Mechanism of degradation of propargyl alcohol by acid-catalysed hydrolysis and corrosion inhibition efficiency of propargyl alcohol intermediates for carbon steel in hydrochloric acid

Abstract

Acetylenic alcohols are one of the most important groups of film-forming corrosion inhibitors for carbon steel in hydrochloric acid (HCl). The inhibition mechanism of acetylenic alcohols has been widely discussed for more than 50 years, however, it is not well-understood, particularly with regard to the chemical degradation reactions of acetylenic alcohols in HCl solution. In the present paper, proton nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) techniques were used to investigate the mechanism and the rate of degradation of propargyl alcohol in 3.6–10.8 M HCl at 20–95 °C. Several water-soluble products, such as 1-hydroxypropan-2-one, 2-chloroprop-2-en-1-ol, 2,2-dichloropropan-1-ol and 2-chloropropane-1,2-diol were formed via the electrophilic acid-catalysed addition of HCl and water to the alkyne. Some intermediates further polymerize to form water-insoluble polymers, such as poly (propargyl alcohol), poly (allyl alcohol) and poly (acrolein). The hydrolysis half-life constants of propargyl alcohol decreased with increasing HCl concentration and temperature. The inhibition efficiency (η) of propargyl alcohol and its precursors for carbon steel in 3.6 M and 7.83 M HCl was studied. The critical role of acrolein intermediates on the corrosion performance of propargyl alcohol in hydrochloric acid was identified. Products of propargyl alcohol hydrolysis, 1-hydroxypropan-2-one and 2-chloroprop-2-en-1-ol are poor corrosion inhibitors. However, acrolein and its reactive intermediates are good corrosion inhibitors with inhibitor efficiency, η in the range from 97.69 ± 0.4 % for fresh acrolein to η = 97.94 ± 0.8 % for aged acrolein.