Electrochemical Equilibrium Pourbaix Diagrams

It was Pourbaix s idea to list aU the chemical and electrochemical reactions that can take place between a metal and water and to define the domains of stability for each chemical species, as a function of the pH for chemical reactions and as a function of the potential for electrochemical reactions. This approach is based on thermodynamics the Nemst 

Pourbaix diagrams cover several domains representing three possible situations  

These are equilibrium diagrams that determine stable species, their domain of stability and the direction of possible reactions. However, they cannot predict the corrosion rate. 

The significance of these diagrams is limited because they refer to an ideal liquid, i.e. chemically pure water at 25 °C, to a metal as pure as possible, and never to an alloy. They do not take into account the possible presence of chloride that plays an important role in pitting corrosion. Furthermore, they do not take into account the nature of the acid and the base that modify the pH value (see Section B.6.1). They do not indicate the risk of cathodic corrosion in the domain of immunity when the potential is highly electronegative (see Section B.5.5). This is due to the method itself, which is based on thermodynamic data without taking into account kinetic data [20]. 

Taking into account these precautions, it is interesting to plot the fi-pH diagram of an 

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Dissolution Potential of Aluminium Electrochemical Equilibrium (Pourbaix) Diagrams The Electrochemical Behaviour of Aluminium Aluminium as a Passive Metal... 

Nickel occupies an intermediate position in the electrochemical series Ni2 /Ni = -0-227 V, so that it is more noble than Zn and Fe but less noble than Sn, Pb and Cu. Figure 4.21 shows a revised potential-pH equilibrium (Pourbaix) diagram for the Ni-H O system at 25°C. The existence of the higher anhydrous oxides Nij04, NijO, and NiOj shown in an earlier diagram appears doubtful in aqueous systems in the absence of positive identification of such species. It is seen that ... 

Potential-pH Equilibrium Diagram (Pourbaix Diagram) diagram of the equilibrium potentials of electrochemical reactions as a function of the pH of the solution. The diagram shows the phases that are thermodynamically stable when a metal reacts with water or an aqueous solution of specified ions. 

The analysis of thermodynamic data obeying chemical and electrochemical equilibrium is essential in understanding the reactivity of a system to be used for deposition/synthesis of a desired phase prior to moving to experiment and/or implementing complementary kinetic analysis tools. Theoretical and (quasi-)equilibrium data can be summarized in Pourbaix (potential-pH) diagrams, which may provide a comprehensive picture of the electrochemical solution growth system in terms of variables and reaction possibilities under different conditions of pH, redox potential, and/or concentrations of dissolved and electroactive substances. 

Figure 1.9 is the Pourbaix diagram for iron and some of its compounds in an aqueous system at 25°C. The equilibrium potential of the reaction Fe° = Fe2+ + 2e falls outside the stability region of water represented by dashed lines. Hence, measurement of the equilibrium electrode potential is complicated by the solvent undergoing a reduction reaction, while the iron is undergoing electrochemical oxidation. This is the basis of the mixed potential model of corrosion. 

A metal CMP process involves an electrochemical alteration of the metal surface and a mechanical removal of the modified film. More specifically, an oxidizer reacts with the metal surface to raise the oxidation state of the material, which may result in either the dissolution of the metal or the formation of a surface film that is more porous and can be removed more easily by the mechanical component of the process. The oxidizer, therefore, is one of the most important components of the CMP slurry. Electrochemical properties of the oxidizer and the metal involved can offer insights in terms of reaction tendency and products. For example, relative redox potentials and chemical composition of the modified surface film under thermodynamically equilibrium condition can be illustrated by a relevant Pourbaix diagram [1]. Because a CMP process rarely reaches a thermodynamically equilibrium state, many kinetic factors control the relative rates of the surface film formation and its removal. It is important to find the right balance between the formation of a modified film with the right property and the removal of such a film at the appropriate rate. 

In chemistry, a Pourbaix diagram, also known as a potential/pH diagram, maps out the possible stable (equilibrium) phases of an aqueous electrochemical system. Predominant ion boundaries are represented by lines. As such, a Pourbaix diagram can be read much like a standard phase diagram with... 

Pourbaix diagrams, or pH-potential diagrams, have been constructed to facilitate the prediction of the various phases (reactions and reaction products) that are stable in an aqueous electrochemical system at equilibrium. " Boundary lines in such diagrams divide the areas of stability for different phases and are derived from the use of Nemst equation... 

Figure 4 is the Pourbaix Diagram for iron and some of its ionic species and compounds in contact with water at 25 C. The equilibrium potential of the iron-ferrous ion reaction falls outside the region of stability of water (dashed lines). Therefore, any attempt to measure the equilibrium potential will fail since the solvent will undergo electrochemical reduc-... 

Formation of ions during equilibrium electrochemical reaction depends on the pH of the solution and electrode potential. The relationship between electrode potential and pH of the solution can be represented by a phase diagram that is known as the Pourbaix diagram [5]. If a metal is made anodic in an aqueous solution, several reactions can occur depending on the change in free energy. For example, if zinc is made anode in water, the following possible reactions may take place ... 

Pourbaix diagram [3] maps out possible stable equilibrium phases of an aqueous electrochemical system and indicates that piue iron is passive at pH values from 9 to 12.5 to form iron hydroxide. Considering the interplay of atmospheric factors this diagram was used as guide to the steel dissolution process to form passivity on WS in laboratory. The passive films formed on pure iron are not so stable and consequently the passivation state of iron is not maintained for prolonged time periods [147, 148]. The mst layers of steels play a role as a barrier against corrosion, and their growth rate is decreased to a rate similar to that of the passive films, when suitable elements are added to the steel [149, 150]. 

Pourbaix Diagrams. Plots of equilibrium pH vs electrochemical potential E describe the effects of aqueous corrosion on borosilicate and silicate glasses. They are applicable to weathering studies and to ground water attack on nuclear waste glasses. The diagrams display any immune zone between active corrosion... 

Pourbaix plotted electrochemical equilibrium diagrams of metals in water as a function of the potential E with respect to the hydrogen electrode, and as a function of pH (Figure B.1.10). Several domains can be identified in these diagrams corrosion, passivation and immunity (see Section B.1.6). 

The main objective of this chapter is to introduce students to one of the most important subjects of the book, equilibrium electrochemistry, which is mainly based on equilibrium thermodynamics. Equilibrium electrochemistry is usually the first and required step in analyzing any electrochemical system. How to estimate the equilibrium potential of a half-reaction and the electric potential difference of an electrochemical cell are described in this chapter. One of the most fundamental equations of electrochemical science and engineering, the Nemst equation, is introduced and anployed for composing the potential-pH (Pourbaix) diagrams. Temperature dependence of the electrode potential and the cell potential difference is also described. 

The hydrogen ion concentration - expressed by pH - is an important parameter for calculating electrochemical equilibrium potentials. The Pourbaix diagram maps in a clear way the influence of the pH value on complicated electrochemical equilibria. The diagram form, which is named after the Belgian corrosion researcher Marcel Pourbaix, was developed in the 1940s. 

The Pourbaix diagram specifies electrochemical equilibrium curves for metals and metal oxides in a voltage vs. pH coordinate system. These curves delimit areas where the metal is immune, where the metal is passivated, and areas where the metal is corrosion active at equilibrium conditions. These equilibrium curves can usually be calculated from the thermodynamic data of the substances areas with passivation or corrosion are determined by tests and from practical experience. 

L. D. Burke and R. A. Scanned, Equilibrium Diagrams Localized Corrosion (Proceeding of an International Symposium Honoring Marcel Pourbaix on his Eightieth Birthday), Ed. by R. P. Frankenthal and J. Kruger, The Electrochemical Society, Pennington, New Jersey, 1984, pp. 135-147. 

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