Fundamental studies of aluminum corrosion in acidic and basic environments: Theoretical predictions and experimental observations
Introduction
Fundamental approaches to corrosion and corrosion inhibition problems [1] are among interesting interdisciplinary topics in the field of materials science in which their investigations are important from both industrial and academic points of view, and have attracted the attention of physical electrochemical scientists in this area [2], [3], [4]. Although the validity of predictions in these studies should be ultimately approved by experiments, the empirical approaches are often time-consuming, wearisome and expensive [5]. Their investigations, particularly those with fundamental basis, need some sophisticated instruments being capable of in situ analyzing the phenomena [6]. Thus, to minimize the experimental efforts, theoretical investigations have been recently proposed as an alternative to predict the phenomena before the direct observations by experiments [7].
The degradation of metals in aggressive environments can be thought as a result of direct interactions of the corrosive agents (e.g. Cl−, OH−, ..) with surface active sites in which the presence of inhibitor compounds can interfere and diminish the rate of corrosion. In theoretical approaches, the adsorption of species on metal surface can be investigated quantum electrochemically through cluster/polarized continuum model [8], i.e. a modified version of cluster model which applies the equilibrium structure of the species optimized at meal/solution interface, rather than in vacuo. Such theoretical investigations are obviously more useful if they can predict the phenomenon before doing experiments.
By considering the remarks pointed above, we have in this work attempted to evaluate the corrosion behavior of an amphoteric metal like as aluminum in both acidic (HCl) and alkaline (NaOH) media, firstly through theory and secondly by means of electrochemical approaches based on potentio-dynamic experiments [9]. It is also worthy to note that this non-transition metal has a wide application in high-tech industries including aerospace, advanced nuclear reactors, metal/air batteries, and ... [10]. This is mainly because of light atomic-mass and capability of the metal to form a protective oxide film on its surface. The passive film is unfortunately degraded in hostile-corrosive environments and hence the inhibitors should be applied. The molecule under consideration is phenol inhibitor [11], [12], introduced by Talati and Modi [13] as aluminum corrosion inhibitor in basic media. For the case of acid, we are also interested here in examining the inhibition properties of the molecule through current theory and experiment.
Section snippets
Theoretical
As implicitly mentioned above and the experimental reports give extra support for that [14], [15], [16], the first step in aluminum corrosion in aggressive solutions involves the contact adsorption of corrosive agents on metal's surface followed by chemical interactions to form soluble products. Theoretically, the interaction of species at metal/solution interface can be investigated by means of cluster model and applying quantum chemical calculations based on density functional theory (DFT)
Experimental
The electrochemical measurements were conducted on a high-purity (99.999) Al rod of 3 mm diameter, purchased from Sigma–Aldrich®, mounted in epoxy resin to expose the cross-sectional area of 7 mm2 to the electrolytes. Before each experiment, the surface of Al electrode was wet ground with successive emery (SiC) papers # 220, 400, 600, 800 and 1000, washed in distilled water and acetone to remove dust and grease. Thereafter, Al electrode was immersed in the alkaline-etching solution (1.5 g
Results and discussion
The interaction of corrosive agents and solvent molecule with aluminum surface was studied. The results are given in Table 1 and depicted in Fig. 2. As it can be seen from the table, the hydroxide anions in comparison with chloride ones adsorb more strongly in less distances. Because of this tenacious attack of the hydroxide corrosive agents (strongly adsorption at short distances), there is no doubt the aluminum corrosion to be more severe in NaOH as compared with that measured in HCl media.
Conclusion
On the basis of present quantum electrochemical approaches, we can conclude that:
- (1)
Hydroxide corrosive agents in comparison with chloride ones adsorb more strongly. This tenacious attack of the hydroxide anions causes the aluminum corrosion in alkaline media to be more severe as compared with that measured in HCl solution.
- (2)
The inhibitor adsorption on aluminum surface has a physical nature in acidic environment. This physical interaction is also in line with inhibitory behavior determined at
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