1. Pourbaix Diagram Pdf

O’Keeke, in, 2001 5 Equilibrium Calculation ExamplesA basic qualitative description of the Pourbaix diagram and the types of equilibrium expressions used to define the various stability regions has been given. The procedures and conventions used to quantify a specific system will now be presented. As with any thermodynamic calculation, the accuracy and precision obtained will depend entirely on the ability to identify the correct phases involved. The second critical item is to insure that the free energy or chemical potential data are available and correct. Pourbaix diagrams offer a large volume of thermodynamic information in a very efficient and compact format. The information in the diagrams can be beneficially used to control corrosion of pure metals in the aqueous environment.

By altering the pH and potential to the regions of immunity and passivation, corrosion can be controlled. For example, on increasing the pH of environment in moving to slightly alkaline regions, the corrosion of iron can be controlled. This can be achieved by water treatment. Similarly, changing the potential of iron to more negative values eliminate corrosion, this technique is called cathodic protection. Also, raising the potentials to more positive values reduces the corrosion by formation of stable films of oxides on the surface of transition metals. Steel in reinforced concrete does not corrode if an alkaline environment is maintained.

On the contrary, an alkaline environment for aluminum is a disaster if the pH exceeds 8.0. The above example clearly demonstrate the merits of Pourbaix diagrams in prediction and control of corrosion.

Pourbaix Diagram Pdf

However, there are several limitations of these diagrams, which are summarized below: 1.These diagrams are purely based on thermodynamic data and do not provide any information on the reactions. The thermodynamic stability may not be achieved to the kinetics of the reaction. No information is provided in the rates of reaction. 2.Consideration is given only to equilibrium conditions in specified environment and factors, such as temperature and velocity are not considered which may seriously affect the corrosion rate. 3.The activity of species is arbitrarily selected as 10 −6 g mol which is not realistic.

4.Pourbaix diagrams deal with pure metals which are not of much interest to the engineers. 5.All insoluble products are assumed to be protective which is not true, as porosity, thickness, and adherence to substrate are important factors, which control the protective ability of insoluble corrosion products.Although the above disadvantages appear to be substantial, the advantages offered by the Pourbaix diagrams far outweigh their limitations. The Pourbaix diagram for aluminum is shown in Figure 26.10. Between pH 4 and 8.5, a thin and very stable film of hydrated aluminum oxide forms, protecting the metal. The corrosion rate of aluminum in pure water is extremely low, even though the driving force for corrosion is very large—about 2.8 volts!

However, over time there is a tendency for attack to occur at weak points in the oxide film, so for most outdoor uses—such as window frames, roofs, and cladding—aluminum is first anodized—given a surface treatment that artificially thickens the oxide film to make it even more protective. In the anodizing process, the metal is put into a bath of water containing various additives to promote compact film growth (e.g., boric acid). It is then made positive electrically, which attracts the oxygen atoms in the polar molecules (see Chapter 4). Tang, in, 2013 12.1.1 Substrate propertyAs seen in the Pourbaix diagram, Mg corrodes in the wide potential range and pH range lower than pH 11.5.

5 This means that a Mg substrate dissolves in a neutral or acidic bath in the coating process. Mg is categorized as a ‘passivation’ metal on which a thick corrosion layer with low corrosion protection forms easily and may prevent metallic binding of the plating layer with the metal substrate. On the other hand, the fact that Mg substrate is easy to dissolve and easy to precipitate as a hydroxide are used in chemical conversion coating. 6–9 It has been reported that the corrosion resistance of pure Mg to atmospheric exposure was high but the contribution of impurities or alloying elements of Mg to corrosion susceptibility is crucial to protect Mg in some corrodible conditions. 10, 11 Mg is generally used in the form of alloys to improve mechanical properties, workability and corrosion resistivity. The AZ series of alloys is amongst the popular Mg alloys composed with Al, Zn and other elements. Depending on composition, concentration and dispersion of these elements, the substrate surface reveals non-uniform activity for reactions in the coating process.

For example, AZ91D has a binary structure composed of a Mg-rich α-phase matrix and an Al-rich β-phase of Mg 17Al 12 eutectic dispersion; the electrochemical activities of these phases are slightly different because of the difference in their compositions. 12–15 The Mg-rich α-phase dissolves more easily than the β-phase surface in a bath at pH lower than 10.5, while the α-phase passivates at pH higher than 11. Such a non-uniform property may induce acceleration of electrochemical reactions, such as rapid and non-uniform metal deposition and gas evolution, and result in not only a non-uniform appearance of the coating but also in a weak binding area of the coating to the substrate.

Suitable coating conditions for substrate materials depending on their alloy composition and impurities must therefore be considered. Lyon, in, 2010 3.15.3.2 Passivity, Corrosion, and Localized CorrosionAs predicted from the Pourbaix diagram, the excellent corrosion resistance of tantalum and niobium is due to the presence of tenacious passive oxide films on the respective metals.

Indeed, unalloyed tantalum is indubitably the ‘most passive’ of all metals just below rhodium and above gold in the Pourbaix practical nobility table. Compared with tantalum, niobium is significantly less passive; however, this is merely relative as niobium is still considerably more corrosion resistant than most other materials. Both the metals spontaneously passivate in almost all environments below 100 °C at atmospheric pressure, except those of low water activity and those containing fluoride, the latter being the metals’ (almost only) Achilles heel.It is extremely difficult to locate literature data on the electrochemistry of dissolution for tantalum or niobium other than in extreme conditions; one must therefore presume that the metals are passive (with consequently low corrosion rates) in most environments. Tantalum is spontaneously passive, with an extremely low corrosion rate in all mineral acids (except HF), at all concentrations and at all temperatures below 100 °C. It is used to condense and concentrate sulfuric acid and exhibits passivity in this medium at a much higher temperature. The corrosion resistance of niobium is generally inferior to that of tantalum, with alloys of niobium and tantalum being intermediate in performance. 26,27Recently, the corrosion of niobium has been re-evaluated in sulfuric (20, 40, and 80%) and hydrochloric (20 and 38%) acids at room temperature, 75 and 95 °C.

28 The metal was found to remain passive under all conditions, but with variations in the passive current density corresponding to changes in the mass-loss (corrosion) rate. Minor pitting was observed only in sulfuric acid at concentrations of 20% and 40%. It was suggested that dissolution of the passive oxide was via a Nb(OH) 4 + species, for which species thermodynamic data had recently been obtained.The pitting (breakdown) potentials of niobium have been investigated in 0.1 M halide solutions. 29 In chloride and iodide up to about 150 V, no pitting or breakdown was observed, and the metal formed an anodic film. In bromide, the breakdown voltage was only 42 V with some slight pitting observed. Likewise, the corrosion behavior of tantalum and niobium was studied in concentrated HBr solution at 25 and 100 °C and, under oxidizing conditions (with added bromine) or reducing conditions (with bubbled hydrogen gas).

30 Tantalum was found to passivate under all conditions studied while niobium corroded slowly at 100 °C with pits of around 5 μm evident. Under reducing conditions, niobium gradually began to corrode actively with hydrogen evolution, while under oxidizing conditions it passivated.

Tantalum is thus remarkable in its resistance to pitting corrosion by chloride, bromide and iodide species, and niobium is scarcely far behind.In contrast to their corrosion resistance in acids, niobium and tantalum corrode at significant rates in strong alkali. Thus, niobium is spontaneously active in NaOH at concentrations greater than 10% and at temperatures above 25 °C. 31 Corrosion leads to the formation of the niobate species, NbO 3 −, with the corrosion rate increasing with concentration and time. Tantalum was found to be significantly more resistant than niobium and remained passive in 10% NaOH up to 75 °C and in 15% NaOH up to 50 °C. 32 At higher concentrations and temperatures, the passive film on tantalum dissolved slowly, forming a polytantalate species.

Tantalum–niobium alloys were found to be intermediate in behavior. Joseph Riskin, in, 2008 5.1.2 Alkaline solutionsAs seen from the Pourbaix diagram ( Figure 1.1), iron and carbon steel are in a passive state in dilute alkaline solutions and, consequently, they maintain their corrosion stability in the absence of attack by external anodic currents 2, 3. Iron passivation in these media is connected with formation of oxide compounds on the metal surface.

The depth of the layer formed by these compounds may be in the order of magnitude of one monolayer, and the passive state can be reached when only a part of the metal surface is covered by the oxide 4.During the anodic polarization of passive iron in alkali solutions, different types of oxide and hydroxide compounds, including Fe 2+ and Fe 3+, ions are formed. The compositions of these compounds depend on the potentials at which the corresponding reactions are proceeding and on the concentrations and temperatures of the solutions. (5.1) 2OH − → 1 2 O 2 + H 2 O + 2e −is thermodynamically possible in 1 N alkali solution at potentials that are more positive than 0.4 V 5.

The overvoltage of the oxygen evolution on iron at low current densities is 0.25 V 6. Therefore, at potentials that are more positive than ∼0.65 V in 1 N alkali solution (and in more concentrated solutions at more negative potentials), the major share of the current will be spent on oxygen evolution. In accordance with the potential–pH diagram obtained for hot alkaline solutions 7, the existence of lepidocrocite (γ-FeOOH) and of magnetite (Fe 3O 4) on the metal surface is possible in this range of potentials. The results of potentiostatic investigations of carbon steel in 1 N NaOH solution at a temperature of 90°C and at a potential scanning rate of 0.72 V/h ( Figure 5.1, curve 1) are in good corroboration with the data in Ref. At potentials from stationary (−0.1 to 0 V), and up to the potential of the start of oxygen evolution (0.7 V), the steel maintained its passive state.

In the area of oxygen evolution, the curve had a Taffel dependence. The current density on the passive steel was only (1–2)×10 −5 A/cm 2. Therefore, it could be expected that even on small values of current density, the potential will shift to the area of oxygen evolution. This was confirmed in experiments that were carried out under conditions of galvanostatic polarization.

The potential shifted to the value 0.73–0.75 V at a current density of 1.5 mA/cm 2. Tests lasting five hours have shown that the weight losses at this current density are insignificant, less than 0.1 g/m 2 h. Anodic polarization plots on carbon steel St 3 in solutions: 1 – 1 N NaOH; 2 − 120 g/l NaOH + 200 g/l NaCl. Temperature 90°C.At higher current densities, the potentials and the weight losses increased significantly. For example, in a 5 h long experiment at current densities of 20 and 50 mA/cm 2, the potentials stabilized at the values 0.80–0.82 V and 0.83–0.87 V, respectively.

The weight losses of carbon steel therewith were, respectively, 0.68 and 1.1 g/m 2 h, or, in current density units, 0.5 and 0.8 mA/cm 2. The shares of the current that were spent on iron dissolution, if the transfer of the Fe 3+ ions into solution is accounted for, were, at 20 and 50 mA/cm 2 respectively, 2.5 and 1.6%. At 50 mA/cm 2, the share of the current spent on iron dissolution was lower than at 20 mA/cm 2, but the absolute value of the weight loss at 50 mA/cm 2 was higher.The corrosion of carbon steel in the area of oxygen evolution can be attributed to the formation of soluble compounds such as FeO 4 2− at high anodic potentials 9.An increase in the corrosion rate in the area of anodic oxygen evolution was observed by many researchers 2, 10.

The stage of adsorption of oxygen contained in the water molecule, onto the metal surface precedes the process of oxygen evolution in the form of bubbles. Following this, further oxygen transfer from the anodically dissolving “cells” of iron oxide to the adsorbed layer of oxygen (which plays the part of a buffer capacity) would be facilitated 11. This is considered to be one of the causes of the growth in the corrosion rate under the conditions of oxygen evolution. The possibility of mechanical destruction of the friable surface layer of magnetite by the bubbles of evolving oxygen also has to be taken into account.In accordance with the existing data 6, the action of external cathodic currents does not lead to the corrosion of carbon steel in alkaline media. Lyon, in, 2010 3.11.3.6 Miscellaneous EnvironmentsAs can be seen from the Pourbaix diagram, lead has no stable passive species at pH  10–11, and hence, lead is not particularly resistant to dilute alkalis, and will dissolve freely as the plumbite oxyanion. Where free access to carbon dioxide is available, a passivating salt film of lead carbonate may form. However, lead is susceptible to lime drips from fresh concrete and cement mortar, which will tend to disrupt the lead carbonate film formation.

Lead can tolerate concentrated alkalis such as KOH to 50% and up to 60 °C and NaOH to 30% and 25 °C, although it is explicitly not used for this purpose.Lead is not generally attacked rapidly by solutions that contain anions, where the lead salt is sparingly soluble, and hence, where lead can passivate by the formation of a salt film. Thus, only nitrates and, to a lesser extent chlorides, are corrosive. The presence of nitrate tends to pit lead, for example, in carbonate solution. 22 In sodium chloride, the corrosion rate increases with concentration to a maximum in 0.05 M solution, then decreases because of the formation of a relatively porous film PbCl 2. Control of the cyclic voltammetry conditions allowed the development of a relatively thick and more protective layer.

66 In potassium bromide, adherent deposits are formed, and the corrosion rate increases with concentration. The attack in potassium iodide is slow in concentrations up to 0.1 M, but in concentrated solutions rapid attack occurs, probably owing to the formation of soluble KPbI 3. In dilute potassium nitrate solutions (0.001 M and below) the corrosion product is yellow and is probably a mixture of Pb(OH) 2 and PbO, which is poorly adherent. At higher concentrations, the corrosion product is more adherent and corrosion is somewhat reduced. Wagh, in, 2016 18.5.1 Case Studies on the Role of Tin Chloride (SnCl 2) in Stabilizing TechnetiumAs one may see from the Pourbaix diagram 24, Tc exists in its oxidation state of + 7 (pertechnetate) state in an alkaline medium. Most of the waste streams in which Tc is found are alkaline, because if they are acidic they are neutralized and, in the process, often they become alkaline rather than being exactly neutral. This is because it is difficult to maintain the neutrality of a waste stream in long-term storage, especially when there exist lots of oxides and minerals within it.

Tc in an alkaline medium is soluble and leaches out easily.Typical CBPC waste forms are not totally neutral and have a pH between 7 and 9. Therefore Tc will exist in the waste form in the + 7 state. 31 carried out a methodical study on the role of the reductant SnCl 2 in reducing and stabilizing technetium in a CBPC matrix. The waste stream they used was a product of a complexation-elution process that separates 99Tc from high-level supernatant from salt waste tanks at Hanford and SRS. A typical waste solution generated during the complexation-elution process contains 1 M NaOH, 1 M ethylenediamine, and 0.005 M Sn 2 +, and Tc content ranged anywhere from 20 to 900 ppm in the waste stream. The results of one of the case studies conducted by Singh et al. 31 are listed in Table 18.10.

Waste forms were fabricated by solidifying partitioned Tc oxide in Ceramicrete. A small amount of SnCl 2 (0.5 wt%) was used to reduce Tc from its + 7 state to its + 4 state; it was added in the binder mixture, which was blended with the waste and water to form paste. After mixing for 20 min, the paste was allowed to set into a hard ceramic waste form.

The hardened form was cured for 3 weeks and then subjected to leaching tests. Concentration of 99Tc (ppm) in waste form (waste loading)41164903Normalized leach rate (g/m 2 per day) in PCT0.070.10.036Leachability index in ANS 16.1 test14.613.314.6Table 18.10 presents the PCT and ANS 16.1 test results for the waste forms. The normalized leaching rate for 99Tc, according to the PCT, was low even at 90°C. This rate is close to the value reported by Ebert et al. 38 for Tc leached from Defense Waste Processing Facility glasses. For all Tc loadings, the LI is consistently high, between 13.3 and 14.6, higher than that of Cs and Sr in glass. The compressive strength of the waste forms was ≈ 30 MPa, and the waste forms were durable in an aqueous environment.

These results demonstrate the effectiveness of a reducing environment in the Ceramicrete matrix.In another study, researchers stabilized debris waste produced from scraping the internal surface of pipes from the Oak Ridge K-25 plant that was destined for demolition at one of the DOE sites 39. The actual work was conducted at ANL.Two waste streams were used in this study. Both were flaky materials, brown in color. The analysis of the first provided by the sponsor, Bechtel Jacobs, showed a Tc level of 33,886 pCi/g (2020 ppm) and a U level of 107.05 pCi/g (40.1 ppm), with 233/234U and 238U as the major contaminants. It contained plastic washers and elongated pieces of materials and could not be crushed easily, so pieces were cut into smaller ones before stabilization. The second debris waste had some metal wire pieces, which were cut into smaller pieces.

The analysis provided by the sponsor showed a Tc level of 1750 pCi/g (104 ppm) and U level of 107.05 pCi/g (96,000 ppm) with 233/234U and 238U as the major phases. The composition used to produce the waste form is shown in Table 18.11. The powders and the waste were added to the water. Again, SnCl 2 was used as the reductant to suitably stabilize Tc. Because the amount of waste was small, the mixing was done by hand with a spatula. The powder and water were mixed for 25 min so that it formed pourable slurry. This slurry was then poured into plastic molds and was allowed to set; it set hard within hours, but was cured in air for 3 weeks so the samples gained nearly full strength and integrity.

ComponentsComposition (wt%)Ceramicrete binder40Class F fly ash19.8Waste40SnCl 20.2Water21% of total powdersThe LIs in the ANS 16.1 study were found to be 12 and 17.7 respectively, which are high and consistent with values obtained for the other waste streams described above. Thus this second example again proves the effectiveness of Ceramicrete with a reductant for stabilization of Tc.This work establishes that SnCl 2 is a potential reductant in a CBPC process to stabilize Tc. This helped in treating actual waste streams in Russian and Ukrainian work, as we shall see in case studies provided on actual streams. The establishment of potential-oxoacidity diagrams (also called Pourbaix diagrams) is useful to determine the electrochemical stability domain of the molten salt and the existence of the stable species at a given activity 12. As in aqueous system, the vertical limits are fixed by the dissociation constant.

In water, the oxidative limit is fixed by the oxidation of water into oxygen. In molten carbonate mixtures, the positive limits of the oxoacidity domain are defined by the oxidation of O(− II) species. This means that carbonate or alkali oxides can be transformed into peroxide ions, superoxide or oxygen. Thermodynamically, the main redox couples to be considered in the oxidation side are Li 2O/O 2 (− II/0), M 2O/MO 2 (− II/− I/2), where M = alkali ions, and Li 2O/Li 2O 2 (− II/− I). All the standard potentials of the different couples are referred to the Li 2O/O 2 system; therefore, its standard potential is equal to zero.

It is possible to express the Nernst potential of each redox couple as a function of the carbon dioxide partial pressure. In molten carbonate mixtures, the limiting reduction reactions are due to the reduction of alkali ions to metals, CO 2 to CO (if the medium is oxoacid) or to C graphite (if it is is oxobasic) and H 2O to H 2.

Only the reduction of gases leads to a variation of the potential with the partial pressure of CO 2. Nernst potential equations have been calculated according to a methodology described by Cassir et al. For instance, a potential-oxoacidity diagram of the eutectic Li-K (62-38 mol%) is presented in Figure 17.1.

A Pourbaix diagram plots the equilibrium potential ( E e) between a metal and its various oxidised species as a function of pH.The extent of half-cell reactions that describe the dissolution of metalM = M z+ + ze -depend on various factors, including the potential, E, pH and theconcentration of the oxidised species, M z+. The Pourbaix diagramcan be thought of as analogous to a phase diagram of an alloy, which plots thelines of equilibrium between different phases as temperature and compositionare varied.To plot a Pourbaix diagram the relevant Nernst equations are used. As the Nernstequation is derived entirely from thermodynamics, the Pourbaix diagram can beused to determine which species is thermodynamically stable at a givenE and pH. It gives no information about the kinetics of thecorrosion process. Constructing a Pourbaix DiagramThe following animation illustrates how a Pourbaix diagram is constructed from first principles, using the example of Zinc.Note: This animation requires Adobe Flash Player 8 and later, which can be.© 2004-2020 University of Cambridge.Except where otherwise noted, content is licensed under a.