**1. Introduction**

504 Mass Transfer - Advanced Aspects

Zolotih, B.N. (1957). About the physical nature of electrospark metal processing. In:

Аcademy of Science of the USSR, Moscow, pp.38-69 (in Russian)

*Electrospark processing of current-carrying materials*, *Publication 1*. Vanity Press

Copper is considered to be among the most important structural elements, just below iron and aluminum. Usually, the properties of this metal improve when combined with other elements (Kear et al., 2004b); the 90% Cu-10% Ni alloy has excellent physical, chemical and mechanical properties that allow it to adapt to different operating conditions (Kutz, 2002; Othmer, 2004). This metal is relatively inexpensive, as a structural element it is aesthetically attractive. It shows good thermal conductivity and a lower electrical resistivity than that observed both in the 70% Cu-30% Ni alloy and in steel; these characteristics make the 90% Cu-10% Ni alloy an efficient and competitive structural element in heat transfer processes (Copper-Nickel Alloys in Marine Environment).

In the last few decades, while trying to establish the dissolution mechanism in the presence of chlorides (Cl¯), this alloy has been the subject of numerous studies (Lee & Nobe, 1984; Crundwell, 1991; Milosev & Metikos, 1997; Kear et al., 2004b), usually at low concentrations and operating conditions close to those in the environment. However, its behavior in the presence of other agents such as bromides (Br¯), has received little attention (Itzhak & Greenberg, 1999; Muñoz-Portero et al., 2005), especially under operating conditions similar to those found in a heat pump that uses the H2O-LiBr pair as a working fluid, which is very attractive because of its thermodynamic properties, however, it is very aggressive to the structural elements of the equipment (Muñoz-Portero et al., 2006).

#### **1.2 Dissolution mechanism**

The kinetics of dissolution of the 90% Cu-10% Ni alloy, in the presence of halides, shows marked similarities to the reaction mechanism of copper (Lee & Nobe, 1984; Crundwell, 1991; Kear et al., 2004a, 2004b).

In the Tafel region, in the vicinity of corrosion potential, three dissolution mechanisms have been proposed. Some researchers (Taylor 1971; Wagner et al., 1998; Kear et al., 2000, cited in Kear et al., 2004a) propose a two step mechanism; the first step consists in an electrochemical reaction, in which the cuprous ion (Cu +) is produced due to the anodic dissolution of metallic copper (Cu). Then, in a chemical process, this species is combined with two chloride ions (Cl- ) to form the cuprous chloride complex ion (CuCl2 -). However, due to thermodynamic matters, this is the least viable of the proposed mechanisms.

Mass Transfer in the Electro-Dissolution of

**-1000**

**-1000**

solution: (a) Copper, (b) 90% Cu-10% Ni

**-800**

**-600**

**-400**

**Potential (mVAg/AgCl)**

**-200**

**0**

**200**

**-800**

**-600**

**-400**

**Potential (mVAg/AgCl)**

**-200**

**0**

**200**

**400**


90% Copper-10% Nickel Alloy in a Solution of Lithium Bromide 507

The system's Ecorr is set in a 72 mV range, reaching at -458 mVAg/AgCl the most oxidizing conditions at 35 °C. The cathodic part of the curve at the analyzed temperature level shows the development of limit currents, reaching its maximum value at 55 °C at

**1x10-5 1x10-3 1x10-1 1x101**

 **25 °C 35 °C 45 °C 55 °C**

**Current Density (mA/cm2)**

(a)

**1x10-6 1x10-4 1x10-2 1x100 1x102**

**)**

 **25 °C 35 °C 55 °C**

**Current Density (mA/cm<sup>2</sup>**

(b)

Fig. 1. Polarization diagrams the absence of oxygen and static conditions in 53% LiBr

In studies carried out by (Walton & Brook, 1977, cited in Dhar et al., 1985) the same mechanism is proposed, concluding that the dissolution of the alloy takes place due to the degradation of a single component: copper. Meanwhile, (Beccaria & Crousier, 1989) speak of a simultaneous dissolution of both components, observing that copper is redeposited in the alloy.

The mechanism proposed by (Lee & Nobe, 1984; Crundwell, 1991; Deslouis et al., 1988a, 1988b) mentions that the alloy is dissolved through a process that takes place in two stages. It begins with the formation of cuprous chloride (CuCl) through an electrochemical reaction between Cu and the Cl¯ ion. Later, in a chemical reaction between CuCl and the Cl¯ ion, the CuCl2¯ ion is formed. This reaction, according to (Kear et al., 2004b, 2007) is partly controlled by a mass transfer process.

Direct dissolution between Cu and two Cl¯ ions through an electrochemical process to produce the CuCl2¯ ion, has been adopted by other researchers (Kato et al., 1980a; Dhar et al., 1985; Muñoz-Portero et al., 2004; Kear et al., 2004b, 2007).
