Corrosion Inhibition of Acidic Corrosion of Mild Steel in Acidic Medium by Cetyl Trimethyl Ammonium Chloride

The inhibition effect of Cetyl trimethyl ammonium chloride (CTMAC) on the corrosion of mild steel in 0.5M hydrochloric acid solution was investigated at different temperatures using weight loss measurement, electrochemical polarization and scanning electron microscopy. Weight loss measurement and electrochemical polarization curves revealed that this surfactant inhibits the mild steel corrosion and inhibition efficiencies up to 98% can be obtained. The inhibition efficiency calculated from these techniques is in reasonably good agreement. The observed corrosion data indicate that the inhibition of mild steel corrosion is due to the adsorption of the inhibitor molecules on the mild steel surface. The surface morphology of mild steel samples in absence and presence of the inhibitor was examined using scanning electron microscopy.

thiadiazole. Many surfactants [7][8][9][10][11] have been synthesized and a considerable number of investigations have been reposted on their unusual physicochemical properties, including their high surface activity, unusual changes of viscosity, unusual micelle structure and aberrant aggregation behavior. The presence of a hydroxyl group can increase the solubility of the inhibitor leading to higher inhibition efficiency 12 . In most inhibition studies the formation of donor acceptor surface complexes between free or Ë electrons of an inhibitor and vacant d-orbital of a metal were postulated 13 . Rozenfeld et al 14 have studied the inhibiting action of amines, monoethanol-amines, diethanolamines and triethanolamines and their mixture with potassium chromate on the corrosion of steel in sodium chloride solutions. Various inorganic acids ethanolamine salts could protect efficiently steel, iron nickel, aluminum, copper and other metals and alloys 4 . Alkanolamine salts, triethanolamine oleiate, tetratriethanolamine mono oleiate, tetratriethanol-amine trioleiate and tetratriethanolamine tetraleiate established their performance in NaCl containing solutions and the efficiency exceeded 95 percent 15 . The present paper deals with the study of inhibiting action of cetyl trimethyl ammonium chloride in acidic solution using weight loss, electrochemical polarization techniques and scanning electron microscopy.

EXPERIMENT
Mild steel specimens with the following composition were used C=0.14, Si=0.03, Mn=0.32, S=0.05, P=0.20, Ni=0.01, Cu=0.01, Cr=0.01 and Fe= Balance (wt %) for weight loss measurement mild steel spiciness of 3.0 cm × 1.5cm size were cut from the 0.25mm thick sheet whereas for electrochemical polarization investigates specimen of sizes 5.0cm × 1.5cm were used. All the samples were polished successively with emery papers of various grades (150,320 and 600) thoroughly cleaned with soap water, rinsed with distilled water and then with alcohol and dried in air. The dried and cleaned specimens were kept in desiccators over silica gel. Cetyl trimethyl ammonium chloride (CTMAC) was used as corrosion inhibitor of mild steel. Name structural formula and molecular mass of the inhibitor are gives in Table 1.
The aggressive solution used were made of AR grade HCl. Appropriate concentrations of the acid were prepared using double distilled water. Inhibitor concentrations ranging from 50 to 250 ppm in 0.5M HCl solutions were prepared.
Weight loss measurements of mild steel samples in the acidic solutions with and without the optimal concentration of the inhibitor was performed at various temperatures ranging from 25 0 to 45 0 C for 48 hours exposure time. The inhibition efficiency (%) of CTMAC was calculated by using the following equation.   For electrochemical polarization studies, mild steel samples of same composition coated with commercially available lacquer (Lakme) with an exposed area of 1.0cm 2 were used and the experiments were carried out in 0.5M HCl solutions at 35 ± 1 0 C with different concentration of CTMAC. Electrochemical experiments were carried out using a potentiostat/ golvanostat PGS 201T (Radiometer Analytical). All the potentials were measured against a saturated calomel electrode. Initially open circuit potential (OCP) was measured as a function of time after that the specimen was polarized in cathodic direction from the OCP value. The specimens were again left under open circuit condition till steady state corrosion potential value was again reached. Then anodic polarization curves was recorded potentiostatically, generally duplicate experiments were performed to confirm the consistency of the results. To study the morphology of corroded surface of metal after exposing it to 0.5M HCl solution and break down of the passive film, scanning electron microscope (JSM -840 J EOL) was used. All the micrographs were taken at magnification of × 400.

RESULTS AND DISCUSSION
Corrosion rate measured by immersion test for mild steel in 0.5M HCl solutions at 25°, 35°a nd 45°C have been recorded in Table 2. It is seen that CTMAC inhibit corrosion of mild steel in 0.5M HCl solutions at all concentration under study. It has been observed that IE (%) increases with increase in concentration as shown in Fig 1. Maximum inhibition efficiency of each compound within the range of chosen concentration was achieved at 250ppm and a further increase in concentration did not show any appreciable change in the performance of the inhibitor. The influence of temperature on IE (%) at 250 ppm of CTMAC is shown in Fig-2. It is observed that IE increases with increase in temperature from 25 to 35°C after that it decreases when the temperature is increased from 35 to 45°C. The decrease of IE (%) with temperature is attributed to desorption of the inhibitor molecules from metal surface at higher temperatures. The polarization behavior of mild steel in 0.5M HCl in the absence and presence of various concentrations of CTMAC is shown in Fig 3. Various parameters like corrosion current density Icorr, anodic tafel's polarization βa, cathodic tafel's polarization βc, corrosion rate IE (%) are recorded in Table 3. The maximum decrease in Icorr was observed at 250 ppm concentration of CTMAC. The inhibition of corrosion of mild steel in the 0.5 M HCl solution can be explained on the basis of adsorption.
In order to evaluate the condition of mild steel surfaces on contact with acid solutions, a superficial analysis was carried out. The SEM micrograph of the specimens in presence of HCl at 35°C is shown in Fig 4. The influence of the CTMAC addition (250 ppm) on the mild steel in HCl solution at 35°C is shown in Fig 5. The surface roughness of the mild steel surface appears lowers with addition of inhibitor than that with out addition. The roughness is found to be more uniform after treatment with acidic solution that contains inhibitor.

CONCLUSION
Cetyl trimethyl ammonium chloride inhibited mild steel corrosion in 0.5M HCl acid solutions. CTMAC act as mixed type of inhibitor for mild steel in 0.5 HCl solutions.
Inhibition efficiency increases up to 35°C and then decreases at higher temperatures. The efficiency of CTMAC increases with concentration up to 250ppm.