Relevant Pourbaix Diagram

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 

FIGURE 21.5 A basic phase diagram the label at each corner represents a component (from Ref. 16). 

FIGURE 21.6 Ternary phase diagram of the sodium octanoate-decanol-water system at 25°C. There are two isotropic solution phases, micellar and reversed micellar (rev mic), and three liquid crystalline phases, hexagonal (hex), lamellar (lam), and reversed hexagonal (rev hex) (from Ref. 17). 

FIGURE 21.7 In a mixture of a cationic and an anionic surfactant, there are two regions of thermodynamically stable vesicles, and V, respectively (from Ref 18). 

FIGURE 21.8 Phase behavior under balanced conditions of a surfactant-water-oil 

---

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. 

Although the basic organization of the book is unchanged from the previous edition, there is in this edition a separate chapter on Pourbaix diagrams, very useful tools that indicate the thermodynamic potential-pH domains of corrosion, passivity, and immunity to corrosion. A consideration of the relevant Pourbaix diagrams can be a useful starting point in many corrosion studies and investigations. As always in corrosion, as well as in this book, there is the dual importance of thermodynamics (In which direction does the reaction go Chapters 3 and 4) and kinetics (How fast does it go Chapter 5). 

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. 

These reaction products have their domains of stability in certain ranges of potential and pH as shown in the Pourbaix diagram in Fig. 2,6 which may have relevance in cases when open-circuit potentials are established at highly negative values [cf. Sections 111(2) and III(5(v))]. 

It is well known that Pourbaix diagrams give the thermodynamic limits of corrosion. However, it is possible that corrosion in a system may be limited by kinetics to rates so low that corrosion that is thermodynamically possible can be neglected under practical circumstances. In this light, (a) construct an Evans diagram, i.e., a plot of the actual relevant electrode potentials against log i for... 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

In order to make a valid assessment of the thermodynamic tendency for corrosion to occur, the chemical potentials of all relevant species that participate in the reaction and their concentrations, as well as the system temperature and pressure, must be known. While thermodynamic prediction methods (i.e., Pourbaix diagrams) can be used to determine whether an electrochemical reaction can occur, information on electrochemical reaction rates is not provided. 

Next we consider the Pourbaix diagram for iron, which is, of course, of paramount importance for the understanding of corrosion of ferrous alloys such as the many types of steel and stainless steel. This is a rather complex diagram, since two oxidation states of iron exist both in the liquid and the solid state and the metal is amphoteric to some extent. Figure 16M is a simplified version of the diagrams shown in the original work of Pourbaix. The two soluble species in acid solutions are Fe and Fe. The relevant equilibria are... 

Although Pourbaix diagrams provide important information regarding the stability of various metal species in different oxidation states, they are of limited use because (1) they contain only thermodynamic information and thus provide no indication of the rate (speed) of the relevant reactions, (2) they usually refer to reactions at 25°C and one atmosphere pressure, and (3) they consider only pure metals. 

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],... 

Although the application of Pourbaix diagrams gives useful information on the behaviour of many systems relevant to environmental and synthesis... 

Pourbaix diagrams of various metals relevant for CMP applications have been compiled elsewhere (Li, 2008). These diagrams are useful as a general guidehne to develop tentative reaction schemes based on the chemical compositions of the specific systems (Nolan and Cadien, 2013). However, since the core of this analysis is based on the Nerstian equilibrium condition, the information about reaction kinetics cannot be readily extracted from the r-pH plots. Furthermore, most traditional Pourbaix diagrams based on the simple description of Eqn (3.17) do not account for the presence of the oxidizers, complexing agents, and other additives commonly used in CMP slurries. Some efforts to address the latter issue have been reported in the context of Cu CMP (Patri et al., 2006). 

Although the zones of corrosion, immunity and passivity are clearly of fundamental importance in corrosion science it must be emphasised again that they have serious limitations in the solution of practical problems, and can lead to unfortunate misconceptions unless they are interpreted with caution. Nevertheless, Pourbaix and his co-workers, and others, have shown that these diagrams used in conjunction with E-i curves for the systems under consideration can provide diagrams that are of direct practical use to the corrosion engineer. It is therefore relevant to consider the advantages and limitations of the equilibrium potential-pH diagrams. 

Selection of environmental conditions the electrol)fte should be selected in view of its known oxidative or reducing power. The test temperature can be ambient temperature or any temperature relevant for the field ap>plication it should reproduce. A decisive parameter in the selection of the electrolyte is its pH. The selection of the pH can be based on pH-potential diagrams (Pourbaix, 1974). In the case of a metallic alloy, a pH range should be selected by paeference where at least one of the constituents passivates. An electrolyte that may cause localized corrosion, like pitting corrosion in particular, should be avoided. 

---