The present research involves potentiostatic investigation of the corrosion behavior of carbon steel alloy in saline (3.5% NaCl) solution.The general pattern and the experimental techniques used could be
classified into three different aspects as follows:
1. The corrosion behavior of carbon steel specimen in saline solution (blank) and in the presence of inhibitors (Val, Ser, Trp and Lys) at pH (2 and 11) with various temperatures ranging between (293313)K from the polarization curves over a range of potentials (700to-300)volt.
2. The inhibiting effect of amino acids (Val, Ser, Trp and Lys) as ecofriendlyinhibitors at different concentrations on the corrosion of carbon steel alloy in the saline solution at pH (2 and 11) with five different temperatures.
3. Computational studies for inhibitors (Val, Ser, Trp and Lys) by using density functional theory (DFT) with 6-31G basis set in gas and aqueous phases.
The main results obtained from experimental studies may be presented as
follows:
1. The corrosion current densities, corrosion rates and penetration rates for an uninhibited and inhibited saline solution are increasing with increasing temperature over a temperature range (293-313) K.
2. The inhibition efficiency values increase with increasing the concentration of inhibitors (Val, Ser, Trp and Lys) due to the adsorption of these compounds on the surface of the metal and decrease with increasing temperature in the range (293-313) K. the sequence of the IE values was for Lys > Ser > Val > Trp at pH 2 Ser > Val > Trp > Lys at pH 11
L-lysine was shown to be more efficient for the protection ofcarbon steel in the acidic saline solution (pH 2). For basic salinesolution, L-serine was the most efficient and suitable inhibitorparticularly at a concentration of 1×10-2 mol.L-1.
3. The amino acids studied (Val, Ser, Trp and Lys) were act as anodic or cathodic or mixed type inhibitors, according to the values ofcorrosion potential.
4. The thermodynamic parameter of corrosion ΔG indicates that theprocess of corrosion spontaneous.
5. Kinetics of the corrosion was controlled by Arrhenius typeequation and the data obtained showed that the activation energy(E ) values are higher in the presence of the inhibitor molecules ascompared with the values of blank solution. The positive sign ofthe activation enthalpies (ΔHa*) means the endothermic nature ofthe activated complex formation which means that required more energy to form its with increasing inhibitors concentrations. Largeand negative change in the values of activation entropy (ΔS)means that the activated complex in the rate determining step represents association rather than the dissociation.
6. The adsorption process obeys Langmuir isotherm and the negative values of ΔGads and ΔHads indicate that the process were spontaneous and exothermic respectively. The values of ΔG ranging between (-40 kJ.mol−1) to (-20 kJ.mol−1) meaning that the adsorption of these inhibitor molecules via chemisorptions and physisorption.The results obtained from theoretical calculations of (Val, Ser, Trp and
Lys) by using the density functional theory (DFT) (B3LYP) with 6–31G basis set. These results indicate that Trp is best inhibitor in aqueous phase and gas phase.The Scanning Electron Microscopy (SEM) images of carbon steel in the presence (1×10-2 M) of amino acids showed that there were much less damage due to the formation of a protective film through adsorption process.
