Passive film structure

Since the initiation of pitting is the localized penetration of the passive film, understanding of this step requires information on the structure of passive films and the mechanisms whereby they can be destroyed locally. Understanding of either of these is complicated by the thinness of the films and the question of the passive film structure when formed by and existing in the aqueous environment as compared with its structure when removed from this environment. The latter is necessary for the use of most of the surface analysis techniques applicable to structure evaluation. As a consequence, specific conclusions as to the structure are frequently inferred rather than more directly established. 

Residual radioactivity accounts for 3 x 10 Cr atoms/cm (1.5 x 10 eq or 0.015 C passive-film substance/cm ). The equation assumes an adsorbed passive-film structure, but the same reasoning applies whatever the structure. 

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

Primers for protective coatings may be divided into three broad classes based on the mechanism of substrate protection barrier primers that function by preventing the ingress of moisture and electrolytes, primers that protect the substrate galvanically in the presence of electrolytes, and primers that contain electrochemical inhibitors to passivate the substrate. Each of these approaches requires a distinct primer film structure due to the different mechanisms of protection. 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

Jin and Atrens (1987) have elucidated the structure of the passive film formed on stainless steels during immersion in 0.1 M NaCl solution for various immersion times, employing XPS and ion etching techniques. The measured spectra consist of composite peaks produced by electrons of slightly different energy if the element is in several different chemical states. Peak deconvolution (which is a non-trivial problem) has to be conducted, and these authors used a manual procedure based on the actual individual peaks shapes and peak positions as recorded by Wagner et al. (1978). The procedure is illustrated in Figure 2.8 for iron. 

The corrosion of stainless steel in 0.1 mol-1 NaCl solutions at open circuit potential was studied in detail by Bruesch et al. [106] using XPS in combination with a controlled sample transfer system [38]. It was verified by XPS analysis that the passivating film contains chromium oxide. The position and the height of the Cr concentration maximum depends critically on the bulk chromium content of the steel. Significant variations in the electrode passivation properties were observed at a Cr concentration of 12%, while the film behaviour was found to be rather independent of the other components like Mo, Ni, Cu. From the fact that the film structures and... 

Especially in conjunction with the detection of water or OH species, ex situ XPS measurements have been critizised because of possible changes occurring during transfer and exposure of the sample to UHV. Kuroda et al. have demonstrated that structural changes of the passive film indeed occur when electron diffraction studies are performed in a hydrated and subsequently in a dehydrated environment. Structural changes, however, do not necessarily cause changes in elemental composition as determined by XPS. 

The study of passive films on electrode surfaces is an area of great fundamental and practical relevance. Despite decades of intensive investigations, there still exists a great deal of controversy as to the exact structural nature of passive films, especially when they are formed in the presence or absence of glass-forming additives such as chromium. 

These results point to the importance of hydration effects on the structure of passive films on iron. However, these results were obtained ex situ and therefore are subject to some uncertainty. 

The Fourier transforms for both films are again quite similar, but as for the ex situ measurements, the chromate-passivated films appear to have a more glassy structure. It should be mentioned that these studies employed a rather limited data range which makes spectral differentiation difficult. 

From these studies, they concluded that the passive film had an Fe—O coordination with six near neighbors at a distance of 2 0.1 A. Although a higher signal-to-noise ratio is required to refine the structure, the approach followed by these authors appears most appropriate since they were able to reduce the deposited films to the metallic state and subsequently oxidize them. It would be most interesting to ascertain how the structure of the passive film varies through sequential reduction/passivation cycles. 

The nature of the passive film has been the object of innumerable further studies with arguments over its thickness, composition, structure and electronic properties raging. What has become very clear is that that removal from solution and drying of the passivated electrode alters the film profoundly and any studies of the film ex situ must be treated with considerable caution. For that reason we will concentrate here primarily on in situ studies or at the least those studies carried out on passive films that retain their hydration. 

The composition of the film changes with potential and with the incorporation of both anions and cations. EXAFS data on the passive films grown on stainless steel or normal steel with Cr04 show considerable incorporation of Cr into the film with further alterations in bond lengths and covalency parameters. Indeed, as the amount of Cr incorporated increases, so does the flexibility of the structure. It is well known, by contrast, that Cl incorporation leads to poorer quality films and to enhanced rates of corrosion. 

The passive film is composed of metal oxides which can be semiconductors or insulators. Then, the electron levels in the passive film are characterized by the conduction and valence bands. Here, we need to examine whether the band model can apply to a thin passive oxide film whose thickness is in the range of nanometers. The passive film has a two-dimensional periodic lattice structure on... 

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