Elsevier

Journal of Molecular Liquids

Volume 282, 15 May 2019, Pages 499-514
Journal of Molecular Liquids

Cassava starch graft copolymer as a novel inhibitor for the corrosion of aluminium in HNO3 solution

https://doi.org/10.1016/j.molliq.2019.03.044Get rights and content

Highlights

  • Cassava starch graft copolymer (CSGC) shows higher inhibition efficiency than CS, AA, or CS/AA mixture.

  • The adsorption of CSGC obeys Langmuir isotherm accompanied by a decrease of entropy.

  • CSGC acts as a mixed-type inhibitor, while mainly retards the anodic reaction.

  • EIS has three time constants, and the inhibited aluminium surface is of hydrophobic nature.

  • The adsorption and inhibition ability follows the order: Glc-AA3 > Glc-AA2 > Glc-AA > Glc.

Abstract

Cassava starch graft copolymer (CSGC) was prepared by grafting acryl amide (AA) onto cassava starch (CS). The inhibition effect of CSGC on the corrosion of aluminium in HNO3 solution was studied by weight loss, electrochemical techniques, scanning electron microscope (SEM), atomic force microscope (AFM), contact angle images and X-ray photoelectron spectroscopy (XPS). Quantum chemical calculation and molecular dynamic simulation were performed to theoretically investigate the adsorption mechanism. CSGC is an effective novel inhibitor, and shows higher inhibition efficiency than either CS or AA. The adsorption of CSGC on aluminium surface obeys Langmuir isotherm. CSGC is a mixed-type inhibitor, while mainly retards the anodic corrosion. EIS consists of a large capacitive loop at high frequencies followed by a small inductive one at middle frequencies and the second capacitive loop at low frequency values, and the impedance strengthens with the concentration of CSGC. SEM and AFM confirms the corrosion of aluminium surface is prominently retarded after adding CSGC to the media. Contact angle image suggests that the inhibited aluminium surface is of hydrophobic nature. XPS provides the evidence of the adsorptive inhibitor of CSGC on aluminium surface. Quantum chemical calculation and molecular dynamic simulation can be well theoretically elucidate that CSGC shows higher adsorption and inhibition ability than CS.

Introduction

Corrosion protection of aluminium in HNO3 water solution has been an important topic in the metal protection area because HNO3 is widely used for cleaning sedimentary scale. Aluminium typically shows a slower dissolving rate in HNO3 than HCl [1], which was attributed to the different corrosion mechanism. Aluminium dissolution mechanism in HCl was largely investigated and almost clearly understood [1]. On the other hand, the corrosion details of aluminium in HNO3 still remain somewhat uncertain though some studies have been dedicated to investigating the corrosion behavior and mechanism of aluminium in HNO3 media [[1], [2], [3], [4], [5]]. Accordingly, the corrosion behavior of aluminium in HNO3 is rather more complex than that in HCl.

To protect aluminium from acid corrosion, inhibitors are always added to the acid solution during cleaning. Comparing with many efficient inhibitor studies for aluminium in HCl, very little attention has been paid to corrosion inhibition of aluminium in HNO3. Most of the efficient inhibitors in HCl solution can lose their inhibitive ability in HNO3 solution [6]. The studies about corrosion inhibitors for aluminium in HNO3 can be dated back to 1970s when Singh et al. [[7], [8], [9]] reported some thiourea and urea derivatives showing corrosion retardancy of aluminium in HNO3. Inhibitive ability followed the general order: naphthyl thiourea > phenyl thiourea > thiourea > urea, and the maximum inhibition efficiency of naphthyl thiourea was around 80% [8]. Afterwards, Yadav et al. [[10], [11], [12]] reported the inhibition effect of benzoic acids and various derivatives on aluminium corrosion in HNO3. Among these compounds, 3,4,5-trihydroxy benzonic acid was found to be the most effective inhibitor with inhibition efficiency of 72% in 20% HNO3. In 2010s, Khaled [13] investigated three sulphur-containing amines, 3-pyridinecarboxaldehyde thiosemicarbazone (META), isonicotinaldehyde thiosemicarbazone (PARA) and 2-pyridinecarboxaldehyde thiosemicarbazone (ORTHO) about the corrosion inhibition of aluminium in 1 0 M HNO3. The results showed that inhibition efficiency followed an order: META > PARA > ORTHO, and the maximum inhibition efficiency of 0.01 M META was 88.4% at 25 °C. Obviously, the efficient inhibitor of aluminium in HNO3 solution is still rather scarce, and it is necessary to explore more inhibitors. These compounds reported in previous literatures [[7], [8], [9], [10], [11], [12], [13]] were typically small organic molecules, and so the question whether polymer compound is well qualified for potential effective inhibitor for aluminium in HNO3 medium has been emerged.

Compared with low molecular inhibitor, the polymer inhibitors were expected to have larger coverage surface, hence better inhibition performance and stability [14]. Some polymers like petrochemical compounds such as tweens [15], polyxoyethylene mono oleates [16] and poly 3-(decyloxy sulfonic acid) aniline [17] as well as natural polymers such as pectin [18], iota-carrageenan [19] and exudate gum [20] were also reported to be able to inhibit aluminium corrosion in HCl solution. These efficient polymer inhibitors of aluminium in HCl do not necessarily exhibit good inhibitive performance in HNO3. Up to now, there is almost no study on using polymer as an inhibitor for aluminium corrosion in HNO3 solution.

Starch is one of the richest natural polymer resources in the world. It is safe, non-toxic and biodegradable with has low-cost. However, starch is seldom used as corrosion inhibitor due to the low corrosion retardency [21]. Recently, we have conducted some works to prepare cassava starch graft copolymer (CSGC) by grafting the vinyl monomer onto cassava starch (CS) with oxidization reduction reaction. To our surprise, CSGC was found to show high efficiency in inhibiting aluminium against in HCl solution [22,23], and the maximum inhibition efficiency was higher than 90% at CSGC concentration of 50 mg L−1. However, the adsorption mechanism of CSGC on aluminium surface needed to be further investigated, and the role of vinyl monomer in CSGC is still uncertain. The corrosion behavior and mechanism of aluminium in HNO3 is quite more complex than that in HCl, so it is still unclear whether CSGC is an effective corrosion inhibitor for aluminium in HNO3 media.

In this study, we firstly reported the inhibition effect of CSGC on the corrosion of aluminium in HNO3 solution. Effects of inhibitor concentration, temperature, immersion time and acid concentration on inhibition efficiency were fully examined by weight loss method. Both adsorption thermodynamic and corrosion kinetic parameters were calculated to assist in understanding the adsorption behavior of CSGC on aluminium surface. Meanwhile, the electrochemical inhibitive mechanism was proposed according to electrochemical techniques. The micro morphologies and compositions of aluminium surface in HNO3 solution were investigated by scanning electron microscope (SEM), atomic force microscope (AFM), contact angle images and X-ray photoelectron spectroscopy (XPS). Moreover, the adsorption of CSGC was theoretically studied by quantum chemical calculations and molecular dynamic (MD) simulations. Lastly, the corrosion reactions for aluminium in HNO3 solution and inhibitive mechanism of CSGC were proposed. It is expected to throw some light on seeking the efficient inhibitor for aluminium in HNO3 solution.

Section snippets

Materials

Aluminium specimen (Al 1060) with the composition (wt%): 0.25 Si, 0.35 Fe,0.05 Cu, 0.03 Mn, 0.03 Mg, 0.05 Zn, 0.03 Ti, 0.05 V and balance Al was obtained Yunnan Aluminium Co. Ltd. The aggressive solutions of 1.0–4.0 M HNO3 were prepared by dilution of analytical (AR) grade 65% HNO3 with distilled water. Industrial-grade CS (about 17% amylose and 83% amylopectin [24,25]) was purchased from Red River Hong Starch Co. Ltd. in Yunnan province of China. AR grade of acryl amide (AA) was obtained from

Raman spectra of CS and CSGC

Raman spectra of CS and CSGC are shown in Fig. 2(a) and (b), respectively. For CS in Fig. 2(a), the absorption peak at 2906 cm−1 was assigned to Csingle bondH antisymmetric and symmetric stretching vibrations. The peaks in the region between 1500 and 1200 cm−1 can provide rich information of functional groups. A band at 1466 cm−1corresponded to Csingle bondH, single bondCH2, and Csingle bondOsingle bondH deformation vibrations. The bands at 1382 and 1341 cm−1 can be assigned to Csingle bondOsingle bondH deformation or Csingle bondO stretching vibration. The peak at 1267 cm−1 was

Conclusions

  • (1)

    The grafting polymer of CSGC behaves as an effective inhibitor for aluminium corrosion in 1 0 M HNO3, and the maximum ηw is 93.3% at 1.0 g L−1 and 20 °C. The inhibitive performance of CSGC is higher than that of CS, AA, or CS/AA mixture. Inhibition efficiency increases with the concentration of CSGC, but decreases with the increase of temperature.

  • (2)

    The adsorption of CSGC on aluminium surface obeys Langmuir adsorption isotherm with ΔG0 of around −20 kJ mol−1, and it is an exothermic process

Acknowledgements

Funding support from the National Natural Science Foundation of China (51561027) and Training Programs of Young and Middle Aged Academic and Technological Leaders in Yunnan Province (2015HB049, 2017HB030) are acknowledged.

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