Anodic polarization of iron

Molybdenum exhibits unusual polarization behavior. The initial portion of the curve, shown dashed in Fig. 5.20, is very difficult to determine experimentally because it occurs at very low current densities indicating that the passive state is very rapidly established by traces of dissolved oxygen or by very low concentrations of other cathodic reactants. In fact, many of the published curves show only the transpassive range over which the current density rapidly increases. The implication is that as long as the potential is below 200 mV (SHE), the corrosion rate of molybdenum would be very low and this is observed. 

The passive films on these metals are either the conventional oxides or species such as FeOOH, which are related chemically. On nickel the film is related to NiO, on chromium to Cr203, and on titanium to Ti02. 

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Very careful measurements of the anodic polarization of iron by Kelly (Ref 7) resulted in the proposal of five kinetics steps, the sum of which result in the simple oxidation reaction, Fe —> Fe2+ + 2e. The pro-... 

Anodic Polarization of Iron-Chromium-Molybdenum Alloys... 

A separate chapter, Chapter 5, is used to introduce the corrosion behavior of active/passive type metals. This allows emphasis on the more complex anodic polarization behavior of these metals and the associated problems in interpreting their corrosion behavior. The chapter is introduced by discussing experimental observations on the anodic polarization of iron as a function of pH and how these observations can be related qualitatively to the iron-water Pourbaix diagram. Pedagogically, it would be desirable to analyze the corrosion behaviors of active/passive metals by relating their anodic polarization curves to curves for cathodic reactions as was done in Chapter 4 for nonpassive alloys. Because of the extreme sensitivity of an experimental curve to the environment, a reasonably complete curve usually can only be inferred. To do so requires understanding of the forms of experimental curves that can be derived from individual anodic and cathodic polar-... 

Manganese, in amounts normally present, effectively decreases acid corrosion of steel containing small amounts of sulfur. Inclusions of MnS have low electrical conductivity compared to FeS in addition, manganese reduces the solid solubility of sulfur in iron, thereby probably restoring the anodic polarization of iron which is lowered by the presence of sulfur [50]. Silicon only slightly increases the corrosion rate in dilute hydrochloric acid (Fig. 7.13). 

FIG. 6—The effect of water content on the anodic polarization of iron in propylene carbonate at 25°C [33]. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 11-7 shows the polarization curve of an iron electrode in an acidic solution in which the anodic reaction is the anodic transfer of iron ions for metal dissolution (Tafel slope 40 mV/decade) the cathodic reaction is the cathodic transfer of electrons for reduction of hydrogen ions (Tafel slope 120 mV /decade) across the interface of iron electrode. 

Vodyanov, a year earlier, had examined the effect of an ultrasonic field on the anodic dissolution of iron in sulphate solutions [119]. The dependence of an ultrasonic field (23 kHz) on the rate of anodic dissolution of Fe was investigated at pH 0.45-2.0 and at a SO concentration ofO. 1-1N. The current versus time curves at controlled potential showed that the ultrasonic field increased anodic polarization. 

Waskaas, M. 1996. Magnetic field effect on electrode reactions III. Effects on the anodic polarization of an iron electrode in an iron(III) chloride solution under potentiostatic conditions. Acta Chemica Scandinavica 50, 526-530. 

From the following data for the polarization of iron, make a reasonable plot of the anodic polarization curve over the current density range from iG Fe to iox Fe = 10+4 mA/m2... 

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

The effect of pH on the polarization of iron is shown in Fig. 5.6. The effect ofpH on the polarization of type 304 stainless steel (nominally 18 to 20 wt% Cr, 8 to 10.5 wt%Ni, 0.08 wt% C maximum) in environments based on 1 M Na2SC>4 with additions of H2SO4 and NaOH to control the pH is shown in Fig. 5.31 (Ref 28). The influence of chromium and nickel in moving the anodic polarization curve of iron to lower current densities persists over the indicated pH range with the corrosion rates being very low for pH >4.0. 

In recent work related to the electrodeposition of PPy from an aqueous pyrrole-oxalic acid solution, the influence of the iron surface pretreatment on the corrosion properties was reported by Van Shaftinghen, Deslouis et al. [79]. The performances of PPy-coated iron samples, obtained through three different electropolymerization techniques were tested (i) electropolymerization at constant current (1 mA cm ) (denoted galstat), (ii) potentiostatic electropolymerization at IV vs. Ag/AgCl in oxalic acid for 600 s followed by addition of pyrrole into the solution (denoted one-step), (iii) potentiostatic polarization in a two-step process identical to the previous one, except that a prehminary polarization at 0 V vs. Ag/ AgCl for 600 s was carried out in the electrolyte alone (denoted two-step) (Figure 16.11). From the anodic curves of iron in oxalic acid alone, a first zone of potential ranging... 

It is a well-known fact that iron dissolution occurs in four different states, namely, the active, passive, transpassive, and brightening states, determined by the nature and kinetics of the reactions involved, which depend in turn on the potential and electrolyte composition. A schematic polarization curve for anodic dissolution of iron in acid solutions is given in Fig. 1. The shape of the curve depends on the nature of the electrolyte, the polarization program, and hydrodynamics. The Fe/Fe, and... 

Anodic polarization of active/passive metals - alloys of nickel, iron, chromium, titanium, and stainless steel in weak-to-extiemely corrosive environments, where economy in consumption of protective currents is required. 

It is worth emphasising too, that the position of those lines representing equilibria with the dissolved species, M, depend critically on the solubility of the ion, which is a continuous function of pH. For example, iron in moderately alkaline solution is expected to be very passive and so it is in borate solutions (in the absence of aggressive ions). However, the anodic polarization curve still shows a small active loop at low potential. 

A limited degree of control over the corrosivity of the product packed is possible. Minor pH adjustments may be helpful, especially in ensuring an anodic relation of tin to steel corrosion promoters, like nitrate, sulphur and copper may be excluded from necessary additives, such as water and sugar, and from sprays applied to crops approaching harvest. The effect of sulphur compounds which may remain from spray residues is complex but often includes reversal of the tin-iron polarity. 

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