Corrosion Protection of Stainless Steel by Organic Inhibitors in Phosphate Industries in 15% H2SO4

Stainless steel is a very important metal for phosphate industry whereas H2SO4 produces hostile environment for surrounding material and it produces several forms of corrosion like galvanic, pitting, crevice, stress, intergranular embrittlment and blistering. Scientists and researchers were used different techniques for corrosion mitigation of materials like organic and inorganic coatings, use organic and inorganic inhibitors, composite materials coating, nanocoating and plasma coatings. Inorganic nanocoating of aluminum phosphate [1], zinc phosphate [2] and magnesium phosphate [3] in presence of DLC (diamond like carbon) filler were used as nanocoating materials in high temperature and acidic environment. Aliphatic and aromatic compounds [3-5] containing amino, hydroxyl and thiol functional groups were applied as corrosion protector in acidic medium. Polymeric coating [6-8] saved material for corrosion but this coating did not produce good results in long duration. Mixed types of organic inhibitors having cathodic [9-11] and anodic [12-15] polarization power used as an inhibitor in acidic condition. Plasma coating [16,17] provided corrosion resistance of metal at high temperature and strong acidic medium. Natural products [18,19] used as inhibitor which is ecofriendly with environment and these products has good inhibition properties.


Introduction
Stainless steel is a very important metal for phosphate industry whereas H 2 SO 4 produces hostile environment for surrounding material and it produces several forms of corrosion like galvanic, pitting, crevice, stress, intergranular embrittlment and blistering. Scientists and researchers were used different techniques for corrosion mitigation of materials like organic and inorganic coatings, use organic and inorganic inhibitors, composite materials coating, nanocoating and plasma coatings. Inorganic nanocoating of aluminum phosphate [1], zinc phosphate [2] and magnesium phosphate [3] in presence of DLC (diamond like carbon) filler were used as nanocoating materials in high temperature and acidic environment. Aliphatic and aromatic compounds [3][4][5] containing amino, hydroxyl and thiol functional groups were applied as corrosion protector in acidic medium. Polymeric coating [6][7][8] saved material for corrosion but this coating did not produce good results in long duration. Mixed types of organic inhibitors having cathodic [9][10][11] and anodic [12][13][14][15] polarization power used as an inhibitor in acidic condition. Plasma coating [16,17] provided corrosion resistance of metal at high temperature and strong acidic medium. Natural products [18,19] used as inhibitor which is ecofriendly with environment and these products has good inhibition properties.

Experimental
Stainless steel (5×3×0.01) square meter size of coupons made for experimental work. Its surface was sharpened 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. Inhibition effect of 2-(aminomethyl)phenol and 2-(aminomethyl) benzenethiol were studied at 50°C, 60°C and 70°C temperatures and 10mM concentration. The corrosion rate was calculated absence and presence of inhibitors by gravimetric method. Stainless steel (316) was used for this and its chemical composition analyzed in Bokaro Steel Plant Jharkhand, India.
The corrosion current and corrosion potential was determined with potentiostatic polarization by using 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.

Results and Discussion
The corrosion rates of stainless steel was calculated at 50°C, 60°C and 70°C temperatures and 10 mM concentration of inhibitors by The corrosion rates of stainless steel without and with inhibitors mentioned in Table 1. The result of Table noticed that corrosion rate of metal is increasing without inhibitors and its values are decreasing with inhibitor as above recorded temperatures and concentration. The graph between log K versus 1/T was looked in Figure 1 shown that the rate of corrosion enhanced at lower temperature to higher temperature and its values suppressed after addition of inhibitors.
The percentage inhibition efficiency and surface coverage area of inhibitors were determined using 2 and equation3.
Where K o is the corrosion rate without coating, K = corrosion rate with coating.
where θ = Surface coverage area, K o = corrosion rate without coating, K = corrosion rate with coating, The percentage inhibition efficiency and surface coverage area values recorded in Table 1 at different temperatures and 10 mM concentration of inhibitors. It depicted that these values were increased with both inhibitors but good results observed with (2-aminomethyl) benzenethiol. The surface coverage area (θ) versus temperatures (T) for both inhibitors plotted in Figure 2. It found that 2-(amino-methyl) benzenethiol occupied more surface area than that of 2-(aminomethyl) phenol. Inhibition efficiency (IE) of both inhibitors versus temperature (T) was represented in Figure 3. This graph indicated that second inhibitor had more inhibition efficiency than that of first.
The activation energy of inhibitors was determined by equation 4 and Figure 1 and its values were recorded in Table 2. Activation energy increased without inhibitors and its values decreased with inhibitors.
These results were shown that inhibitors adhered with surface of base metal. d /dt (logK) = E a / R T 2 (4) where T is temperature in Kelvin and E a is the activation energy of the reaction.
The heat of adsorption was calculated with help equation 5 and their values were mentioned in Table 2. The graph plotted between log (θ/1-θ) vs. 1/T was shown straight line in Figure 4 and its negative values indicated that inhibitors bind with metal by physical forces.
where T is temperature in Kelvin and Q ads heat of adsorption Free energy was determined by equation 6 and its values were recorded in Table 3. Its values mentioned in Table 2 depicted that after addition of inhibitor exothermic reaction occurred.     The energy of enthalpy and entropy were calculated by transition state equation 7 and it values were recorded in Table 2. These values noticed that inhibitors were produced an exothermic reaction and they bonded with metal chemical adsorption. Thermodynamical values of Ea, Qads. , ∆G, ∆H and ∆S versus surface coverage area (θ) plotted in Figure 5 indicated that inhibitors adsorbed with metal by physisorptionchemisorption.
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.
The corrosion current density is determined in the absence and presence of inhibitors the help of equation 8 and their values are recorded in Table 3. ∆E/∆I = β a β c / 2.303 I corr (β a + β c ) (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/cm 2 .
The metal penetration rate (mmpy) was calculated by equation 9 in absence and presence of inhibitors.
The observation of the results of Table 3, it was noticed that corrosion current increased without inhibitors and it reduced after addition of inhibitors. Figure 6 indicated that Tafel graph has plotted between electrode potential and corrosion current density in the absence and presence of inhibitors and this figure was shown that anodic potential, corrosion current density and corrosion rate increased without inhibitors but after addition of inhibitors these values decreased and inhibition efficiency increased.

Conclusion
Observation of results of inhibitors 2-(aminomethyl) phenol and 2-(aminomethyl) benzenethiol indicated that inhibition efficiency and surface coverage area at different temperatures in H 2 SO 4 medium produced corrosion resistance effect for stainless steel. The inhibitors adhered with metal surface by physisorption-chemisorption adsorption. This adsorption phenomenon confirmed by activation energy, heat of adsorption, free energy, enthalpy and entropy. Potentiostat results for both inhibitors indicated that these inhibitors produced high polarization current and nullify the attack of H + ions.