Passivating oxide potential

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

At high temperature, siUcon carbide exhibits either active or passive oxidation behavior depending on the ambient oxygen potential (65,66). When the partial pressure of oxygen is high, passive oxidation occurs and a protective layer of Si02 is formed on the surface. 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

The process of formation of a passivating oxide film is an anodic one the driving force for its formation is raised by raising the potential anodically... 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

Tie has good resistance to sulfuric acid.l l A passivating oxide layer is formed up to a potential of 1.8 V at which point corrosion becomes severe. TiC is also very resistant to sea water, neutral industrial waste waters, and human sweat. Cr7C3 is even more corrosion resistant and is used extensively as a passivation interlayer. 

Pourbaix diagrams illustrate graphically the dominant solution or precipitate species of a component or components as a function of pH and oxidation potential (Jj. They are particularly useful for defining the conditions for selective precipitation or solution in hydrometallurgical extraction (2j and for passivation of metals. However, they are tedious to produce manually, especially when a number of components are present. 

Aqueous electrolytes of high pH etch silicon even at open circuit potential (OCP) conditions. The etch rate can be enhanced or decreased by application of anodic or cathodic potentials respectively, as discussed in Section 4.5. The use of electrolytes of high pH in electrochemical applications is limited and mainly in the field of etch-stop techniques. At low pH silicon is quite inert because under anodic potentials a thin passivating oxide film is formed. This oxide film can only be dissolved if HF is present. The dissolution rate of bulk Si in HF at OCP, however, is negligible and an anodic bias is required for dissolution. These special properties of HF account for its prominent position among all electrolytes for silicon. Because most of the electrochemistry reported in the following chapters refers to HF electrolytes, they will be discussed in detail. 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

In the range of potential of the passive state the passive oxide film is in the state of band edge level pinning at the film/solution interface hence, the potential A( )h across the film/solution interface remains constant irrespective of the electrode potential of the passive metal. With increasing anodic polarization and in the... 

Fig. 11-13. Anodic polarization curve of a metallic nickel electrode in a sulfuric add solution transpassivation arises at a potential relatively dose to the flat band potential because of p-type nature of the passive oxide film. [From Sato, 1982.]...

A1 is thermodynamically unstable, with an oxidation potential at 1.39 V. Its stability in various applications comes from the formation of a native passivation film, which is composed of AI2O3 or oxyhydroxide and hydroxide.This protective layer, with a thickness of 50 nm, not only stabilizes A1 in various nonaqueous electrolytes at high potentials but also renders the A1 surface coating-friendly by enabling excellent adhesion of the electrode materials. It has been reported that with the native film intact A1 could maintain anodic stability up to 5.0 V even in Lilm-based electrolytes. Similar stability has also been observed with A1 pretreated at 480 °C in air, which remains corrosion-free in LiC104/EC/ DME up to 4.2 However, since mechanical... 

In addition to the above thermodynamic consideration, kinetics also play an important role in determining the anodic stability of these salts. For example, some salts whose decomposition products are polymeric moieties were found to passivate the electrode surface effectively." Therefore, although the intrinsic oxidation potentials for these anions were not as high ( 4.0 V), they showed stability up to 4.50 V in subsequent scans. It should be cautioned here, though, as the passivation was only observed on an inert electrode surface, whether similar passivations would occur on an actual cathode surface... 

The third aspect to consider is the electrochemical stability of the material used. For the oxygen reduction reaction, the electrode potential is highly anodic and at this potential, most metals dissolve actively in acid media or form passive oxide films that will Inhibit this reaction. The oxide forming metals can form non-conducting or semi-conducting oxide films of variable thickness. In alkaline solutions, the range of metals that can be used is broader and can include non-precious or semi-precious metals (Ni, Ag). 

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