Formation of Anodic Oxides

Anodization systems of various electrolyte compositions and polarization conditions have been explored as can be seen in Table 3.1. They are characterized by the yield (thickness per volt), current efficiency (CE ionic current responsible for oxide growth), and maximum voltage, (highest attainable anodic voltage). Several general remarks may be made based on the data in Table 3.1. (1) Ahigh field is required for the growth of anodic film, on the order of 10 -IO V/cm. (2) Ionic current efficiency 

TABLE 3.1. Anodization of Silicon under Different Polarization Conditions in Various Electrolytes 

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X-ray photoelectron spectroscopy (XPS) of electrodes was first applied to the oxidation of noble metal electrodes. Kim and Winograd investigated in 1971 the electrochemical formation of anodic oxides on Pt [10] and later on Au electrodes [60]. The electrochemical parameters of oxide formation on these noble metal electrodes were well characterized and enabled a direct correlation between ex situ XPS and in situ electrochemical analysis. 

The second current maximum J3 corresponds to an oxide thickness at which tunneling of charge carriers becomes negligible, as shown in Fig. 4.7. At the bias corresponding to J3 the formation of anodic oxides in electrolyte-free HF shows a change of growth kinetics, as shown in Fig. 5.2. 

Parhutik VP, Makushok IE, Kudriavtsev E, Sokol VA, Khodan AN (1987) An X-ray electronic study of the formation of anodic oxide films on aluminium in nitric acid. Electrochemistry (Elektrokhymia) 23 1538-1544 Kundu M, Khosravi AA, Kulkami SK (1997) Synthesis and study of organically capped ultra small clusters of cadmium sulphide. J Mater Sci 32 245-258... 

There are few data on the effect of substrate on the formation of anodic oxide. In 2M KOH, the formation characteristics of anodic oxides on (100) and (111) surfaces in the potential range from 6 to 15V are the same. Ion implantation of substrate silicon has been found to greatly affect the anodization behavior of silicon. An increase in the anodization rate occurs after implantation of C, N", P, As", Ar, ... 

The EV range is obviously affected by the level of selfdischarge. The latter is mainly due to the reduction of Pb02 by lead of the grid and to the formation of anodic oxidation products (e.g., of Sb) which diffuse to the negative plate. Their deposition decreases the hydrogen overpotential and results in corrosion of... 

Nigam, R.K., and Chaudhary, R.S. (1970) Steady state kinetics of formation of anodic oxide films on tantalum in molten electrolytes of LiNOs - NaNOs - KNO3, Ind. J. Chemistry 8, 343-346... 

Strehblow H.-H., Mellior-Smith C.M. and Augustyniak WM. (1978b), Examination of aluminium-copper films during the galvanostatic formation of anodic oxide , J. Electrochem. Soc., 125, 915-19. 

Figure 21. Decay of carrent density during potetiostatic oxidation at 0.80 V vs. Ag/AgCl/Sat. KCl in pH 8.4 borate solution containing Fe at 0.0, 0.25, and 0.50 mM. Reprint from T. Ohtsuka and H. Yamada, Effect of Ferrous Ion in Solution on the Formation of Anodic Oxide Film on Iron , Corrosion Sci., 40 (1998) 1131, Copyright 1998 with permission from Elsevier Science.

Under cyclic or repeated stress conditions, rupture of protective oxide films that prevent corrosion takes place at a greater rate than that at which new protec tive films can be formed. Such a situation frequently resiilts in formation of anodic areas at the points of rupture these produce pits that serve as stress-concentration points for the origin or cracks that cause ultimate failure. 

A consequence of ion migration is electrolytic blister formation. In the case of anodic blisters the coated surface shows pitting, whereas in the case of cathodic blisters there is no change in the metal surface or there is merely the formation of thin oxide layers with annealing color. 

Figure 18. Dependence of activation barrier A f for the nucleation of a thin oxide film on the metal surface as a function of electrode potential. Ey is the equilibrium potential of anodic oxide formation.7 The solid line represents the value of A against and the dotted line corresponds to the critical potential for the film formation. AE = 0.2 V, Cd= -1Fm-2, am = 0.411 m 2, a -0.01 J m-2,...

The mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain. Several groups have reported that the voltammetry of Au in acidic electrolytes is straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966 Gibbs et al., 1977 Kita et al., 1985 Sun et al., 1999]. There is, however, no voltammetric evidence for the adsorption of CO on the Au surface, and spectroscopic studies indicate only a weak interaction of CO with poly crystalline Au surfaces in acidic solutions [Kunimatsu et al., 1986 Cuesta et al., 2003]. Moreover, there is little evidence for the formation of oxidizing species at the potential where the oxidation process is observed. Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly... 

Raja KS, Misra M, Paramguru K (2005) Formation of self-ordered nano-tubular structure of anodic oxide layer of titanium. Electrochim Acta 51 154—165... 

The true role of incorporation of anions in the formation of anodic alumina is being intensively discussed. Baker and Pearson183 have considered the anion effect in modifying the structure of anodic oxides to be due to the coordinative ability of anions to replace alumina tetrahedra in the body of the oxides. Dorsey184,185 has postulated that in porous oxides, anions stabilize the network of alumina tetrahedra and octahedra. 

Specific structural features are observed in the formation of composite oxides. Kobayashi, Shimizu, and their co-workers have, in a series of papers, reported studies of the structure of barrier alumina films, anodically formed on aluminum covered by a thin (5 nm) layer of thermal oxide.198,199 Their experiments have shown that the thermally oxidized thin layer generally contains y- alumina crystals of about 0.2 nm size. This layer does not have a pronounced effect on ionic transport in the oxide during anodization. Also, islands of y -alumina are formed around the middle of anodic barrier oxides. They are nucleated and developed from tiny crystals of y -Al203 and grow rapidly in the lateral direction under prolonged anodization.198,199... 

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

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