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    Der Pharma Chemica, 2015, 7(10):155-168 (http://derpharmachemica.com/archive.html)

    ISSN 0975-413X CODEN (USA): PCHHAX

    155 www.scholarsresearchlibrary.com

    Electrochemical and thermodynamic investigation of benzenamine as corrosion inhibitor of austenitic stainless steel (type 304) in hydrochloric acid

    solution

    Roland Tolulope Lotoa,b*, Cleophas Akintoye Lotoa,b, Ayodele Samuel Onawunmia,c and Williams Kupolati d

    aDepartment of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria

    bDepartment of Chemical, Metallurgical & Materials Engineering, Tshwane University of Technology, Pretoria, South Africa

    cDepartment of Mechanical Engineering, Ladoke Akintola University of Technology, Ogbomoso, Oyo State, Nigeria dDepartment of Civil Engineering, Tshwane University of Technology, Pretoria, South Africa

    _____________________________________________________________________________________________ ABSTRACT The inhibiting action of benzenamine (BEZ) on the corrosion of type 304 stainless steel in 3M hydrochloric acid was investigated through electrochemical tests. Results show BEZ to be an excellent inhibitor. The compound was observed to be of mixed type in action. Inhibition efficiency increased with increase in BEZ concentration giving an optimal value of 96.14% at 6.25% BEZ concentration from weight loss and 88.7% at 5% BEZ concentration from polarization tests. X-Ray diffractometry revealed the absence of corrosion products. Statistical analysis using ANOVA showed the statistical significance of BEZ concentration on its inhibition efficiency performance. The compound obeyed Langmuir adsorption isotherm. Keywords: corrosion; benzenamine; inhibition; pitting; austenitic stainless steel; hydrochloric acid _____________________________________________________________________________________________

    INTRODUCTION

    Acid solutions are commonly employed in industry for cleaning, acid decaling and pickling of steel, which often lead to corrosion of the alloy. Hydrochloric acid is frequently applied in industrial operations such as chemical cleaning and pickling to abstract oxide scales and rusts from the metal surfaces. A cost effective technique for corrosion control in harsh conditions is the augmentation of chemical additive known as inhibitors to the acidic solution in contact with the metal surface. Effective inhibitors perform excellently even in aggressive environments and high temperatures between 60 to 95 °C. The most efficient corrosion inhibitors used in hydrochloric acid solutions consists of heteroatoms such as sulphur, nitrogen and oxygen [1 - 6].Organic compounds [7 - 13]have been reported for their corrosion inhibition performance for steel in acidic environments. They typically function via adsorption of the molecules on the metal surface, forming a protective film to corrosion attack. This research is to further establish the usefulness and economic viability of organic derivatives as for corrosion prevention. Therefore, the present study aims to investigate the inhibition effect and adsorption properties of benzenamine on Type 304 stainless steel corrosion in hydrochloric acid. Benzenamine is an aromatic amine compound comprising of a phenyl group connected to an amino group. Benzenamine is the prototypical aromatic amine. Being an antecedent to multifarious industrial chemicals, its major application is in the production of precursors to polyurethane.

  • Roland Tolulope Loto et al Der Pharma Chemica, 2015, 7 (10):155-168 _____________________________________________________________________________

    156 www.scholarsresearchlibrary.com

    MATERIALS AND METHODS

    Material Commercially available Type 304 austenitic stainless steel was used for all experiments of average nominal composition; 18.11%Cr, 8.32%Ni and 68.32%Fe. The material is cylindrical with a diameter of 18mm. Inhibitor Benzenamine(BEZ) a brownish, translucent liquid, obtained from SMM Instruments, South Africa is the inhibitor used. The structural formula of BEZis shown in Fig. 1. The molecular formula is C6H5NH2while the molar mass is 93.13gmol−1.

    Figure1 Chemical structure of Benzenamine(BEZ)

    BEZ was prepared involume concentrations of 1.25%, 2.5%, 3.75%, 5%, 6.25% and 7.5% per 200 ml of the acid solution. Test Media 3M hydrochloric acid with 3.5% recrystallised sodium chloride of Analar grade were used as the corrosion test media. Preparation of Test Specimens The cylindrical stainless steel (18mm dia.) was mechanically cut into a number of test specimens of dimensions in length ranging from 17.8mm and 18.8mm coupons. The two surface ends of each of the specimen were ground with Silicon carbide abrasive papers of 80, 120, 220,800 and1000 grits. They were then polished with 6 μm to 1 μm diamond paste, washed with distilled water, rinsed with acetone, dried and stored in a dessicator for further weight- loss test and linear polarization. Weight-loss Experiments Weighted test species were fully and separately immersed in 200ml of the test mediaat specific concentrations of the BEZ for 360h at ambient temperature of 25oC. Each of the test specimens was taken out every 72 h, washed with distilled water, rinsed with acetone, dried and re-weighed. Plots of weight-loss (mg)and corrosion rate (mm/y)versus exposure time (h)(Fig.2&3)for the test media and those of percentage inhibition efficiency(%IE)(calculated) versusexposure time (h) and percentage BEZ concentration(Fig.4 & 5) were made from table 1. The corrosion rate (R) calculation is from this equation 1:

    R= ���.���� (1) WhereW is the weight loss in milligrams, D is the density in g/cm2, A is the area in cm2, and T is the time of exposure in hours. The %IE was calculated from the relationship in equation 2.

    %IE = ����� �� ˟ 100 (2) The %IE was calculated for all BEZ concentration every 72 h during the course of the experiment, while the surface coverage is calculated from the relationship:

    � � �1 � � �� (3)

  • Roland Tolulope Loto et al Der Pharma Chemica, 2015, 7 (10):155-168 _____________________________________________________________________________

    157 www.scholarsresearchlibrary.com

    Where� is the substance amount of adsorbate adsorbed per gram (or kg) of the adsorbent. W1and W2 are the weight loss of austenitic stainless steel coupon in free and inhibited acid solutions respectively. Open Circuit Potential Measurement A two-electrode electrochemical cell with a silver/silver chloride was used as reference electrode. The measurements of OCP were obtained with Autolab PGSTAT 30 ECO CHIMIE potentiostat. Resin mounted test electrodes/specimenswith exposed surface of 254 mm2 were fully and separately immersed in 200ml of the test media (acid chloride) at specific concentrations of BEZ for a total of 288 h. The potential of each of the test electrodes was measured every 48 h. Plots of potential (mV) versus immersion time (h) (Fig. 6) for the two test media were made from the tabulated values in Table 2. Linear Polarization Resistance Linear polarization measurements were carried out using, a cylindrical coupon embedded in resin plastic mounts with exposed surface of 254 mm2. The electrode was polished with different grades of silicon carbide paper, polished to 6µm, rinsed by distilled water and dried with acetone. The studies were performed at ambient temperature with Autolab PGSTAT 30 ECO CHIMIE potentiostat and electrode cell containing 200 ml of electrolyte, with and without BEZ inhibitor. A graphite rod was used as the auxiliary electrode and silver chloride electrode (SCE) was used as the reference electrode. The steady state open circuit potential (OCP) was noted. The potentiodynamic studies were then made from -1.5Vversus OCP to +1.5 mV versus OCP at a scan rate of 0.00166V/s and the corrosion currents were registered. The corrosion current density (jcorr) and corrosion potential (Ecorr) were determined from the Tafel plots of potential versus log I. The corrosion rate (R),the degree of surface coverage (θ) and the percentage inhibition efficiency (%IE) were calculated as follows

    R = �.��� � ∗�����∗��

    � (4) Where Jcorr is the current density in uA/cm

    2, D is the density in g/cm3;Eq is the specimen equivalent weight in grams; The percentage inhibition efficiency (%IE) was calculated from corrosion rate values using the equation.

    %IE = 1 –����� ˟ 100(5) where C1and C2 are the corrosion rates in absence and presence of BEZ inhibitors, respectively. X-Ray Diffraction Analysis X-ray diffraction (XRD) patterns of the film formed on the metal surface without BEZ addition was analyzed using a Bruker AXS D2 phaser desktop powder diffractometer with monochromatic Cu Kα radiation produced at 30kV and 10mA, with a step size of 0.03o 2θ. The Measurement program is the general scan xcelerator. Analysis of the steel sample inhibited with BEZ was done with PANalyticalX’Pert Pro powder diffractometer with X’Celerator detector and variable divergence- and receiving slits with Fe filtered Co-Kα radiation. The phases were identified using X’PertHighscore plus software. Statistical Analysis Two-factor single level statistical analysis using ANOVA test (F-test) was performed so as to investigate the significant effect of BEZ inhibitor concentration and exposure time on the inhibition efficiency values of BEZ in the

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