Titanium pourbaix diagram
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The numerical value of the electrode potential E for the dissolution of Al as Al3+ ions dependson the concentration of the dissolved ion. Then, forįrom Table 6.1, Eo = 1.663 V vs. (1) is a reaction involving only the electrodepotential E (but not the pH). The dissolution of aluminum as Al3+ions by Eq. Thus, in the case of the Al/H2O system, the various chemical entities and their chemical potentialsare as follows :ĭissolved substances o(cal/mol)Al3+ 115,000AlO2 200,710H+ 0Solid substancesAl 0Al2O3 384,530Liquid substancesH2O 56,690The chemical reactions involving these species are In neutral or nearly neutral solutions, aluminum is covered with a protectiveoxide film. Aluminum undergoes dissolution in acid solutions as Al3+ ions and in basic solutions asĪluminate ions (AlO2). It is firstnecessary to assemble the appropriate chemical information about the specific metal being con-sidered. The Pourbaix dia-gram for aluminum serves as a simple example as to how these diagrams are developed. The assumptions made are that (1) the Pourbaixdiagram for pure aluminum also holds for the dilute Al alloy (1% Mn) and (2) the system is inequilibrium after the relatively short immersion period of 24 h.Ĭonstruction of the Pourbaix Diagram for AluminumPourbaix diagrams are constructed from the first principles of thermodynamics. 6.1, it is seen that this electrode potentialat a pH of 3.5 corresponds to a region of corrosion. Solution: First convert the electrode potential to the standard hydrogen scaleįrom the Pourbaix diagram for pure aluminum in Fig. What behavior is expected in terms ofcorrosion, passivity, or immunity? What assumptions are you making in your analysis? Potential at a pH of 7.6 corresponds to a region of passivity.Įxample 6.2: An Al1% Mn alloy immersed in 0.1 M sodium citrate solution (pH 3.5) for 24 hdisplayed an electrode potential of 1.25 V vs. SHE = 1.47 V + 0.242 V = 1.23 VBy referring to the Pourbaix diagram for pure aluminum in Fig. 6.1 Pourbaix diagram for aluminum at 25C. McCafferty, Introduction to Corrosion Science, DOI 10.1007/978-1-4419-0455-3_6,C Springer Science+Business Media, LLC 2010ĩ6 6 Thermodynamics of Corrosion: Pourbaix Diagramsįig. Solution: First convert the electrode potential to the standard hydrogen scale. What behavior is expected in terms ofcorrosion, passivity, or immunity? Įxample 6.1: After immersion in natural seawater (pH 7.6) for 70 days, the electrode potential ofpure aluminum was observed to be 1.47 V vs. Kruger has describedthe Pourbaix diagram as being a map of the possible. When the stable species is theunreacted metal species itself, the region is labeled as a region of immunity. When the stable species is either a solid oxide or asolid hydroxide, the region on the Pourbaix diagram is labeled as a region of passivity, in whichthe metal is protected by a surface film of an oxide or a hydroxide. When the stable species is a dissolved ion, the region on the Pourbaixdiagram is labeled as a region of corrosion. 6.1 identifies the various regions where the species Al (solid), Al2O3 (solid ), Al3+ ions, andAlO2 ions are each stable. Pourbaix diagrams also contain regions or fields between the various lines where specific chem-ical compounds or species are thermodynamically stable. The pH),(2) Vertical lines, which are for reactions involving only the pH (but not the electrode potential E),(3) Slanted lines, which pertain to reactions involving both the electrode potential E and the pH. In a Pourbaix diagram, there are three possible types of straightlines:(1) Horizontal lines, which are for reactions involving only the electrode potential E (but not The ordinate is the electrode potential E, which is a measure ofthe electrochemical environment. The abscissa in the diagram is the pH of the aqueous solution, which is a mea-sure of the chemical environment. 6.1, which shows the Pourbaix diagramfor aluminum. Īn example of a Pourbaix diagram is given in Fig. Pourbaix diagrams are available forover 70 different metals. Such diagrams are usually called Pourbaixdiagrams but are sometimes called equilibrium diagrams because these diagrams apply to con-ditions where the metal is in equilibrium with its environment. These diagrams indicatecertain regions of potential and pH where the metal undergoes corrosion and other regions of poten-tial and pH where the metal is protected from corrosion. Marcel Pourbaix has developed a unique and concise method of summarizing the corrosion thermo-dynamic information for a given metal in a useful potentialpH diagram. Chapter 6Thermodynamics of Corrosion: Pourbaix Diagrams