Passive films concept

The concepts and basic approach used in studies of electrical fluctuations in corrosion processes proved to be very successful as well in mechanistic studies of electrode reactions taking place at materials covered by passivating films. A typical example is the electrochemical dissolution of silicon. From an analysis of the noise characteristics of this process, it has been possible to identify many features as well as the conductivity of the nanostructures of porous silicon being formed on the original silicon surface. 

Bonhoeffer, Vetter, and others (63) have made extensive studies on iron which indicate that the passive film is composed of one or more oxides of iron. Young (64). Vermilyea (65) and Johansen et at, (57) have shown that the Mott-Cabrera concepts are applicable for the thin films on Ta, Ti, Hf, and Hb. Petrocelli (58) has shown evidence that the dissolution of aluminum In sulfuric acid takes place through a thin film and that the process appears to follow the Motr-Cabret a theory. Stern (66) reports data indicating that the kinetic. for the anodic oxidation of stainless steel are similar to those for aluminum apd tantalum (67). Pryor (68) has recently reviewed the work on passive films on iron and suggests a single passive film of y contains non-uniform defect concentra-... 

If the passive film cannot be reestablished and active corrosion occurs, a potential drop is established in the occluded region equal to IR where R is the electrical resistance of the electrolyte and any salt film in the restricted region. The IR drop lowers the electrochemical potential at the metal interface in the pit relative to that of the passivated surface. Fluctuations in corrosion current and corrosion potential (electrochemical noise) prior to stable pit initiation indicates that critical local conditions determine whether a flaw in the film will propagate as a pit or repassivate. For stable pit propagation, conditions must be established at the local environment/metal interface that prevents passive film formation. That is, the potential at the metal interface must be forced lower than the passivating potential for the metal in the environment within the pit. Mechanisms of pit initiation and propagation based on these concepts are developed in more detail in the following section. 

This system permits brief descriptions of some key concepts encountered in corrosion phenomena electrode potentials, exchange currents, mixed potentials, corrosion potentials, passive films, as well as leading to thermodynamic descriptions of systems.- ... 

Adherents to this theory have different opinions on the potential at which the film forms. Its thickness, the mechanism of formation, and, most Important, the "cause of passivity. In the earlier theories It was postulated that the passivation follows the formation of a "primary layer" of small conductivity, x lth porous character, which Is sometimes due to precipitation of metal salt on and near the electrode.(32) In the pores the current Increases, and by polarization at an "Umschlagspotentlal" (Vj, = V, Figure 1) an actual passive layer is formed. Thus the essential concept of the passivation process Is connected with the change of the properties (chemical or physical) of the primary film at a certain potential. The passive film Is free from pores and presents a barrier between the metal and the environment. It is electronically conductive and slowly corrodes In solution.(6,8,24,37)... 

The selection made covers the first efficient and stable system based on the ternary chalcopyrite CulnSe2, an electrochemical treatment to avoid a toxic etching step in solid-state CIS device fabrication, the first stable and efficient liquid-junction solar cell (InP), and a novel concept where nanoemitters, interspersed in a nanoporous passivating film, are used to scavenge excess minority carriers. 

Anodic protection was developed using the principles of electrode kinetics and is difficult to understand without introducing advanced concepts of electrochemical theory. Briefly, anodic protection is controlled by the formation of protective passive film on metals and alloys using an externally applied potential. Anodic protection is used to a lesser degree because of the limitations on metal-environment systems for which anodic protection is viable. In addition, it is possible to accelerate corrosion if proper controls are not implemented during anodic protection. 

In modem technology an increasing number of nonmetallic materials, such as semiconductors, oxides, ionic crystals, and polymers, is employed, which corrode or degrade via chemical rather than electrochemical mechanisms. Corrosion protection of these materials by inhibitors is currently only marginally studied and will be an important future challenge for inhibitor science. For the important case of oxides, similar concepts as employed for the stabilization of passive films in the inhibition of localized corrosion should be applicable. 

The rate of this process in aprotic electrolytes is rather high the exchange current density is fractions to several mA/cm. As pointed out already, the first contact of metallic lithium with electrolyte results in practically the instantaneous formation of a passive film on its surface conventionally denoted as solid electrolyte interphase (SEI). The SEI concept was formulated yet in 1979 and this film still forms the subject of intensive research. The SEI composition and structure depend on the composition of electrolyte, prehistory of the lithium electrode (presence of a passive film formed on it even before contact with electrode), time of contact between lithium and electrolyte. On the whole, SEI consists of the products of reduction of the components of electrolyte. In lithium thionyl chloride cells, the major part of SEI consists of lithium chloride. In cells with organic electrolyte, SEI represents a heterogeneous (mosaic) composition of polymer and salt components lithium carbonates and alkyl carbonates. It is essential that SEI features conductivity by lithium ions, that is, it is solid electrolyte. The SEI thickness is several to tens of nanometers and its composition is often nonuniform a relatively thin compact primary film consisting of mineral material is directly adjacent to the lithium surface and a thicker loose secondary film containing organic components is turned to electrolyte. It is the ohmic resistance of SEI that often determines polarization of the lithium electrode. 

Easier diffusion of B produces features such as a smoother dissolution front, internal vacancy clusters (polyatomic voids), and islands of A-type atoms hindered from dissolution. Qualitatively similar conclusions are drawn on 3D lattices except for the specific generation of pores with easier diffusion of B atoms as predicted [201 ] by a nonstochastic approach. This tendency to generate a tunneling attack at the cost of only surface diffusion could be considered as a likely explanation of pit nucleation at the atomic level, with no need for the concept of passive film breakdown. 

Corrosion is an irreversible surface modification of a material due to chemical reaction with the environment that results in the formation of metal ions dissolved in the liquid (material loss) and, in the case of passive metals, of surface oxide films. A preliminary attempt to include particle flow in tribocorrosion was already proposed by Stemp [11] and Mischler et al [9] to explain the discrepancy mentioned above between first body degradation and mechanical wear. This paper is aimed at developing a phenomenological model of tribocorrosion by combining electrochemical corrosion effects with the third body concept of wear. The approach is applied to three electrochemically controlled wear situations, i.e. wear under cathodic protection (absence of corrosion), wear in presence of passive films and wear combined with metal dissolution. The proposed concepts are compared to already published results concerning carbon steel and stainless steels and their merits are discussed. 

The development of acidity within an occluded cell is by no means a new concept, and it was used by Hoar s as early as 1947 in his Acid Theory of Pitting to explain the pitting of passive metals in solutions containing Cl ions. According to Hoar the Cl ions migrate to the anodic sites and the metal ions at these sites hydrolyse with the formation of HCl, a strong acid that inhibits the formation of a protective film of oxide or hydroxide. Edeleanu and Evans followed the pH changes when aluminium was made anodic in Cl solutions and found that the pH decreased from 8-8 to 5-3. 

These facts are different demonstrations of the same event degradation reactions occur simultaneously with electropolymerization.49-59 These reactions had also been called overoxidation in the literature. The concept is well established in polymer science and consists of those reactions between the pristine polymer and the ambient that promote a deterioration of the original polymeric properties. The electrochemical consequence of a strong degradation is a passivation of the film through a decrease in the electrical conductivity that allows a lower current flow at the same potential than the pristine and nondegraded polymer film did. Passivation is also a well-established concept in the electrochemistry of oxide films or electropainting. 

Other Models. In addition to Besenhard s model, the other models were mainly modifications developed from the original Peled s concept for lithium electrode passivation, with surface reaction as the major process, and emphasis was placed upon the composition and structure of the precipitated film or the interaction between the precipitated products and the bulk electrolyte components. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

While these six generalizations are not all encompassing, in that exceptions may exist, they are sufQcient to differentiate between various theories that have been proposed for the growth of barrier oxide layers on metals and alloys. A number of models that have been developed to describe the growth of anodic oxide films on metals are listed in Table 4.4.2, together with some of their important features and predictions. Of the models listed, which were chosen because they make analytical predictions that can be tested and because they introduced new concepts into the theory of passivity, only the point defect model (PDM) in its latest form (D. Macdonald [1999], Pensado-Rodriguez et al. [1999a,b]) accounts for all of the observations summarized above. 

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