Alloys equilibrium potential

Corrosion of the positive grid [Eq. (28)1 occurs equivalent to about 1 mA/lOOAh at open-circuit voltage and intact passivation layer. It depends on electrode potential, and is at minimum about 40-80mV above the PbS04/Pb02 equilibrium potential. The corrosion rate depends furthermore to some extent on alloy composition and is increased with high anti-monial alloys,... 

Silver deposition on polycrystalline Pt electrodes at potentials positive to the equilibrium potential gave 2.5 atomic layers. Two binding types of Ag layers were found by anodic stripping the first Ag layer deposited on Pt, which seems to form an alloy of Ag-Pt, on which the second Ag deposition takes place in the Ag underpotential deposition region. STM images from the underpotential to the overpotential deposition region were observed for Cu underpotential deposition on Au(l 11) in sulfuric acid solution, where Cu underpotential deposition does not affect overpotential deposition, although the latter always takes place on the surface with Cu underpotential deposition and a metal. ... 

In electroplating industrial iron metals, zinc metal electrodeposition is accompanied by the formation of Zn-Ni, Zn-Co, and Zn-Fe alloys, where zinc electrodeposition is known to be anomalous in some cases. The ratio of zinc metal to iron metal in those alloys is sometimes higher than that of the electroplating bath solution, and zinc ions occasionally deposit at potentials positive to the equilibrium potential of zinc ions on zinc metal although is very negative to the equilibrium potentials of iron metals. It can be seen from the study of underpotential deposition of zinc ions " that this is not anomalous, but could be explained as an underpotential deposition phenomenon, to be clarified in further work. 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibility for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibility depends on pH. The susceptibility temperature increases parabolically from 65°C as pH is increased from zero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equilibrium potential into the passive region where a protective film is formed and maintained. 

The simplest material that can change composition is a binary mixture of two atomic species an example would be a copper-tin alloy. Let a mixture of this type contain atomic species A and B then for the chemical potential of species A, dn fdC = RTfC, where is the mole fraction or number ratio of atoms of A to total number of atoms in a sample, n l(n -l- n ), and dfi /dP = the partial molar volume of species A. If stress is nonhydrostatic, the associated equilibrium potential of A for direction n follows a similar relation djx jda = V. ... 

Although most metals display an active or activation controlled region, when polarised anodically from the equilibrium potential, many metals and perhaps even more so alloys developed for engineering applications, produce a solid corrosion product. In many examples the solid is an oxide that is the stable phase rather than the ion in solution. If this solid product is formed at the metal surface and has good intimate contact with the metal, and features low ion-conductivity, the dissolution rate of the metal is limited to the rate at which metal ions can migrate through the film. The layer of corrosion product acts as a barrier to further ion movement across the interface. The resistance afforded by this corrosion layer is generally referred to as the passivity. Alloys such as the stainless steels, nickel alloys and metals like titanium owe their corrosion resistance to this passive layer. 

Complete equilibrium of the alloy electrode will only be established if the equilibrium potentials of both reactions, as given by the Nernst equations (see Chapter 1.2)... 

In the case of a large difference of the standard potentials of the components, — eI, the equilibrium potential of the alloy electrode, Eab > with A being... 

The equilibrium potential of this cell is the difference between the potential of the metal in the alloy state minus the potential of the pure metal... 

Equilibrium potential, standard potential, and the standard Gibbs energy depend on the alloy composition. 

CPs are redox-active materials having a positive equilibrium potential with respect to those of iron, aluminium and other alloying elements (Table 10.1). This suggests anodic protection as more expected corrosion protection mechanism. For CPs electroactivity, it exists a range of potential values because reduction potential depends on kind and level of doping. 

The use of CPs as anticorrosion materials has several advantages. CPs are suitable to replace chromate and other hazardous materials. They are cheap and easily deposited on protecting substrate by electrochemical or chemical synthesis methods. Moreover, CPs have the equilibrium potential positive relative to those of iron, aluminium and some other metals, so they can protect effectively ferrous and non-ferrous alloys. Although these beneficial properties, the interactions between CP and metal are numerous and complex, and the mechanisms of the corrosion protection are not yet fully clarified. 

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