Type of Passivating Film

TYPES OF PASSIVATING FILMS ON COPPER SURFACE UNDER OXDIZING CONDITIONS... 

Correlation of the observed onset of Wagner s passivity on alloys like Ni-Cu, Nl-Zn-Cu, and Cu-Ni-Al to the occupancy of the d levels of the alloys is given in support of the theory. According to the theory, the same type of passive film (l.e., M-O-O ) is formed in solutions, interposing a stable barrier between metal and electrolyte, displacing adsorbed H2O and increasing the activation energy for the hydration and dissolution of the metal lattice. Such films... 

The type of passive film formed is important, as the breakdown of a passive film results in the onset of crevice corrosion. 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

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... 

One of the methods used in corrosion control is anodic inhibition. The method applies in particular to iron and its steels. The electrode is moved in the anodic direction (at first stimulating the corrosion rate), but soon an oxide film forms and reduces the dissolution current. There are certain types of oxide film, passive films, that are particularly protective. Indeed, such films are involved in the way metals preserve themselves in nature. There is much to be found out about these films (why they are so protective) and some of the material that allows us to understand them and their eventual breakdown by aggressive ions such as chloride, has been given in this chapter. 

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... 

Fig. 7.10 (a) Partially perforated passive film on pit in type 304 stainless steel, (b) Fragment of passive film over edge of pit. 0.4 M FeCI3. 

Other Types of Passive Aluminum Oxide Films, Anof, Modified Aluminum, Alloys If aluminum is polarized to higher potentials, breakdown of the film can occur. Optical observation shows small sparks on the surface indicating local... 

The oxide film is often the first line of defense a material has against the corrosive influences of its environment. A passive oxide film forms upon oxygen adsorption from the environment. These films can be beneficial, serving to slow or block metal dissolution and further corrosion. This effect is called oxide passivation. On the other hand, metals may continue to oxidize/corrode, even after an oxide film has formed or this film may break down leading to active corrosion. Because of these drastically different outcomes, an understanding of how these films are formed under different environmental conditions is needed to understand how to mitigate corrosion. The focus of this chapter is the use of quantum mechanics simulations to understand the thermodynamics of passive film formation. In particular, we look at calculations of first-principles phase diagrams as a function of environmental conditions and how these studies fit with experimental data. We focus on Pd, Mg, and Pt metal systems as they are well studied, represent different types of oxide film formation, and fall within the authors areas of expertise. 

The diagnostic features of this analysis have been used by Chao et al. [1982] in their investigation of the growth of passive films on nickel and Type 304 stainless steel in borate and phosphate buffer solutions. Typical complex plane and... 

Figure 3-7. Schematic representation of the bands of an n-type semiconducting passive film a) at the flat band potential, and b) under anodic polarization with respect to the flat band potential. As the passive film is ultra-thin and the density of states is low, the band bending extends over the whole thickness of the passive film.

Figure 3-8. Schematic representation of the principle of photoelectrochemical measurements for an n-type semiconducting passive film under anodic polarization. The photon illumination forms electron and hole pairs whose charge can be consumed in the oxidation of redox couples at the surface of the film, giving rise to a photocurrent.

A typical example of the application of EIS is the investigation of passive films on Zn, Zn-Co, and Zn-Ni (Fig. 7-18), which were carried out to explain the difference in the corrosion behavior of pure and low-alloyed zinc by the possible formation of electron traps through the incorporation of cobalt or nickel into the oxide film (Vilche et al., 1989). Passive films of zinc in alkaline solutions are known to be n-type semiconductors with a band gap Eg = 3.2 eV (Vilche et al., 1989). The n-type character arises from an excess of zinc atoms in the nonstoichiometric oxide. The impedance measurements in 1 N NaOH solution were carried out at potentials at which Faraday reactions like transpassive dissolution and oxygen evolution do not interfere. The passive layer was formed for 2 h at positive potential before the potential was swept in the negative direction for the impedance meas-... 

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. 

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