Ferrous ion concentration

During acid cleaning or severe acid upsets, ferric-ion concentration may increase, albeit much more slowly than ferrous-ion concentration, to high levels. Resulting corrosion can be severe. Iron is oxidized, and... 

By contrast, it appears from Figs. 10.4 and 10.6 that a potential measurement would be more reliable. Specifically that (the equilibrium potential for the iron/ferrous-ion electrode given a suitably low ferrous ion concentration) would always represent the achievement of full protection for iron. 

It must not be assumed that the protection potential is numerically equal to the equilibrium potential for the iron/ferrous-ion electrode (E ). The standard equilibrium potential (E ) for iron/ferrous-ion is -0-440V (vs. the standard hydrogen electrode). If the interfacial ferrous ion concentration when corrosion ceases is approximately 10 g ions/1 then, according to the Nernst equation, the equilibrium potential (E ) is given by ... 

A simple calculation based on the solubility product of ferrous hydroxide and assuming an interfacial pH of 9 (due to the alkalisation of the cathodic surface by reaction ) shows that, according to the Nernst equation, at -0-85 V (vs. CU/CUSO4) the ferrous ion concentration then present is sufficient to permit deposition hydroxide ion. It appears that the ferrous hydroxide formed may be protective and that the practical protection potential ( —0-85 V), as opposed to the theoretical protection potential (E, = -0-93 V), is governed by the thermodynamics of precipitation and not those of dissolution. 

Microsensors have been used to develop profiles in mixed species biofilms. Figure 10 shows concentration profiles of sulfide, oxygen, and pH in a biofilm accumulated on the surface of a mild steel corrosion coupon. The concentration of sulfide is highest near the metal surface, where iron sulfide forms quickly and covers the steel surface if both ferrous and sulfide ions are available. At low ferrous ion concentrations, adherent and temporarily protective films of iron sulfides are formed on the steel surface, with a consequent reduction in corrosion rate. High rates of SRB-induced corrosion of mild steel are maintained only in high concentrations of ferrous ion. 

When the methyl viologen reached its reductive equilibrium, noted by the start of hydrogen evolution, the enzyme in the side arm was tipped into the main compartment, and the reaction stopped after 10 min. The solution in the main compartment of the flasks were analyzed for ferrous ion concentration, which is a measurement for metal corrosion. 

The upper dose limit may be extended to more than 3 X 106 rads by increasing the ferrous ion concentration from 0.001M to 0.01M. The range may be increased if the ferrous concentration is further increased (9). 

Effect of Ferrous Ion Concentration. The reactions involved in the ferrous-cupric dosimeter, as described by Hart (1) are independent of oxygen concentration, and one would not expect to observe a change in the ferric yield ( (Fe3+) in this system when increasing the dose, as occurs in the Fricke dosimeter. This would indicate that the dose limit of this dosimeter is a function of the initial ferrous ion concentration and is not influenced by the oxygen concentration. To explore this idea, we increased the ferrous ion concentration from 0.001M to 0.01M while keeping the... 

Figure 3. Effect of initial ferrous ion concentration on linear portion of absorbance-irradiation time (dose) relationship...

In view of these considerations, one can adopt practical and reasonable, though arbitrary, criterion A ferrous ion concentration of 10 6 mol liter 1 and higher implies the occurrence of considerable dissolution, i.e., of corrosion. With these considerations as background, it is conventional in using a potential-pH diagram for deciding whether a metal can possibly corrode, to calculate the equilibrium potential for the M + + ne M reaction for a metal-ion concentration of 10 6 mol liter 1. 

At very low ratios of peroxide to ferrous ion concentrations (Ri) only steps (o) and (1) occur and hence n = 0.5 and the rate of ferrous oxidation is given by... 

The experimental data of Lamb and Elder (96), however, are not in agreement with this predicted rate expression for they find the initial rate proportional to the square of the ferrous ion concentration and directly proportional to the oxygen pressure. This has recently been confirmed by the author (97), and it would appear that either the autoxidation is subject to a true catalysis by trace impurities (an induced reaction is excluded by the total ferrous ion oxidized being large, about M/20) or the actual mechanism is different from that suggested by Weiss. 

The reaction is fast [81], and the absorption process benefits from this until the ferrous ion concentration is nearly depleted. Usually, chlorine is sparged into a pool of FeCh solution. The reaction tank therefore must be vented to a scrubber to prevent the release of chlorine to the atmosphere. FRP of the proper grade again is satisfactory, and again the specific gravity of the product is quite high (1.4—1.5, depending on concentration). 

The graft copolymerizations were conducted in a three-neck flask fitted with a nitrogen bubbler, stirrer, and dropping funnel. In a typical reaction, dextran (M 100,000-200,000) was dissolved in distilled water under a nitrogen atmosphere with stirring for 10 minutes. The ferrous ion solution was added and 30 minutes were allowed for adsorption. Acrylamide monomer was added and stirred for 10 minutes and then 50 ml of hydrogen peroxide solution were added dropwise for 20 minutes. The variations of reaction parameters are shown in Table 4. Reaction series PA, PB, PC, PC, and PE correspond to variations of ferrous ion concentration, hydrogen peroxide concentration, reaction time, monomer concentration, and dextran substrate concentration, respectively. 

In this case n = 2, z+ = 2 and z = -1. Figure 4.43 schematically shows the concentration profile of Fe and Cl ions. The electroneutrality condition requires that the chloride concentration at each point in the diffusion layer is equal to twice the ferrous ion concentration. At the limiting current, one finds thus for the surface concentration ... 

It may be worth noting that researchers such as Smith and Miller in their review of the corrosive effects of sulphides on ferrous metals have reported that in media with high ferrous ion concentration, most of the corrosion of mild steel in biotic (bacterial) cultures can be attributed to the ferrous sulphide produced by the bacteria. In other words, it seems that when SRB are present, the iron sulphide produced by their interactions could be more corrosive than chemically (no bacteria) prepared iron sulphide. See [94] (The author thanks Dr. Peter Farinha s for his remarks regarding this paper and his kindness in providing the author with this paper). 

The first criterion is probably the best known and widely used in industry due to its ease of apphcation. Using the Nemst equation and a ferrous ion concentration of 10 M (a criterion commonly used to define negligible corrosion in thermodynamics), a potential for steel of -930 mV w.r.t. Cu/CuS04 can be derived, which is somewhat more negative than this criterion. The satisfactory performance imder the less stringent potential requirement may be related to the formation of protective ferrous hydroxide on the surface. Strictly speaking, potential protection cri-... 

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