alloy specimens were investigated in 0.5 M NaOH solution containing various concentrations of MAMT at different temperatures using Tafel polarization technique.
As can be seen from the data, addition of MAMT decreases the corrosion of the base alloy sample.
Table 3: Activation parameters for the corrosion of 6061 Al alloy in 0.5 M sodium hydroxide solution in the presence of MAMT Inhibitor [E.sub.a] [DELTA] [H.sun.a] - [DELTA] [S.sub.a] concentration (kJ mo[l.sup.-1]) (kJ mo[l.sup.-1]) (J mo[l.sup.-1] (ppm) [k.sup.-1]) 0 17.79 15.19 160.98 5 24.70 22.10 143.12 10 24.85 22.25 143.37 25 27.65 25.05 135.06 50 30.83 28.23 125.83 The effect of chemically stable surface active inhibitors is to increase the energy of activation and to decrease the surface area available for corrosion.30 The results in Table 3 indicate that the value of activation energy [E.sub.a], activation enthalpy [DELTA] [H.sub.a], and activation entropy [DELTA] [S.sub.a] are given in Table 3.
(33) The decrease in the inhibition efficiency of MAMT with increase in temperature can be attributed to an appreciable decrease in the adsorption of the inhibitor on the metal surface with increase in temperature and corresponding increase in corrosion rate because a greater area of the metal is exposed to the corrosion medium.
(42) In the present study, the value of enthalpy is -13.30 kJ mol-1 which suggests that the adsorption of MAMT on 6061 Al alloy involves a physisorption phenomenon.
Table 4: Thermodynamic parameters for the adsorption of MAMT on 6061 Al alloy surface in 0.5 M sodium hydroxide solution Temperature log k -[DELTA] [G -[DELTA] [H -[DELTA] ([degrees]C) [degrees].sub.ads] [degrees].sub.ads] [S[degrees] (KJ mo[l.sup.-1]) (KJ mo[l.sup.-1]) sub.ads] (KJ mo[l .sup.-1] [k.sup.-1]) 30 0.2400 11.51 35 0.2030 11.48 40 0.1701 11.47 13.30 58.70 45 0.1306 11.42 50 0.0989 11.40 The negative values of AG[degrees]ds indicate the spontaneity of the adsorption process and stability of the adsorbed layer on the metal surface.
In order to get more information about the corrosion inhibition of 6061 Al alloy specimens in 0.5 M NaOH containing different concentrations of MAMT, EIS measurements were carried out at different temperatures.
AC impedance data of 6061 Al alloy in 0.5 M NaOH in the absence and presence of different concentrations of MAMT are given in Table 5.