Metallic dissolution products

Figure 9.13 Metallic dissolution products released from a polished Co-Cr-Mo alloy after 550 h in 0.17 M NaCl+2.7xlO m EDTA solution at 37°C. uncoated TiN coated. (Wisbey et al, 1987.)...

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

Because electrons are neither products nor reactants in chemical reactions, the two processes are interdependent and neither can occur alone. The zinc metal dissolution must furnish electrons for the copper metal plating. The reaction of zinc and copper sulfate solution is a spontaneous reaction involving a transfer of electrons, i.e., is a spontaneous redox process. The spontaneity of the reaction is commonly explained by saying that zinc loses electrons more readily than copper or, alternatively, that Cu2+ ions gain electrons more readily than Zn2+ ions. 

The electrode is generally, though not necessarily, a good conductor, being the source or sink of electrons and the site of reaction, a reactant in the case of metal dissolution, a product in metal deposition, etc. It must be borne in mind that, in electrode reactions, one of the reactants is the electron, the concentration and energetics of which may be controlled through the electrode—electrolyte interfacial electrical potential difference. 

It must not be imagined that the ultimate product of the metal-dissolution reaction is always an ionic species, e.g., M —> M,i+ + ne. Often it is a solid oxide or hydroxide. 

Since the metal-dissolution current is equal to the product of the corresponding current density [Pg.142]

Chromium passivates very effectively down to very negative potentials even in strongly acidic electrolytes (Fig. 5). The cathodic current density of hydrogen evolution is followed by a small potential range of E = —0.4 to O V of anodic metal dissolution where Cr dissolves as Cr2+. At E > 0 V Cr passivates with a drop of the current density to less than 0.1 pA cm 2. In this potential range Cr3+ is the corrosion product. RRD studies have been applied to determine quantitatively the formation of Cr3+ ions. In principle the dissolution of Cr3+ at a Cr disc may be studied with two concentric analytical rings with their reduction to Cr2+ at the inner ring and its... 

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

Fig. 7. Cross-sectional view of a pit. A, metal dissolution reaction, e.g. A1 -> Al3t + 3e, acid chlorides form, which produce hydrochloric acid and aluminium hydroxide on hydrolysis B, 2 Hh +2e- H2 C, porous corrosion product restricting oxygen access D, passive layer on the metal surface and E, inclusion acting as a local cathode.

The transport steps may be controlling either in bringing reactant material to the reaction site (the metal-solution interface) from either phase, or in removing products from the site into the liquid phase,. Accepting the fact that metal reactions in conducting solutions are electrochemical in nature, it follows that control may reside in transport with respect to the cathodic process, or with respect to the anodic process, or with respect to both simultaneously a much less likely possibility. In metal dissolution reactions, the steps can be described still less equivocally for the first case, the transfer of reducible species from the solution to the electrode is involved for the second case, the removal of oxidized species from the electrode is involved. In the latter instance complications are usually caused by the formation of solid reaction products. 

The theories proposed to explain the formation of passivation film are salt-film mechanism and acceptor mechanism [21]. In the salt-film mechanism, the assumption is that during the active dissolution regime, the concentration of metal ions (in this case, copper) in solution exceeds the solubility limit and this results in the precipitation of a salt film on the surface of copper. The formation of the salt film drives the reaction forward, where copper ions diffuse through the salt film into electrolyte solution and the removal rate is determined by the transport rate of ions away from the surface. As the salt-film thickness increases, the removal rate decreases. In the acceptor mechanism, it is assumed that the metal-ion products remain adsorbed onto the electrode surface until they are complexed by an acceptor species like water or anions. The rate-limiting step is therefore the mass transfer of the acceptor to the surface. Recent studies confirmed that water may act as an acceptor species for dissolving copper ions [22]. 

On the other hand, the existence of a gap between the TE and the WP reduces the accuracy of TE reproduction. The distribution of metal dissolution rate over the machining surface is determined by the distribution of the electric field in the complex-shaped interelectrode space, which is filled with an electrolyte with varying conductivity. The electrolytes cause corrosion of equipment. In some cases, the TE is complex and expensive. A special TE has to be developed for each product therefore, the ECM processes are best suited to large-lot production applications. 

The metal dissolution rate in the ECM varies over a wide range, but frequently it is within 0.1-1.0 mm min-1. The impediments to the ECM productivity are frequently associated not with the electrode reactions, but with the processes in the interelectrode space. In the design of the ECM processes, especially for... 

The development of methods for calculating the distribution of local rates of metal dissolution over the machining surface and, consequently, the distribution of gap width. The methods of ECM modeling, which enable one to determine the WP shape after the machining with known TE or, conversely, to determine the TE shape and the machining parameters for the production of the required WP, are discussed in Sect. 12.5. 

The application of air-electrolyte mixtures as a working medium for ECM enables one to raise the localization of metal dissolution in places with the smallest gaps and, thus, to enhance the accuracy of electrochemical reproduction of TE on the WP. To achieve the highest efficiency of this method, several conditions should be fulfilled. The air-electrolyte mixture should be formed in the immediate vicinity of the gap inlet and the flow rate should be adequately high. A ratio between gas and liquid amounts from 3.0 to 3.5 is considered to be most preferable. Rigid stabilization of all the process parameters is required. The design and size of all parts of the mixing device and the values of inlet and outlet pressures are important. To avoid a considerable decline in productivity, the main metal stock should be removed in the air-free... 

In electrochemical grinding, the mechanical removal of both the passive anodic film and the metal proceeds concurrently with the anodic dissolution. In this method, normally electrolytes, in which the metal dissolution is localized only on the areas of abrasive depassivation, are used. This enhances the machining accuracy in relation to the ECM. As compared with mechanical grinding, the combined method is characterized by a significantly lower tool wear and a high productivity. 

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