influence of welding on the dissolution of api- 5 l x60 in simulated soil solution- corrosion protection by phosphoric compound- dft calculations

The present work aims to examine the susceptibility to soil corrosion of base metal (bm) and heat affected zone (haz) of api l5 x60 steel pipeline, and to assess the ability of di-(2-ethylhexyl) phosphoric acid (d2ehpa) to inhibit the corrosion of the two samples. an overview of the literature reported almost no studies related to the corrosion inhibition of pipeline in soil solutions. the experiments were carried out in a simulated soil solution (ns3) using electrochemical methods, a thermodynamic approach, and surface analysis. the results demonstrated that d2ehpa is a potent inhibitor for both steels in the soil solution. indeed, its efficiency increased with the increase of its concentration, exceeding 98 % at the optimal concentration, even for haz which is less resistant to corrosion than bm, due to the coarsening  of α-ferrite grains. polarization curves showed that d2ehpa acts as an anodic-type inhibitor, and the calculated standard free adsorption energy values deduced by langmuir isotherm indicated that the phosphoric compound adsorbs via electrostatic and chemical bindings. the stability of the adsorbed d2ehpa layer, on both the surfaces of bm and haz that were immersed in the inhibitive solution for 168 h, has been proven by eis studies. moreover, the effective adsorption of d2ehpa at the steel/sn3 interface is clearly highlighted by scanning electron microscopy (sem) and ft-ir spectra. theoretical dft calculations were also performed to determine some electronic properties of the studied molecule and to find a correlation between the inhibitive effect and the electronic structure of the neutral form and the deprotonated form of d2ehpa.