Corrosion standard potential

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

The real electrical potential of various metals and their alloys may, under practical boiler operating conditions, be considerably different from their standard potential under ideal conditions. Thus, a reversal of potential may take place in the boiler plant system, with unexpected forms of galvanic corrosion occurring. 

Nevertheless, the calculated standard potential given in Eq. (2) remains valid with a good precision. When pH value is higher than 12, the solubility of silicate ion grows rapidly and then corrosion process... 

Again, the standard potential is negative enough to induce the mechanism of electrochemical corrosion. We just have to point out that the kinetics of corrosion is very sensitive to the surface structure of the material and the presence of defects or impurities. 

A bare surface of silicon can only exist in fluoride containing solutions. In reality, in these media, the electrode is considered to be passive due to the coverage by Si— terminal bonds. Nevertheless, the interface Si/HF electrolyte constitutes a basic example for the study of electrochemical processes at the Si electrode. In this system, the silicon must be considered both as a charge carrier reservoir in cathodic reactions, and as an electrochemical reactant under anodic polarization. Moreover, one must keep in mind that, according to the standard potential of the element, both anodic and cathodic charge transfers are involved simultaneously (corrosion process) in a wide range of potentials. 

Although the nickel-containing systems have been extensively studied also by electrochemical methods [1] due to their practical importance, for example, in electrochemical power sources (Ni—Fe, Ni—Cd, Fi—NiF2 batteries), in corrosion-resistant alloys (tableware, coins, industrial instruments) as well as due to their interesting (magnetic, spectral, catalytic) properties most of the standard potentials of electrode... 

The standard potential series can be used as only a rough guide with respect to the ability of a metal to resist corrosion. In most of the corrosion reactions, the potential values shown in the table are not applicable because of the presence of a film on the metal surface, and the change in potential because the activity of metal ions is less than unity. 

Data from A. J. Bard, R. Parsons, and J. Jordan, eds.. Standard Potentials in Aqueous Solution, Marcel Dekker, New York, 1985 A. Kaczmarcyzk, W. C. Nichols, W. H. Stockmayer, and T. B. Ames, Inorg-Chem., 1968, 7, 1057 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, 2d ed., translated by J. A. Franklin, National Association of Corrosion Engineers, Houston, XX, 1974. 

D25.12 Corrosion is an electrochemical process. We will illustrate it with the example of the rusting of iron, but the same principles apply to other corrosive processes. The electrochemical basis of corrosion that occurs in the presence of water and oxygen, is revealed by comparing the standard potentials of the metal reduction, such as... 

Another form of surface coating is provided by galvanizing, the coating of an iron object with zinc. Because the latter s standard potential is —0.76 V, which is more negative than that of the iron couple, the corrosion of zinc is thermodynamically favored and the iron survives (the zinc survives because it is protected by a hydrated oxide layer). 

Individual half cell potentials on the hydrogen scale may be compared to all other half cells referred to the hydrogen scale. Table 10.1 lists some standard electrode potentials for some common metals. The sign allocated to the standard potentials is positive but the opposite sign is often used by corrosion engineers. As long as consistency is observed these differences present no difficulties. 

The free corrosion potential, Ecott, lies in between the standard potentials of the two electrochemical systems. 

Zinc-manganese This alloy received attention when an US Air Force report was published [50]. In contact with metals more negative in the list of standard potentials than zinc, such as aluminum or magnesium, a better corrosion behavior is expected. Although the system found new interest in recent years [51,... 

Galvanic corrosion by electrical contact between metals with different standard potentials in an electrolyte... 

Both Sn and Sn in the corrosion reaction are at standard state. The potential measured for the Sn Sn electrode is 0.138 V with Sn Sn negative vs. the hydrogen reference electrode. Because the potential of the SHE is defined as zero, the standard potential of the Sn I Sn redox system is e° =—0.138 V, as presented in Table 2.2. Electrodes shown... 

Corrosion will occur with the displacement of silver ions from the electrolyte in the presence of Ni metal, which has more negative standard potential than silver. 

The pressure to stop the Ni corrosion cell is much less than the pressure to stop the iron corrosion ceU. Iron is more thermodynamically active with standard potential of -0.44 V vs. SHE when compared with nickel with standard electrode potential ofe° = -0.250V vs. SHE. 

The reversible potentials can be used to predict the corrosion tendency of the metal when the metal and the electrolyte are under standard thermodynamic conditions described in Chapter 2, Section 2.12.2. Table 6.1 is written as reduction reactions following the guidelines suggested by the International Union of Pure and Applied Chemistry (lUPAC) during the Stockholm Convention in 1953. The procedure for estimating half-cell potential is presented in Chapter 2. In an electrochemical ceU, the electrode with a smaller standard potential in Table 6.1 undergoes oxidation and transfer electrons to the electrode with a larger standard potential, which is reduced at the interface. In redox sys-... 

The corrosion behavior of metals cannot be predicted from the position of their standard potentials in the electrochemical series because the potential of an electrode changes with the current density. If an electrode in which only one electrode process takes place is termed a working electrode and the resultant potential, a working potential, then the differences between working potential and the Nernst equilibrium potential is called an overpotential, that is caused by reaction restraints. In general, polarization is defined as the shift in potential of working electrodes within a corrosion element. In such an element, at least two electrode reactions occur whose overpotentials are superimposed, resulting in the polarization effect. 

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