Propargyl as Corrosion Inhibitor for Al-5%Si/15%SiC Composite in 0.5molar Sodium Hydroxide

The corrosion characteristic of Al-5%Si/15%SiC composite in 0.5molar sodium hydroxide solution (ca--ustic soda) using propargyl as corrosion inhibitors was investigated employing gravimetric and potential measurements. The research was carried out at different inhibitor concentrations, time and temperature ran--ges of 0.5 – 2.5% v/v, 1– 5 hours and 30 – 70 ° C respectively. Results obtained revealed that propargyl is a moderate corrosion inhibitor for the composite, with maximum inhibition efficiency of 59.23% at 30 ° C and inhibitor concentration of 1.5% v/v. Thermodynamic parameters such as heat of adsorption, free energy and activation energy were obtained from experimental data and the mechanism of inhibition was elucidated. The inhibitor is physically and chemically absorbed unto the surface of the composite.


Introduction
Aluminium Matrix Composites (AMCs) are gain-ing considerable amount of industrial importance be-cause of their excellent combination of physical, me-chanical and electrochemical properties. Due to their interesting combination of properties, these materials are being used in many engineering applications like pistons, track shoes, brake drums, cylinder liners etc in automobile sectors [1], marine [2,3], mining and mineral processing applications [4,5].
One of the major limitations of AMCs has been corrosion in aqueous environment and several inv-estigations have been carried out extensively on the corrosion of AMCs with common reinforcements such as SiC and, Al 2 O 3 . Fang et al. [6] studied the synergistic effect of wear and corrosion on Al 2 O 3 particulate reinforced 6061 Aluminium Matrix co-mposite, and reported that under wear-corrosion co-ndition, the corrosion potential shifted to the active side while the current density increased with the de-crease of Al 2 O 3 volume fraction, and the incorporati-on of reinforcement was detrimental to the corrosion resistance of the aluminium metal matrix. Ramach-andra and Radhakrishna, [3] worked on the sliding wear, slurry erosive wear, and corrosive wear of al--uminium/SiC composite and observed that the corr-osion resistance of reinforced matrix decreased with increasing SiC content. A comparative study of the corrosion behaviour of Al -Si/ SiC composite and cast iron in three different media, namely synthetic mine water, 3.5%NaCl solution and 3.5% NaOH so-lution through immersion technique was studied by Saraswathi et al, [2] they reported that the corrosion rate of Al -Si/ SiC composite in 3.5%NaCl soluti-on was minimum followed by synthetic mine water while that of 3.5% NaOH solution was the highest.
One of the methods of combating corrosion in aqueous environment is the application of corrosion inhibitors. Most of the well known inhibitors are or-ganic compounds containing nitrogen, sulphur and/ or oxygen atoms [7]. Zaki and Abdul [8] worked on the degradation of Al 6013/SiC composites in salt water and its control, and reported that the localized attack was concentrated mainly on the Al 6013/SiC interface where addition of cerium chloride drastic-ally suppressed the rate of corrosion. Monticelli et al. [9] studied corrosion and corrosion inhibition of alumina particulate/aluminium alloys metal matrix composites in neutral chloride solutions and report-ed that among tungsten and molybdenum-containi-ng inorganic salts tested as corrosion inhibitors, only ammonium tetrathiotungstate afforded good inhibi-ting properties, particularly towards the AA 2014based MMC. Mishra et al. [10] studied the corrosion *corresponding author. E-mail: asukef@yahoo.com inhibition of 6061-SiC by rare earth chlorides (lant-hanum and cerium chlorides), and reported that the polarization resistance increased after addition of LaCl 3 and CeCl 3 , with maximum increase noticed at 250 ppm LaCl 3 and 1,000ppm CeCl 3 . CeCl 3 addition displayed better improvement in polarization resist-ance. Also rare earth chloride addition resulted in an increase in resistance on both cathodic intermetallic sites and the pitted regions by formation of preci-pitates of their oxide/hydroxide on those locations, resulting in high pitting nucleation resistance as well as improved corrosion resistance. Hence this resea-rch aims at investigating the possibility of using pr-opargyl as corrosion inhibitor for Al-5%Si/15%SiC composite in 0.5molar sodium hydroxide solution

Propargyl Alcohol
Propargyl alcohol has chemical formula HC≡C-

Materials
High purity aluminium electrical wires were ob-tained from Northern Cable Company (NOCACO) Kaduna, silicon, silicon carbide with average par-ticle size of 10mm, silica sand, bentonite, distilled water, ethanol, sodium hydroxide, and Propargyl.

Methods
The synthesis of the metal matrix composite used in this study was carried out using the stir-casting method at the foundry shop of the National Metall-urgical Development Center, (NMDC) Jos, Nigeria, by adding 15% SiC to the Al -5%Si alloy. The co-mposite produced is of composition Al-5% Si/15%--SiC, after casting, the samples were machined into standard corrosion coupons.

Corrosion testing
The coupons were polished, degreased in absolu-te ethanol, dried, weighed and stored in a dessicator.
The coupons were immersed in 0.5M NaOH with varying time and temperature. The first set of tests were carried out without inhibitor to serve as contr-ol. Then in solutions containing propargyl of conce-ntrations 0.5, 1.0, 1.5, 2.0 and 2.5% v/v. The weight loss and potential methods were used. The weight loss and potential of each sample were measured af-ter 1 hour over a period of 5 hours. From the date obtained, corrosion rate, inhibitor efficiency, degree of surface coverage, free energy, activation energy and heat of absorption were determined.

Corrosion rate and Inhibitor Efficiency (%)
The weight loss was determined by finding the difference between the initial weight of the coupons and the new weight after 1hour using the relations-hip [12 and 13] (Eq.1) The corrosion rate was determined from standard expression for measurement of corrosion rate in mil-ls per year (mpy) [12 and 13] (Eq. 2) Where W = Weight loss (mg), D = Density of mate-rial (g/cm 3 ), T = Time of exposure (hours), A = Total surface area (in 2 ).

Measurement of Potential
After the test coupons were immersed in the so-lution a reference electrode (platinum) was also im-mersed in the solution, the electrode and coupons were then connected to a potentiostat, which was used to measure the corrosion potential with respect to the reference electrode.

Inhibitor Efficiency
The inhibitor efficiency (IE) was computed using the relationship [13 and 14] Inhibitor Efficiency (IE) = (Eq. 3) Where CR and CR o are the corrosion rates with and without the inhibitors respectively.

Degree of Surface coverage
The degree of surface coverage(θ) was calcula-ted from the equation Where CR and CR o are the corrosion rates with and without the inhibitors.

Reaction kinetics
The activation energy Ea of the corrosion rea-ction was calculated using the Arrhenius equation given by [13] (Eq. 5 ) Where R1 and R2 are the corrosion rates at any given two different temperatures T1 and T2 while the free energy of adsorption was determined using the given relationships (Eq. 6)

Where
Where K = Equilibrium constant, θ = is the deg-ree of surface coverage, C= is the concentration of inhibitor (% v/v) Also the heat of adsorption ∆Hads of the inhib-itors was calculated using the equation [13] (Eq. 7) Where θ1 and θ2 are degree of surface coverage at any two temperatures T1 and T2.

Microstructure of the produced composite
The microstructure of the produced composite is given in figure 1  The Microstructure shows the presence of mat-rix phase (white) and the SiC reinforcement (dark patches) The variation of the corrosion rate with time of exposure is given by Figures 2-4 Figure 1 shows the produced composite in the as cast condition, it shows the reinforcement (SiC) uniformly distributed in the matrix (Al-Si alloy).

Discussion
Visual observation of coupons (without and with inhibitor) after 5 hours of exposure reveals changes in colour of the coupons from bright shiny surfaces to dull ones showing uniform corrosion. Pits were also observed on the samples which are indications of cor-rosion attack by the alkaline media. However the ch-anges in colour were more intense with the solutions without inhibitor so was the presence of pits.  show the variation of corrosion rate with time in the presence of propargyl as corrosion inhibitor at temperatures of 300C, 500C and 700C. In these graphs the trend showed a decrease in co-rrosion rate with variation in concentration from 0.5-1.5%v/v, while from 2.0-2.5%v/v there was an increase in corrosion rate. Figures 5 -7 showed the variation of potential difference with time. These graphs showed that the highest potentials were associated with 1.5% v/v which is in line with the observation from the grav-imetric measurement. It was evident from           show the variation of inhibitor eff-iciency with time. The maximum inhibitor efficie-ncies were obtained at a concentration of 1.5% v/v. however the highest inhibitor efficiency of 59.23% was obtained at 30°C and the least inhibitor efficie-ncy of 32.89% was obtained at 70°C. this is also in line with the results obtained from gravimetric and potential measurements.

Corrosion inhibition of Propargyl
The corrosion inhibition of propargyl is attribut-ed to the presence of multiple bond in its molecular structure which enhance the deposition of its mole-cules on the surface of the composite. Figures 11 show the variation of inhibitor effi-ciency with temperature. The graph shows that the highest inhibitor efficiency is obtained at 300C wh-ile the least obtained at 700C, showing that the in-hibitor acts best at 300C. The decrease in inhibitor efficiency with increase in temperature is attributed to the fact that at lower temperature the inhibitor molecules absorb onto the composite surface, while at a higher temperature desorption of the molecules from the composite surface occur as a result of diss-ociation of constituents of the inhibiting substance.

Reaction kinetics
The free energy of adsorption (ΔGads), the heat of adsorption (ΔHads), and activation Energy (Ea) were calculated. Figure 12, tables 1 and 2 show the results obtained respectively.  Table 2.
Activation Energy (-Ea) The negative values of the free energy of adso-rption (see figure 12) indicates a strong interaction of the inhibitor molecules on the surface of the co-mposite. Table 1 show the values of heat of adsorption (ΔHads) obtained in this work. The nature of ad-sorption depends on the values of ΔHads: thus if │ΔHads│< 10KJ/mol the adsorption is physical adsorption and if │ΔHads│> 10KJ/mol the adsor-ption is chemical [13]. From the values obtained in this work, both physical and chemical adsorption is attained.
The activation energy of the inhibited system were higher than those of the uninhibited system (see table 2), this indicates that the presence of the inhibitors caused a change in the values of the appar-ent activation energies, this indicates a change in the rate determining step brought about by the adsorpti-on of propargyl onto the surface of the composite.