Elsevier

Electrochimica Acta

Volume 56, Issue 9, 30 March 2011, Pages 3322-3327
Electrochimica Acta

Theoretical challenges in understanding the inhibition mechanism of aluminum corrosion in basic media in the presence of some p-phenol derivatives

https://doi.org/10.1016/j.electacta.2011.01.021Get rights and content

Abstract

Using quantum electrochemical/thermodynamical approaches based on coupled cluster/polarized continuum models and density functional theory (CM/PCM–DFT), we investigated the corrosion inhibition mechanism of Al/NaOH system in the presence of some p-phenol derivatives. The influencing parameters on inhibitory action, i.e. charges on oxygen and hydrogen atoms of hydroxyl group, charge transfer, interaction energy, molecular activity and softness, electric dipole moment and de-solvation free energy, were determined for both neutral and deprotonated species at metal|solution interface. A good correlation was observed between these parameters and inhibition efficiency data reported in the literature. By introducing an appropriate thermodynamic procedure, we also determined the proton-loss tendency of the molecules nearby interface. The results were amazing and revealed a complicated protonation/deprotonation cycle for inhibitor species inside electrical double layer; the corrosive agents in the vicinity of metal surface become locally neutralized and pushed away.

Research highlights

► Quantum electrochemical/thermodynamical models for inhibition mechanism. ► A new insight based on local deactivation of corrosive agents at interface region. ► Protonation/deprotonation cycle of phenolic species inside electrical double layer.

Introduction

Corrosion is a destructive phenomenon by which the metallic structures are destroyed gradually through chemical or electrochemical reactions. Therefore, various attempts must be employed to prevent or retard these undesired reactions. The application of inhibitors, viz. specific organic compounds containing heteroatoms, aromatic rings or pi electrons, is one of the most practical methods to combat corrosion and in some situations, it is the only choice [1].

Organic inhibitors are often weak acids/bases, de-/protonating in aggressive media. Regardless which form of the molecules (neutral or charged) is responsible for the inhibitory action, it has been generally accepted that the inhibitor species should firstly diffuse towards the metal|solution interface, then the rate of degradation will be diminished by subsequent adsorption of the species, blocking the active-sites of the metal surface [2]. The mechanism may alternatively be thought of the result of local neutralization of the corrosive agents at the interface region. This conjecture is the issue of present paper and seems to play well for amphoteric systems, especially for aluminum corrosion in alkaline media, in which the rate of degradation is highly pH-dependent and the hydroxide anions are corrosive agents causing the metal atoms to be dissolved as aluminates [3]; the cathodic reaction is water reduction:Al+4OHAlO2+2H2O+3e3H2O+3e3OH+32H2Fundamental approaches to interfacial problems (e.g. corrosion inhibitors at interface region) are among highly interdisciplinary topics that offer a mechanistic perspective for understanding the phenomena from molecular viewpoints [4]. The objective can be achieved through quantum-oriented physico-electrochemical models developed in this area, with considering the effect of solvent, substrate and electric field of the region [5]. Using these quantum-electrochemical models and calculations based on density functional theory (DFT), we have proposed an inhibition mechanism for some ortho-substituted aniline derivatives in acidic solutions [6]. Through a new inspiration, however, the present author has attempted to prove the conjecture mentioned above and reveal how phenol derivatives prohibit the aluminum corrosion in alkaline media.

Section snippets

Corrosion inhibitors at metal|solution interface: CM/PCM approach

The inhibitory action of molecules in aggressive solutions is a direct result of complicated interactions with corroding metal atoms, solvent molecules and electric field of the interface [2], [5]. The interactions can be investigated quantum mechanically via cluster/polarized continuum model [5]. Briefly, in this method of investigation (see Fig. 1), the metal surface is treated as a set of discrete points (atoms) in the Euclidean space (cluster model; CM approach [4], [7]) and the corrosion

Proton-loss tendency inside electrical double layer (EDL)

Phenol inhibitors are among weak acids de-protonating in basic media and appear as anionic species, named phenolate or phenoxide. Thus, the negative form of these compounds should also be taken into consideration in alkaline solutions.

To calculate the proton-loss tendency (pKa) of a molecule in aqueous media, one may use the thermodynamic cycle [11], [12] (see Scheme 1).In this cycle, ΔGaq is calculated from Eq. (3):ΔGaq=ΔGg+ΔΔGsolvwhere the notation ΔΔGsolv is standing forΔΔGsolv=[ΔGsolv(ϕ-O)+

Results and discussion

Table 1 includes the molecules under consideration along with their inhibitory performance calculated using Eq. (9):ε(%)=1ΔMΔM0×100where ΔM and ΔM0 are the extent of metal's degradation (mass-loss [8]) measured gravimetrically in the presence and absence of the inhibitors, respectively.

The results of quantum electrochemical investigations are summarized in Table 2, Table 3. Using data listed in these tables, we examined the effect of molecular parameters on inhibitory performance through

Inhibition mechanism

On the basis of current theory (CM/PCM–DFT), we suggest the following mechanism for phenol corrosion inhibition system in basic medium:

The step d is not elementary but consists of the following reactions:ϕ-OHEDL+OHadsϕ-OEDL+H2Oadsϕ-OEDL+H2Oadsϕ-OEDL+H2OEDL(d)(OHadsneutralization and phenoxide replacement)The annihilation of hydroxide anions on metal surface (OHads  H2O) may alternatively proceed as follows:
By decreasing the number of corrosive agents on metal surface, the rate of

Acknowledgements

Special gratitude is due to Prof. George C. Shields (Hamilton College) and Prof. Paul G. Seybold (Wright State University) for their useful comments on experimental values of pKa in bulk solution.

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