Anodic dissolution mechanisms

Competitive Reactivity Studies of Anodic Dissolution Mechanisms... 

T. Tsum, Anodic dissolution mechanisms of metals and alloys. Mater. Sci. Eng. A 146A (1991). 

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

May was the first to stress the important role pf CujClj within the pits on the mechanism, and he considered that it acted as a screen that prevented dissolved oxygen gaining access to the bottom of the pit thus preventing the formation of a protective CujO film the low solubility of CU2CI2 also maintained the activity of copper ions at a low value and thus facilitated anodic dissolution of the copper. 

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

Corrosion (from Latin corrodere, gnaw to pieces ) of metals is the spontaneous chemical (oxidative) destruction of metals under the elfect of their environment. Most often it follows an electrochemical mechanism, where anodic dissolution (oxidation) of the metal and cathodic reduction of an oxidizing agent occur as coupled reactions. Sometimes a chemical mechanism is observed. 

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

Mandich, N.V. and Vyazovikina, N.V., Kinetics and mechanisms of the chromium anodic dissolution in the chromium plating solution in the transpassive range, 47th Meeting of International Society for Electrochemistry, Veszprem, Hungary, September 1996. 

Volkov S., Tumanova N., Kochetova S., Buryak N. The polysurface Mechanism of Ta and Ti Anodic Dissolution in Low-Temperature Carbamide-NELtCl Melt. Z. Naturforsh,... 

The same two-step mechanism of metal dissolution has also been delivered for the anodic dissolution of nickel in acid solutions [Sato-Okamoto, 1964]. The mechanistic concepts for iron dissolution other than the two-step mechanism have also been presented in the literature [Heusler, 1958], 6ind the mechanism of metal dissolution is still a subject of research [Plonski, 1996]. 

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Corrosion can be controlled by Isolation of the metal from the corrosive environment by suppression of the anodic dissolution of metal and by suppression of the corresponding cathodic reaction. Isolation of corrosion prone metals from corrosive environments is probably the most general mechanism of the corrosion protection afforded by paint films, sealers, and similar polymer-based materials. Effective isolation requires that polymeric materials have good barrier properties and remain adherent in the presence of water and the products of metallic corrosion. Barrier properties and adhesion aspects of corrosion control are discussed in detail in subsequent sections. 

The anodic dissolution of zinc [233, 264] was investigated in solution H3BO3 + NH4CI + Na2S04 at pH 4.4 using electrochemical impedance spectroscopy and EQCM. In the same solution, zinc anodic dissolution of different galvanized steel sheets of zinc [265] was studied. The postulated [41, 266] mechanism of Zn oxidation in acidic solution corresponds to two consecutive monoelectronic transfers. 

The mechanism of anodic dissolution of the intermetallic compound CdSb was studied in concentrated KOH solutions [215]. 

The role of chloride ions in the anodic dissolution of Au(lll) in perchloric acid solutions has been studied in Ref. 54. The mechanism of anodic dissolution was discussed in relation to the structure of the chloride adlayer on the Au(lll) electrode surface. 

The mechanism of anodic dissolution of silver in cyanide solutions has been studied by Bek and coworkers [378-380]. For example, using [379] the rotating disc electrode and pulse potentiostatic method, it has been found that the limiting step involved the formation, at the electrode surface, of the adsorbed complex with two... 

There is no stable entity Al2+(aq) to compare with Fe2+(aq) consequently, the mechanism that causes rust to be nonprotective because of migration of Fe2+(aq) through the water before precipitation as FeO(OH) does not apply to aluminum, on which Al(OH)3 or AIO(OH) forms, at once, on the anodic site. Conversely, removal of the protective aluminum oxide film cannot occur by the reductive dissolution mechanism described for iron. 

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. 

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