Effect of the molecular structure on the inhibitor properties of azoles on mild steel corrosion in 1 M hydrochloric acid
Introduction
The electrochemical corrosion proceeds as a heterogeneous reaction at the interface metal/solution. Therefore adsorption processes influence to a great extent the electrode kinetics. By adding appropriate chemical compounds to the system, corrosion inhibition interferes in the adsorption pattern in order to decrease the reaction rate. A closer insight in adsorption phenomena is hence essential to understand the mechanism of corrosion inhibition [1], [2]. The theoretical consideration for a correlation between the molecular structure of organic compounds and their ability to adsorb on the metal surface and hence to inhibit the corrosion process is aimed at in a series of studies [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25].
In the conception of “molecular structure”, two components can be distinguished. The electronic structure is determined by the electronic density at the reaction centre in the molecule while the chemical structure is determined by the volume (area) and the steric distribution of eventual substituents. Different approaches have been applied to find correlations between the inhibitor properties and the molecular structure – Hammet-like substituents’ constants using of linear free energy relations [3], [4], [5], [6], [7], Taft constants and pKa [8], ionisation potential and electron mobility [9], [10], highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital energy [11], [12], electrodonor properties of different functional groups [13], [14], molecular area [9], [15], position of the substituents and steric factors [16], [17].
In our previous investigations 15 different benzimidazole derivatives were tested as corrosion inhibitors of mild steel in 1 M HCl [26], [27]. We found that the efficiency depends on both the electronic and the chemical structure (molecular area and steric distribution of the substituents) of the molecules, the influence of the latter being better expressed.
In this study different azoles were chosen, whose molecules have almost the same planar structure and area. As the chemical structure is almost the same for all compounds the differences in the inhibition properties should be due mainly to the different electronic structure of these compounds.
This work is concerned with inhibition of mild steel corrosion in 1 M HCl by bicyclic compounds containing a benzene ring fused to five-membered heterocycles (Table 1): indole (I, one N atom), benzimidazole (BI, two N atoms), 1,3-benzothiazole (BNS, one N and one S atom), 1H-benzotriazole (BTA, three N atoms) and 2,1,3-benzothiadiazole (BTD, two N and one S atom).
Section snippets
Experimental
The mild steel used had the following chemical composition (wt. %): C – 0.16; Mn – 0.35; Si – 0.06; P – 0.01: S – 0.029; Cr – 0.06; Cu – 0.10; Fe – balance. All investigated compounds were commercial products: indole (EGA–Chemie, ⩾99%), benzimidazole (Fluka AG, ⩾98%), 1,3-benzothiazole (Fluka AG, ⩾95%), 1H-Benzotriazole (Fluka AG, ⩾99%), 2,1,3-benzothiadiazole (Fluka AG, ⩾98%).
Two classic techniques (gravimetric and potentiodynamic polarisation), were used to determine the corrosion inhibition
Gravimetric tests
The average corrosion rate w was determined for every concentration after 24 h exposure in the test solution. The highest concentration is sometimes limited by the solubility of the compound. If at two consecutive concentrations the inhibitor efficiency showed no further increase, higher concentrations were not tested. The concentration at which the IE was less than 5% was chosen as lower concentration limit.
The inhibitor efficiency was calculated according to Eq. (1). In Fig. 1 are presented
Adsorption
The adsorption isotherm can be determined if the inhibitor effect is due mainly to the adsorption on the metal surface (i.e., to its blocking). The type of the adsorption isotherm can provide additional information about the properties of the tested compounds. The fractional surface coverage θ can be easily determined from Eq. (1), if one assumes that the values of IE do not differ substantially from θ [14], [22], [23], [28], [29].
According to Damaskin [30] the experimental data are most often
Molecular structure and inhibitive properties
The molecular structure is one of the major factors influencing the adsorption of the organic molecules on the metal surface, and hence the inhibitor properties, especially in the case of chemisorption, which involves charge sharing or charge transfer from the inhibitor molecules to the metal to form coordinate type of bonds. The electronic density of the atoms, acting as reaction centres of the molecules, determines the adsorption bond.
All investigated compounds are bicyclic, containing a
Conclusions
The investigated compounds inhibit the corrosion processes of mild steel in 1 M HCl. An exception is 2,1,3-benzothiadiazole, which acts as a stimulator. The inhibiting properties increase with concentration. The same ranking is obtained from gravimetric and potentiometric measurements.
The adsorption of BI and BNS is well described by the Frumkin adsorption isotherm. The interaction parameter a shows, that in the case of BI the interaction in the adsorbed layer is attraction, while for BNS it is
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