1. Introduction

The Corrosion of metal is not fully control but its effect can be minimized by application suitable methods e.g. give proper design and shape of operational metals ^[1], take care of surrounding operating temperatures and atmosphere ^[2], used different types of coatings ^[3], addition of inorganic and organic inhibitors as cathodic and anodic protection ^[4] and applied nanocoating ^[5]. When operational equipments came in contact of acidic environment ^[6], they exhibited several types of corrosion problems like galvanic, pitting crevice, stressed, intergranular, blistering, embritlement. Different types of coatings methods used for corrosion alleviation of metal such coatings were metallic coating ^[7], inorganic coating ^[8], organic coating ^[9], painting coating, polymeric coating, and nanocoatings ^[10]. These coating did not provide good support for metal in acidic environment because porosities were developed on the surface of base metal during coatings. The H⁺ ions entered into porosities of coating materials by the process of diffusion and it developed corrosion cell on the surface base metal.

Various types of inhibitors like inorganic, organic and mixed types used to control the corrosion of metal. Organic inhibitors which possessed nitrogen, oxygen, sulphur, silicon, phosphorous, methyl, phenyl, primary, secondary and tertiary alkyl groups whereas these organic compounds have high electron rich functional groups and they have capacity to produce thin film on surface of metal. Inhibitors were bonded with metals by physical-chemical adsorption. Aromatic and heterocyclic organic compounds containing above mentioned functional groups produced anticorrosive effect in acidic medium.

Nanocoatng of Zn₃(PO₄)₂ ^[11], Mg₃(PO₄)₂ ^[12] and AlPO₄ ^[13] in presence of DLC (diamond like carbon) controlled high temperatures corrosion and minimize hydrogen ions attack. Plasma and composite coating gave corrosion protection of metal in acidic environment. Inhibitor 1-(2-chlorophenyl)methanamine and 1-(2-bromophenyl)methanamine used for this work. These inhibitors contained electron rich functional which minimize the attack of H⁺ ions and forming thin film and it also increased surface coverage area and efficiency. The thermodynamical results noticed that these inhibitors have good adsorption capability.

2. Experimental Procedure

Stainless steel coupons were cut into size of (5 x 3) cm². Its surface was rubbed with emery paper and samples were washed with double distilled water. Finally it was rinsed with acetone and dried with air dryer and kept into desiccator. Test sample dipped into 250ml biker with support glass hook and corrosion rate metal determined absence and presence of inhibitors 1-(2-chlorophenyl)methanamine and 1-(2-bromophenyl)methanamine at different temperatures 333⁰K, 343⁰K and 353⁰K and 15mM concentration and thermostat used to mention temperature. The corrosion rate was measured by gravimetric method.

The corrosion current density and corrosion rate were calculated by potentiostatic polarization technique with help of an EG & G Princeton Applied Research Model 173 Potentiostat. A platinum electrode was used as an auxiliary electrode and a calomel electrode was used as reference electrode with stainless steel coupons. The used inhibitors structure mentioned as:

3. Results and Discussion

The corrosion rates of stainless steel without and with inhibitors 1-(2-chlorophenyl)methanamine and 1-(2-bromophenyl) methanamine were determined by equation1 and its results were mentioned in Table 1.

(1)

where W = weight loss of test coupon expressed in gm, A = Area of test coupon in square centimeter, D = Density of the material in g/cm³.

The surface coverage areas (θ) and the inhibition efficiencies (IE) occupied by inhibitors were calculated equation 2 and 3 and their results were also written in Table 1.

(2)

where θ = Surface coverage area, K_o = corrosion rate without inhibitor, K = corrosion rate with inhibitor

(3)

where K_o is the corrosion rate without inhibitor, K= corrosion rate with inhibitor

Table 1. Corrosion of stainless steel at different temperatures without and with inhibitors in 15% H₂SO₄

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The results of Table 1 observed that corrosion rate increased in acidic medium without addition of inhibitors but its values decreased after addition of addition of inhibitors. The results of surface coverage area and inhibition efficiency with 1-(2-chlorophenyl)methanamine and 1-(2-bromophenyl)methanamine enhanced at different temperatures and it looked in Figure 1 plot between θ (surface coverage area) versus T⁰K and Figure 2 IE (inhibition efficiency) versus T⁰K. These results indicated that used inhibitors produced anticorrosive effect against acid.

Figure 1. θ (surface coverage area) versusT⁰K for stainless steel at different tenpertaures

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Figure 2. IE(%) versus T⁰K for stainless steel at different temperatures

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Figure 3. log K versus 1/T for stainless steel at different temperatures

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Figure 4. log(θ/1-θ) versus 1/T for stainless steel at different temperatures

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Figure 5. Thermodynamical values of different inhibitors versus θ(surface coverage area) at different temperatures

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The activation energy, heat of adsorption, free energy, enthalpy and entropy of inhibitors 1-(2-chlorophenyl)methaneamine and 1-(2-bromophenyl)methaneamine were calculated by equation4, equation 5, equation 6 and equation 7 and their values were recorded in Table 2. The activation energy increased without inhibitors and its values decreased with inhibitors and its values were determined by plot of logK versus 1/T in Figure 3. It indicated that inhibitors bonded with base metal. Heat of adsorption found to be negative which indicating that inhibitors adhered with metal by physio-chemio adsorption. Its values were calculated by plot of log(θ/1-θ) versus 1/T in Figure 4. The results of free energy, enthalpy and entropy values were shown negative sign which depicted that adsorption occurred on the surface of metal and the graph of all thermodynamical values (Ea, Q_ads, ΔG,_, ΔH and ΔS) versus θ (surface coverage area) were presented in Figure 5 .

(4)

where T is temperature in Kelvin and E_a is the activation energy

(5)

where T is temperature in Kelvin and Q_ads. heat of adsorption

(6)

where T is temperature in Kelvin and ΔG free energy

(7)

where N is Avogadro’s constant, h is Planck’s constant, ΔS^# is the change of entropy activation and ΔH ^# is the change of enthalpy activation.

Table 2. Thermodynamical values of inhibitors in 15% H₂SO 4 for stainless steel

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The corrosion current density determined in the absence and presence of inhibitor with the help of equation 8 and their values were recorded in Table 3.

(8)

where ∆E/∆I is the slope which linear polarization resistance (R_p), β_{a and} β_c are anodic and cathodic Tafel slope respectively and I_corr is the corrosion current density in mA/cm².

The metal penetration rate (mmpy) was determined by equation9 in absence and presence of inhibitors.

(9)

where I_corr is the corrosion current density ρ is specimen density and Eq.Wt is specimen equivalent weight.

The results of Table 3 indicated that corrosion current increase without inhibitors and its values reduced after addition of inhibitors because these inhibitors enhanced cathodic current so corrosion current and corrosion rate minimized. Tafel graph was plotted in Figure 6 between electrode potential and corrosion current density in the absence and presence of inhibitors.

Table 3. Potentiostatic polarization of inhibitors in 15mM concentration and 15% of H₂SO₄

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Figure 6. Plot of E (mV) Vs. I (mA) for stainless steel with 15mM concentration of inhibitors

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4. Conclusion

These inhibitors possessed electron releasing functional which had capability to enhance electron charge density towards corred metal and protected base metal by formation of thin film. The results of surface coverage area and inhibition efficiency for both inhibitors indicated that both inhibitors adhered with the surface of metal. The results of activation energy, heat of adsorption, free energy, enthalpy and entropy were shown both inhibitors bonded with base metal physical-chemical adsorption.

Acknowledgement

I am thankful to UGC, New Delhi for providing me financial support for this work. The author is thankful to Professor Sanjoy Misra, Department of chemistry, Ranchi University, Ranchi who provide me suggestion and guidance. I am also thankful to the department of chemistry, Ranchi University, Ranchi and the department of applied Chemistry Indian school of Mines, Dhanbad for providing laboratory facilities.

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