Pourbaix diagram, 6.27

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 

Copper polish slurry formations may contain dissolved NH, ig) to complex the copper ions and increase copper solubility. In 1 vol% NH4OH, for example, Cu ions are complexed by NHj according to  

The second reason the region of stability is important is that if the metal falls in the region of immunity (the Cu region), it is thermodynamically impossible for the metal to dissolve into solution or form a solid oxidation product. If there is no oxidation of the metal, there is no chemical component to the polish. Without a chemical component, the polish becomes strictly mechanical polishing (MP). An example of a slurry where copper is immune to corrosion is given in Section 4.2.4. 

There are several difficulties with using the Pourbaix diagram to predict the stability of copper during polishing, specifically  

Complexing ions alter the Pourbaix diagram by moving the boundaries of the stability regions and adding new stability regions/  

The constmction of the E-pH Pourbaix diagram for water and oxygen is straightforward. Consider the hydrogen evolution reaction at standard conditions (T = 25 °C, Po = 1 atm) 

From eq. (2.32), the Nemst equation gives the hydrogen potential as 

Example 2.10 Drive eq. (2.48) for an alkaline solution and compute the Eh and Eq. for a pH = 10 under standard conditions. 

It can be concluded that eq. (2.48) gives the same result if either acid or alkaline solution is selected. 

A great deal of information about redox systems can be conveyed efficiently by Pourbaix diagrams in which pE° values for various oxidation states are plotted against pH or another relevant variable such as log[L], 

The resultant diagrams consist of a series of appropriately drawn line segments. The areas between the lines represent regions in which one species predominates and are labeled accordingly. Let us consider the effect of pH on pEjjj . The following equations will be considered. 

The pE° will remain unchanged with pH until the value at which Ni(OH)2(s) begins to precipitate from a solution in which [Ni ] = 1 

The pE° change with pOH and therefore, with pH, with a slope of unity because although there is a two-electron change, two OH are produced. 

Fill column A with pH values from 0 to 14 in steps of 1, adding the value of 6.1. If necessary, conduct a sort in A to place 6.1 properly. In column B, place the value -4.34 from 0 to 6.0. At pH = 6.1 enter the formula, -4.34 - (pH - 6.1) using the appropriate A cell number for pH. Fill the remainder of B cells by the block copy command. Add a vertical line at pH 6.1 for values of -4.34 of pE° (either by hand or, if using QPRO, by the Annotate command, /GA). 

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. 

Primarily connected to corrosion concepts, Pourbaix diagrams may be used within the scope of prediction and understanding of the thermodynamic stability of materials under various conditions. Park and Barber [25] have shown this relevance in examining the thermodynamic stabilities of semiconductor binary compounds such as CdS, CdSe, CdTe, and GaP, in relation to their flat band potentials and under conditions related to photoelectrochemical cell performance with different redox couples in solution. 

Pourbaix diagrams for the aqueous Cd-S, Cd-Te, Cd-Se, Cu-In-Se, and Sb-S systems have been compiled and discussed by Savadogo [26] in his review regarding chemically and electrochemically deposited thin Aims for solar energy materials. Dremlyuzhenko et al. [27] analyzed theoretically the mechanisms of redox reactions in the Cdi xMn , Te and Cdi- , Zn i Te aqueous systems and evaluated the physicochemical properties of the semiconductor surfaces as a function of pH. 

These authors constructed Pourbaix diagrams for the MnTe, ZnTe, Cdi , Mn i Te, and Cdi cZn cTe systems and argued that the related analysis is an effective approach to determine conditions for selective etching, chemical polishing, passivation, and self-metallization of ZnTe, MnTe, and their solid solutions. 

Equilibria considerations on solution-grown zinc chalcogenide compounds have been put forward by Chaparro [28] who examined the chemical and electrochemical reactivity of solutions appropriate for deposition of ZnS, ZnSe, ZnTe (and the oxide ZnO) in order to explain the results of recipes normally used for the growth of such thin films. The author compared different reaction possibilities and analyzed the composition of solutions containing zinc cations, ammonia, hydrazine, chalcogen anions, and dissolved oxygen, at 25 °C, by means of thermodynamic diagrams, applicable for concentrations usually employed in most studies. 

When an electrochemical reaction is perturbed from its equilibrium state, the relative stabilities of the species in the reaction are changed. The change due to the perturbation is reflected in the measured electrode potential, which differs from the equilibrium 

When the measured potential is negative with respect to the equilibrium value, the reaction favours the reduced form. 

Consider water, which is an electrochemically active species. The electrochemical reactions involving water are  

For solutions in which the activity of water and the fugacities of oxygen and hydrogen gases are unity, the equilibrium electrode potentials for the above reactions at 25 °C are  

The equilibrium electrode potential for the copper-cupric ion reaction is located within the region of stability of water represented by dashed lines. Thus, the 

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Figure C2.8.1. Simplified /pH diagram (Pourbaix diagram) for the iron-water system at 25°C. The diagram is...

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Fig. 1. Pourbaix diagram for the iron—water system at 25°C, considering as solid substances only Fe, Fe(OH)2, and Fe(OH)2, where (-), line a, represents...

Pourbaix diagrams are only thermodynamic predictions and yield no information about the kinetics of the reactions involved nor are the influences of other ionic species which may be present in the solution included. Complexing ions, particularly haUdes, can interfere with passivation and can influence... 

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electro deposition, and other phenomena in aqueous solutions.Erom the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

Verink, E. D., Simplified Procedure for Constructing Pourbaix Diagrams , Corros. Sci., 23, 371 (1967)... 

Lennox, T. J., Limitations on the Use of Pourbaix Diagrams to Predict De-alloying or Other Corrosion Characteristics , Discussion, Corrosion, 26, 397 (1970)... 

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 ... 

The Pourbaix diagram" for tin (Fig. 4.41) refers only to solutions in which formation of soluble tin complexes or protective layers of insoluble salts does not occur. There are few instances of the formation of protective layers other than oxide on tin, and although the formation of soluble complexes is more common, the diagram provides a useful general indication of the... 

The Pourbaix diagram indicates the possibility of attack by solutions of pH values above about 9-5, but the position of this limit is influenced by temperature, by the constitution of the solution, and by the surface condition of the metal. Corrosion will ensue if the surface oxide can be dissolved this will invariably take place if the pH exceeds 12, and may occur even at pH values between 10 and 12. 

As with the chemical behaviour of the noble metals in aqueous solutions, their anodic behaviour closely follows the predictions of the Pourbaix diagrams if due allowance is made for the formation of complexes. 

Essentially, the pH is controlled to suppress the hydrogen evolution cathodic reaction- The Pourbaix Diagram for iron indicates that high pH values as well as low values may lead to corrosion. The construction of these diagrams for higher than ambient temperatures - shows how the area of the alkaline zones increases considerably under boiler conditions, so that the risk of corrosion is correspondingly higher. Many feed systems contain copper alloys. 

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. 

A survey of the thermodynamic situation is provided by so-called Pourbaix diagrams [10], which show equilibrium potentials versus the pH value. Figure 1 shows such a diagram for lead and its oxides in a very simplified form that considers only the standard concentrations of the dissolved components. The complete diagram contains a great number of parallel lines that express the various concentrations. 

Potential sweep measurements, with microwave frequency effects, 455 Pourbaix diagrams, applied to adlayers on copper, 93... 

It is considered useful to include here the potential-pH diagram for some redox systems related to oxygen (Fig. 2.1) [4]. Lines 11 and 33 correspond to the (a) and (b) dashed lines bounding the stability region of water, as depicted in all the subsequent Pourbaix diagrams. 

In the Pourbaix diagram, solid sulfur appears to be stable in a very narrow triangular domain, which lies completely within the stability domain of water. Sulfur is therefore stable in the presence of water and in acid solutions free from oxidizing agents. It is unstable, however, in alkaline solutions, in which it tends to disproportionate to give HS , (and polysulfides), SO , and other oxidation products. In... 

The thermodynamic principles of the Cd-Te-water system are depicted in the Pourbaix diagram of Fig. 3.5 [82]. The corresponding electrochemical reactions of CdTe reduction and oxidation are shown in Table 3.1. 

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