Corrosion inhibition of steel in chloride-containing alkaline solutions

Abstract

Some organic inhibitors of steel corrosion in saturated calcium hydroxide solutions containing 0.1 M chlorides were investigated to elucidate their interactions with the steel surface. The compounds studied were dicyclohexylammonium nitrite (DCHAMN), dicyclohexylamine (DCHA), sodium β-glycerophosphate (GPH) and 5-hexyl-benzotriazole (C6BTA). Sodium nitrite (SN) was also studied as a reference. The techniques applied were electrochemical impedance spectroscopy (EIS), polarization curves, cyclic voltammetry (CV) and Fourier transform infrared (FTIR) spectroscopy. FTIR spectra showed that DCHAMN, DCHA, GPH and C6BTA interact with the steel surface by chemisorption and some information about the mechanism also emerged. From DCHAMN solution, DCHA chemisorption is induced by slow salt hydrolysis. CV tests show that, among chemisorbed substances, only GPH avoids chloride penetration on cycling, most likely due to a quick chemisorption, while DCHAMN can only enlarge the passive potential range. Addition of SN also prevents chloride attack on cycling. Twenty days of immersion in inhibited solutions revealed that, besides SN, GPH and DCHAMN also form an impervious surface film on steel, which blocks any localized corrosion attack, whereas in the case of DCHA and C6BTA solutions, pitting corrosion is slowed down, but not avoided. DCHAMN exhibits the highest inhibiting efficiency at long immersion times, as a result of a synergetic inhibitive action which develops between nitrite and chemisorbed DCHA.