drawn for other concentrations. If the diagram involves the equilibrium between a dissolved species and a gas, the pressure is usually set to P0 = 1 atm = , the minimum pressure required for gas evolution from an aqueous solution at standard conditions. In addition, changes in temperature and concentration of solvated ions in solution will shift the equilibrium lines in accordance with the Nernst equation. The diagrams also do not take kinetic effects into account, meaning that species shown as unstable might not react to any significant degree in practice. A simplified Pourbaix diagram indicates regions of "immunity", "corrosion" and "passivity", instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while corrosion shows that general attack will occur. Passivation occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of aluminium because of the alumina layer formed on its surface when exposed to air. Applicable chemical systems While such diagrams can be drawn for any chemical system, it is important to note that the addition of a metal binding agent (ligand) will often modify the diagram. For instance, carbonate () has a great effect upon the diagram for uranium. (See diagrams at right). The presence of trace amounts of certain species such as chloride ions can also greatly affect the stability of certain species by destroying passivating layers. Pourbaix diagrams are an useful tool in understanding the stability of different oxidation states in a solution, and can be applied to the analysis of organic leaching processes for metals such as iron. The diagrams provide valuable information for optimizing the leaching process and advancing our understanding of organometallic species in these systems. Limitations Even though Pourbaix diagrams are useful for a metal corrosion potential estimation they have, however, some important limitations: Equilibrium is always assumed, though in practice it may differ. The diagram does not provide information on actual corrosion rates. Does not apply to alloys. Does not indicate whether passivation (in the form of oxides or hydroxides) is protective or not. Diffusion of oxygen ions through thin oxide layers are possible. Excludes corrosion by chloride ions (, etc.). Usually applicable only to temperature of , which is assumed by default. The Pourbaix diagrams for higher