Effective inhibition of corrosion of steel in phosphoric acid solution

Some common known inhibitors namely bexamine (HAM), gelatin (GEN), ammonium thiocyanate (ATC) and tri-benzyl methyl ammonium chloride (TBA) provide poor inhibition towards corrosion of steel in phosphoric acid solutions. Polymeric reaction product of tetra-hydro pyrimidine and formaldehye (THP), however, provides superior protection toward the system in comparison to the other tested inhibitors. This inhibitor when tested in combination of the other surface active agents furnishing anionic and 4 cationic species in the solution, provides improved inhibition. THP in combination of ATC and TBA in an optimized composition (OC) affords the protection of the order of 98% to steel in 10% H3PO4 (phosphoric acid) solution at 25 C. Rise in temperature from 25 to 90 C has negligible deteriorating effect on the performance of the OC and THP. Electrochemical polarization, Zeta potential, EDAX and reflectance studies have been performed to ascertain the mechanism of functioning of the inhibitors. EDAX studies and XRD have confirmed that an insoluble FePO4 film forms on the steel surface exposed to H3PO4 solutions. In the presence of THP and OC, however, no such film is detected. This is further corroborated by the measurement of reflectance and polarization studies on the surface after exposing them in different compositions of the solutions.