Corrosion thermodynamics Gibbs free-energy

Most metals tend to corrode in an environment of air and/or water, forming metal oxides or hydrated oxides. Whether or not such a reaction is possible is dictated by the thermodynamics of the corrosion reaction. If the reaction has a negative Gibbs free energy of formation, then the reaction is thermodynamically favoured. While thermodynamics determines whether a particular reaction can occur or not, the rate of the corrosion reaction is determined by kinetic factors. A number of variables can affect the corrosion rate, including temperature, pH and passivation, which is the formation of a thin protective film on a metal surface. Passivation can have a tremendous influence on the corrosion rate, often reducing it to a negligible amount. 

The Gibbs free energy of formation values. Figs. 10.9 and 10.10, can be used to estimate the dissociation pressure or the gas composition where surface scale formation will be thermodynamically feasible. Assuming the formation of the corrosion product follows a similar reaction as the decomposition of palladium-sulfide (Eq. 10.10), the correlation between the equilibrium constant and Gibbs free energy of formation is illustrated by Eq. 10.11. 

McNeil and Odom [16] developed a thermodynamic model to predict metal susceptibility to MIC by 8RB. If the reaction to produce the sulfide from the oxide has a negative Gibbs free energy, the reaction will take place. If the value is positive, the metal is immune to derivation by sulfides and will not be vulnerable to corrosion by 8RB. The model is limited to thermodynamic predictions as to whether a reaction will take place and does not consider metal toxicity to the organisms, tenacity of the resulting sulfide or others factors that influence corrosion rate. The following is a summary of mineralogical products... 

When a metal (M) is immersed in a solution containing its ions (M ), several reactions may occur. The metal may lose an electron (corrosion) to form metal ions or the metal ions in solution gain electrons (reduction) and enter the solid metal state. The equilibrium across the metal-solution interface controls which reaction, if any, will occur at the metal-electrolyte interface. Because the equilibrium is determined by the equality of the partial Gibbs free-energy or chemical potentials (//) on either side of the electrode interface (i.e., Absolution=A dectrode). when any metal is immersed in the electrolyte, thermodynamics... 

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