Passivity anodic oxidation, alkaline solutions

In alkaline solutions, the effective dissolution valence at OCP, as shown in Table 5.1, is zero and changes only very slightly with anodic polarization before the passivation peak it is less than 0.4 at V. The dissolution reaction below the passivation potential is almost completely chemical. ° ° The dissolution valence in the passive region in alkaline solutions, which is not found in the literature, is likely close to 4 since the growth of anodic oxide films should be identical to that in HF solutions (see Chapter 3). 

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. 

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... 

Etching of silicon in alkaline solutions occurs under evolution of hydrogen with a ratio of two molecules H2 per dissolved Si atom. This ratio is found to be reduced under positive bias [Pa6] or by addition of oxidizing agents like H202 [Sc6], If the anodic bias is increased beyond the passivation potential (PP), the dissolution rate is reduced by orders of magnitude. 

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. 

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. 

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). 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

In nonalkaline and nonfluoride aqueous solutions, silicon substrates behave as essentially inert electrodes due to the presence of a thin oxide film. Even in alkaline solutions, silicon is passivated by an oxide film at anodic potentials beyond the passivation peak. Very small current can pass through the passivated silicon surface of n- or p-type materials in the dark or under illumination. Depending on the pH of the electrolyte, oxidized surface sites Si—OH are more or less ionized into anionic species Si—0 owing to the acido-basic properties of such radicals so that the passivation current can vary in a wide range from a few... 

The passivating layer formed on Cd electrode in alkaline solution in the presence of Na2S was studied voltammetri-cally [323]. At low Na2S concentrations, CdO, Cd(OH)2, and CdS layer were produced during the anodic oxidation of the Cd electrode. At higher Na2 S content, a few monolayers of thick CdS film were formed. 

Another process of physical protection is the formation of an oxide layer that makes the metal passive. This procedure is used for aluminium. Aluminium is normally anodized in 10 per cent sulphuric acid with steel or copper cathodes until an oxide thickness of 10-100 pm is obtained. As the more superficial part of the oxide layer has a fairly open structure it is possible to deposit metals (cobalt, nickel, etc.) or organic pigments in the pores and seal with boiling water or with an alkaline solution. The colours after metallic deposition are due to interference effects. Chromic and oxalic acids are also used significantly as electrolyte. 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

Exception to this oxide theory, however, has been taken by Muller and Konigsberger,5 on the ground that the reflecting power of iron, rendered passive anodically when immersed in alkaline solution, remains undimmed, whereas if a layer of oxide were formed an alteration would be expected. But it is not necessary to postulate the formation of a thick layer of oxide. If of merely molecular dimensions, it would still preserve the underlying metal from attack, whilst a thickness comparable with that of the length of a light wave would be necessary to affect the reflecting power.6... 

The passivating effect is attributed to the formation of an insoluble oxide film on the silicon. The chemical attack by the hot alkaline solution on the silicon does not resume immediately after the anodic bias is removed. Several minutes are required before the passive layer breaks down and chemical attack resumes. A brief cathodic treatment, however, reactivates the silicon immediately. 

Molybdenum and tungsten are rendered passive more readily in acid than in alkaline solution this is the reverse of the behavior exhibited by chromium and the iron-group metals. Although oxidizing agents generally favor passivity, such is not the case with a tin anode in this instance, too, chloride ions do not have the inhibiting effect they have in other cases. It is apparent, therefore, that each metal requires its own specific conditions in order that it may be rendered passive. 

Anodic behavior of sihcon can best be characterized by i-V curves. Neglecting the details associated with a silicon substrate such as doping, the current-potential relationship of silicon in aqueous solutions can be considered to be principally determined by the pH and HE concentration as illustrated in Pig. 5.1. In nonalkaline and nonfluoride aqueous solutions, silicon as an electrode is essentially inert, showing a very small current at anodic potential due to the presence of a thin oxide film. In alkaline solutions, silicon is also passivated by an oxide film at anodic potentials but is active below the passivation potential, Vp. In fluoride solutions, the silicon electrode is active in the whole anodic region as shown by the large anodic current. 

The anodic behavior of silicon electrodes in alkaline solutions is characterized by the formation of an oxide film and passivation of the surface. Typical i-V curves of n- and p-type silicon in KOH solution are shovra in Fig. 5.9. Silicon surface is passivated at an overpotential marked by a small current peak. The potential at the current peak is called the passivation potential, Vp, and in the systems shown in Fig. 5.9 it is -0.92 and -0.71 V ce for n- and p-type silicon, respectively. The small current at... 

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