INTRODUCTION:
Several industries use acid solutions for cleaning, de-scaling and pickling of steel structures. These processes are normally accompanied by considerable dissolution of the metal. Corrosion inhibitors are used to reduce the loss due to metal dissolution. Compounds containing hetero atoms like N, S, O and inorganic compounds like chromates, phosphates, molybdates etc are being investigated as corrosion inhibitors1-6. The organic compounds can adsorb on the metal surface, block the active sites and thereby reduce corrosion rate. As most of the synthetic compounds are toxic and cause severe environmental hazards, use of plant extract7-14 as eco-friendly, non-toxic and harmless corrosion inhibitor is of significance.
This study is aimed at investigating the effectiveness of Caesalpinia pulcherrima (CP) flowers as corrosion inhibitor for mild steel in 1N hydrochloric acid by weight loss, polarization, impedance and SEM techniques.
EXPERIMENTAL:
Material preparation:
Mild steel sheet of about 1.5 mm thickness was mechanically cut to form coupons each of dimension 5 × 1cm2. The surface preparation of the mechanically polished specimen was carried out using different grades of emery paper and then degreasing with acetone.
They were finally washed with deionized water, dried and stored in a desiccator. Accurate weights of the samples were taken using electronic balance. The exposed area for polarization studies of the mild steel specimen was 1cm2.
Inhibitor preparation:
The stock solution of the inhibitor material (Caesalpinia pulcherrima extract) was prepared by refluxing 15g of dry CP powdered flowers with 500ml of 1N hydrochloric acid (Analar grade) for 3 hours. The refluxed solution was allowed to stand overnight and filtered. The residue was repeatedly washed with small quantities of 1N hydrochloric acid and the filtrate was made up to 500ml. From this stock solution different concentrations of the extract from 0.006 % to 0.6 % v/v were prepared.
Weight loss Method:
Weighed mild steel specimens were suspended in 1N hydrochloric acid in triplicate in the absence and presence of various concentrations of the inhibitor for different intervals of time (Table -1). The coupons were then removed from the test solution, thoroughly washed with sodium bicarbonate solution, then with deionized water, dried well and then weighed. The inhibition efficiency (%IE) was calculated using the formula (1).
Temperature study:
Weight loss determinations were carried out at different temperatures viz 303K, 313K, 323K, 333K, 343K, 353K. After initial weighing, the specimens were immersed in 100 ml of blank 1N hydrochloric acid and in 1N hydrochloric acid containing different concentrations (0.006% to 0.6% v/v) of the plant extract at the above temperatures. The thermostat was set to the appropriate temperature and after 1hr immersion the specimens were removed, washed, dried, and reweighed.
Calculation of corrosion parameters:
1. Inhibitor efficiency (IE %):
The Inhibitor Efficiency was calculated using the formula
IE (%) =
Weight loss without inhibitor – Weight loss
with inhibitor×100 (1)
Weight loss without inhibitor
2. Corrosion rate (CR):
The corrosion rate was determined using the formula
Corrosion rate = 534 × Weight loss × 1000 (2)
DAT
Where, D= Density (7.9 g/cm3 for mild steel).
A= Area in square inches.
T= Time in hours.
3. Electrochemical studies:
Electrochemical experiments were performed in a conventional three electrodes cell assembly with mild steel as working electrode, a platinum electrode as counter electrode and saturated calomel electrode as reference electrode.
The polarization measurements were performed with Parstat 2273. Potentiodynamic anodic and cathodic polarisation curves were obtained in the potential range -0.2mv to -1mv with a scan rate of 2mv/s relative to the corrosion potential (Ecorr). Extrapolation of the cathodic branch of the polarization curve back to Ecorr gave values for corrosion current density (Icorr). Measurements of Rp in the vicinity of Ecorr were also carried out. Impedance spectra (EIS) were recorded at Ecorr in the frequency range 0 to 10000 Hz.
SEM Studies:
The surface morphology of the specimen after immersion for 1 hr at 30oC in 1N blank hydrochloric acid and in 1N hydrochloric acid containing 0.6% v/v of inhibitor was performed on a JSM-6390 scanning electronic microscope.
RESULTS AND DISCUSSION
The IE of the plant extract in 1N hydrochloric acid as calculated by gravimetric method with different inhibitor concentrations and different immersion periods is indicated in Table1
The IE was found to increase from 21.73% at inhibitor concentration of 0.006% v/v up to 95.65% at inhibitor concentration of 0.6% for 1 hour immersion (Figure-1). Above 0.6% inhibitor concentration there was not much change in efficiency. Corrosion rate is found to decrease with increase in inhibitor concentration and with increased immersion time in acid. The corrosion inhibition may be due to adsorption of the phytoconstituents on the metal surface making a barrier for mass and charge transfers and thus protecting the metal surface from corrosion. The degree of protection increases with increasing surface coverage by the adsorbed molecules.
Fig. 1 Variation of inhibitor efficiency with concentration at various immersion periods in presence of Caesalpinia pulcherrima in 1N HCl
The decrease of IE with increase in immersion time (Figure-2) from 1 hr to 24 hrs may be due to physisorption and desorption of the phytoconstituents which happens with increase in time whereby the metal surface becomes exposed.
Fig. 2 Variation of inhibitor efficiency with immersion time in presence of Caesalpinia pulcherrima extract in 1N HCl
Fig. 3 Variation of corrosion rate with temperature at different concentrations of Caesalpinia pulcherrima extract in 1N HCl