Anodic oxides passivation

A corrosion-inhibiting admixture is a chemical compound which, when added in small concentrations to concrete or mortar, effectively checks or retards corrosion. These admixtures can be grouped into three broad classes, anodic, cathodic and mixed, depending on whether they interfere with the corrosion reaction preferentially at the anodic or cathodic sites or whether both are involved [48]. Six types of mechanisms, viz. anodic (oxidizing passivators), anodic (non-oxidizing passivators), cathodic, precipitation... 

The MIS detector comprises a gate 308, a gate dielectric 306 of ZnS, an anodic oxide passivation 303, a layer 332 of Hgo.73Cdo.27Te and a layer 334 of Hgo.8Cdo 2Te. 

This review of STM studies of thin anodic oxide (passive) films formed on metals and alloys shows that important results have been obtained by direct imaging of the sur ce structure, providing direct evidence on (for example), the crystallinity of passive films and the nature of defects. The fully crystalline character of the film on Ni has been demonstrated by STM. The nature of defects (steps, kinks, vacancies, points of reduced thickness) has been elucidated. This is important for a better understanding of the breakdown of passive films. The unique protectiveness of fire film on Cr may be related to the observed structure with oxide nanocrystals cemented by a noncrystalline hydroxide. Many more results are expected to be produced, in the future, on the atomic structure of passive films, including the local interactions of impurities and anions with passive films and especially with surface defects, file local conductivity of passive films derived fi om I-V curves at specific sites, and chemical features derived fixim spectroscopic imaging. All these data should drastically improve our understanding of the relation between structure and properties of passive films. 

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

Films, anodic oxide Films, passivating Films, plastic Film theory Film wrappers Filter Filter aid Filter aids Filter fabrics Filtering centrifuges Filter media Filters... 

Oxidation and tarnishing active dissoiution in acids anodic oxidation and passivity chemicai and eiectrochemical polishing atmospheric and immersed corrosion in certain cases Crevice corrosion hiiform corrosion deposit attack bimetaliic corrosion intergranuiar corrosion weid decay... 

Formation of the first layer (a monolayer) of passivating oxide film on a denuded metal surface occurs very simply by the loss of protons from the adsorbed intermediate oxidation products, such intermediates being common to both dissolution and passivation processes . Thus for example, the first oxidative step in the anodic oxidation of nickel is the formation of the unstable adsorbed intermediate NiOH by... 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

CV of solutions of lithium bis[ salicy-lato(2-)]borate in PC shows mainly the same oxidation behavior as with lithium bis[2,2 biphenyldiolato(2-)-0,0 ] borate, i.e., electrode (stainless steel or Au) passivation. The anodic oxidation limit is the highest of all borates investigated by us so far, namely 4.5 V versus Li. However, in contrast to lithium bis[2,2 -biphenyl-diolato(2-)-0,0 Jborate based solutions, lithium deposition and dissolution without previous protective film formation by oxidation of the anion is not possible, as the anion itself is probably reduced at potentials of 620-670 mV versus Li, where a... 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

In the first group the titrant is generated either directly from a participating or active electrode, or indirectly from an inert or passive electrode, in which case it is necessary to add previously an auxiliary substance that generates the titrant by either cathodic reduction or anodic oxidation the end-point detection is usually potentiometric or amperometric. The following selected examples are illustrative of the first group in non-aqueous media ... 

If the electrode is covered with a film, then anodic oxidation of the metal does not involve facetting. The surface of the metal either becomes more rough (if the film is discontinuous) or becomes very lustrous (continuous films). Films, especially continuous films, retard the electrode reaction of metal oxidation. The metal is said to be in its passive state. 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Mechanism 3 involves NiOH in at least three reactions, and Ni(OH)2 as the active Ni reactant in solution. Since increasing the concentration of the complex-ant(s) in solution will reduce the concentration of both unhydrolyzed and hydrolyzed metal ions, arguments of complexation cannot be readily employed to either support or discount this mechanism. However, it has been this author s experience in formulating electroless Co-P solutions with various complexants for Co2+ that improper complexation which results in even a faint precipitate of hydrolyzed cobalt ions yields an inactive electroless Co-P solution. Furthermore, anodic oxidation of hypo-phosphite at Ni anodes does not proceed at a significant rate under conditions where the surface is most probably covered with a passive film of nickel oxide [48], e.g. NiO.H20, which would be expected to oxidize the reducing agent via a cyclic redox mechanism. 

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. 

Under anodic potentials in acidic electrolytes free of fluoride, silicon is passivated by formation of an anodic oxide under comsumption of four holes (h+), according to the reaction ... 

At higher anodic potentials an anodic oxide is formed on silicon electrode surfaces. This leads to a tetravalent electrochemical dissolution scheme in HF and to passivation in alkaline electrolytes. The hydroxyl ion is assumed to be the active species in the oxidation reaction [Drl]. The applied potential enables OH to diffuse through the oxide film to the interface and to establish an Si-O-Si bridge under consumption of two holes, according to Fig. 4.4, steps 1 and 2. Details of anodic oxide formation processes are discussed in Chapter 5. This oxide film passivates the Si electrode in aqueous solutions that are free of HF. 

In alkaline electrolytes, in contrast, silicon is readily dissolved at OCP. Under cathodic conditions the dissolution becomes reduced, while under low anodic potentials the dissolution rate is enhanced compared to the OCP rate. If, however, the anodic potential becomes larger than the PP, the silicon electrode is passivated due to a thin anodic oxide film and the dissolution rate becomes negligible. The current density needed to keep the electrode in the passive state corresponds to the dissolution rate of the anodic oxide in the alkaline electrolyte used and is usually very low. 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

Anodic oxidation of phenols gave the corresponding poly(1,4-phenyleneoxide)s by selecting the electrolysis conditions to prevent passivation of the electrode. 

A special problem can be the passivation of the electrode surface by insulating layers, for example, formation of oxides on metals at a too high anodic potential or precipitation of polymers in aprotic solvents from olefinic or aromatic compounds by anodic oxidation. As a result, the effective surface and the activity of the... 

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