Propanoyl(1Z)-N-(2,6-dimethylphenyl)-2-oxopropanehydrazonoate as inhibitor for corrosion of 6061 Al alloy15 % (v) SiC(p) composite in hydrochloric acid media

ABSTRACT : The corrosion inhibition effect of Propanoyl(1Z)-N-(2,6-dimethylphenyl)-2-oxopropanehydrazonoate (PDOH) in the corrosion of 6061 Aluminium alloy-15%(v) SiC (p)  composite in 0.5 and 1M hydrochloric acid medium at four different temperatures (30, 40, 50 and 60 ⁰C) was investigated using potentio-static polarization (Tafel extrapolation and Linear polarization) and weight loss methods. The results obtained reveal that PDOH is an efficient corrosion inhibitor with around 96% inhibition efficiency within the range of temperature studied. Leftward and downward shifts in Tafel plots were observed with the addition of the inhibitor, indicating that PDOH inhibits the corrosion process effectively, and that it is a cathodic inhibitor. Corrosion rate increases and inhibition efficiency decreases with increase in temperature. Results obtained by Tafel extrapolation, linear polarization, and weight loss methods are in agreement. The adsorption of the inhibitor onto the surface of the 6061 Al alloy 15 % (v) SiC (p)  composite is found to obey Temkins' adsorption isotherm that verifies the assumption of mono-layer adsorption on a uniform homogeneous composite surface with an interaction in the adsorption layer. The inhibition is therefore governed by the physisorption mechanism. ABSTRAK: Kesan perencatan kakisan Propanoyl(1Z)-N-(2,6-dimethylphenyl)-2-oxopropanehydrazonoate (PDOH) ke atas kakisan komposit aloi Al 6061-15%(v) SiC (p) dalam media asid hidroklorik 0.5 dan 1M pada suhu-suhu yang berbeza (30, 40, 50 dan 60⁰C) telah dikaji menggunakan polarisasi statik-upaya (extrapolarisasi Tafel dan polarisasi Linear) dan kaedah kehilangan berat. Keputusan diperolehi menunjukkan PDOH adalah perencat yang efisien dengan 96 % keupayaan perencatan bagi julat suhu yag dikaji. Anjakan ke kiri dan ke bawah plot Tafel dapat dilihat dengan penambahan perencat, menunjukkan PDOH merencatkan proses kakisan dengan berkesan, dan ia adalah perencat katodik. Dengan peningkatan suhu, kadar kakisan meningkat dan keberkesanan perencatan menurun. Keputusan dari ekstrapolasi Tafel, polarisasi linear dan kehilangan berat.adalah saling bersetuju antara satu sama lain. Penjerapan perencat ke atas permukaan komposit aloi 6061 Al alloy 15 % (v) SiC (p) memenuhi penjerapan Temkins’ isoterma yang mengesahkan andaian penjerapan lapisan-mono ke atas permukaan komposit homogenus yang seragam melalui interaksi pada lapisan penjerapan. Perencatan dikawal oleh mekanisma fizijerapan.


INTRODUCTION
Aluminium and its alloys and composites represent an important category of materials due to their high technological value and wide range of applications, especially in aerospace industry and military applications.The use of these materials in light weight applications because of high strength to weight ratio is widespread and these usages expose them to environments that could be acidic or alkaline [1].Generally, aluminium alloys and composites are highly resistant to corrosion owing to the formation of a protective oxide layer on their surface [2].Corrosion does take place when this protective Al 2 O 3 -3H 2 O layer is either unstable or defective.Corrosion in Al alloys and its composites is either pitting or inter-granular corrosion.Pitting takes place in environments where the protective Al 2 O 3 -3H 2 O film is partially stable.In these cases, corrosion usually originates adjacent to the flaws in the microstructure of the film [3].Inter-granular corrosion results in selective corrosion at the grain boundaries or any precipitate free zones that might be found adjacent to them, with the remainder of the matrix undergoing very little corrosion.Inter-granular corrosion occurs because of the formation of precipitates along the grain boundaries of Al and its alloys which are more anodic with respect to the interior of the grains.As a result, preferential dissolution occurs at these sites where these precipitates or precipitate free zones undergo anodic reactions [4].
Hydrochloric acid solutions are used for pickling, chemical and electrochemical etching and in various chemical process industries wherein Al alloy composites are used.In such cases it becomes very important to use corrosion inhibitors so as to protect the material against excessive corrosion [5].The effect of any particular organic compound on the corrosion behaviour of metals in acid system depends on the type of metal, environment contamination and its interaction with the surface of the metal.The inhibiting action of organic compounds has been mainly attributed to adsorption of the inhibiting molecule on the surface of the material.This adsorption affects the electrochemical behaviour involved in the corrosion process of metals/alloys/composites [6].Organic compounds are widely used in various industries as corrosion inhibitors in acidic environments.The effective organic inhibitors contain nitrogen, sulphur and oxygen atoms.These heteroatoms are capable of forming coordinate covalent bond with metal owing to their free electron pairs and thus act as inhibitor.Compounds with π-bonds also generally exhibit good inhibitive properties due to interaction of π orbital with metal surface.The organic inhibitors function through adsorption on the metal surface blocking the active sites by displacing water molecules and forming a compact barrier film to decrease the corrosion rate.
The inhibition effect of 3-methyl-4-amino-5-mercapto-1,2,4-triazole on the corrosion behaviour of 6061 Al alloy 15 % (v) SiCp composite has been reported [7].This triazole compound was found to act as an anodic inhibitor with a maximum efficiency of 58 % at 30 °C.Allyl thiourea was used as an inhibitor for the corrosion of 6061 Al alloy 15 % (v) SiCp composite in HCl medium and showed a maximum efficiency of 70% in the temperature range of 30 -50 °C [8].
In the present work, inhibitive action of Propanoyl(1Z)-N-(2,6-dimethylphenyl)-2oxopropanehydrazonoate (PDOH) on the corrosion behavior of 6061 Al alloy 15 % (v) SiC (p) composite in 0.5 and 1 M hydrochloric acid medium at four different temperatures (30 -60 °C) has been investigated using Tafel extrapolation technique.The results obtained are cross checked by Linear polarization and weight loss methods.

EXPERIMENTAL METHOD 2.1 Specimen
6061 Al alloy 15 % (v) SiC (p) composite having aluminum alloy 6061 as the matrix and 15 vol.% of silicon carbide particles of mean diameter 25 µm are manufactured in the form of cylindrical bars of length 120 mm and diameter 40 mm at Vikram Sarabhai Space Centre (VSSC), Trivandrum by stir casting process.Specimen of required length is cut from the extruded bar and machined to the required diameter.The specimen with an exposed bottom surface area of 0.95 cm 2 is polished with emery papers of different grades degreased with acetone and then rinsed with distilled water and finally dried in air.Then the specimen surface is covered with Teflon tape, to enable the required surface getting exposed to the media.Specimen acts as the working electrode in the three electrode cell system and is connected to the potentiostat along with the reference and counter electrodes.This composite material has the chemical composition (% wt): 0.25 Cu; 1.0 Mg; 0.60 Si; 0.20 Cr and balance Al.

Inhibitor
is prepared by dissolving 2,6-dimethylaniline (2.26g, 0.01 mol) in dilute hydrochloric acid (22.0 ml i.e 9.0 ml 12 M HCl dissolved in 13.0 ml water) and cooled to 0°C in an ice bath.To this, a cold solution of sodium nitrite (3.2 g, 0.0462 mol in 10.0 ml of water) is added, as the temperature of the reaction mixture maintained below 5 °C.The resulting diazonium salt solution is filtered into a cooled solution of ethylacetoacetate (3.4 ml) and sodium acetate (7.0 g) in ethanol (100 ml).The resulting yellow-orange solid is filtered, washed with ice cold water, dried in air and recrystallized from methanol.[Yield 3.65 g, 86.5 %; Mol.wt.278.30].Purity of the compound is checked by its melting point (346 -348 ⁰C) and elemental analysis [9].

Medium
Analytical reagent grade HCl (Merck) and double distilled water are used for preparing test solutions of 0.5 and 1 M HCl.

Tafel Extrapolation Studies
The polarization studies are performed using a Wenking Potentiostat (LB 95L) and a three electrode cell system.The steady state Tafel extrapolation studies are made from -250 mV versus OCP to +250 mV versus OCP in steps of 20 mV from the cathodic side and the corrosion currents are noted.The Tafel plots of potential versus logI are drawn and the corrosion current density (I corr ) and the corrosion potential (E corr ) are determined.The corrosion rate, the degree of surface coverage (θ) and the percentage inhibition efficiency (% IE) are calculated.The experiments are performed with 1 M HCl at four different temperatures (30-60 °C).Temperature is maintained within ±1 °C.

Linear Polarization Method
Linear polarization studies are carried out in the potential (E) range of -20 mV versus OCP to +20 mV vs OCP in steps of 5 mV from the cathodic side and the steady state corrosion currents (I) are noted.The slope of E vs I are used to determine the corrosion current density (I corr ) and the corrosion rate (CR).

Weight Loss Method
Specimen surface of 1 cm 2 is exposed to 100 ml of HCl solution for 6 hours.Prior to exposure, the surface is polished with different grades of emery paper and rinsed with distilled water and acetone, then dried and finally accurately weighed.After exposure, again the specimen is gently polished, rinsed with distilled water and acetone, then dried and accurately weighed.The difference in weight gives the weight loss.Experiments are conducted with 0.5 and 1 M HCl in the presence and absence of inhibitor at 30 °C.

RESULTS AND DISCUSSION
The electrochemical parameters for the inhibiting action of PDOH on the corrosion of 6061 Al alloy 15 % (v) SiC (p) composite in 0.5 and 1 M HCl solution are studied by potentiostatic polarization technique at four temperatures.The results are presented in Table 1 and 2. The corrosion rate, and the percentage inhibitor efficiency (% IE) are calculated by using the following relations [10].
where, I corr is the corrosion current density in µA/cm 2 , Eq. wt is the specimen equivalent weight in g, D is the specimen density in g/cc and 0.1288 is the metric and time conversion factor.
where, I corr and I corr(inh) are the corrosion current densities in µA/cm 2 in the absence and presence of the inhibitor respectively.
The inhibiton efficiency by weight loss method is calculated by using the following expression [11].
where, W and W inh are the values of weight loss of the specimen after immersion in test solution without and with inhibitor, respectively.
It is observed from the polarization results (Tables 1 & 2) and the polarization curve (Fig. 2) that there is a large negative shift in the corrosion potential (E corr ) and a drastic reduction in the corrosion current density (I corr ) and corrosion rate (CR) values.The shift in corrosion potential in the negative direction indicates that PDOH is an efficient cathodic inhibitor.A comparison of the inhibition efficiency obtained from Tafel extrapolation, Linear polarization techniques and weight loss method at 30 °C is given in Table 3.The results obtained by all the three methods are in agreement.
The % IE increases with increase in concentration of PDOH up to 4.5×10 -4 molL -1 (Table 1 and 2).The increase in % IE may be due to the blocking effect of the surface by both adsorption and film forming mechanism which decreases the effective area of attack.The results indicate that PDOH is an effective corrosion inhibitor which gives efficiency values as high as 96 %.The inhibiting effect of PDOH may be due to its parallel adsorption at the 6061 Al alloy/SiCp composite-acid solution interface.The parallel adsorption takes place owing to the presence of one or more active centers of adsorption.PDOH might have displayed high inhibition efficiency because of the presence of hetero atoms of N in their structure as well as the presence of N rich azo group and O rich ester (carbonyl) multiple functional groups in its structure.The surface analysis of 6061 Al alloy 15 % (v) SiC (p) composite is carried out using scanning electron microscope (JEOL Model 8340 LA).The scanning electron micrograph of a fresh specimen is shown in Fig. 3a.The micrographs for the specimens immersed in 1 M HCl solution at 30° C in the absence and presence of inhibitor are shown in Fig. 3b and 3c respectively.The corrosion of 6061 Al alloy 15 % (v) SiC (p) in HCl medium is presumably due to the anodic dissolution either at the grain boundaries or at the metalmedia interface.It is seen from the Fig. 3c that the surface of the composite exposed to solution with inhibitor is smoother than of that exposed to acid solution without inhibitor (Fig. 3c).These observations suggest that the inhibitor forms a protective layer on the composite surface, which prevents the attack of acid on the surface.
To understand the mechanism of corrosion inhibition, the adsorption behaviour of the inhibitor compound on the composite surface must be known.The surface coverage values for different concentrations of PDOH from the acid solution are calculated.The surface coverage values (θ) are tested graphically by fitting a suitable adsorption isotherm.The plot of θ versus log c (Fig. 4) for different concentrations of PDOH shows a straight line indicating that the adsorption of the compound on the composite surface follows Temkin's adsorption isotherm.The applicability of Temkin's adsorption isotherm verifies the assumption of mono-layer adsorption on a uniform homogeneous composite surface with an interaction in the adsorption layer [12,13].The thermodynamic parameters for the corrosion of 6061 Al alloy 15 % (v) SiC (p) in the presence of PDOH are studied (Table 4).
The values of activation energy (E a ) are calculated using Arrhenius equation [14] ln ቀ where, r 2 and r 1 are the corrosion rates at temperatures T 2 and T 1 respectively, R is the gas constant, T 1 and T 2 are the two temperature levels and ΔT is the difference in temperatures.
The equilibrium constant (K) is determined by the relation: where, θ is the degree of surface coverage on the metal surface and c is the concentration of the inhibitor in molL -1 .
The free energy of adsorption (∆G ads ) is calculated from the following equation [15,16].
Δ‫ܩ‬ ௗ௦ ൌ െܴ݈ܶ݊ሺ55.5‫ܭ‬ሻሺ6ሻ where 55.5 is the concentration of water in solution in molL -1 and T is the temperature in Kelvin.
The negative values of ∆G ads indicate spontaneous adsorption and strong interaction of inhibitor molecules on to the surface of the composite.The ∆G ads values for the optimal concentration (4.5×10 -4 molL -1 ) of PDOH are less than -40 kJ mol -1 .This indicates that the inhibition is governed by physical adsorption mechanism [17].

CONCLUDING REMARKS
a) The corrosion rate decreased drastically in the presence of PDOH in both 0.5 and 1 M hydrochloric acid media over the temperatures studied (30-60 °C).b) PDOH has proved to be an efficient cathodic inhibitor for corrosion of 6061 Al alloy 15 % (v) SiC (p) composite in 0.5 and 1 M hydrochloric acid media.c) The adsorption of the compound on the surface of 6061 Al alloy 15 % (v) SiC (p) composite obeys Temkin's isotherm and is governed by physical adsorption mechanism.d) % IE obtained by polarization techniques and weight loss method are in agreement.

Table 1 :
Electrochemical parameters for the corrosion inhibition of 6061 Al alloy 15 % (v) SiC (p) in 0.5 M HCl medium.

Table 2 :
Electrochemical parameters for the corrosion inhibition of 6061 Al alloy 15 % (v) SiC (p) in 1 M HCl medium.