Corrosion passive region

The titanium oxide film consists of mtile or anatase (31) and is typically 250-A thick. It is insoluble, repairable, and nonporous in many chemical media and provides excellent corrosion resistance. The oxide is fully stable in aqueous environments over a range of pH, from highly oxidizing to mildly reducing. However, when this oxide film is broken, the corrosion rate is very rapid. Usually the presence of a small amount of water is sufficient to repair the damaged oxide film. In a seawater solution, this film is maintained in the passive region from ca 0.2 to 10 V versus the saturated calomel electrode (32,33). 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibiUty for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibiUty depends on pH. The susceptibiUty temperature increases paraboHcaHy from 65°C as pH is increased from 2ero. 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 equiUbrium potential into the passive region where a protective film is formed and maintained. 

Figure 2-13 shows the potential dependence of the corrosion rate in the passive region (Fig. 2-2) for the system Fe/H2S04. kiU < = 0.8 V, the transition... 

Cid etal. studied the corrosion resistance of Ni, 5% Fe-Ni and 10% Fe-Ni alloys in the trans-passive region in sulphuric acid. For a given acid concentration the addition of iron reduced the corrosion rate. It was concluded that the addition of small percentages of Fe was doubly beneficial, decreasing both general and intergranular corrosion. 

The significance of the Flade potential Ef, passivation potential pp, critical current density /pn, passive current density, etc. have been considered in some detail in Sections 1.4 and 1.5 and will not therefore be considered in the present section. It is sufficient to note that in order to produce passivation (a) the critical current density must be exceeded and b) the potential must then be maintained in the passive region and not allowed to fall into the active region or rise into the transpassive region. It follows that although a high current density may be required to cause passivation ) only a small current density is required to maintain it, and that in the passive region the corrosion rate corresponds to the passive current density (/p, ). 

On a clean surface of an Fci7Cr alloy in a 0.5M NaCl electrolyte, corrosion is accelerated as pitting corrosion when a potential pulse of 1 s duration extending from the passive region and above the pitting potential is applied. Gugler et al. ° showed by in situ AFM that in this case the pitting corrosion was initiated close to an inclusion on the surface (Fig. 8). Such a surface defect may act as a center for pit nucleation, as was... 

This makes clear how the oil-sealed pump is protected against corrosion the concentration of the oxidation agent in the oil is negligible and thus the opportunity for the metal to release electrons is equally small. This also makes it clear that the use of so-called non-rusting or stainless steels does not make sense since oxidation is necessary for the passivation of these steels, in order to reach the so-called passive region for these steel compounds. The aitical passivation current density will normally not appear in oil-sealed pumps. 

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

One must be wary of the use of anodic protection, in that any area that is not polarized completely into the passive region will dissolve at a high rate. The optimum protection range is shown in Fig. 16. Therefore anodic protection is more susceptible to the presence of crevices, deposits, or poor placement of polarizing electrodes than is cathodic protection. If a component is cathodically under protected, the maximum rate at which the unprotected area corrodes is the normal open circuit corrosion rate in anodic protection, underprotection results in high rate dissolution of the unprotected area and can therefore can lead to unexpected career changes. Understanding the manner in which current from an anodic protection system is distributed across a surface is important in such installations. The issues involved in current distribution are discussed in detail in Chapter 4. 

Temperature has been used in conjunction with electrochemical control to quantify the resistance of materials to localized corrosion. Kearns (26) has reviewed the different critical temperature tests in some detail. Electrochemical critical temperature testing consists of holding a material exposed to a solution of interest potentiostatically at a potential in its passive region while increasing the temperature of the solution either intermittently (54) or continuously (55). An example of the results of the latter type of testing is shown in Fig. 48. In this... 

In the passive region, values of the passive corrosion rate could be obtained from a polarization curve, though there is no guarantee that it would represent... 

The corrosion potential, Ecoa for iron in aerated water is in the range of —600 to —700 mV at pH 7 against the silver-silver chloride reference electrode (Point 0). By decreasing the pH below 7.0 the system is unaffected and corrosion persists. Increase in pH moves the system into a passive region, but this type of perturbation of the system is not the focus of the present discussion. By applying a more negative potential it is possible to move the system into the region of immunity which means the corrosion susceptibility is reduced. 

The more negative the potential, the greater the cathodic reaction and the smaller the anodic reaction the metal is more cathodic, which is the basis of cathodic protection of metals. By applying more positive potentials the system moves into the passive region where the corrosion rate may be reduced. This is particularly the case for some steels in particular environments and other metals, which forms the basis of anodic protection of metals. Thus, it is seen that changing the potential of a system in an environment which cannot be altered leads to effective corrosion control by cathodic or anodic protection as the case may be. 

Flade potential electrode potential F of a metal in contact with a corrosive electrolyte solution where the current associated with the anodic metal dissolution (- corrosion, active region) drops to very small values. See also - Flade, and -> passivation. 

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