Elsevier

Corrosion Science

Volume 76, November 2013, Pages 206-218
Corrosion Science

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

https://doi.org/10.1016/j.corsci.2013.06.044Get rights and content

Highlights

  • 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.

Abstract

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.

Introduction

The industrial application of nonmetallic materials is almost always precluded because of problems arising from the severity of the environment and the aggressive conditions of temperature, pressure and wear. On the other hand, in several industrial environments most metals and alloys are affected by different forms of corrosion attack, particularly dangerous if localized. For these reasons the industrial use of corrosion inhibitors is often a good solution to prevent corrosion phenomena and to provide a more acceptable lifetime of metallic structures [1], [2], [3], [4].

Many compounds with polar group (containing N-, O-, S-, etc. atom/s and/or π-electrons) are known to act as efficient corrosion inhibitors. These organic molecules can be adsorbed on the metal surface because they are able to establish coordinative interactions between free N electron pairs (eventually enforced by π-retrodonation) and the oxidized metal surface: as a result, the corrosive attack is reduced in acidic media [5], [6], [7], [8], [9], [10], [11], [12].

The corrosion of iron or mild steel in acidic media has been studied extensively, especially for its industrial relevance. In fact, acid solutions widely used in cleaning, descaling, pickling, and oil well acidizing, require the use of corrosion inhibitors to reduce their corrosion attack on metallic structures or materials [13], [14], [15], [16], [17], [18], [19].

In the last decades this research field focused on corrosion inhibitors that act in particularly aggressive corrosive media such as acidic media in the presence of chlorides [18], [19], [20], [21], [22]. Regarding this medium, the literature reports cases of inhibitors acting by physisorption or chemisorption on the metal surface, depending on the nature of the metal’s surface charge, the aggressiveness of the medium, and the chemical structure of the inhibitors [23], [24], [25].

It is reported that isoxazolidines are good corrosion inhibitors in 1 M HCl at 60 °C [26], but the 2-butyl-hexahydropyrrolo[1,2-b][1,2]oxazole (BPOX) has not yet been considered as a corrosion inhibitor. In fact, it was already known from the 1990s, but it was mainly used in reaction mechanisms and structural studies, and in some syntheses of products of pharmaceutical interest [27], [28], [29].

This paper reports the results of an inhibition efficiency study carried out on MS specimens in aerated 0.5 M hydrochloric acid solutions in the presence of BPOX in the 30–60 °C temperature range, using electrochemical, WL and other spectroscopic methods.

The tests were carried out after 2 (electrochemical tests) and 48 h (electrochemical tests and WL tests) of immersion time also to study the BPOX inhibition efficiency in a longer period of time.

The BPOX adsorption behavior was also studied to determine the adsorption isotherm and thermodynamic data.

Section snippets

Specimens

Corrosion inhibition tests were performed using specimens prepared from a sheet of mild steel (MS) classified as J55, having the composition (wt%): C: 0.33; Si: 0.32; Mn: 0.9; P:<0.015; S: 0.02; Cr: 0.10; Ni: 0.12; Mo: 0.02; Cu: 0.2; Al: 0.01; Fe balance. The samples used in electrochemical tests (polished specimens) were abraded with emery paper (until 1200 grade) and then with diamond paste (until 1 μm) to reach a polished surface. Some potentiodynamic tests were also carried out with a

Potentiodynamic tests

Polarization curves obtained in 0.5 M HCl for MS (polished surface) in BPOX solutions in the 30–60 °C temperature range gave the electrochemical results reported in Table 1 [bC, the cathodic Tafel slope, Ecorr, the free corrosion potential, bA, the anodic Tafel slope, icorr, the corrosion current density, and ηp, the inhibition efficiency obtained by the Tafel curves].

In Fig. 4a, Fig. 4b some typical Tafel curves obtained for MS specimens (polished surface) in 0.5 M HCl, containing different

MS corrosion reaction in the inhibited HCl solution

The efficiency of an organic compound as an inhibitor (Org) is mainly dependent on its ability to get adsorbed on metal surface. This process generally consists in the replacement of water molecules at a corroding metal surface, following the equilibrium (5):Org(sol)+nH2O(ads)Org(ads)+nH2O(sol)

The adsorption of this organic compound is influenced by electronic structure, steric factor, electronic density at donor site, presence of functional groups, molecular area and molecular weight [47],

Conclusions

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 HCl in the 30–60 °C temperature range. Its inhibition effectiveness was assessed after short (2 h) and longer (48 h) immersion time tests, through potentiodynamic, EIS and WL measurements. The conclusions of this work can be summarized as follows:

  • (1)

    at low immersion times (2 h) the potentiodynamic tests indicated that

Acknowledgements

This research was supported by the Ministry of the University and of the Scientific Technological Research (MURST) of Italy through the Dept. of Macromolecular Sciences and Nanosystems under 2010 Grants.

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