Passive layers on iron

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

Fig. 34. Deconvoluted Ols core level of a passive layer on iron in borate buffer at 0.65 Vrhe. After [108],...

Passive layers on iron are often only a few molecular layers thick and hence, are optically invisible. They are protective because they are nonporous, uniform and adhere firmly to the metal. There is still some uncertainty about the composition of the pas-... 

D. Rahner, W. Forker, Effect of convection conditions on the stabihty and in-situ analytics of passive layers on iron in neutral media, Z. Phys. Chem. 18 (1978) 344. 

Finally, we should briefly discuss the rate of steady state dissolution of a compact corrosion product in aqueous solution. In order to calculate this stationary corrosion rate, it is necessary to know the rate-determining step for the removal of metal ions from the corrosion product into the electrolytic solution. It has been found that the stationary corrosion rate of a passivated layer on iron between 0.7 and 3.9 in an aqueous H2SO4 solution does not depend upon the stirring rate, nor upon the Fe /Fe ratio in the solution, nor, most importantly, does it depend upon the electrode potential h [34]. From these observations it has been concluded that the rate-controlling step is a chemical reaction between the iron ions, which are leaving the corrosion product, and the solvent ions to form complex ions immediately next to the surface. Thus, in the case of iron under the given conditions, this step determines the overall corrosion rate and thus it also determines the stationary thickness of the passivating corrosion product. 

Figure 1-24. Angular resolved XPS analysis of a bilayer structure. Fe(II)/Fe(III) passive layer on iron developing at =-0.16 V in 1 M NaOH with passivation time tp (Haupt and Strehblow, 1987a).

Exceeding the Flade potential in the positive direction causes the formation of passive layers on iron, while in the negative direction it causes dissolution of the metal. The potential range between Ep and cor-... 

I. A. Ammar and S. Darwish, Growth of passive layers on iron and nickel, Electrochim. Acta 12 225 (1967). 

Hydrogenis prevented from forming a passivating layer on the surface by an oxidant additive which also oxidizes ferrous iron to ferric iron. Ferric phosphate then precipitates as sludge away from the metal surface. Depending on bath parameters, tertiary iron phosphate may also deposit and ferrous iron can be incorporated into the crystal lattice. When other metals are included in the bath, these are also incorporated at distinct levels to generate species that can be written as Zn2Me(P0 2> where Me can represent Ni, Mn, Ca, Mg, or Fe. 

Thus inhibitive anions can retard the dissolution of both the T-FejO, and the magnetite layers of the passivating oxide layer on iron. This has the dual effect of preventing breakdown of an existing oxide film and also of facilitating the formation of a passivating oxide film on an active iron surface, as discussed in the previous section. 

In acidic electrolytes only lead, because it forms passive layers on the active surfaces, has proven sufficiently chemically stable to produce durable storage batteries. In contrast, in alkaline medium there are several substances basically suitable as electrode materials nickel hydroxide, silver oxide, and manganese dioxide as positive active materials may be combined with zinc, cadmium, iron, or metal hydrides. In each case potassium hydroxide is the electrolyte, at a concentration — depending on battery systems and application — in the range of 1.15 - 1,45 gem"3. Several elec-... 

The Natural Passivation and Corrosion of Metals and Alloys XPS studies of the air-formed natural passive layer on aluminium surfaces have identified a number of hydroxides as well as alumina (Barr, 1977). The oxidation of pure iron and of stainless steels and other iron alloys have also been extensively... 

Corrosion-inhibitive properties of the compound Na2P03F have been tested by Andrade et al., either by incorporating it in a mortar or as a penetrant[68]. This compound, which is currently available as a proprietary product, is reported to act as an anodic inhibitor, possibly with some cathodic action. The minimum required ratio of phosphate to chloride was suggested as 1 1. The mechanism of action of this admixture is to stabilize the passive layer of iron oxide on the steel and also increase the density of concrete, thus decreasing the permeability... 

Potassium formate and a few other carboxylic acid salts are one result of the breakdown of the DEA molecule. These salts are benign at low concentrations. However, when at concentrations of 5% or more, they interfere with operations by altering the physical properties of the carbonate solution. Some amine degradation compounds are even considered to be corrosion accelerators. They may solubilize iron, keep it in solution and prevent it from forming the passivation layer on the pipe and equipment. UOP s ACT-1 activator is an amine that contains a more-stable molecule that is more resistant to degradation206. 

Corrosion and Passivity. The inhibition of dissolution is important m the corrosion of metals and building materials. Passivity is imparted to many metals by overlying oxides, the so-called passive films the inhibition of the resolution of these passive layers protects the underlying material. Figure 13 eves a schematic model of the hydrated passive film on iron. 

A further interesting feature of Figure 46 occurs at t = 38 s, where the tip potential was adjusted to 0.0 V, causing the cessation of Cl production. At this point, the substrate current also stopped fluctuating, providing further evidence for the importance of Cl in the breakdown of the passive layer of iron. Light microscopy of the substrate revealed a small (9 /xm X 14 /xm) pit on the iron surface where the tip had been located, which was similar in size to that predicted based on the charge passed in the current spikes on the substrate. 

This is explained by the formation of an oxide layer, the so-called passive layer, which forms on the metallic surface. However, the exact composition of this passive layer is not known and descriptions in the literature are very contradictory. Photoelectrochemical measurements [1] lead to the following model describing the formation of a passive layer on stainless steel and iron. 

The oxide layer on iron in a boron buffer solution mainly consists of Fe203. Fe " ions in the film act as misfits. They disturb bondings within the film and lead to dangling, bonds. These misfits strongly influence the electronic behavior of the semiconducting passive layer. 

As an alternative approach, gold particles have been deposited on iron electrodes in studies of passive films on iron [470]. A layered structure was found. The inner layer is composed mostly of Fe304 the outer layer contains Fe(III) species and may also contain y-Fc203. 

Several experimental results support the adsorption mechanism for stationary conditions of the passive layer. Even the stationary passive current density depends on the composition of the electrolyte. For iron in 0.5 M H2SO4, the passive current density is 7 pA cm , whereas less than lpAcm is detected in 1 M HCIO4. From these observations, a catalysis for the transfer of Fe + from the passive layer to the electrolyte by S04 ions was concluded [55, 56]. Similarly, the dissolution Ni + from passive nickel and nickel base alloys is accelerated by organic acids hke formic acid and leads to a removal of NiO from the passive layer [57]. Additions of citrate to the electrolyte cause the thinning of passive layers on stainless steel and increase its Cr content [58]. Apparently Fe and Ni ions are complexed at the surface of the passive film, which causes an enhancement of their dissolution into the electrolyte. It should be mentioned that the dissolution of Cr " " apparently is not catalyzed by these anions and remains... 

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