Corrosion inhibition of the mild steel in 0.5 M HCl by 2-butyl-hexahydropyrrolo[1,2-b][1,2]oxazole

• 2-butyl-hexahydropyrrolo[1,2-b][1,2]oxazole was used as a MS corrosion inhibitor in 0.5 M HCl.
• DC, EIS and gravimetric measurements were performed in the 30–60 °C temperature range.
• After 48 h, the inhibitor is inefficient below 30 °C; in the 40–60 °C the ηWL reached a >95% at 40 °C.
• In acid, the synergistic effect of Cl− with BPOX was demonstrated by UV and PZC methods.

The use of 2-butyl-hexahydropyrrolo[1,2-b][1,2]oxazole (BPOX) as a corrosion inhibitor of mild steel (MS) – with a polished or a pre-corroded (for 2 h) surface – was tested in 0.5 M aerated hydrochloric acid in the 30–60 °C temperature range. Its inhibition effectiveness (IE) was assessed after short (2 h) and longer (48 h) immersion time tests, through potentiodynamic, EIS and weight loss (WL) measurements. At low immersion times (2 h) the potentiodynamic tests indicated that BPOX inhibited corrosion reaction of the polished MS surface at all temperatures, acting as a mixed-type inhibitor and reaching, in many experimental conditions, inhibitor efficiency values ηp > 95%. This behavior was confirmed by the EIS results, with the exception of those obtained at 40 °C, where the ηEIS did not decrease dramatically, ranging between 55% and 81%. These results were not confirmed by the WL experiments carried out on pre-corroded MS specimens at longer immersion time (48 h): BPOX did not act as an inhibitor until 30 °C, and in the 40–60 °C the ηWL reached a maximum at 40 °C and 5 × 10−3 M (ηWL > 95%), then decreasing but not so dramatically until 60 °C. Starting from these results, it was demonstrated that after 48 h the adsorption mechanism visibly changed between 30 and 40 °C. Moreover, 40 °C is the optimum temperature for the inhibitive BPOX action by means of potentiodynamic test carried out after 48 h. The MS potential of zero charge (EZC) in 5 × 10−3 M BPOX at 40 °C was found to be −530 mVSCE: it was demonstrated that the presence of Cl− in acidic medium promoted the BPOX corrosion inhibition of the positively charged MS surface. Potentiodynamic experiments carried out in the 10−3 M BPOX presence (40 °C) showed that ηp value obtained in 0.5 M HCl (ηp = 94% after 24 h) was even passed in 0.5 M H2SO4 + 0.5 M NaCl (ηp => 95%). The negative ΔGads° values calculated (approximately −30 to −34 kJ/mol), lower than −20 kJ/mol but not as low as −40 kJ/mol, indicated the stability of the adsorbed layer and that the BPOX adsorption mechanism was more than a physisorption, but not a true chemisorption.