Corrosion and corrosion inhibition of Cu–20%Fe alloy in sodium chloride solution

doi:10.1016/j.corsci.2007.11.018

Corrosion Science

Volume 50, Issue 4, April 2008, Pages 928-937

https://doi.org/10.1016/j.corsci.2007.11.018 Get rights and content

Abstract

The electrochemical behavior of copper (Cu), iron (Fe) and Cu–20%Fe alloy was investigated in 1.0 M sodium chloride solution of pH 2. The effect of thiourea (TU) addition on the corrosion rate of the Cu–20%Fe electrode was also studied. Open-circuit potential measurements (OCP), polarization and electrochemical impedance spectroscopy (EIS) were used. The results showed that the corrosion rates of the three electrodes follow the sequence: Cu < Cu–20%Fe < Fe. Potentiostatic polarization of the Cu–20%Fe electrode in the range −0.70 V to −0.45 V (SCE), showed that iron dissolves selectively from the Cu–20%Fe electrode surface and the rate of the selective dissolution reaction depends on the applied potential. At anodic potential of −0.45 V, thiourea molecules adsorb at the alloy surface according to the Langmuir adsorption isotherm. Increasing thiourea concentration (up to 5 mM), decreases the selective dissolution reaction and the inhibition efficiency η reach 91%. At [TU] > 5 mM, the dissolution rate of the Cu–20%Fe electrode increases due to formation of soluble thiourea complexes. At cathodic (−0.6 V), the inhibition efficiency of thiourea decreases markedly owing to a decrease of the rate of the selective dissolution reaction and/or desorption of thiourea molecules. The results indicated that thiourea acts mainly as inhibitor of the selective dissolution reaction of the Cu–20%Fe electrode in chloride solution.

Introduction

Cu–Fe alloys are materials that are more and more extensively applied in industry. Firstly, they are utilized as master alloys to produce new copper alloys for very special purposes. They are used as materials for electrical device components, for example, semiconductor lead frames, electrical connectors, and electrical fuses [1]. The phase diagram of the Cu–Fe system shows that copper solubility in iron is practically high at high temperatures. However, in the range of low temperatures, the copper solubility significantly drops to 1.88 at.% at the eutectoid temperature of 850 °C. A new melting process has been used to prepare Cu–Fe alloys with 10, 20, and 30 wt% Fe, but these alloys show a tendency to segregate [1]. On determining and applying the proper parameters of the melting process, it is possible to produce Cu–Fe alloys with such high iron content [1].

Studies of the corrosion behavior of Cu–Fe alloys in aqueous media seem to be rare. It has been reported that iron-alloying addition to copper alloys may be used to improve their corrosion resistance [2]. The mechanisms of the dissolution and passivation of bulk polycrystalline icosahedral Al₆₃Cu₂₅Fe₁₂ specimens during electrolytic corrosion in sodium hydroxide and sulfuric acid solutions were studied [3]. Selective dissolution of Al and Fe from the alloy surface was found to occur at the open-circuit potential, which leads to precipitation of porous layer of re-crystallized copper. After anodic polarization, the dissolution of the alloy is followed by re-deposition of Cu and formation of Cu₂O [3].

The adsorption of thiourea onto metal surfaces in aqueous solutions of electrolytes is of great interest since it is known to be effective as a corrosion inhibitor in acid media [4]. It has been found that thiourea is strongly chemisorbed and the adsorption characteristics are similar to those of halide ions, that is involving sharing or donation of electron pairs [5], [6]. As with other organic molecules, the extent of thiourea adsorption depends on several factors including temperature, applied potential, type of electrolyte and thiourea concentration. Previous work showed contradictory results. Most of the authors agreed that the quantity of thiourea adsorbed increases as the potential becomes more positive and the extent of adsorption decreases at negative potentials [7]. The results of the SERS studies by Brown et al. [5], on the other hand, showed the opposite trend. These authors found strong thiourea adsorption at the copper surface polarized at negative potentials and this behavior was observed for potentials up to −0.70 V. The reactions involved in the electro-oxidation of copper in aqueous thiourea-containing solutions were investigated by Bolzan et al. [8]. In the range −0.30 ⩽ E ⩽ +0.07 V (vs. SCE), the main electrochemical reactions are the electro-oxidation of thiourea to formamidine disulphide FDS, and of copper to several Cu(I)–TU complex ions [8]. At E ⩾ +0.07 V, electro-decomposition of FDS and Cu(I)–TU complex ions takes place [8]. At potentials less than −0.30 V, oxidation of thiourea to FDS is not expected and thiourea is assumed to be chemisorbed at the electrode surface as molecular (TU) or protonated (TUH⁺) species [9].

Thiourea interacts strongly with the surfaces of d-metals (e.g. iron). This interaction has the character of chemisorption. The structure of the adsorbed over-layer, the orientation of the adsorbed particles and the dissociative or associative character of the adsorption were determined [10], [11]. Adsorption of thiourea molecules on the surfaces of group IB metals (e.g. copper), is on the other hand, much weaker than with d-elements, and the adsorption process has a reversible character [12]. The effect of adsorption of thiourea on the corrosion kinetics of pure copper and iron samples has been investigated in detail [12]. Investigation of the adsorption and inhibition effects of thiourea on the Cu–20%Fe alloy seems, therefore to be very interesting.

The objective of the present work is to investigate the corrosion rate of the Cu–20%Fe electrode in aqueous 1.0 M sodium chloride solution of pH 2. The effects of the polarization potential and thiourea concentration on the corrosion rate of the Cu–20%Fe electrode were studied. The data were compared with those of the constituent elements, namely copper and iron. In these investigations, conventional electrochemical techniques were used including open-circuit potential measurements (OCP), polarization and the electrochemical impedance spectroscopy (EIS).

Section snippets

Experimental details

The copper and iron test samples were cut from spectroscopically pure rods (Johnson & Matthey). The copper–iron alloy with chemical composition (Cu–20%Fe), was one of the products of the company of the ferrous and non-ferrous metals and alloys, Helwan, Egypt. All electrodes were prepared with a constant surface area of 0.196 cm² as described elsewhere [13], [14], [15]. Before each experiment, the working electrode was polished with emery papers of different grades, wrapped against smooth cloth,

Open-circuit potential measurements

Fig. 1 shows the variation in open-circuit potentials of the mechanically polished copper, iron and the Cu–20%Fe electrodes with time in 1.0 M sodium chloride solution of pH 2. For all electrodes, the open-circuit potentials drift with time in the active direction and tend to stabilize within ∼60 min. This negative shift of potentials is related to active dissolution of the different electrodes in the sodium chloride solution. The steady state open-circuit potential of the copper electrode in the

Summary

1.
The corrosion rates of copper, iron and the Cu–20%Fe electrodes in 1.0 M sodium chloride solution of pH 2 follow the sequence Cu < Cu–20% Fe < Fe.
2.
The Cu–20%Fe electrode corrodes in sodium chloride solution by selective dissolution of the more active, Fe, component and the rate of the selective dissolution reaction increases with increasing applied potential.
3.
Under the present experimental conditions, thiourea molecules act both as inhibitor and accelerator. At [TU] < 5 mM, it inhibits the selective

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Corrosion and corrosion inhibition of Cu–20%Fe alloy in sodium chloride solution

Abstract

Introduction

Section snippets

Experimental details

Open-circuit potential measurements

Summary

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