A Study of Mild Steel Corrosion Using Adhatoda Vasica ( AV ) Extract as Inhibitor in Different Acid Medium

Corrosion behavior of mild steel in acidic medium using Adhatoda vasica (AV) extract was investigated. The inhibitive effective of Adhatoda vasica on the corrosion of mild steel in different acidic medium has been studied by weight loss and polarization methods. The Ecorr values are shifted slightly towards negative side in presence of inhibitors which indicate the inhibitors inhibit the corrosion of mild steel in acids solution by controlling both anodic and cathodic reactions due to the blocking of active sites on the metal surface. It is evident that inhibitors bring about considerable polarization of the cathode as well as anode. It was, therefore, inferred that the inhibitive action is of mixed type.


Introduction
Mild steel is widely used in most of the chemical industries due to its low cost and easy availability for fabrication of various reaction vessels, tanks, pipes, etc. Since it suffers from severe corrosion in aggressive environments, it has to be protected. Acids have been used for drilling operations, pickling baths and in de-scaling processes. To reduce the corrosion problems in these environments inhibitive effects of various organic compounds have been tried so far. Organic compounds having hetero atoms are found to have higher basicity and electron density and thus assist in corrosion inhibition 1 . There are numerous naturally occurring substances like Embellica officianalis, Terminalia chebula, Terminalia bellirica 2 , a mixture of latter three, Sapindus trifolianus, Capparis decidua 3 , Prosopis juliflora 4 and Quinoline based Cinchona alkaloids 5 have been evaluated as effective corrosion inhibitors. Due to the bio-degradability, eco-friendliness, cost-effectiveness, less toxicity and easy availability of this material, the trend of using them have become increasingly important in the recent years. To evaluate the extract of Adhatoda vasica (AV) probable acid corrosion inhibitor in mineral acid for mild steel.

Experimental
According to ASTM method as reported already, all the test specimens having the size of 5 cm × 1 cm (Fe = 99.687, Ni = 0.012, Mo = 0.015, Cr = 0.043, S = 0.014, P = 0.009, Si = 0.007, Mn = 0.196 and C = 0.017) were used for the weight loss measurements. Mild steel specimen was polished using 1/0, 2/0, 3/0 and 4/0 emery papers and finally degreased with the organic solvent trichloroethylene. The specimens were weighed. After weighing the specimens were immersed in 1 M sulphuric acid with and without inhibitor. After 1 h, the specimens were washed with distilled water, dried and again weighed. The weight loss was noted. From this weight loss value, corrosion rate and inhibition efficiency were determined.
Corrosion rate has been determined from the following relationship.  Where, W 0 is the weight loss without inhibitor and W e is the weight loss with inhibitor.

Electrode surface preparation
The mild steel rod with an exposed area of 0.785 cm 2 was polished using 1/0, 2/0, 3/0 and 4/0 emery papers and finally degreased with the organic solvent trichloroethylene and immediately used for the experiments.

Electrode cell assembly
Electrochemical measurements were carried out in a glass cell with a capacity of 100 mL. A platinum electrode and a saturated calomel electrode were used as a counter electrode and reference electrode respectively. The mild steel electrode was then placed in the test solution (uninhibited and inhibited solutions) for 10-15 minutes before electrochemical measurements.
Electrochemical impedance spectroscopy (EIS) and Tafel polarization studies were conducted in a electrochemical measurement unit (Model 1280 B Solartron, UK). The EIS measurements were made at corrosion potentials over a frequency range of 10 kHZ to 0.01 Hz with signal amplitude of 10 mV. The Tafel polarization measurements were made after EIS for a potential range of -200 mV to +200 mV with respect to open circuit potential, at a scan rate of 1 m V/sec. The I corr , E corr , R ct and C dl values were obtained from the data using the corresponding "Corr view" and "Zview" softwares. The inhibition efficiency from potentiodynamic polarization was calculated from the value I corr by using the formula. Where, I corr(blank) is the corrosion current in the absence of inhibitor. I corr(inh) is the corrosion current in the presence of inhibitor. The inhibition efficiency from impedance measurements was calculated using the formula Where, R ct(inh) is the charge transfer resistance in the presence of inhibitor. R ct(blank) is the charger transfer resistance in the absence of inhibitor.

Results and Discussion
Mild steel corrosion has been investigated by weight loss method from 1 N HCl, 1 N H 2 SO 4 and 1 N H 3 PO 4 (1 h). From the weight loss values the corrosion rate and inhibition efficiency were calculated and tabulated in Table 1. From the table, it could be observed that corrosion rate decreased with increase in concentration of inhibitor. The corrosion rate of mild steel in 1 N HCl (blank) = 180 mmpy, 1 N H 2 SO 4 (blank) =196 mmpy and 1 N H 3 PO 4 (blank) =210 mmpy were obtained. From the table, it could be calculated that inhibition efficiency increased with increase in concentration of inhibitor. Maximum inhibition efficiency of AV (0.1%) was 77.77% for 1 N HCl. It is evident from the graph obtained by plotting concentration of inhibitor against inhibition efficiency as shown in Figure 1.     Popova et al 7 . also interpreted the decrease of I.E value with temperature increase as an indication for a physical or columbic type of adsorption. The inhibitive effect of substituted 1,3,4-thiadiazol-2-amines on the corrosion of mild steel in 1 M H 2 SO 4 has been studied by weight loss and electrochemical methods 8 . Therefore it can be conclude that thiadiazolines are adsorbed on the mild steel surface by physisorption. It could be observed from the table that the E corr values are shifted slightly towards negative side in presence of inhibitors suggesting that the inhibitors inhibit the corrosion of mild steel in acids solution by controlling both anodic and cathodic reactions due to the blocking of active sites on the metal surface. It is evident that inhibitors bring about considerable polarization of the cathode as well as anode. It was, therefore, inferred that the inhibitive action is of mixed type. The non-constancy of Tafel slopes for different inhibitor concentration reveals that the inhibitor action due to the interference in the mechanism of the corrosion processes at cathode as well as anode. The I corr values were decreased with increase in concentration of inhibitor. The inhibition efficiencies were determined from the values of corrosion current density and the inhibition efficiency values were found to show good agreement with those obtained from weight loss measurements.

Conclusion
Adhatoda vasica (AV) was acted as good inhibitor in HCl, H 2 SO 4 and H 3 PO 4 for mild corrosion. Since, it contains alkaloids. Corrosion rate decreased with increase in concentration of inhibitor in 1 N HCl, 1N H 2 SO 4 and 1 N H 3 PO 4 acid solutions. The inhibition efficiency increased with increase in concentration of inhibitor. Maximum inhibition efficiency was obtained for 1 N HCl (1 h) = 77.77% (0.1% of AV). In this investigation AV acted as good inhibitor in HCl than H 2 SO 4 and H 3 PO 4 . From the potentiostatic polarization studies curves, AV acted as mixed type inhibitor.