ReviewInhibition of copper corrosion by 1,2,3-benzotriazole: A review
Introduction
Corrosion plays a very important role in diverse fields of industry and, consequently, in economics. The protection of metals and alloys is thus of particular interest. The goal of studying the processes of corrosion is to find methods of minimizing or preventing it. One approach is the use of corrosion inhibitors. The mechanism of how organic corrosion inhibitors work is usually not known. In most cases, empirical testing has provided information on the effectiveness of a particular molecule as a corrosion inhibitor for a certain substrate in a certain medium. Knowledge of the surface chemistry of adsorbed organics is important for elucidating the mechanism of inhibitor action and, to achieve this, surface analytical techniques are required.
Copper is one of the most important nonferrous materials, with long life properties in neutral water solution, such as in water distribution networks. Strehblow and Titze [1] showed that the Cu passive layer consists of a duplex structure of oxides, with an inner cuprous oxide and an outer cupric hydroxide. Copper will not corrode in non-oxidizing acidic environments, since hydrogen evolution is not a part of its corrosion process. However, when oxygen or other oxidants such as Fe3+ and ions are present, corrosion becomes important. Copper oxides are stable only in the pH range of 8–12 [2], but not in acidic solutions (Eqs. (1), (2)) in which surface roughening can occur:
Moreover, Cu+ ions can undergo disproportionation according to Eq. (3) [3].
The potentiodynamic behaviour of pure Cu in near neutral and slightly alkaline solutions exhibits three anodic peaks associated with the formation of Cu2O, CuO and Cu(OH)2. Cu2O is first formed (Eq. (4)), which subsequently oxidizes to CuO (Eq. (5)) or, at more positive potentials, to Cu(OH)2 (Eq. (6)) [1], [4].
It has been known since 1947 [5] that 1,2,3-benzotriazole (BTAH, C6H5N3) is an effective corrosion inhibitor for copper and its alloys by preventing undesirable surface reactions. Since then BTAH has been included in a number of patents ([6], [7] and references therein). BTAH prevents discoloration of copper surfaces under atmospheric and immersed conditions. It has been used to protect artefacts of archaeological and historical concern [8], [9], [10]. It is known that a protective barrier layer, consisting of a complex between copper and BTAH, is formed when Cu is immersed in a solution containing BTAH. However, contradictory mechanisms and models of its action have been proposed. The reason probably lies in the insolubility of Cu-BTAH complexes in aqueous and many organic solutions, which precludes detailed structural and chemical studies that could lead to better understanding of its surface behaviour. The mechanism by which BTAH becomes connected to the surface on copper is also not completely understood, in spite of the application of various techniques. Powerful in situ techniques with high surface sensitivity are thus required to provide specific molecular information concerning the metal surfaces.
The most important application of BTAH is to protect copper and its alloys under immersed conditions. However, BTAH can also be adsorbed from the gas phase (then the surface chemistry is different [11], [12]) and, due to its low vapour pressure (8 × 10–6 torr), it can be used as a vapour phase inhibitor. It can also be used as an important additive for Cu plating baths [13].
BTAH (CAS RN 95–14-7) is a heterocyclic organic compound with a molar mass of 119.124 g/mol (Fig. 1), produced by reaction of orthophenylenediamine with sodium nitrite and acetic acid (Eq. (7)) [14], giving a whitish powder at room temperature.
It is sufficiently soluble in water solutions for use as a corrosion inhibitor. The melting and boiling points of BTAH are around 100 °C and 350 °C. It is only slightly toxic – the oral LD50 value in white rats is quoted as 560 mg/kg [9] – so it does not constitute a major environmental hazard. The form of the molecule in aqueous solution depends on the pH and can be either neutral (BTAH), negatively charged (BTA¯) or protonated () [15], [16], [17].In slightly acidic environments benzotriazole is present mainly in the undissociated form as BTAH, whilst at alkaline pH the molecule is predominantly present in the BTA¯ form.
In this review Cu-complexes will be designated as Cu(I)BTAH and Cu(II)BTAH when there is no clear evidence for removal of the N–H hydrogen from the BTAH molecule. Cu(I)BTA and Cu(II)BTA represent complexes with known Cu oxidation state and not involving this hydrogen atom. Cu-BTA refers to the molecule with no N–H hydrogen atom, but with unknown Cu oxidation state. Finally, the term Cu-BTAH is used when the structure of the complex is not a matter of discussion. We have followed studies of BTAH action from the early studies up to the present, drawing attention to differences in interpretation. The pioneering studies are described, followed by an account of the most important results concerning the structure, composition, mechanism, inhibitory effectiveness, synergistic effects of BTAH and the impact of benzotriazole derivatives. The most important models and mechanisms proposed for the BTAH action are presented.
Section snippets
Pioneering research on benzotriazole as a corrosion inhibitor for copper
Cotton and his co-workers [18], [19], [20] pioneered research in the field of BTAH as a copper corrosion inhibitor. They showed that the Cu surface pretreated with BTAH induces long-lasting prevention of staining. Dugdale and Cotton [19] explained the BTAH inhibitory action in terms of a physical barrier, i.e. a surface complex of Cu-BTAH. It was proposed that this structure was formed on immersion of copper in BTAH containing solution, where soluble copper ions were produced. This surface
Surface structure and composition of the BTAH treated Cu
Chadwick and Hashemi [35], [36] showed that the Cu(I)BTA complex, whether adsorbed on the Cu surface or synthesized as a Cu(I)BTA crystal, is unstable when exposed to air and starts to oxidize to Cu(II)BTA. In contrast to the previous observations of Roberts [34], only Cu(I)BTA complex was formed during longer immersion in acidic chloride solution. Cu(II) species were formed only after removal from the liquid phase. Based on X-ray Photoelectron Spectroscopy (XPS) depth profiling, it was
Mechanism of benzotriazole inhibition
Evans [68] sought to explain the inhibition by BTAH in chloride solutions by the formation of basic copper chloride on the Cu surface that interacts with BTAH molecules. This blocking substance would thus be responsible for the high degree of inhibition. According to this theory BTAH would not be effective in other solutions, or the mechanism for corrosion inhibition should change when chlorides are not present.
Ogle and Poling [69] classified the formation of naturally growing thin and thick
Inhibitory effectiveness of benzotriazole
The effectiveness of inhibition of corrosion by BTAH, using various conditions and procedures, is summarized in Table 1.
Ross and Berry [24] investigated BTAH action on Cu in 10% H2SO4 solution under dynamic conditions. At low flow rates (Reynolds number < 500) they reported maximum IE at 10 mM BTAH. IE was not dependent on aeration of the solution. On the other hand, at higher flow rates (Reynolds number of 10,000) the presence of O2 increased the corrosion rate.
Walker [7] showed that 0.12 g/L
Synergistic and accelerating effects
A synergistic effect of benzylamine and BTAH for Cu protection was reported by Fleischmann et al. [111]. The layer formed in the BTAH/benzylamine solution was thought to be composed mainly of a [Cu(II)BTA2]n polymeric network. The layer was more protective when formed at anodic potentials.
Wu et al. [112] reported that iodide ions and BTAH together improve corrosion protection of Cu in sulphuric acid. It was suggested that adsorbed iodide ions on the Cu surface preferentially adsorb an
Derivatives of benzotriazole
Fleishmann et al. [43] showed that BTAH in 1 M KCl completely displaces 1-hydroxybenzotriazole (BTAOH) from the Cu surface after prolonged exposure, indicating its higher inhibitory potential. Musiani et al. [105] demonstrated that BTAH is displaced by the more protective inhibitor, 2-mercaptobenzothiazole, in chloride solution at pH 1. Naphthotriazole is more effective than benzotriazole against Cu corrosion [122], but its use is precluded by the fact that it is far more expensive.
Törnkvist et
Conclusions
Benzotriazole (BTAH) is one of the best corrosion inhibitors for Cu and its alloys. It is effective in preventing copper corrosion under stationary as well as dynamic conditions. Its inhibitory effectiveness increases with increased concentration and time of immersion. It was shown that BTAH acts as a mixed type inhibitor, but its predominant effect is on inhibition of anodic reaction. The Cu(I)BTA surface complex, involving Cu–N bonds, is well recognized, but the exact structure of the complex
Acknowledgment.
This work has been supported by the Slovene Research Agency (grants No. P2-0148 and J1-9516). The authors thank Sebastijan Peljhan for constructing the figures in XCrySDen software.
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