Poly Acryl Amide as Acid Corrosion Inhibitor of C-steel in 15% HCl

The effect of poly acryl amide in the 15% HCl solution of carbon steel (class L-80) was studied using weight loss method , the efficiency of inhibitor increased by increasing inhibitor concentration and decreasing temperatures. Inhibition attributed to insoluble complex adsorbed on the coupon (L-80) carbon steel surface. The iron complex was identified by FTIR. And the mechanism of inhibition was explained as Langmuir adsorption isotherm. The thermodynamic parameters of adsorption operation was calculated and explained.


No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
temperature on carbon steel (L-80) was studied in exist of poly acryl amide and some thermodynamic parameters were founds.

Method:
Used (60) gr of carbon steel (L-80) has ingredients (wt%) 0.019 Cr, 1.41Mn , 0.353 Si, 0.263 C, 0.159 Ni, 0.03 P, 0.001 S and the remained Fe. with dimension of (2.5 x 4.5 x 0.7) cm. C-steel coupons surface was polished with smoothing paper, degreased with acetone, and rinsed with distilled water. the cleaned C-steel coupons were weighed before and after immersion in (50) ml of (%15) HCl solution at the times (3,4,5,6) hr. The weight loss of coupon in experiments was taken in grams. The temperature (40, 60, 80, and 90) o C was adjusted using air thermostat. poly acryl amide (C 3 H 5 NO) n used as inhibitor was prepared by Fluka company and has the following structure:

Results and Discussion
Figures (1,2,3,4) illustrates clearly that corrosion and weight loss of carbon steel (L-80) became very low in the exists of poly acryl amide by adsorbing and making a thin film on the surface of the coupon objected the corrosion process [18], the linearity follows Langmuir equation . Table (1) has inhibition efficiency (%E) and surface coverage (ϴ) surface covered by the inhibitor molecules as in equation (1,2). %E = (1-W inh. / W free ) X 100 ………… (1) ϴ = (1-W inh. / W free ) ………….. (2) Where W free and W inh. Are the weight losses of C-steel in the absence and exists of inhibitors the weight loss decreases as the concentration of poly acryl amide was increased.

Effect of Temperature:
Corrosion rates increases with increasing temperature in this study corrosion rate measured by the following equation [19]: Where Δw is the loss of weight S is the surface area (cm 2 ) and t is the displayed time (hr).
The data listed in Table (1) illustrate that the highest inhibition efficiency is happened at 40°C by using poly acryl amide as inhibitor in (%15) HCl . The activation energy Ea* of the corrosion operation calculated using Arrhenius equation [20].
log R corr = log A-Ea* / 2.303 RT ……….… (4) Where R corr is the rate of corrosion from weight loss, A is Arrhenius constant, R is the gas constant (8.31J.K -1 .mol -1 ) and T is absolute temperature. Figures (5,6,7,8) shows Arrhenius curves between (log R corr ) and (1/T ) in the exist or absence of different concentrations of poly acryl amide and from the slope of the straight lines activation energy Ea* can be calculated from the slope of curve and in the increasing of activation energy at exists of inhibitor means decrease in the adsorption operation on coupon surface, the data listed in Tables (2, 3 , 4, 5) illustrates by increasing in temperature the corrosion rate increase and surface area of coupon that is displayed to acid became greater [21]. in our study Ea * increases with increasing the poly acryl amide concentration

Effect of adsorption:
Poly acryl amide adsorbed on coupon and covered (ϴ) part of its surface in different concentration, temperature and time and the data are listed in Table (1) and it was found the value of (ϴ) increase with increasing the concentration of poly acryl amide and decrease with rise in temperature from 40 to 90°C.

Thermodynamic parameter:
The adsorption equation formulated on Langmuir as: Where K is the equilibrium constant for adsorption operation. Figures (13,14,15,16) shows the curve of log C/ ϴ agonist log C for poly acryl amide the striate line indicates that the adsorption on surface of (L-80) carbon steel obey Langmuir absorption isotherm. The result states clearly there is no reaction between the adsorbed species. The equilibrium constant (K) for adsorption operation can be calculated from the equation (7). The thermodynamic parameters for adsorption operation. (ΔG) inh. and entropy (ΔS) inh of the investigated poly acryl amide can be obtained from the thermodynamic relations ( 8,9) .
The ΔG inh and (ΔS) inh values at temperature range from 40 to 90°C are given in Tables   (2, 3

Mechanism of inhibition:
The action of poly acryl amide as a corrosion inhibitor on carbon steel in acid media can be attributed to some factors such as structure, nature of molecule, types of adsorption sites and the ability to form complexes [23]. The inhibition mechanism of poly acryl amide is happened as a result of complex formation between Fe 2+ ion, and poly acryl amide. The iron complexes adsorbed on coupon of (L-80) carbon steel and isolated the surface metal from more corrosion happens and these complexes forming by an electron donating groups NH 2 and electron withdrawing group C=O that order the inhibition efficiency of the poly acryl amide .

Spectra (FTIR):
The FTIR spectrum of poly acryl amide complex that adsorbed on (L-80) carbon steel coupon shows an intense peak at ~3100 cm -1 which is a characteristic feature of N-H stretching frequency and peak at ~ 1600 cm -1 for C=O stretching [23][24][25].

Conclusions:
1-Poly acryl amid can be used as acid corrosion inhibitor.

2-
The efficiency of poly acryl amide depends on concentration of poly acryl amide and temperature of the medium.
3-Poly acryl amide makes stable and insoluble complexes on carbon steel surface.

4-
The chemical adsorption of poly acryl amide on coupon of (L-80) carbon steel follows Langmuir equation (absorption isotherm).