Predictions from Potential-pH Diagrams

The use of potential-pH diagrams in the discussion of corrosion was introduced by Pourbaix [1]. The reversible potential and the pH are considered independent variables for reactions of the type  

Here X represents the oxidized form and W the reduced form of the substance in question. The equilibrium of reaction 1 may be expressed at 25 °C  

If x = w, the simplification requires only equal activities (a = ayf). The drawing of lines according to Eq. 3 in the potential-pH diagram allows [1,2] to establish domains of relative predominance of dissolved species or domains of relative stability of solid substances at equilibrium. By superimposing the diagrams for dissolved species and solid substances, the conditions of potential and pH are found under which a given 

Corrosion by dissolution Corrosion by gasification Passivation by oxide or hydroxide layer X//y///A Passivation by hydroxide layer i - - I Immunity 

The following points should be considered when the diagrams in Fig. 83 are used  

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Stability relatiMshipa and the sequence of formation of surface reaction layers can he predicted from potential-pH diagrams. The Cu-O-S-HjO system will be used as an example- Figure 93-6 represents the Cu-O-S-H.O system at S = 10". Stable regions for Cu2S and CuS indicate that sulfur films will not form adjacent to CujS since the reaction... 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

The form of Figure 1.43 is common among many metals in solutions of acidic to neutral pH of non-complexing anions. Some metals such as aluminium and zinc, whose oxides are amphoteric, lose their passivity in alkaline solutions, a feature reflected in the potential/pH diagram. This is likely to arise from the rapid rate at which the oxide is attacked by the solution, rather than from direct attack on the metal, although at low potential, active dissolution is predicted thermodynamically The reader is referred to the classical work of Pourbaix for a full treatment of potential/pH diagrams of pure metals in equilibrium with water. 

There is a second group of metals like Fe, Cr, Ni and their alloys, which do not follow all predictions of their potential-pH diagrams. As an example, the Pourbaix Diagram for iron of Fig. 3 predicts corrosion for all potentials in strongly acidic electrolytes. However, experiments show that it is passive for potentials above a potential of Ep = 0.58 — 0.059 pH. For these conditions the passive layer is far from any dissolution equilibrium and its protecting properties have to be related to its slow dissolution kinetics. The same arguments hold for the passivation of Cr, Ni and their alloys. 

Dissolution of the chlorides from the corrosion products is an essential part of the conservation process. It is essential that the artefact is immersed in an electrolyte that will not corrode the metal any further, while this dissolution is taking place. Corrosion scientists have developed redox potential - pH diagrams from thermodynamics in order to predict the most stable form of the metal. These diagrams are divided into three zones. Where metal ions are the most stable phase, this is classed as a zone of corrosion. If the metal itself is the most stable species, this is said to be the zone of immunity. The third zone is where solid metal compounds such as oxides, hydroxides, etc, are the most stable and may form a protective layer over the metal surface. This zone is termed passivity and the metal will not corrode as long as this film forms a protective barrier. The thickness of this passive layer may only be approximately 10 nm thick but as long as it covers the entire metal surface, it will prevent further corrosion. 

Reymond, F. Steyaert, G. Carrupt, P.A. Testa, B. Girault, H. Ionic partition diagrams a potential-pH representation. J. Am. Chem. Soc. 1996,118, 11,951-11,957. Tsantili-Kakoulidou, A. Panderi, I. Csizmadia, F. Darvas, F. Prediction of distribution coefficient from structure, II. Validation of prologD, an expert system. J. Pharm. Sci. 1997, 86, 1173-1179. 

These diagrams allow us to predict the i -pH conditions in which sulfur is adsorbed on a surface of Fe, Ni, Cr, or Cu in water by oxidation of sulfides or reduction of thiosulfates. In aqueous solution, when the potential is increased in the anodic direction, the adsorbed water molecules are replaced by sulfur atoms adsorbed from H2S. s and HS . At higher potentials, sulfur is oxidized in HS2O3 or... 

The Af-HjO diagrams present the equilibria at various pHs and potentials between the metal, metal ions and solid oxides and hydroxides for systems in which the only reactants are metal, water, and hydrogen and hydroxyl ions a situation that is extremely unlikely to prevail in real solutions that usually contain a variety of electrolytes and non-electrolytes. Thus a solution of pH 1 may be prepared from either hydrochloric, sulphuric, nitric or perchloric acids, and in each case a different anion will be introduced into the solution with the consequent possibility of the formation of species other than those predicted in the Af-HjO system. In general, anions that form soluble complexes will tend to extend the zones of corrosion, whereas anions that form insoluble compounds will tend to extend the zone of passivity. However, provided the relevant thermodynamic data are aveiil-able, the effect of these anions can be incorporated into the diagram, and diagrams of the type Af-HjO-A" are available in Cebelcor reports and in the published literature. 

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