Intermediates anodic dissolution

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

If the anodic potential is increased, the current density becomes larger than JPS and dissolution occurs via an intermediate anodic oxide film. Hence the reaction can be separated into electrochemical oxide formation according to reaction (4.2) and chemical dissolution of the oxide due to HF, (HF)2 or HF2 [So2] ... 

Such a transfer reaction of metallic ions in a series connection of two elementary steps involving an intermediate of an adsorbed metallic ion complex may be illustrated by the anodic dissolution of metallic iron in acidic solution [Despic, 1983] as shown in Eqn. 9-21 ... 

Fig. 9-6. Adaorption coverage of a reaction intermediate of hydroxo-complezes in anodic dissolution of a metallic iron electrode as a function of electrode potential in acidic sulfate solutions at pH 1.0, 2.0 and 3.0 solution is 0.5 M (Ns2S04 + H2SO4) at room temperature. Oqh adsorption coverage of reaction intermediates FeOH,4 and PeOH Vss = volt referred to the saturated silver-silver chloride electrode. [Prom Tsuru, 1991.]...

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

However, there is a strong likelihood of a soluble intermediate in the formation of CdfOHty Cadmium has an appreciable solubility in alkaline solutions 2 x 10-/ mol/L in 8 M potassium hydroxide at room temperature. In general, it is believed that the solution process consists of anodic dissolution of cadmium ions in the form of complex hydroxides. 

Several conclusions may be drawn from the results discussed in this section. Firstly, it appears that in almost all cases studied, the stabilization reaction involves decomposition intermediates instead of free holes. We will not comment on this point here (for a discussion, see ref. [52]). Similarly, we will not enlarge on the observation that in certain cases, Xj and in other cases X2 intermediates are involved, as these problems are beyond the scope of the present paper, which essentially pertains to anodic dissolution and etching. As far as this subject is concerned, two important points emerge, i. e., the fact that, due to the interconnection between stabilization and dissolution, the latter reaction tends to dominate at sufficiently high current densities, and the fact that, depending on the semiconductor and on the circumstances, dissolution either occurs by the DH or by the DX mechanism. In what follows, independent information on the latter point will be gathered, and the factors which determine the dissolution mechanism will be investigated. 

It has been suggested [38] that quaternary ammonium amalgams are intermediates in the hydrodimerization of AN in an acetonitrile-water-C02 medium other explanations [36,37,40], however, have also been considered. A layer of quaternary ammonium (Q ) salts could first be produced by anodic dissolution of a sufficient amount of Q" " from the amalgam. 

Electroless deposition of Au in KAu(CN)2 -I- HF can be controlled by both the kinetic process and the diffusion process. The deposition is a two-step process, with initial diffusion-limited deposition of the intermediate species, followed by surface-limited reduction of this species. For electroless deposition of Pt, it has been reported that the rate-determining step is the deposition on n-Si, whereas it is the dissolution of silicon on p-Si. Electroless copper deposition does not occur on Si02-covered silicon surface due to the lack of anodic dissolution of silicon In a non-HF solution, the deposition of copper on a bare silicon surface results in the formation of oxide aroimd the metal particles. In HF solutions, the deposition of copper proceeds very slowly in the dark on both p-Si and n-Si samples due to the lack of carriers. The... 

Reaction (6.6) represents creation of an intermediate by a hole, which in the case of n-Si is provided by the reduction of Cr ". Reactions (6.7) and (6.8) represent two possible reaction routes for the intermediate Si, one by electron injection into the conduction band and the other by a chemical reaction with hydrogen ions. The injection of electrons into the conduction band accounts for the anodic current on n-Si in the dark. The reaction with hydrogen ions accounts for the hydrogen evolution during the anodic dissolution of -Si. 

Fig. 8.2 Formation of intermediate states (surface states) during the anodic dissolution of germanium...

We see in literature [79,80] that the anodic dissolution of iron in acidic solution is likely to occur through a series of steps involving adsorbed intermediates ... 

It is suggested that the anodic dissolution will be inhibited if the adsorbed anion and the reaction intermediate are stable and hardly dissolve in aqueous solution. On the contrary, if the reaction intermediate is relatively unstable and readily dissolves into aqueous solution, the anion will function as an electrocatalyst accelerating the metal dissolution rate. It is now common knowledge that hydroxide ions, OH, catalyze the anodic dissolution of metallic iron and nickel in acid solution [81,82]. It is also known that chloride ions inhibit the anodic dissolution of iron in acidic solution [83]. No clear-cut understanding is however seen in literature on why hydroxide ions catalyze but chloride ions inhibit the anodic dissolution of iron, even though the two kinds of anions are in the same group of hard base. We assume that the hardness level in the Lewis base of adsorbed anions will be one of the most effective factors that determine the catalytic activity of the adsorbates. Further clarification on the catalytic characteristics will require a quantum chemical approach to the adsorption of these anions on the metal surface. 

Fig. 45. Coverage of surface by the adsorbed intermediate as a function of potential in the anodic dissolution of iron. Adsorption follows a Langmuir isotherm at low coverages (—) and a Tempkin isotherm at higher coverages. The data are as calculated by Tsuru et al. [129].

Cathodic polarization of /7-type InP single crystals in 1N H2SO4 leads to phosphane and metallic indium [31] according to InP + 3H+ + 3e" PH3 + In. In IN KOH or IN KF, formation of PH3 and indium is only observed at current densities 10 A/cm. Anodic dissolution of p-InP in KOH solution yields PH3, probably by disproportionation of the intermediately formed H3PO3. Phosphane also forms in acidic (1N H2SO4,1N NH4F HF, 1N HF) or neutral electrolytes (0.8N KF, IN NH4F) [31]. 

Inadequate surface preparation of titanium before coating can result in surface oxides of Ti with the O content approaching two. Also, if the anode potential is high, the oxide films on the Ti can break down, leading to the anodic dissolution of Ti. It is essential to ensure that the intermediate layer containing mixed oxides of Ti and Ru is conductive. This can be done by proper thermal treatment of the coating. Otherwise, the anode potential will be high from the start. 

Evidence exists that platinum dissolves in both acid and alkaline elec-trolytes at potentials of —1.0 V vs. RHE. Thus the anodic dissolution of Pt may be the steady state complementary reaction. The dissolution may proceed through the direct dissolution of the platinum or through a Pt-OH or Pt-O intermediate. 

The Nature of the Intermediate Surface Species in Anodic Dissolution... 

It is now generally accepted that anodic dissolution involves the existence of intermediate surfece bonds between the metallic state and the solution species. The nature and kinetic behavior of these entities are inferred from classical transient techniques relating the time or frequency response of the current, or of the potential, to the relaxation of fiieir surface concentrations. 

The detailed anodic dissolution process of Mg involves intermediate steps. It is well known that one electron transfer is much easier than two electron transfers in an electrochemical reaction. The above reaction is more likely to realize through an intermediate step involving mono-valence Mg " ... 

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