A Comparative Study of Inhibitive Effects of Some Schiff Bases on Mild Steel in Acidic Media by Mass Loss Techniques

Mass loss techniques have been employed to study the corrosion inhibition of some newly synthesized Schiff bases. viz. N-4-(Diethyl amino) salicylidine-2-amino-5-chloropyridine (SB1), N-(4-ethylbenzalidine)-2-amino-5chloropyridine (SB2), N-(4-ethylbenzalidine)-2-amino-5-bromopyridine (SB3), N-4-(Diethyl amino) salicylidine-2-amino-5-bromopyridine (SB4) for mild steel in HCl and H2SO4 solutions. Results of inhibition efficiencies from the mass loss technique show that Schiff bases are good inhibitors in both the acidic solutions. Inhibition efficiencies increase with the increase in the concentration of acids as well as those of inhibitors. Maximum efficiency for H2SO4 was found 99.78% and for HCl it was 97.92%.


Introduction
Mild Steel is an important metal regarding to its wide applications in industry in various mechanical and structural purposes, like in bridgework, industrial parts, steam engine parts, automobiles, for ship hulls and off shore drilling platforms etc. Mild steel is passive towards an alkali but it is prone to corrosion in acids like HCl and H 2 SO 4 solutions.These acids are generally used for drilling operations, Pickling baths and in descaling processes 1 .
Corrosion of mild steel is very common and serious problem, which causes considerable economic loss through out the world.Although it is inevitable but proper maintenance, good design and effective inhibitors may control it.Corrosion of mild steel and it alloys in different acid media has been extensively studied [2][3][4][5] .

Experimental
A rectangular specimen of mild steel having dimensions 2.0 cm x 2.0 cm x 0.021 cm containing a small hole of about 0.02 cm diameter near the upper edge were taken.The approximate composition of the specimen was 99.7% Fe, 0.11% C, 0.5% Mg, 0.13% Si, 0.06% S. Specimens were cut from a sheet and thoroughly cleaned by buffing to produce a spotless finish and then degreased.Finally each specimen was washed with acetone and dried.All chemicals used for the synthesis of Schiff bases were of analytical reagent grade and solutions of HCl and H 2 SO 4 were prepared in double distilled H 2 O.All the Schiff bases were prepared by conventional method i.e. by refluxing equimolar quantities of respective aldehydes and amines.Each specimen was weighed accurately with a digital balance up to the accuracy of 0.1 mg and then suspended in a borosilicate glass beaker of 50 mL capacity containing test solution, by a V-shaped glass hook made by capillary tubes at room temperature.After the test, Specimens were cleaned with running H 2 O and dried with hot air drier and then weighed again.The Percentage inhibition efficiency (η%) was calculated as 16 .

(
) 100 Where ∆M u = Mass loss of specimen in uninhibited solution, ∆M i = Mass loss of specimen in inhibited solution.The corrosion rate in mm/yr (milli mils per year) can be obtained by following equations 17 .
C.R (mm/yr) = ATD M 6 .87 × ∆ ∆M=Mass loss in mg, A=Exposed area of metal surface in cm 2 , T=Time of exposure in hours, D=Metal density in gm/cm 3 and 1 mil=0.001inch.Surface coverage (θ) of metal specimen by inhibitor was calculated as ( ) Where ∆M u and ∆M i have same significances as given in the formula of inhibition efficiency (η %).

Results and Discussion
Mass loss (∆M) and percentage inhibition efficiencies (η%) for different concentrations of HCl and inhibitors are shown in Table 1 and those of for the H 2 SO 4 are shown in Table 2.It is observed that percentage inhibition efficiency (η%) increases with increase in the concentrations of both the acids and also with the increase in the concentration of inhibitors.All the four Schiff bases show maximum inhibition efficiency at the highest concentration of both the acids 2 N at their highest concentration i.e. 0.7%.The maximum efficiency was shown by N-(4-ethylbenzalidine)-2-amino-5-bromopyridine (SB 3 ) in HCl (97.72%) and in H 2 SO 4 (99.78%).
Corresponding corrosion rate (mm/yr) and surface coverage (θ) for HCl solutions are depicted in Table 3 where as for H 2 SO 4 they are shown in Table 4.It is observed from both the tables that corrosion rate of mild steel decreases with the increase in the concentrations of inhibitors where as corrosion rate increases with the increase in the strength of HCl and H 2 SO 4 solutions.
Corrosion rate of mild steel is much higher in H 2 SO 4 than in HCl.It means H 2 SO 4 has more adverse effect on mild steel in comparison to HCl.Values of surface coverage indicate that inhibitors are in general more effective in H 2 SO 4 than in HCl.Surface coverage (θ) of metal specimen by inhibitors increases with the increase in the acid strength as well as with the increase in the concentration of inhibitors.Maximum surface coverage is observed at the highest concentration (2N) of acids at maximum concentration (0.7%).Surface coverage (θ) and log [θ/1-θ] values of mild steel in HCl solutions are depicted in Table 5.Where as, for H 2 SO 4 , these are shown in Table 6.It is observed from both the tables that as Surface coverage (θ) increases, the value of log [θ/1-θ] also increases.

Table 2 .
Mass loss ∆M & inhibition efficiency η% for mild steel in H 2 SO 4 solution with

Table 6 .
Surface coverage (θ) and log Langmuir adsorption isotherms for mild steel in 1.0 N H 2 SO 4ConclusionA study of five synthesized bases has shown them effective corrosion inhibitors for mild steel in HCl and H 2 SO 4 acid solutions.Mass loss method has shown that efficiencies of inhibitors increase with increase in the concentration of acids as well as those of inhibitors.Among the four Schiff bases under study maximum inhibition efficiency was shown by SB 3 in both acid solutions at 0.7% concentrations at the highest (2 N) strength.It has been observed that corrosion rate of mild steel is much higher in H 2 SO 4 than in HCl which gives the conclusion that H 2 SO 4 is much more corrosive for mild steel than HCl.Results of surface coverage indicate that synthesized Schiff bases are more effective in H 2 SO 4 than in HCl.